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The straight dope on cholesterol – Part
I
I’ve been planning to write at length about this topic for a few
months, but I’ve been hesitant to do so for several reasons:
To discuss it properly requires great care and attention (mine
and yours, respectively).
1.
My own education on this topic only really began about 9
months ago, and I’m still learning from my mentors at a
geometric pace.
2.
This topic can’t be covered in one post, even a Peter-Attia-
who-can’t-seem-to-say-anything-under-2,000-word post.
3.
I feel a bit like an imposter writing about lipidology because my
mentors on this topic (below) have already addressed this topic
so well, I’m not sure I have anything to add.
4.
But here’s the thing. I am absolutely – perhaps pathologically –
obsessed with lipidology, the science and study of lipids.
Furthermore, I’m getting countless questions from you on this
topic. Hence, despite my reservations above, I’m going to give this
a shot.
A few thoughts before we begin.
I’m not even going to attempt to cover this topic entirely in this
post, so please hold off on asking questions beyond the
scope of this post.
1.
Please resist the urge to send me your cholesterol2.
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numbers. I get about 30 such requests per day, and I cannot
practice medicine over the internet. By all means, share your
story with me and others, but understand that I can’t really
comment other than to say what I pretty much say to everyone:
standard cholesterol testing (including VAP) is largely irrelevant
and you should have a lipoprotein analysis using NMR
spectroscopy (if you don’t know what I mean by this, that’s ok…
you will soon).
This topic bears an upsettingly parallel reality to that of nutrition
“science” in that virtually all health care providers have no
understanding of it and seem to only reiterate conventional
wisdom (e.g., “LDL is bad,” “HDL is good”). We’ll be blowing
the doors off this fallacious logic.
3.
By the end of this series, should you choose to internalize this
content (and pick up a few homework assignments along the way),
you will understand the field of lipidology and advanced lipid testing
better than 95% of physicians in the United States. I am not being
hyperbolic.
One last thing before jumping in: Everything I have learned and
continue to learn on this topic has been patiently taught to me by
the words and writings of my mentors on this subject: Dr. Tom
Dayspring, Dr. Tara Dall, Dr. Bill Cromwell, and Dr. James Otvos. I
am eternally in their debt and see it as my duty to pass this
information on to everyone interested.
Are you ready to start an exciting journey?
Concept #1 – What is cholesterol?
Cholesterol is a 27-carbon molecule shown in the figure below.
Each line in this figure represents a bond between two carbon
atoms. Sorry, I’ve got to get it out there. That’s it. Mystery over.
All this talk about “cholesterol” and most people don’t actually know
what it is. So there you have it. Cholesterol is “just” another
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organic molecule in our body.
I need to make one important distinction that will be very important
later. Cholesterol, a steroid alcohol, can be “free” or “unesterified”
(“UC” as we say, which stands for unesterified cholesterol)
which is its active form, or it can exist in its “esterified” or storage
form which we call a cholesterol ester (“CE”). The diagram above
shows a free (i.e., UC) molecule of cholesterol. An esterified
variant (i.e., CE) would have an “attachment” where the arrow is
pointing to the hydroxyl group on carbon #3, aptly named the
“esterification site.”
Since cholesterol can only be produced by organisms in the Animal
Kingdom it is termed a zoosterol. In a subsequent post I will write
about a cousin of cholesterol called phytosterol, but at this time I
think the introduction would only confuse matters. So, if you have a
question about phytosterols, please hang on.
Concept #2 – What is the relationship between the cholesterol
we eat and the cholesterol in our body?
We ingest (i.e., take in) cholesterol in many of the foods we eat and
our body produces (“synthesizes”) cholesterol de novo from various
precursors. About 25% of our daily “intake” of cholesterol –
roughly 300 to 500 mg — comes from what we eat (called
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exogenous cholesterol), and the remaining 75% of our “intake” of
cholesterol — roughly 800 to 1,200 mg – is made by our body
(called endogenous production). To put these amounts in context,
consider that total body stores of cholesterol are about 30 to 40 gm
(i.e., 30,000 to 40,000 mg) and most of this resides within our cell
membranes. Every cell in the body can produce cholesterol and
thus very few cells actually require a delivery of cholesterol.
Cholesterol is required by all cell membranes and to produce
steroid hormones and bile acids.
Of this “made” or “synthesized” cholesterol, our liver synthesizes
about 20% of it and the remaining 80% is synthesized by other cells
in our bodies. The synthesis of cholesterol is a complex four-step
process (with 37 individual steps) that I will not cover here (though I
will revisit), but I want to point out how tightly regulated this process
is, with multiple feedback loops. In other words, the body works
very hard (and very “smart”) to ensure cellular cholesterol levels are
within a pretty narrow band (the overall process is called
cholesterol homeostasis). Excess cellular cholesterol will crystalize
and cause cellular apoptosis (programmed cell death). Plasma
cholesterol levels (which is what clinicians measure with standard
cholesterol tests) often have little to do with cellular cholesterol,
especially artery cholesterol, which is what we really care about.
For example, when cholesterol intake is decreased, the body will
synthesize more cholesterol and/or absorb (i.e., recycle) more
cholesterol from our gut. The way our body absorbs cholesterol is
so amazing, so I want to spend a bit of time discussing it.
In medical school, whenever we had to study physiology or
pathology I always had a tendency to want to anthropomorphize
everything. It’s just how my brain works, I guess, and understanding
cholesterol absorption is a great example of this sort of thinking.
The figure below shows a cross-section of a cell in our small
intestine (i.e., our “gut”) called an enterocyte that governs how stuff
in our gut actually gets absorbed. The left side with the fuzzy
border is the side facing the “lumen” (the inside of the “tube” that
makes up our gut). You’ll notice two circles on that side of the cell,
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a blue one and a pink one.
[What follows is a bit more technical than I would have liked, but I
think it’s very important to understand how this process of
cholesterol absorption works. It’s certainly worth reading this a few
times to make sure it sinks in.]
The blue circle represents something called a Niemann-Pick
C1-like 1 protein (NPC1L1). It sits at the apical surface of
enterocytes and it promotes active influx (i.e., bringing in) of gut
luminal unesterified cholesterol (UC) as well as unesterified
phytosterols into the enterocyte. Think of this NPC1L1 as the
ticket-taker at the door of the bar (where the enterocyte is the
“bar”); he lets most cholesterol (“people”) in. However, NPC1L1
cannot distinguish between cholesterol (“good people”) and
phytosterol (“bad people” – I will discuss these guys later, so no
need to worry about it now) or even too much cholesterol (“too
many people”). [I can’t take any credit for this
anthropomorphization – this is how Tom Dayspring explained it
to me!]
The pink circle represents an adenosine triphosphate (ATP)-
binding cassette (ABC) transporters ABCG5 and ABCG8.
This complex promotes active efflux (i.e., kicking out) of
unesterified sterols (cholesterol and plant sterols – of which over
40 exist) from enterocytes back into the intestinal lumen for
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excretion. Think of ABCG5,G8 as the bouncer at the bar; he
gets rid of the really bad people (e.g., phytosterols as they serve
no purpose in humans) you don’t want in the bar who snuck
past the ticket-taker (NPC1L1). Of course in cases of
hyperabsorption (i.e., in cases where the gut absorbs too much
of a good thing) they can also efflux out un-needed cholesterol.
Along this analogy, once too many “good people” get in the bar,
fire laws are violated and some have to go. The enterocyte has
“sterol-excess sensors” (a nuclear transcription factor called
LXR) that do the monitoring and these sensors activate the
genes that regulate NPC1L1 and ABCG5,G8).
There is another nuance to this, which is where the CE versus UC
distinction comes in:
Only free or unesterified cholesterol (UC) can be absorbed
through gut enterocytes. In other words, cholesterol esters
(CE) cannot be absorbed because of the bulky side chains they
carry.
Much (> 50%) of the cholesterol we ingest from food is esterified
(CE), hence we don’t actually absorb much, if any,
exogenous cholesterol (i.e., cholesterol in food). CE can be
de-esterified by pancreatic lipases and esterolases – enzymes
that break off the side branches and render CE back to UC —
so some ingested CE can be converted to UC.
Furthermore, most of the unesterified cholesterol (UC) in our gut
(on the order of about 85%) is actually of endogenous origin
(meaning it was synthesized in bodily cells and returned to the
liver), which ends up in the gut via biliary secretion and
ultimately gets re-absorbed by the gut enterocyte. The liver is
only able to efflux (send out via bile into the gut) UC, but not CE,
from hepatocytes (liver cells) to the biliary system. Liver CE
cannot be excreted into bile. So, if the liver is going to excrete
CE into bile and ultimately the gut, it needs to de-esterify it using
enzymes called cholesterol esterolases which can convert liver
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CE to UC.
Also realize that the number one way for the liver to rid itself of
cholesterol is to convert the cholesterol into a bile acid, efflux
that to the bile (via a transporter called ABCB11) and excrete
the bile acids in the stool (typically most bile acids are
reabsorbed at the ileum).
Concept #3 – Is cholesterol bad?
One of the biggest misconceptions out there (maybe second only to
the idea that eating fat makes you fat) is that cholesterol is “bad.”
This could not be further from the truth. Cholesterol is very
good! In fact, there are (fortunately rare) genetic disorders in which
people cannot properly synthesize cholesterol. Once such disease
is Smith-Lemli-Opitz syndrome (also called “SLOS,” or
7-dehydrocholesterol reductase deficiency) which is a metabolic
and congenital disorder leading to a number of problems including
autism, mental retardation, lack of muscle, and many others.
Cholesterol is absolutely vital for our existence. Let me repeat:
Cholesterol is absolutely vital for our existence. Every cell in
our body is surrounded by a membrane. These membranes are
largely responsible for fluidity and permeability, which essentially
control how a cell moves, how it interacts with other cells, and how
it transports “important” things in and out. Cholesterol is one of the
main building blocks used to make cell membranes (in particular,
the ever-important “lipid bilayer” of the cell membrane).
Beyond cholesterol’s role in allowing cells to even exist, it also
serves an important role in the synthesis of vitamins and steroid
hormones, including sex hormones and bile acids. Make sure you
take a look at the picture of steroid hormones synthesis and
compare it to that of cholesterol (above). If this comparison doesn’t
convince you of the vital importance of cholesterol, nothing I say
will.
One of the unfortunate results of the eternal need to simplify
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everything is that we (i.e., the medical establishment) have done
the public a disservice by failing to communicate that there is no
such thing as “bad” cholesterol or “good” cholesterol. All
cholesterol is good!
The only “bad” outcome is when cholesterol ends up inside of the
wall of an artery, most famously the inside of a coronary artery or
a carotid artery, AND leads to an inflammatory cascade which
results in the obstruction of that artery (make sure you check out
the pictures in the links, above). When one measures cholesterol in
the blood – we really do not know the final destination of those
cholesterol molecules!
And that’s where we’ll pick it up next time – how does “good”
cholesterol end up in places it doesn’t belong and cause “bad”
problems? If anyone is looking for a little extra understanding on
this topic, please, please, please check out my absolute favorite
reference for all of my cholesterol needs, LecturePad. It’s designed
primarily for physicians, but I suspect many of you out there will find
it helpful, if not now, certainly once we’re done with this series.
To summarize this somewhat complex issue
Cholesterol is “just” another fancy organic molecule in our body,
but with an interesting distinction: we eat it, we make it, we
store it, and we excrete it – all in different amounts.
1.
The pool of cholesterol in our body is essential for life. No
cholesterol = no life.
2.
Cholesterol exists in 2 forms – UC and CE – and the form
determines if we can absorb it or not, or store it or not
(among other things).
3.
Most of the cholesterol we eat is not absorbed and is excreted
by our gut (i.e., leaves our body in stool). The reason is it not
only has to be de-esterified, but it competes for absorption with
the vastly larger amounts of UC supplied by the biliary route.
4.
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Re-absorption of the cholesterol we synthesize in our body is
the dominant source of the cholesterol in our body. That is,
most of the cholesterol in our body was made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will discover that synthesis
and absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
up.
7.
(To Part II »)
25
APR
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The straight dope on cholesterol – Part
II
In this post I’m going to tackle the next set of logical (at least in my
mind) questions to follow up on last week’s post, Part I in this
series.
Last week we addressed these 3 concepts:
#1 — What is cholesterol?
#2 — What is the relationship between the cholesterol we eat
and the cholesterol in our body?
#3 — Is cholesterol bad?
This week we’ll address the following concept:
#4 — How does cholesterol move around our body?
I want to thank folks for doing their best to resist the following two
urges:
Please resist asking me questions beyond the scope of this
post. If it’s not in here, it will probably be in a subsequent post
in this series.
1.
Please resist sending me your cholesterol numbers. Share
your story with me and others, but understand that I can’t really
comment other than to say what I pretty much say to everyone:
standard cholesterol testing (including VAP) is of limited value
and you should have a lipoprotein analysis using NMR
spectroscopy (if you don’t know what I mean by this, that’s ok…
2.
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you will soon). I can’t practice medicine over the internet.
Remember last week’s take away messages:
Cholesterol is “just” another fancy organic molecule in our body
but with an interesting distinction: we eat it, we make it, we
store it, and we excrete it – all in different amounts.
1.
The pool of cholesterol in our body is essential for life. No
cholesterol = no life.
2.
Cholesterol exists in 2 forms – unesterified or “free” (UC) and
esterified (CE) – and the form determines if we can absorb it
or not, or store it or not (among other things).
3.
Much of the cholesterol we eat is in the form of CE. It is not
absorbed and is excreted by our gut (i.e., leaves our body in
stool). The reason this occurs is that CE not only has to be
de-esterified, but it competes for absorption with the vastly
larger amounts of UC supplied by the biliary route.
4.
Re-absorption of the cholesterol we synthesize in our body
(i.e., endogenous produced cholesterol) is the dominant source
of the cholesterol in our body. That is, most of the cholesterol
in our body was made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will discover that synthesis
and absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
7.
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up. To see an important reference on this topic, please look
here.
Concept #4 – How does cholesterol move around our body?
To understand how cholesterol travels around our body requires
some understanding of the distinction between what is hydrophobic
and hydrophilic. A molecule is said to be hydrophobic (also called
nonpolar) if it repels water, while a molecule is said to be
hydrophilic (also called polar) if it attracts water. I could spend a lot
of time getting in to the nuances of these properties, but I think it’s
best to just focus on the major issues. Think of your veins, arteries,
and capillaries as the “waterways” or rivers of your body.
BONUS concept: Another important concept is that cell
membranes are lipid bilayers (which are hydrophobic) as I wrote
about last week. Hence, a hydrophilic substance cannot pass
through lipid membranes. Substances that can pass through lipid
membranes are said to be lipophilic. A substance that has both
polar (hydrophilic) and nonpolar (hydrophobic) properties is called
amphipathic. The fact that unesterified cholesterol (UC) is an
amphipathic molecule is a crucial property for its location in cell
membranes. CE in which the –OH group has been replaced by a
long chain fatty acid is a very nonpolar or hydrophobic molecule.
If a molecule needs to travel from your gastrointestinal tract (A) to,
say, a cell in your quadriceps muscle (B) it needs to get on the river
and travel from point A to point B. Because blood is effectively
water, (the “water” part of blood is called plasma, an aqueous
solution with a bunch of “stuff” in it (e.g., red blood cells, white blood
cells, other proteins, ions) there are two ways to move down the
river – swim or hitch a ride on a boat.
If a molecule is hydrophilic, it can be transported in our bloodstream
without any assistance – sort of like swimming freely in the river –
because it is not repelled by water. Conversely, if a molecule is
hydrophobic, it must have a “transporter” to move about the river
because the plasma (water) wants to repel it. I know this seems
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like a strange concept, but if you think about it, you’ve already seen
great examples in your day-to-day life:
Sugar and salt will easily dissolve in water. They are, therefore,
hydrophilic. Oil does not dissolve in water. It is, therefore,
hydrophobic.
By extension, a molecule of glucose (sugar) or sodium and chloride
ions (salt), because of their chemical properties which I won’t detail
here, will travel through plasma without assistance. A lipid will not.
All of this is a long way of saying that sterol lipids (of which
cholesterol ester is the predominant form in plasma), because they
are hydrophobic, need to be carried around our bloodstream.
They can’t move from one place to the next without a protein
transporting molecule.
In other words, cholesterol doesn’t exist in our bloodstream
without something to carry it from point A to point B.
So what are these “transporting molecules” called?
The proteins that traffic collections of lipids are called apoproteins.
Once bound to lipids they are called apolipoproteins, and the
protein wrapped “vehicle” that transports the lipids are called
lipoproteins. Many of you have probably heard this term before,
but I’d like to ensure everyone really understands their important
features. A crucial concept is that, for the most part, lipids go
nowhere in the human body unless they are a passenger inside a
protein wrapped vehicle called a lipoprotein. As their name
suggests lipoproteins are part lipid and part protein. They are
mostly spherical structures which are held together by a
phospholipid membrane (which, of course, contains free
cholesterol). The figure below shows a schematic of a lipoprotein.
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You will also notice variable-sized proteins on the surface of the
lipid membrane that holds the structure together. The most
important of these proteins are called apolipoproteins, as I alluded
to above. The apolipoproteins on the surface of lipoprotein
molecules serve several purposes including:
Assisting in the structural integrity and solubility of the
lipoprotein;
1.
Serving as co-factors in enzymatic reactions;2.
Acting as ligands (i.e., structures that help with binding) for
situations when the lipoprotein needs to interact with a receptor
on a cell.
3.
Apolipoproteins come in different shapes and sizes which
determine their “class.” Without getting into the details of protein
structure and folding, let me focus on two important classes:
apolipoprotein A-I and apolipoprotein B. Apoprotein A-I
(abbreviated apoA-I), which is composed of alpha-helicies, form
lipoproteins which are higher in density. (The “A” class designation
stems from the fact that apoA’s migrate with alpha-proteins in an
electrophoretic field). Conversely, apoprotein B (abbreviated
apoB), which is predominantly composed of beta-pleated-sheets,
form lipoproteins which are lower in density. (The “B” class
designation stems from the fact that apoB’s migrate with
beta-proteins in an electrophoretic field.)
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Virtually all apoB in our body is found on low-density lipoprotein –
LDL, while most apoA-I in our body is found on high-density
lipoprotein – HDL. Going one step further, the main structural
apoprotein on the LDL is called apoB100 (though we often shorten
this to just “apoB”), and there is only one apoB molecule per
particle. It’s starting to come together now with “high” and “low”
density lipoproteins, isn’t it?
But there’s actually more to it.
Everything I just described above deals with the structure and
surface of the lipoprotein molecule – sort of the like the hull of the
ship. But, what about the cargo? Remember what started this
discussion. It’s all about transporting cholesterol (and lipids) which
can’t freely travel in the bloodstream. The “cargo” of these ships,
what they actually carry both on their surface [molecules of
cholesterol and phospholipids] and in their core [cholesteryl esters
(CE) and triglycerides (TG, or triacylglycerols)] is what we’ll now
turn our attention to.
The ratio of lipid-to-protein in the lipoprotein structure determines its
density – which is defined as mass per unit volume. Something
that has a high density is heavier for a given volume than
something with a low density. The table in this link (which I’ve also
included below) shows the relative density of the five main classes
of lipoproteins (from most dense to least dense) as they were
originally discovered using ultracentrifugation: high density
lipoprotein (HDL), low density lipoprotein (LDL), intermediate
density lipoprotein (IDL), very low density lipoprotein (VLDL), and
chylomicron.
Note the very subtle difference in density between the most and
least dense lipoprotein – about 10 or 15%. Conversely, note the
very large difference in diameter between each lipoprotein – as
much as 2 orders of magnitude. Later in this series, when we start
to talk about the volume of a lipoprotein particle, this difference will
be amplified 1,000 times (because volume is calculated to the third
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power of diameter).
Below is a figure I’ve borrowed graciously from one of Tom
Dayspring’s remarkable lectures which gives you a sense of the
diversity of each of these classes of lipoproteins as well as the
subclasses within each class. If this topic wasn’t confusing enough,
there are actually multiple nomenclatures for the HDL subparticles.
Originally, nomenclature was based on their buoyancy. Today
nomenclature is based on the following methods, dependent on the
technology used to measure them:
Particle separation using gradient gel electrophoretic
fractionation (deployed by Berkeley Heart Lab).
1.
Magnetic resonance assaying of lipid terminal methyl groups,
called Nuclear Magnetic Resonance, or NMR (deployed by
Liposcience).
2.
Two-dimensional gradient gel electrophoresis and apoA-I
staining (deployed by Boston Heart Lab).
3.
We’ll cover this later, but I want to point this out now to avoid
(unnecessary) confusion in the figure below, which uses the first
two of these.
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A few things probably jump out as you look at this figure:
ApoA-I lipoproteins (i.e., HDLs) are tiny compared to ApoB
lipoproteins (i.e., VLDL’s, IDL’s, and LDL’s) [this figure is not
actually to scale – the “real” difference is even more
pronounced.]
1.
As a general rule (with pathological exceptions), as particles
move from being larger to smaller, the relative content of
triglycerides (TG) goes down while the relative content of
protein goes up, hence the density change.
2.
Actual cholesterol mass is greatest in the LDL particle.3.
Each specific lipoprotein has a different core make up –
meaning the variable ratio of TG to cholesterol ester changes. A
particle of VLDL has 5 times more TG than CE whereas a
particle of LDL typically has 4 or more times more CE than TG
(i.e., ratio > 4:1), and an HDL has 90-95% CE and < 10% TG in
its core.
4.
The TG trafficking lipoproteins are chylomicrons from the
intestine and VLDLs from the liver.
5.
Deep breath. Anyone left wondering why this topic is NOT
covered in medical school? I think I can conservatively say
95% to 99% of physicians do not know what you have just
learned — not because they aren’t “smart,” but because this
topic is simply not covered in medical school, and the pace at
which the field is developing is too great for most doctors to
keep up with.
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Why is cholesterol concentration increasing and triglyceride
concentration decreasing as lipoproteins progress from larger
to smaller?
The liver exports VLDL which, after chylomicrons (used to get
triglycerides to muscles and adipocytes and cholesterol from the
gut to the liver) are the largest of the lipoprotein particles. VLDL
particles “give up” some of their triglycerides in the form of free fatty
acids and shrink as they also release surface phospholipids. Once
a certain size or buoyancy is reached it is called a “VLDL remnant”
and ultimately an IDL. Some (though not all) of the IDL particles
undergo continued lipolysis to reduce in size and become the
famous (or infamous) LDL particles. However, most of the IDL
particles are actually cleared by liver LDL receptors and do not
become LDL particles.
All along this process, the larger particles “shed” phospholipids and
fatty acids and thus become cholesterol-rich. It is the LDL particle
that is the ultimate delivery vehicle of cholesterol back to the liver in
a process now called “indirect reverse cholesterol transport.”
However, under certain circumstances the LDL will penetrate and
deliver its cholesterol load to the artery walls. THIS IS EXACTLY
WHAT WE DON’T WANT TO HAPPEN. (Sorry for the bold ALL
CAPS – I know some of you may have fallen asleep by now, but I
didn’t want anyone missing the punch line.) Because almost all
cells in the body de-novo synthesize all the cholesterol they need,
LDLs are not actually needed to deliver cholesterol to most cells.
The final important point I want to make about cholesterol transport
is that it goes BOTH ways. Lipoprotein particles carry triglycerides
and cholesterol from the gut and liver to the periphery (muscles
and adipocytes – fat cells) for energy, cellular maintenance, and
other functions like steroid creation (called “steroidogenic” purposes
– remember the figure last week showing a cholesterol molecule
and steroid molecule). Historically this process of returning
cholesterol to the liver was thought to be performed only by HDL’s
and has been termed reverse cholesterol transport, or RCT (you’ll
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need to subscribe — for free — to lecturepad.org to access this last
link, which is well worth the time).
This RCT concept is outdated as we now know LDL’s actually
perform the majority of RCT. While the HDL particle is a crucial part
of the immensely complex RCT pathway, a not-so-well-known fact
is that apoB lipoproteins (i.e., LDL’s and their brethren) carry most
of the cholesterol back to the liver. In other words, the “bad”
lipoprotein, LDL, does more of the cleaning up (i.e., taking
cholesterol back to the liver) than the “good” lipoprotein, HDL!
The problem, as we’ll discuss subsequently, is that LDL’s actually
do the bad stuff, too – they dump cholesterol into artery walls.
Let’s put this all together to summarize how cholesterol gets
around our body
Cholesterol and triglycerides are not soluble in plasma (i.e.,
they can’t dissolve in water) and are therefore said to be
hydrophobic.
1.
To be carried anywhere in our body, say from your liver to your
coronary artery, they need to be carried by a special protein-
wrapped transport vessel called a lipoprotein.
2.
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As these “ships” called lipoproteins leave the liver they undergo
a process of maturation where they shed much of their
triglyceride “cargo” in the form of free fatty acid, and doing so
makes them smaller and richer in cholesterol.
3.
Special proteins, apoproteins, play an important role in moving
lipoproteins around the body and facilitating their interactions
with other cells. The most important of these are the apoB
class, residing on VLDL, IDL, and LDL particles, and the apoA-I
class, residing on the HDL particles.
4.
Cholesterol transport occurs in both directions, towards the
periphery and back to the liver.
5.
The major function of the apoB-containing particles is to traffic
energy (triglycerides) to muscles and phospholipids to all
cells. Their cholesterol is trafficked back to the liver. The apoA-I
containing particles traffic cholesterol to steroidogenic tissues,
adipocytes (a storage organ for cholesterol ester) and
ultimately back to the liver, gut, or steroidogenic tissue.
6.
All lipoproteins are part of the human lipid transportation system
and work harmoniously together to efficiently traffic lipids. As
you are probably starting to appreciate, the trafficking pattern is
highly complex and the lipoproteins constantly exchange their
core and surface lipids. This is a big reason why measuring
how much cholesterol is within various lipoprotein species
will in many circumstances be so misleading, as we’ll
discuss subsequently in this series.
7.
This was a bit of a tough one, so let’s stop there. Next week we’ll
discuss how to actually measure cholesterol levels. In other words,
if you’re looking at the river, with all its floating ships carrying their
cargo, how do we measure the amount of cargo actually contained
within the ships? Furthermore, is this the most important thing to
be measuring? Ironically, it’s easier to measure the cargo in the
ships, but more important to know the number of ships in the river.
The straight dope on cholesterol – Part II
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But now I’m getting ahead of myself.
P.S. Happy Birthday Dad (now I’ll know if you’re reading my
blog!)
(To Part III »)
3
MAY
The straight dope on cholesterol – Part II
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The straight dope on cholesterol – Part
III
Previously, in Part I and Part II of this series, we addressed 4
concepts:
#1 — What is cholesterol?
#2 — What is the relationship between the cholesterol we eat and
the cholesterol in our body?
#3 — Is cholesterol bad?
#4 — How does cholesterol move around our body?
This week we’ll address the following concept:
#5 – How do we measure cholesterol?
Quick refresher on take-away points from previous posts,
should you need it
Cholesterol is “just” another fancy organic molecule in our body
but with an interesting distinction: we eat it, we make it, we
store it, and we excrete it – all in different amounts.
1.
The pool of cholesterol in our body is essential for life. No
cholesterol = no life.
2.
Cholesterol exists in 2 forms – unesterified or “free” (UC) and
esterified (CE) – and the form determines if we can absorb it
or not, or store it or not (among other things).
3.
The straight dope on cholesterol – Part III
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Much of the cholesterol we eat is in the form of CE. It is not
absorbed and is excreted by our gut (i.e., leaves our body in
stool). The reason this occurs is that CE not only has to be
de-esterified, but it competes for absorption with the vastly
larger amounts of UC supplied by the biliary route.
4.
Re-absorption of the cholesterol we synthesize in our body
(i.e., endogenous produced cholesterol) is the dominant source
of the cholesterol in our body. That is, most of the cholesterol
in our body is made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will see that synthesis and
absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
up. To see an important reference on this topic, please look
here.
7.
Cholesterol and triglycerides are not soluble in plasma (i.e.,
they can’t dissolve in water) and are therefore said to be
hydrophobic.
8.
To be carried anywhere in our body, say from your liver to your
coronary artery, they need to be carried by a special protein-
wrapped transport vessel called a lipoprotein.
9.
As these “ships” called lipoproteins leave the liver they undergo
a process of maturation where they shed much of their
triglyceride “cargo” in the form of free fatty acid, and doing so
10.
The straight dope on cholesterol – Part III
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makes them smaller and richer in cholesterol.
Special proteins, apoproteins, play an important role in moving
lipoproteins around the body and facilitating their interactions
with other cells. The most important of these are the apoB
class, residing on VLDL, IDL, and LDL particles, and the apoA-I
class, residing for the most part on the HDL particles.
11.
Cholesterol transport in plasma occurs in both directions,
from the liver and small intestine towards the periphery and
back to the liver and small intestine (the “gut”).
12.
The major function of the apoB-containing particles is to traffic
energy (triglycerides) to muscles and phospholipids to all
cells. Their cholesterol is trafficked back to the liver. The apoA-I
containing particles traffic cholesterol to steroidogenic tissues,
adipocytes (a storage organ for cholesterol ester) and
ultimately back to the liver, gut, or steroidogenic tissue.
13.
All lipoproteins are part of the human lipid transportation system
and work harmoniously together to efficiently traffic lipids. As
you are probably starting to appreciate, the trafficking pattern is
highly complex and the lipoproteins constantly exchange their
core and surface lipids. This is a big reason why measuring
how much cholesterol is within various lipoprotein species
will in many circumstances be so misleading, as we’ll
discuss subsequently in this series.
14.
Concept #5 – How do we measure cholesterol?
All this talk about cholesterol probably has some of you wondering
how one actually measures the stuff. Much of the raw content I’m
going to present here is actually material I’ve had to learn recently.
One of the best resources I’ve found on this topic is the text book
Contemporary Cardiology: Therapeutic Lipidology, in particular,
chapter 14 by Tom Dayspring and chapter 15 by Bill Cromwell and
Jim Otvos. Anyone aspiring to be a lipid savant like these three
pioneers probably ought to get a copy. The other book that tells
The straight dope on cholesterol – Part III
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this story well is The Cholesterol Wars: The Skeptics versus the
Preponderance of Evidence. For most folks, however, I’m hoping
this series is sufficient and I’ll do my best to get the important points
across.
As far back as the 1940’s scientists understood that cholesterol and
lipids could not simply travel freely within the bloodstream without
something to carry them and obscure their hydrophobicity, but it
certainly wasn’t clear what these carriers looked like.
The initial breakthrough came during the Second World War when
two researchers, E.J. Cohn and J.L. Oncley at Harvard developed a
complex and elaborate technique to fractionate (i.e., separate)
human serum (serum is blood, less the cells and clotting factors)
into two “classes” of lipoproteins: those with alpha mobility and
those with beta mobility. [“Alpha” versus “beta” mobility describes a
pattern of movement seen by different particles, relative to fluid,
under a uniform electric field, which is the essence of
electrophoresis.]
You’ll recall that LDL particles are also called “beta” particles and
HDL particles are also called “alpha” particles. Now you see why.
This work set the stage for subsequent work, by a physicist named
John Gofman, using the techniques of preparative and analytic
ultracentrifugation to fully classify the major classes of human
lipoproteins. The table below summarizes what was gleaned by
these experiments.
The straight dope on cholesterol – Part III
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Cool, huh? Well, sort of. While this was an enormous
breakthrough scientifically, it didn’t really have an inexpensive and
quick test that could be used clinically the way, say, one could
measure glucose levels or hemoglobin levels in patients routinely.
What became crucial with Gofman’s discovery is that lipoproteins
were now a recognized entity and they got their names according to
their buoyancy: very low density, intermediate density, low density
and high density.
There is more interesting history to this tale, but let’s fast-forward to
where we are today. When you go to your doctor to have your
cholesterol levels checked, what do they actually do?
Let’s start at the finish line. What do they report? The figure below
is a representative result. It reports serum cholesterol (in total),
serum triglycerides, HDL cholesterol (i.e., HDL-C), LDL cholesterol
(i.e., LDL-C) and sometimes non-HDL-C (i.e., LDL-C + VLDL-C).
But where do these numbers come from?
Blood is drawn into a tube called a serum separator tube (SST) and
immediately spun in centrifuge to separate the blood from “whole
blood” into serum (normally clear yellow, top) and blood cells (dark
red, bottom). A gel film, from the SST, separates the serum and
blood cells, as shown below. The tube is kept cool and sent from
the phlebotomy lab to the processing lab.
The straight dope on cholesterol – Part III
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As early as the 1950’s scientists figured out clever chemical tricks
to directly measure the content of total cholesterol in the serum.
The chemical details probably are not interesting to non-chemists,
but I was able to find a great paper from 1961 that details the
methodology. The point is this: initially it was only possible to
measure the total content of cholesterol (TC), or concentration to
be technically correct, in plasma. By that I mean it is the total mass
(weight of all the cholesterol molecules) of cholesterol trafficked
within all of the lipoprotein species that exist in a specified unit of
volume: in the United States, we measure this in milligram of
cholesterol per deciliter of plasma abbreviated as mg/dL, or in the
rest of the world as mmol/Liter or mmol/L. Why? Think back to our
analogy from last week:
Cholesterol is a passenger on a ship — the “ship,” of course, being
a lipoprotein particle. The early methods of measuring cholesterol
had to break apart the hull of the ship to quantify the cargo. The
assays to do so, like the one described above, were pretty harsh. If
you had a bunch of LDL ships, HDL ships, VLDL ships, and IDL
ships, these assays ripped them all apart and told you the sum total
of the cargo. Obviously this was a great breakthrough in the day,
but it was limited. From this assay, one could conclude, for
example, that a person had 200 mg/dL of cholesterol hiding out in
all their lipoprotein particles.
Good to know, but what next? It turns out there were two other
important factors that could be measured directly in blood:
The straight dope on cholesterol – Part III
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triglycerides and the cholesterol content within the HDL
particle, HDL-C. Early on laboratories could easily separate
apoA-I-containing particles (i.e., HDL) from the apoB-containing
particles (i.e., VLDLs, IDLs and LDLs), but they could not easily and
economically separate the various apoB-containing particles from
one another. A full description of these methods is not necessary
to appreciate this discussion, but for those interested,
methodologies can be found here (TG) and here (HDL-C).
Important digression for context
What becomes critical to understand for our subsequent
discussions is that the apoB particles have the potential to deliver
cholesterol into an artery wall (the problem we’re trying to avoid),
and 90-95% of the apoB particles are LDL particles. Hence, it is
LDL particle number (LDL-P or apoB) that drives the
particles into the artery wall. Thus, physicians need to be able
to quantify the number of LDL particles present in a given
individual. For decades there was no way of doing that. Then
LDL-C (read on) became available and it served as a way (not
entirely accurate, but nonetheless a way) of quantitating LDL
particles.
Back to the story
How can one figure out the concentration of cholesterol in the LDL
particle? As you may recall from last week, LDL is the “ship” that
carries the most cholesterol cargo. More importantly, as I
mentioned above, it is also the key ship that traffics cholesterol
directly into the artery wall. Thus, there has always been an
enormous interest in knowing how much cholesterol is trafficked
within LDL particles.
For a long time it was not possible to directly measure LDL-C, the
cholesterol content of an LDL particle. However, we did know the
following had to be true:
The straight dope on cholesterol – Part III
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TC = LDL-C + HDL-C + VLDL-C + IDL-C + chylomicron-C
+ remnant-C + Lp(a)-C
where X-C denotes the cholesterol content of a respective
cholesterol-carrying particle. There are 2 particles in the equation
above that I didn’t specifically mention last week, the remnant
particle and the Lp(a) particle (pronounced “EL – pee – little – a,”
which sounds less silly than, “Lip-a”). Lp(a) is an LDL-like particle
but with a special apoprotein attached to it, aptly called
apoprotein(a), which is actually “attached” to the apoB molecule of
a standard LDL particle. Think of Lp(a) as a “special” kind of LDL
particle. As we’ll learn later in this series, Lp(a) particles are bad
dudes when it comes to atherosclerosis.
“Remnants” are nearly-empty-of-triglyceride particles of
chylomicrons and VLDL. In essence they are larger TG-rich
particles that have lost a lot of their TG core content as well as
surface phospholipids and are thus smaller than, or remnants of,
their “parent particles.” Hence,they are cholesterol-rich particles.
Under fasting conditions, in a not-too-terribly-insulin-resistant
person, IDL-C, chylomicron-C, and remnant-C are negligible.
Furthermore, in most people Lp(a)-C does not exist or is not very
high.
So we’re left with this simplification:
TC ~ LDL-C + HDL-C + VLDL-C
which is clearly an improvement in convenience over the first
equation. But what to do about that pesky VLDL-C?
There are a number of variations, but essentially a breakthrough
(mid 1970s) formula, called the Friedewald Formula, estimates
VLDL-C as one-fifth the concentration of serum triglycerides (some
variants use 0.16 instead of one-fifth, or 0.20). This assumes all
TG are trafficked in one’s VLDL particles and that a normally
composed VLDL contains five times more TG than cholesterol.
The straight dope on cholesterol – Part III
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Rearranging the above simplified formula we have:
LDL-C ~ TC – HDL-C – TG/5
Let’s plug in the numbers from the above figure, as an example.
TC = 234 mg/dL; HDL-C = 48 mg/dL, and TG = 117 mg/dL. Hence,
LDL-C is approximately 234 – 48 – 117/5 = 163 mg/dL.
Kind of a long run for a short slide, huh? Perhaps, but it is important
to understand that when you go to your doctor and get a
“cholesterol test,” odds are this is exactly what you’re getting.
Therefore LDL-C can be estimated knowing just TC, HDL-C, and
TG, assuming LDL-C matters (hint: it doesn’t matter much in
many folks).
Furthermore, what if the LDL particle is cholesterol-depleted
instead of its normal state of being cholesterol-enriched?
Unfortunately, especially in an insulin resistant population (i.e., the
United States), both TG content within lipoproteins and the
exchange of TG for cholesterol esters between particles is very
common, and using this formula can significantly underestimate
LDL-C. Worse yet, LDL-C becomes less meaningful in predicting
risk, as I will address next week.
What about direct measurement of LDL-C?
To chronicle the entire history of direct LDL-C measurement is
beyond the scope of this post. Many companies have developed
proprietary techniques to measure LDL-C directly, along with apoB,
and ultimately LDL-P. I’ll address two “major players” here:
Atherotech and LipoScience.
Atherotech developed an assay, called a VAP panel (VAP stands
for Vertical Auto Profile) to do everything described above, but also
to directly measure the amount of cholesterol contained within the
LDL particle. Furthermore, they have developed assays to directly
measure the cholesterol in IDL particles, VLDL particles, and even
The straight dope on cholesterol – Part III
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Lp(a) particles. Below is a snapshot of how VAP reporting looks.
A couple of things are worth mentioning:
Subparticle cholesterol content information is also generated,
including 2 different classes of HDL particles (HDL-2, HDL-3)
and 4 different classes of LDL particles (LDL-1, LDL-2, LDL-3,
LDL-4).
1.
LDL particles, based on the subparticle information, are
classified as “pattern A,” “pattern B,” or “pattern A/B.” Pattern A
implies more large, buoyant LDL particles, while pattern B
implies more small, dense LDL particles.
2.
Remember, though, while cholesterol mass concentration numbers
may correlate with the number of particles, they often do not. They
only convey the mass concentration of cholesterol molecules within
all of the particle subtypes per unit of volume. VAP tests do not
report the number of LDL or HDL particles, but they do attempt to
estimate atherogenic particle number (apoB) using a proprietary
formula based on subparticle cholesterol concentration and particle
sizes. I should point out that the formula, to my knowledge, has not
been validated in any study and not published in a peer reviewed
journal.
The straight dope on cholesterol – Part III
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A high estimate of apoB100 (i.e., what the VAP reports) is said to
correlate with the actual measurement of apoB. Since apoB is
found on each LDL particle, this serves as a proxy of LDL-P. The
American Diabetic Associate and the American College of
Cardiology Consensus Statement on Lipoproteins and the new
National Lipid Association biomarker paper stipulates that apoB
must be done using a protein immunoassay, not an estimate, such
as that of VAP.
But how can one actually count the number of LDL particles
and HDL particles?
There are several methods of doing this, but only one company,
LipoScience, has the FDA approved technology to do so using
nuclear magnetic resonance spectroscopy, or NMR for short. The
other available methodologies are ion mobility transfer and
ultracentrifugation (by Quest) and separation of LDL particles with
particle staining (by Spectracell). Virtually all guidelines (e.g., ADA,
ACC, AACC and NLA) only advise LDL-P via NMR at this time.
NMR, which is the basis for not only how to count lipoprotein
particles, but also diagnostic tests such as MRI scans, is really one
of my favorite technical topics. In residency I wrote a surgical
handbook and on page145-146, if you’re interested, you can read a
brief description of how MRI technology works, which will explain
how NMR technology can actually count lipoprotein particles.
As an aside, and just to give you an idea of what a great sport my
wife is, I wrote this surgical handbook over the course of a year
while in residency. To do so, I had to read approximately 8,000
pages of surgical textbooks and try to distill them down to just this
160 page summary. Doing so required reading about 22 pages
every day while working about 110 hours per week, typical of a
surgical residency “back in the day.” Besides exercising, I spent
every single moment of my “free” time reading for and writing this
handbook. Finally, a few months into it, my wife asked, “Why the
hell are you doing this? You never watch TV, you never go out,
The straight dope on cholesterol – Part III
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you never do anything else!” I responded that it was the best way
I could learn this material, but also, that I wanted to have a legacy
when I left residency. Half joking, I asked her, “What’s your
legacy?” Blank stare. A few months later, for Valentine’s Day, she
gave me this t-shirt. I think it’s safe to say not a single person has
read this handbook. So much for my legacy…
A brief explanation of how NMR works to count (and measure)
particles can also be found here.
Below is a snapshot of how NMR reporting looks. This particular
report is from Health Diagnostics Laboratory (HDL), Inc.
LipoScience performs the actual NMR test, but HDL, Inc. runs a
number of complimentary biomarkers I will discuss in subsequent
posts. I now use the HDL, Inc. test exclusively for reasons I will
explain later.
The straight dope on cholesterol – Part III
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In addition to counting the actual total number of LDL particles
(LDL-P) and HDL particles (HDL-P) per liter, HDL, Inc. (not
LipoScience) directly measures apoB and apoA-I. Furthermore, the
size of each particle is measured using NMR in nanometers (to give
you a sense of how small these things are, and why we need to use
nanometers to measure them, about 1.3 million LDL particles
stacked side-by-side would measure only one inch).
The final point I’ll make about the value of NMR reported
subparticle sizes and diameters is particularly telling when it comes
to insulin resistance. In the panel below, you can see that this
person has small VLDL particles, small HDL particles, and LDL
particles. Why is this interesting? The presence of increased large
VLDL-P, large VLDL size, increased small LDL- P, small LDL size,
reduced large HDL-P, small HDL size are early markers for insulin
resistance, and such findings may actually precede more
conventional signs of insulin resistance (insulin levels, glycemic
abnormalities) by several years. In other words, the number and
size of the lipoprotein particles is perhaps the earliest warning
sign for insulin resistance.
The straight dope on cholesterol – Part III
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In summary
The measurement of cholesterol has undergone a dramatic
evolution over the past 70 years with technology at the heart of
the advance.
1.
Currently, most people in the United States (and the world for
that matter) undergo a “standard” lipid panel which only
directly measures TC, TG, and HDL-C. LDL-C can be
measured directly, but is most often estimated.
2.
More advanced cholesterol measuring tests do exist to directly
measure LDL-C (though none are standardized), along with the
cholesterol content of other lipoproteins (e.g., VLDL, IDL) or
lipoprotein subparticles.
3.
The most frequently used and guideline recommended test that
can count the number of particles is the NMR LipoProfile. In
addition to counting the number of particles – the most
important predictor of risk – NMR can also measure the size of
each lipoprotein particle, which is valuable for predicting
insulin resistance in drug naïve patients, before changes are
noted in glucose or insulin levels.
4.
I know some of you are getting antsy. I thank you for your patience,
and I hope you appreciate that it was a necessary step to get
through this somewhat technical material and nomenclature. Next
week we’ll get to the “fun” stuff – what does all of this cholesterol
have to do with heart disease?
In addition, we’ll get further into the importance of using LDL-P as
the best predictor of risk. If anyone wants to read up on another
very important topic, especially for understanding why LDL-P is
more important to know than LDL-C, get familiar with the concepts
The straight dope on cholesterol – Part III
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of discordant and concordant variables. You’ll be hearing a lot
about these.
(To Part IV »)
10
MAY
The straight dope on cholesterol – Part III
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The straight dope on cholesterol – Part
IV
Previously, in Part I, Part II and Part III of this series, we addressed
these 5 concepts:
#1 — What is cholesterol?
#2 — What is the relationship between the cholesterol we eat
and the cholesterol in our body?
#3 — Is cholesterol bad?
#4 — How does cholesterol move around our body?
#5 –How do we measure cholesterol?
In this post we’ll continue to build out the story with the next
concept:
#6 – How does cholesterol actually cause problems?
Asked another way, how does someone end up with a coronary
artery that looks like the one in the picture above?
Quick refresher on take-away points from previous posts,
should you need it:
Cholesterol is “just” another fancy organic molecule in our body
but with an interesting distinction: we eat it, we make it, we
store it, and we excrete it – all in different amounts.
1.
The pool of cholesterol in our body is essential for life. No2.
The straight dope on cholesterol – Part IV
1 of 13
cholesterol = no life.
Cholesterol exists in 2 forms – unesterified or “free” (UC) and
esterified (CE) – and the form determines if we can absorb it
or not, or store it or not (among other things).
3.
Much of the cholesterol we eat is in the form of CE. It is not
absorbed and is excreted by our gut (i.e., leaves our body in
stool). The reason this occurs is that CE not only has to be
de-esterified, but it competes for absorption with the vastly
larger amounts of UC supplied by the biliary route.
4.
Re-absorption of the cholesterol we synthesize in our body
(i.e., endogenous produced cholesterol) is the dominant source
of the cholesterol in our body. That is, most of the cholesterol
in our body was made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will discover that synthesis
and absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
up. To see an important reference on this topic, please look
here.
7.
Cholesterol and triglycerides are not soluble in plasma (i.e.,
they can’t dissolve in water) and are therefore said to be
hydrophobic.
8.
To be carried anywhere in our body, say from your liver to your9.
The straight dope on cholesterol – Part IV
2 of 13
coronary artery, they need to be carried by a special protein-
wrapped transport vessel called a lipoprotein.
As these “ships” called lipoproteins leave the liver they undergo
a process of maturation where they shed much of their
triglyceride “cargo” in the form of free fatty acid, and doing so
makes them smaller and richer in cholesterol.
10.
Special proteins, apoproteins, play an important role in moving
lipoproteins around the body and facilitating their interactions
with other cells. The most important of these are the apoB
class, residing on VLDL, IDL, and LDL particles, and the apoA-I
class, residing for the most part on the HDL particles.
11.
Cholesterol transport in plasma occurs in both directions,
from the liver and small intestine towards the periphery and
back to the liver and small intestine (the “gut”).
12.
The major function of the apoB-containing particles is to traffic
energy (triglycerides) to muscles and phospholipids to all
cells. Their cholesterol is trafficked back to the liver. The apoA-I
containing particles traffic cholesterol to steroidogenic tissues,
adipocytes (a storage organ for cholesterol ester) and
ultimately back to the liver, gut, or steroidogenic tissue.
13.
All lipoproteins are part of the human lipid transportation system
and work harmoniously together to efficiently traffic lipids. As
you are probably starting to appreciate, the trafficking pattern is
highly complex and the lipoproteins constantly exchange their
core and surface lipids.
14.
The measurement of cholesterol has undergone a dramatic
evolution over the past 70 years with technology at the heart of
the advance.
15.
Currently, most people in the United States (and the world for
that matter) undergo a “standard” lipid panel which only
16.
The straight dope on cholesterol – Part IV
3 of 13
directly measures TC, TG, and HDL-C. LDL-C is measured or
most often estimated.
More advanced cholesterol measuring tests do exist to directly
measure LDL-C (though none are standardized), along with the
cholesterol content of other lipoproteins (e.g., VLDL, IDL) or
lipoprotein subparticles.
17.
The most frequently used and guideline-recommended test that
can count the number of LDL particles is either
apolipoprotein B or LDL-P NMR which is part of the NMR
LipoProfile. NMR can also measure the size of LDL and other
lipoprotein particles, which is valuable for predicting insulin
resistance in drug naïve patients (i.e., those patients not on
cholesterol-lowering drugs), before changes are noted in
glucose or insulin levels.
18.
Concept #6 – How does cholesterol actually cause problems?
If you remember only one factoid from the previous three posts on
this topic, remember this: If you were only “allowed” to know one
metric to understand your risk of heart disease it would be the
number of apoB particles (90-95% of which are LDLs) in your
plasma. In practicality, there are two ways to do this:
Directly measure (i.e., not estimate) the concentration of apoB
in your plasma (several technologies and companies offer such
an assay). Recall, there is one apoB molecule per particle;
1.
Directly measure the number of LDL particles in your plasma
using NMR technology.
2.
If this number is high, you are at risk of atherosclerosis.
Everything else is secondary.
Does having lots of HDL particles help? Probably, especially if they
are “functional” at carrying out reverse cholesterol transport, but it’s
not clear if it matters when LDL particle count is low. In fact, while
The straight dope on cholesterol – Part IV
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many drugs are known to increase the cholesterol content of HDL
particles (i.e., HDL-C), not one to date has ever shown a benefit
from doing so. Does having normal serum triglyceride levels
matter? Probably, with “normal” being defined as < 70-100 mg/dL,
though it’s not entirely clear this is an independent predictor of low
risk. Does having a low level of LDL-C matter? Not if LDL-P (or
apoB) are high, or better said, not when the two markers are
discordant.
But why?
As with the previous topics in this series, this question is sufficiently
complex to justify several textbooks – and it’s still not completely
understood. My challenge, of course, is to convey the most
important points in a fraction of that space and complexity.
Let’s focus, specifically, on the unfortunately-ubiquitous clinical
condition of atherosclerosis – the accumulation of sterols and
inflammatory cells within an artery wall which may (or may not)
narrow the lumen of the artery.
Bonus concept: It’s important to keep in mind that this disease
process is actually within the artery wall and it may or may not
affect the arterial lumen, which is why angiograms can be normal
in persons with advanced atherosclerosis. As plaque progresses
it can encroach into the lumen leading to ischemia (i.e., lack of
oxygen delivery to tissues) as the narrowing approaches 70-75%,
or the plaque can rupture leading to a thrombosis: partial leading
to ischemia or total leading to infarction (i.e., tissue death).
To be clear, statistically speaking, this condition (atherosclerotic
induced ischemia or infarction) is the most common one that will
result in the loss of your life. For most of us, atherosclerosis (most
commonly within coronary arteries, but also carotid or cerebral
arteries) is the leading cause of death, even ahead of all forms of
cancer combined. Hence, it’s worth really understanding this
problem.
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In this week’s post I am going to focus exclusively on what I like to
call the “jugular issue” – that is, I’m going to discuss the actual
mechanism of atherosclerosis. I’m not going to discuss
treatment (yet). We can’t get into treatment until we really
understand the cause.
“It is in vain to speak of cures, or think of remedies, until
such time as we have considered of the causes . . . cures
must be imperfect, lame, and to no purpose, wherein the
causes have not first been searched.”
—Robert Burton, The Anatomy of Melancholy, 1621
The sine qua non of atherosclerosis is the presence of sterols in
arterial wall macrophages. Sterols are delivered to the arterial wall
by the penetration of the endothelium by an apoB-containing
lipoprotein, which transport the sterols. In other words, unless an
apoB-containing lipoprotein particle violates the border
created by an endothelium cell and the layer it protects, the
media layer, there is no way atherogenesis occurs.
For now, let’s focus only on the most ubiquitous apoB-containing
lipoprotein, the LDL particle. Yes, other lipoproteins also contain
apoB (e.g., chylomicrons, remnant lipoproteins such as VLDL
remnants, IDL and Lp(a)), but they are few in number relative to
LDL particles. I will address them later.
The endothelium is the one-cell-thick-layer which lines the lumen
(i.e., the “tube”) of a vessel, in this case, the artery. Since blood is
in direct contact with this cell all the time, all lipoproteins – including
LDL particles – come in constant contact with such cells.
So what drives an LDL particle to do something as sinister as to
leave the waterway (i.e., the bloodstream) and “illegally” try to park
at a dock (i.e., behind an endothelial cell)? Well, it is a gradient
driven process which is why particle number is the key driving
parameter.
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As it turns out, this is probably a slightly less important question
than the next one: what causes the LDL particle to stay there? In
the parlance of our metaphor, not only do we want to know why the
ship leaves the waterway to illegally park in the dock, but why
does it stay parked there? This phenomenon is called “retention.”
Finally, if there was some way an LDL particle could violate the
endothelium, AND be retained in the space behind the cell (away
from the lumen on the side aptly called the sub-endothelial side)
BUT not elicit an inflammatory (i.e., immune) response, would it
matter?
I don’t know. But it seems that not long after an LDL particle gets
into the sub-endothelial space and takes up “illegal” residence (i.e.,
binds to arterial wall proteoglycans), it is subject to oxidative forces
and as one would expect an inflammatory response is initiated.
The result is full blown mayhem. Immunologic gang warfare breaks
out and cells called monocytes and macrophages and mast cells
show up to investigate. When they arrive, and find the LDL particle,
they do all they can to remove it. In some cases, when there are
few LDL particles, the normal immune response is successful. But,
it’s a numbers game. When LDL particle invasion becomes
incessant, even if the immune cells can remove some of them, it
becomes a losing proposition and the actual immune response to
the initial problem becomes chronic and maladaptive and expands
into the space between the endothelium and the media.
The multiple-sterol-laden macrophages or foam cells coalesce,
recruit smooth muscle cells, induce microvascularization, and
before you know it complex, inflamed plaque occurs.
Microhemorrhages and microthrombus formations occur within the
plaque. Ultimately the growing plaque invades the arterial lumen or
ruptures into the lumen inducing luminal thrombosis. Direct luminal
encroachment by plaque expansion or thrombus formation causes
the lumen of the artery to narrow, which may or may not cause
ischemia.
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Before we go any further, take a look at the figure below from an
excellent review article on this topic from the journal Circulation –
Subendothelial Lipoprotein Retention as the Initiative Process in
Atherosclerosis. This figure also discuss treatment strategies, but
for now just focus on the pathogenesis (i.e., the cause of the
problem).
In this figure you can see the progression:
LDL particles (and a few other particles containing apoB) enter
the sub-endothelium
1.
Some of these particles are retained, especially in areas where
there is already a bit of extra space for them (called pre-lesion
susceptible areas)
2.
“Early” immune cells initiate an inflammatory response which
includes the direct interaction between the LDL particle and
proteins called proteoglycans.
3.
The proteoglycans further shift the balance of LDL particle
movement towards retention. Think of them as “cement”
keeping the LDL particles and their cholesterol content in the
sub-endothelial space.
4.
More and more apoB containing particles (i.e., LDL particles
and the few other particles containing apoB) enter the
sub-endothelial space and continue to be retained, due to the
existing “room” being created by the immune response.
5.
As this process continues, an even more advanced form of
immune response – mediated by cells called T-cells – leads to
further retention and destruction of the artery wall.
6.
Eventually, not only does the lumen of the artery narrow, but a
fibrous cap develops and plaques take form.
7.
It is most often these plaques that lead to myocardial infarction,8.
The straight dope on cholesterol – Part IV
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or heart attacks, as they eventually dislodge and acutely
obstruct blood flow to the portion of muscle supplied by the
artery.
Another way to see this progression is shown in the figure below,
which shows the histologic progression of atherosclerosis in the
right coronary artery from human autopsy specimens. This figure is
actually a bit confusing until you understand what you’re looking at.
Each set of 3 pictures shows the same sample, but with a different
kind of histological stain. Each row represents a different
specimen.
The column on the left uses a stain to highlight the distinction
between the intimal and media layer of the artery call. The
intima, recall, is the layer just below the endothelium and should
not be as thick as shown here.
The middle column uses a special stain to highlight the
presence of lipids within the intimal layer.
The right column uses yet a different stain to highlight the
The straight dope on cholesterol – Part IV
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presence of macrophages in the intima and the media.
Recall, macrophages are immune cells that play an important
role of the inflammatory cascade leading to atherosclerosis.
What is particularly compelling about this sequence of figures is
that you can see the trigger of events from what is called diffuse
intimal thickening (“DIT”), which exacerbates the retention of
lipoproteins and their lipid cargo, only then to be followed by the
arrival of immune cells, which ultimately lead the inflammatory
changes responsible for atherosclerosis.
Why LDL-P matters most
You may be asking the chicken and egg question:
Which is the cause – the apoB containing LDL particle OR the
immune cells that “overreact” to them and their lipid cargo?
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You certainly wouldn’t be alone in asking this question, as many
folks have debated this exact question for years. The reason, of
course, it is so important to ask this question is captured by the
Robert Burton quote, above. If you don’t know the cause, how can
you treat the disease?
Empirically, we know that the most successful pharmacologic
interventions demonstrated to reduce coronary artery disease are
those that reduce LDL-P and thus delivery of sterols to the artery.
(Of course, they do other things also, like lower LDL-C, and maybe
even reduce inflammation.)
Perhaps more compelling is the “natural experiment” of people with
genetic alterations leading to elevated or reduced LDL-P. Let’s
consider an example of each:
Cohen, et al. reported in the New England Journal of Medicine
in 2006 on the cases of patients with mutations in an enzyme
called proprotein convertase subtilisin type 9 or PCSK9 (try
saying that 10 times fast). Normally, this proteolytic enzyme
degrades LDL receptors on the liver. Patients with mutations
(“nonsense mutations” to be technically correct, meaning the
enzyme is somewhat less active) have less destruction of
hepatic LDL receptors. Hence, they have more sustained
expression of hepatic LDL receptors, improved LDL clearance
from plasma and therefore fewer LDL particles. These patients
have very low LDL-P and LDL-C concentrations (5-40 mg/dL)
and very low incidence of heart disease. Note that a reduction
in PCSK9 activity plays no role in reducing inflammation.
1.
Conversely, patients with familial hypercholesterolemia (known
as FH) have the opposite problem. While there are several
variants and causes of this disease, the common theme is
having decreased clearance of apoB-containing particles, often
but not always due to defective or absent LDL receptors,
leading to the opposite problem from above. Namely, these
patients have a higher number of circulating LDL particles, and
2.
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as a result a much higher incidence of atherosclerosis.
So why does having an LDL-P of 2,000 nmol/L (95th percentile)
increase the risk of atherosclerosis relative to, say, 1,000 nmol/L
(20th percentile)? In the end, it’s a probabilistic game. The more
particles – NOT cholesterol molecules within the particles and not
the size of the LDL particles – you have, the more likely the chance
a LDL-P is going to ding an endothelial cell, squeeze into the
sub-endothelial space and begin the process of atherosclerosis.
What about the other apoB containing lipoproteins?
Beyond the LDL particle, other apoB-containing lipoproteins also
play a role in the development of atherosclerosis, especially in an
increasingly insulin resistant population like ours. In fact, there is
some evidence that particle-for-particle Lp(a) is actually even more
atherogenic than LDL (though we have far fewer of them). You’ll
recall that Lp(a) is simply an LDL particle to which another protein
called apoprotein(a) is attached, which is both a prothrombotic
protein as well as a carrier of oxidized lipids – neither of which you
want in a plaque. The apo(a) also retards clearance of Lp(a) thus
enhancing LDL-P levels. It may foster greater penetration of the
endothelium and/or greater retention within the sub-endothelial
space and/or elicit an even greater immune response.
In summary
The progression from a completely normal artery to an
atherosclerotic one which may or may not be “clogged” follows a
very clear path: an apoB containing particle gets past the
endothelial layer into the sub-endothelial space, the particle and
its cholesterol content is retained and oxidized, immune cells
arrive, an initially-beneficial inflammatory response occurs that
ultimately becomes maladaptive leading to complex plaque.
1.
While inflammation plays a key role in this process, it’s the
penetration of the apoB particle, with its sterol passengers, of
the endothelium and retention within the sub-endothelial space
2.
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that drive the process.
The most numerous apoB containing lipoprotein in this process
is certainly the LDL particle, however Lp(a) (if present) and
other apoB containing lipoproteins may play a role.
3.
If you want to stop atherosclerosis, you must lower the LDL
particle number.
4.
(To Part V »)
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The straight dope on cholesterol – Part
V
In Part I, Part II, Part III and Part IV of this series, we addressed
these 6 concepts:
#1 — What is cholesterol?
#2 — What is the relationship between the cholesterol we eat
and the cholesterol in our body?
#3 — Is cholesterol bad?
#4 — How does cholesterol move around our body?
#5 – How do we measure cholesterol?
#6 – How does cholesterol actually cause problems?
In this post we’ll continue to build out the story with the next
concept:
#7 – Does the size of an LDL particle matter?
Quick refresher on take-away points from previous posts,
should you need it:
Cholesterol is “just” another fancy organic molecule in our body
but with an interesting distinction: we eat it, we make it, we
store it, and we excrete it – all in different amounts.
1.
The pool of cholesterol in our body is essential for life. No
cholesterol = no life.
2.
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Cholesterol exists in 2 forms – unesterified or “free” (UC) and
esterified (CE) – and the form determines if we can absorb it
or not, or store it or not (among other things).
3.
Much of the cholesterol we eat is in the form of CE. It is not
absorbed and is excreted by our gut (i.e., leaves our body in
stool). The reason this occurs is that CE not only has to be
de-esterified, but it competes for absorption with the vastly
larger amounts of UC supplied by the biliary route.
4.
Re-absorption of the cholesterol we synthesize in our body
(i.e., endogenous produced cholesterol) is the dominant source
of the cholesterol in our body. That is, most of the cholesterol
in our body was made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will discover that synthesis
and absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
up. To see an important reference on this topic, please look
here.
7.
Cholesterol and triglycerides are not soluble in plasma (i.e.,
they can’t dissolve in water) and are therefore said to be
hydrophobic.
8.
To be carried anywhere in our body, say from your liver to your
coronary artery, they need to be carried by a special protein-
wrapped transport vessel called a lipoprotein.
9.
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As these “ships” called lipoproteins leave the liver they undergo
a process of maturation where they shed much of their
triglyceride “cargo” in the form of free fatty acid, and doing so
makes them smaller and richer in cholesterol.
10.
Special proteins, apoproteins, play an important role in moving
lipoproteins around the body and facilitating their interactions
with other cells. The most important of these are the apoB
class, residing on VLDL, IDL, and LDL particles, and the apoA-I
class, residing for the most part on the HDL particles.
11.
Cholesterol transport in plasma occurs in both directions,
from the liver and small intestine towards the periphery and
back to the liver and small intestine (the “gut”).
12.
The major function of the apoB-containing particles is to traffic
energy (triglycerides) to muscles and phospholipids to all
cells. Their cholesterol is trafficked back to the liver. The apoA-I
containing particles traffic cholesterol to steroidogenic tissues,
adipocytes (a storage organ for cholesterol ester) and
ultimately back to the liver, gut, or steroidogenic tissue.
13.
All lipoproteins are part of the human lipid transportation system
and work harmoniously together to efficiently traffic lipids. As
you are probably starting to appreciate, the trafficking pattern is
highly complex and the lipoproteins constantly exchange their
core and surface lipids.
14.
The measurement of cholesterol has undergone a dramatic
evolution over the past 70 years with technology at the heart of
the advance.
15.
Currently, most people in the United States (and the world for
that matter) undergo a “standard” lipid panel, which only
directly measures TC, TG, and HDL-C. LDL-C is measured or
most often estimated.
16.
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More advanced cholesterol measuring tests do exist to directly
measure LDL-C (though none are standardized), along with the
cholesterol content of other lipoproteins (e.g., VLDL, IDL) or
lipoprotein subparticles.
17.
The most frequently used and guideline-recommended test that
can count the number of LDL particles is either
apolipoprotein B or LDL-P NMR, which is part of the NMR
LipoProfile. NMR can also measure the size of LDL and other
lipoprotein particles, which is valuable for predicting insulin
resistance in drug naïve patients, before changes are noted in
glucose or insulin levels.
18.
The progression from a completely normal artery to a “clogged”
or atherosclerotic one follows a very clear path: an apoB
containing particle gets past the endothelial layer into the
subendothelial space, the particle and its cholesterol content is
retained, immune cells arrive, an inflammatory response ensues
“fixing” the apoB containing particles in place AND making more
space for more of them.
19.
While inflammation plays a key role in this process, it’s the
penetration of the endothelium and retention within the
endothelium that drive the process.
20.
The most common apoB containing lipoprotein in this process is
certainly the LDL particle. However, Lp(a) and apoB containing
lipoproteins play a role also, especially in the insulin resistant
person.
21.
If you want to stop atherosclerosis, you must lower the LDL
particle number.
22.
Concept #7 – Does the size of an LDL particle matter?
There are few, if any, topics in lipidology that generate more
confusion and argument that this one. I’ve been leading up to it all
month, so I think the time is here to address this issue head on.
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I’ve read many papers and seen many lectures on this topic, but
the one that stole my heart was a lecture given by Jim Otvos at the
ADA 66th Scientific Sessions in Washington, DC. Some of the
figures I am using in this post are taken directly or modified from his
talk or subsequent discussions.
At the outset of this discussion I want to point out two clinical
scenarios to keep in mind:
The most lethal lipoprotein disorder is familial
hypercholesterolemia, which I have discussed in previous
posts. Such patients all have large LDL particles, but most of
these patients die in childhood or early adulthood if not treated
with medications to reduce particle number.
1.
Conversely, diabetic patients and other patients with
advanced metabolic syndrome have small LDL particles, yet
often live well into their 50s and 60s before succumbing to
atherosclerotic diseases.
2.
The common denominator is that both sets of patients in (1) and
(2) have high LDL-P. What I’m going to attempt to show you today
is that once adjusted for particle number, particle size has no
statistically significant relationship to cardiovascular risk. But first,
some geometry.
“Pattern A” versus “Pattern B” LDL
The introduction of gradient gel electrophoresis about 30 years ago
is what really got people interested in the size of LDL particles.
There is no shortage of studies of the past 25 years demonstrating
that of the following 2 scenarios, one has higher risk, all other
things equal. [This is a big disclaimer and I went back and forth
for a while before deciding to include this point. It is an
uncharacteristic oversimplification. If you’ve been reading this
blog for a while, you’ll know I’m rarely accused of that sin – but I’m
about to be].
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Here’s the example: Consider 2 patients, both with the same total
content of cholesterol in their LDL particles, say, 130 mg/dL.
Furthermore, assume each has the “ideal” ratio of core cholesterol
ester-to-triglyceride (recall from Part I and III of this series, this ratio
is 4:1). I’m going to explain in a subsequent post why this
assumption is probably wrong as often as it’s right, but for the
purpose of simplicity I want to make a geometric point.
LDL-C = 130 mg/dL, Pattern A (large particles) – person on the
left in the figure below
1.
LDL-C = 130 mg/dL, Pattern B (small particles) – person on the
right in the figure below
2.
Under the set of assumptions I’ve laid out, case #2 is the higher risk
case. In other words, at the same concentration of cholesterol
within LDL particles, assuming the same ratio of CE:TG, it is
mathematically necessary the person on the right, case #2, has
more particles, and therefore has greater risk.
Bonus concept: What one really must know is how many
cholesterol molecules there are per LDL particle. It always
requires more cholesterol-depleted LDL particles than
cholesterol-rich LDL particles to traffic cholesterol in plasma, and
the number of cholesterol molecules depends on both size and
core TG content. The more TG in the particle, the less the
cholesterol in the particle.
So why does the person on the right have greater risk? Is it
because they have more particles? Or is it because they have
smaller particles?
This is the jugular question I want to address today.
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If you understand that the person on the right, under the very
careful and admittedly overly simplified assumptions I’ve given, is at
higher risk than the person on the left, there are only 4 possible
reasons:
Small LDL particles are more atherogenic than large ones,
independent of number.
1.
The number of particles is what increases atherogenic risk,
independent of size.
2.
Both size and number matter, and so the person on the right is
“doubly” at risk.
3.
Neither feature matters and these attributes (i.e., size and
number) are markers for something else that does matter.
4.
Anyone who knows me well knows I love to think in MECE terms
whenever possible. This is a good place to do so.
I’m going to rule out Reason #4 right now because if I have not yet
convinced you that LDL particles are the causative agent for
atherosclerosis, nothing else I say matters. The trial data are
unimpeachable and there are now 7 guidelines around the world
advocating particle number measurement for risk assessment. The
more LDL particles you have, the greater your risk of
atherosclerosis.
But how do we know if Reason #1, #2, or #3 is correct?
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This figure (one of the most famous in this debate) is from the
Quebec Cardiovascular Study, published in 1997, in Circulation.
You can find this study here.
This is kind of a complex graph if you’re not used to looking at
these. It shows relative risk – but in 2 dimensions. It’s looking at
the role of LDL size and apoB (a proxy for LDL-P, you’ll recall from
previous posts). What seems clear is that in patients with low
LDL-P (i.e., apoB < 120 mg/dl), size does not matter. The relative
risk is 1.0 in both cases, regardless of peak LDL size. However, in
patients with lots of LDL particles (i.e., apoB > 120 mg/dl), smaller
peak LDL size seems to carry a much greater risk – 6.2X.
If you just looked at this figure, you might end up drawing the
conclusion that both size and number are independently
predictive of risk (i.e., Reason #3, above). Not an illogical
conclusion…
What is not often mentioned, however, is what is in the text of the
article:
“Among lipid, lipoprotein,and apolipoprotein variables, apo B
[LDL-P] came out as the best and only significant predictor of
ischemic heart disease (IHD) risk in multivariate
stepwiselogistic analyses (P=.002).”
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“LDL-PPD [peak LDL particle diameter] — as a continuous
variable did not contribute to the risk of IHD after the contribution
of apo B levels to IHD risk had been considered.”
What’s a continuous variable? Something like height or weight,
where the possible values are infinite between a range. Contrast
this with discrete variables like “tall” or “short,” where there are only
two categories. For example, if I define “tall” as greater than or
equal to 6 feet, the entire population of the world could be placed in
two buckets: Those who are “short” (i.e., less than 6 feet tall) and
those who are “tall” (i.e., those who are 6 feet tall and taller). This
figure shows LDL size like it’s a discrete variable – “large” or
“small” – but obviously it is not. It’s continuous, meaning it can
take on any value, not just “large” or “small.” When this same
analysis is done using LDL size as the continuous variable it is,
the influence of size goes away and only apoB (i.e., LDL-P)
matters.
This effect has been observed subsequently, including the famous
Multi-Ethnic Study of Atherosclerosis (MESA) trial, which you can
read here. The MESA trial looked at the association between
LDL-P, LDL-C, LDL size, IMT (intima-media thickness – the best
non-invasive marker we have for atherosclerosis), and many other
parameters in about 5,500 men and women over a several year
period.
This study used the same sort of statistical analysis as the study
above to parse out the real role of LDL-P versus particle size, as
summarized in the table below.
The straight dope on cholesterol – Part V
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This table shows us that when LDL-P is NOT taken into account
(i.e., “unadjusted” analysis), an increase of one standard deviation
in particle size is associated with 20.9 microns of LESS
atherosclerosis, what one might expect if one believes particle size
matters. Bigger particles, less atherosclerosis.
However, and this is the important part, when the authors adjusted
for the number of LDL particles (in yellow), the same phenomenon
was not observed. Now an increase in LDL particle size by 1
standard deviation was associated with an ADDITIONAL 14.5
microns of atherosclerosis, albeit of barely any significance
(p=0.05).
Let me repeat this point: Once you account for LDL-P, the
relationship of atherosclerosis to particle size is abolished (and
even trends towards moving in the “wrong” direction – i.e., bigger
particles, more atherosclerosis).
Let me use another analysis to illustrate this point again. If you
adjust for age and sex, but not LDL-P [left graph, below], changes
in the number of LDL particles (shown in quintiles, so each group
shows changes by 20% fractions) seem to have no relationship
with IMT (i.e., atherosclerosis).
However, when you adjust for small LDL-P [right graph, below], it
becomes clear that increased numbers of large LDL particles
significantly increase risk.
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I’ve only covered a small amount of the work addressing this
question, but this issue is now quite clear. A small LDL particle is
no more atherogenic than a large one, but only by removing
confounding factors is this clear. So, if you look back at the figure I
used to address this question, it should now be clear that Reason
#2 is the correct one.
This does not imply that the “average” person walking around with
small particles is not at risk. It only implies the following:
The small size of their particles is probably a marker for
something else (e.g., metabolic derangement due to higher
trafficking of triglycerides within LDL particles);
1.
Unless you know their particle number (i.e., LDL-P or apoB),
you actually don’t know their risk.
2.
Let’s wrap it up here for this week. Next week we’ll address
another question that’s probably been on your mind: Why do we
need to measure LDL-P or apoB? Isn’t the LDL-C test my doctor
orders enough to predict my risk?
Summary
At first glance it would seem that patients with smaller LDL
particles are at greater risk for atherosclerosis than patients with
large LDL particles, all things equal. Hence, this idea that
Pattern A is “good” and Pattern “B” is bad has become quite
popular.
To address this question, however, one must look at changes in
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cardiovascular events or direct markers of atherosclerosis (e.g.,
IMT) while holding LDL-P constant and then again holding
LDL size constant. Only when you do this can you see that
the relationship between size and event vanishes. The only
thing that matters is the number of LDL particles – large, small,
or mixed.
“A particle is a particle is a particle.” If you don’t know the
number, you don’t know the risk.
(To Part VI »)
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The straight dope on cholesterol – Part
VI
Previously, in Part I, Part II, Part III, Part IV and Part V of this
series, we addressed these 7 concepts:
#1 — What is cholesterol?
#2 — What is the relationship between the cholesterol we eat
and the cholesterol in our body?
#3 — Is cholesterol bad?
#4 — How does cholesterol move around our body?
#5 – How do we measure cholesterol?
#6 – How does cholesterol actually cause problems?
#7 – Does the size of an LDL particle matter?
In this post we’ll continue to build out the story with the next
concept:
#8 – Why is it necessary to measure LDL-P, instead of just
LDL-C?
(Not so) quick refresher on take-away points from previous
posts, should you need it:
Cholesterol is “just” another fancy organic molecule in our body
but with an interesting distinction: we eat it, we make it, we
store it, and we excrete it – all in different amounts.
1.
The straight dope on cholesterol – Part VI
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The pool of cholesterol in our body is essential for life. No
cholesterol = no life.
2.
Cholesterol exists in 2 forms – unesterified or “free” (UC) and
esterified (CE) – and the form determines if we can absorb it
or not, or store it or not (among other things).
3.
Much of the cholesterol we eat is in the form of CE. It is not
absorbed and is excreted by our gut (i.e., leaves our body in
stool). The reason this occurs is that CE not only has to be
de-esterified, but it competes for absorption with the vastly
larger amounts of UC supplied by the biliary route.
4.
Re-absorption of the cholesterol we synthesize in our body
(i.e., endogenous produced cholesterol) is the dominant source
of the cholesterol in our body. That is, most of the cholesterol
in our body was made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will discover that synthesis
and absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
up. To see an important reference on this topic, please look
here.
7.
Cholesterol and triglycerides are not soluble in plasma (i.e.,
they can’t dissolve in water) and are therefore said to be
hydrophobic.
8.
The straight dope on cholesterol – Part VI
2 of 17
To be carried anywhere in our body, say from your liver to your
coronary artery, they need to be carried by a special protein-
wrapped transport vessel called a lipoprotein.
9.
As these “ships” called lipoproteins leave the liver they undergo
a process of maturation where they shed much of their
triglyceride “cargo” in the form of free fatty acid, and doing so
makes them smaller and richer in cholesterol.
10.
Special proteins, apoproteins, play an important role in moving
lipoproteins around the body and facilitating their interactions
with other cells. The most important of these are the apoB
class, residing on VLDL, IDL, and LDL particles, and the apoA-I
class, residing for the most part on the HDL particles.
11.
Cholesterol transport in plasma occurs in both directions,
from the liver and small intestine towards the periphery and
back to the liver and small intestine (the “gut”).
12.
The major function of the apoB-containing particles is to traffic
energy (triglycerides) to muscles and phospholipids to all
cells. Their cholesterol is trafficked back to the liver. The apoA-I
containing particles traffic cholesterol to steroidogenic tissues,
adipocytes (a storage organ for cholesterol ester) and
ultimately back to the liver, gut, or steroidogenic tissue.
13.
All lipoproteins are part of the human lipid transportation system
and work harmoniously together to efficiently traffic lipids. As
you are probably starting to appreciate, the trafficking pattern is
highly complex and the lipoproteins constantly exchange their
core and surface lipids.
14.
The measurement of cholesterol has undergone a dramatic
evolution over the past 70 years with technology at the heart of
the advance.
15.
Currently, most people in the United States (and the world for16.
The straight dope on cholesterol – Part VI
3 of 17
that matter) undergo a “standard” lipid panel, which only
directly measures TC, TG, and HDL-C. LDL-C is measured or
most often estimated.
More advanced cholesterol measuring tests do exist to directly
measure LDL-C (though none are standardized), along with the
cholesterol content of other lipoproteins (e.g., VLDL, IDL) or
lipoprotein subparticles.
17.
The most frequently used and guideline-recommended test that
can count the number of LDL particles is either
apolipoprotein B or LDL-P NMR, which is part of the NMR
LipoProfile. NMR can also measure the size of LDL and other
lipoprotein particles, which is valuable for predicting insulin
resistance in drug naïve patients, before changes are noted in
glucose or insulin levels.
18.
The progression from a completely normal artery to a “clogged”
or atherosclerotic one follows a very clear path: an apoB
containing particle gets past the endothelial layer into the
subendothelial space, the particle and its cholesterol content is
retained, immune cells arrive, an inflammatory response ensues
“fixing” the apoB containing particles in place AND making more
space for more of them.
19.
While inflammation plays a key role in this process, it’s the
penetration of the endothelium and retention within the
endothelium that drive the process.
20.
The most common apoB containing lipoprotein in this process is
certainly the LDL particle. However, Lp(a) and apoB containing
lipoproteins play a role also, especially in the insulin resistant
person.
21.
If you want to stop atherosclerosis, you must lower the LDL
particle number.
22.
The straight dope on cholesterol – Part VI
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At first glance it would seem that patients with smaller LDL
particles are at greater risk for atherosclerosis than patients with
large LDL particles, all things equal.
23.
“A particle is a particle is a particle.” If you don’t know the
number, you don’t know the risk.
24.
To address this question, however, one must look at changes in
cardiovascular events or direct markers of atherosclerosis (e.g.,
IMT) while holding LDL-P constant and then again holding
LDL size constant. Only when you do this can you see that
the relationship between size and event vanishes. The only
thing that matters is the number of LDL particles – large, small,
or mixed.
25.
Concept #8 – Why is it necessary to measure LDL-P, instead of
just LDL-C?
In the growing list of reasons why I used to refer to myself as
“chick-repellant” in college, I have a confession to make: I find the
topic of statistical concordance and discordance to be so
exciting, I sometimes have a hard time containing myself. This may
explain the paucity of girlfriends in college. Let me use an example
to illustrate the distinction between these terms. Let’s say you want
to predict the change in home prices in the following year (I used to
model this for a living). There are at least a dozen parameters
linked to this, including: GDP growth, unemployment, interest rates
(both short term and long term, though to different degrees),
housing inventory (i.e., how many houses are on the market),
housing absorption (i.e., how quickly houses go from being on the
market to being sold), major stock indices, and consumer
confidence. Historically, from the mid-1990’s until about the fourth
quarter of 2006, this worked like clockwork. While each of these
variables had differing strengths of predicting changes in home
prices, they all moved together. For example, when GDP growth
was robust, unemployment was low, interest rates were modest,
housing inventories were about 60 to 90 days, etcetera. All of
The straight dope on cholesterol – Part VI
5 of 17
these variables pointed to a predictable change in home values.
Around Q42006 (i.e., last 3 months of 2006), one of these variables
began to deviate from the others. The details aren’t important, but
the point is one variable began to suggest home prices would fall
while the others all pointed to a continued rise. Prior to Q42006
these parameters were said to be concordant – they all predicted
the same thing – either up or down. By 2007, they became
discordant – one variable said the sky was falling while others said
everything was fine.
This was true on the “micro” level, too. [What I described above is
called “macro” level.] As a lender, it should be very important to
know the risk of each and every loan you make (clearly this was
part of the root problem in the age of mass securitization). Will this
person pay the loan back or will they default?
Same game here, but now a new set of even greater variables. As
a lender, if I want to know if YOU will default, I will want to know a
lot of things about you, such as your agency credit risk scores, your
bank account activity, payroll activity, how much you’re borrowing
relative to the value of your house, where your house is located,
and about 50 other things (literally).
Not surprisingly, the same thing that happened on the macro side
happened on the micro side. It became difficult to predict who
would default and would not default because there were so many
variables to consider and lenders didn’t know which ones were still
predictive. The models that predict default are very sensitive to the
balance of these inputs. When all of the variables are concordant,
their accuracy is prophetic, as was the case from the mid-1990s
until late 2006. When some variables become discordant with each
other, especially variables that were historically concordant with
each other, really bad stuff happens, as became evident to me,
personally, one Thursday afternoon in November 2007. It became
clear the sky was about to fall. And, of course, it did.
The straight dope on cholesterol – Part VI
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What does real estate have to do with atherosclerosis?
Fortunately, predicting heart disease is a little easier than predicting
changes in home prices. It’s not perfect, of course, but it’s pretty
good. Why is it not perfect? For one thing, we can’t do the
“perfect” experiment. The “perfect” experiment would look
something like this:
Take 100,000 people and randomize them into four matched
groups, A, B, C, and D. Wave a magic wand (you can see why
this experiment hasn’t been and won’t be done) and give the folks
in Group A an LDL particle concentration of, say, 700 nmol/L;
those in Group B you give 1,200 nmol/L; those in Group C you
give 1,600 nmol/L; and those in Group D get 2,000 nmol/L.
In our dream world, due to the randomization process, these four
groups would be statistically identical in every way except one –
they would, thanks to our magic wand, have a different number of
LDL particles. We would follow them without further intervention for
10 years and then compare their rates of heart disease, stroke, and
death.
There are some areas in medicine where we can do such
experiments. But, we can’t do this experiment for this question.
Even when we do the next best thing — give people a drug that
lowers their LDL-P and measure the impact of this intervention —
there is always a chance we’ve done something in addition to “just”
lowering LDL-P. If you’ve been reading this series, you no doubt
know my thoughts on this: while other factors are likely to be
involved the pathogenesis of atherosclerosis (e.g., endothelial
“health”, normal versus abnormal inflammatory response) the
primary driver of atherosclerosis is the number of apoB
trafficking lipoproteins in circulation, of which LDL particles
are the vast majority.
The data below should further clarify this association.
What do concordant LDL-C and LDL-P values look like?
The straight dope on cholesterol – Part VI
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Among the two largest studies tracking the association between
cholesterol and atherosclerotic mortality are the Framingham study
and the MESA trial (the two largest trials were AMORIS and
INTERHEART). The figure below, which I’ve graciously borrowed
from Jim Otvos, shows the risk stratification of LDL-C (top) and
LDL-P (bottom) from the Framingham study and MESA trial,
respectively. As you can see, conveniently, LDL-C values in mg/dL
are about 10x off from LDL-P values in nmol/L. In other words, in
the Framingham population, the 20th percentile value of LDL-C was
100 mg/dL, while the MESA trial found the 20th percentile of the
population to have an LDL-P concentration of 1,000 nmol/L. As
you will see by the end of this post, this “rule of the thumb” should
never be used to infer LDL-P from LDL-C.
If this were always the case – that is, if LDL-C and LDL-P were
always concordant – we could conclude that LDL-C and LDL-P
would be of equal value in predicting heart disease. Obviously this
is not the case, or I wouldn’t be making such a fuss over the
distinction. But how bad is it?
What do discordant LDL-C and LDL-P values look like?
The figure below, from the Journal of Clinical Lipidology, shows the
The straight dope on cholesterol – Part VI
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cumulative incidence of cardiovascular events (e.g., myocardial
infarction, death) over time in three sub-populations:
Those with concordant LDL-P and LDL-C (black line);1.
Those with discordant LDL-P and LDL-C (LDL-P>LDL-C,
shown by the red line);
2.
Those with discordant LDL-P and LDL-C (LDL-P<LDL-C,
shown by the blue line).
3.
This analysis was done using a Cox proportional hazard model and
was adjusted for age, sex, and race. The steeper the line the more
people in that sub-population died or experienced adverse cardiac
events relative to other sub-populations. In other words, the folks in
the red group had the worst outcomes, followed by the folks in the
black group, followed by the folks in the blue group.
What can we infer from these data?
First, we confirm what I alluded to above. Namely, that a non-zero
percent of the population do not have LDL-C and LDL-P values that
The straight dope on cholesterol – Part VI
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predict the same level of risk. However, and perhaps more
importantly, we get another look at an important theme of this
series: LDL-P is driving atherosclerotic risk, not LDL-C. If
LDL-P and LDL-C were equally “bad” – even when discordant –
you would expect the blue line to be as steep as the red line (and
both to be steeper than the black line). But this is not the case.
Let’s look at these data parsed out another way. Below we see the
four possible subgroups, from the top:
Not low LDL-P, low LDL-C (red line);1.
Not low LDL-P, not low LDL-C (yellow line);2.
Low LDL-P, low LDL-C (black line); and3.
Low LDL-P, not low LDL-C (blue line).4.
Note that “low” is defined below the 30th percentile and “not low” is
defined as greater than 30th percentile for each variable. This
figure is even more revealing than the one above. Again, it
demonstrates the frequency of discordance (about 20% in this
population with these cut-off points), and it shows the importance of
LDL-P’s predictive power, relative to that of LDL-C.
In fact, though not statistically significant, the highest risk group
has high LDL-P and actually has low LDL-C (I’ll give you a hint of
why, below) while the lowest risk group has low LDL-P and
not-low LDL-C. *This is not a typo.
The straight dope on cholesterol – Part VI
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The highest risk and lowest risk groups are those with discordant
LDL-C and LDL-P. The high risk group has high LDL-P and low
LDL-C, while the lowest risk group has high LDL-C with low
LDL-P. Only a minority of physicians would know that there is a
segment of the population with elevated LDL-C who are at low risk!
The same conclusion will be drawn from the next study.
Let’s look at an even longer-term follow up study, below. This study
followed a Framingham offspring cohort of about 2,500 patients
over a median time period of almost 15 years in each of the four
possible groups (i.e., high-high, high-low, low-high, and low-low)
and tracked event-free survival. In this analysis the cut-off points
for LDL-P and LDL-C were the median population values of 1,414
nmol/L and131 mg/dL, respectively. So “high” implies above these
values; “low” implies below these values. Kaplan-Meier survival
curves are displayed over a 16 year period – the steeper the slope
of the line the worse the outcome (survival).
The straight dope on cholesterol – Part VI
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The same patterns are observed:
LDL-P is the best predictor of adverse cardiac events.1.
LDL-C is only a good predictor of adverse cardiac events when
it is concordant with LDL-P; otherwise it is a poor predictor of
risk.
2.
Amazingly the persons with the worst survival had low (below
median) LDL-C but high LDL-P. The patients most likely to have
high LDL-P with unremarkable or low LDL-C are those with either
small LDL particles, or TG-rich / cholesterol poor LDL particles, or
both (e.g., insulin resistant patients, metabolic syndrome patients,
T2DM patients). This explains why small LDL particles, while
no more atherogenic on a per particle basis than large
particles, are a marker for something sinister.
Populations where LDL-P and LDL-C discordance are even
more prevalent
As I described above, the discordance between LDL-P and LDL-C
is exacerbated in patients with metabolic syndrome. The figure
below, MESA data, again borrowed from Jim Otvos, presents this
difference in an elegant way. The horizontal axes show LDL-P
concentration in the usual units, nmol/L.
The straight dope on cholesterol – Part VI
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Patients with LDL-C between 100 and 118 mg/dL (i.e., second
quartile of risk: 25th to 50th percentile) are shown without metabolic
syndrome (top) and with metabolic syndrome (bottom). In the
patients without metabolic syndrome, LDL-C under-predicts
cardiac risk 22% of the time, consistent with the population data I
have shown you earlier. However, when you look at the patients
with metabolic syndrome, you can see that 63% of the time their
risk of cardiac disease is under-predicted. Again, not a typo.
There are so many subsets and cut-off points that I could devote
ten more posts to showing you every one of these analyses. Let
me finish this point with the most recent, hot-off-the-press (actually,
still in press in the American Journal of Cardiology, though you can
get a preprint here) analysis of which Tom Dayspring is one of the
authors.
The straight dope on cholesterol – Part VI
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These data were collected from nearly 2,000 patients with diabetes
who presented with “perfect” standard cholesterol numbers: LDL-C
< 70 mg/dL; HDL-C > 40 mg/dL; TG <150 mg/dL. However, only
in 22% of cases were their LDL-P concordant with LDL-C. That is,
in only 22% of cases did these patients have an LDL-P level below
700 nmol/L.
Remember, LDL-C < 70 mg/dL is considered VERY low risk – the
5th percentile. Yet, by LDL-P, the real marker of risk, 35% of these
patients had more than 1,000 nmol/L and 7% were high risk. When
you do this analysis with the same group of patients stratified by
less stringent LDL-C criteria (e.g., <100 mg/dL) the number of
patients in the high risk group is even higher.
The real world tragedy: 90-95% of physicians, including
cardiologists, would bet their own lives that persons with an
LDL-C < 70 mg/dL have no atherosclerotic risk.
Tim Russert, shortly before his death, had his LDL-C level
checked. It was less than 70 mg/dL. Sadly, his doctors didn’t
realize they should also have been checking his LDL-P or apoB.
The figure below, which is from one of Tom Dayspring’s
presentations, shows data from this study of nearly 137,000
patients hospitalized for coronary artery disease between 2000 and
2006. As you can see, LDL-C fails to even reasonably predict
cardiovascular disease in a patient population sick enough to show
up in the hospital with chest pain or outright myocardial infarction.
The straight dope on cholesterol – Part VI
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Why are LDL-C and LDL-P so often discordant?
Think back to what you learned in a previous post in this series.
LDL particles traffic not only cholesterol ester but also triglycerides.
Each and every LDL particle has a variable number of cholesterol
molecules which, because of constant particle remodeling, is
constantly changing. In other words, of the several quadrillion LDL
particles floating in your plasma, no two are carrying the exact
same number of cholesterol molecules. It takes many more
cholesterol-depleted LDL particles than cholesterol-rich LDL
particles to traffic a given cholesterol mass (i.e., number of
cholesterol molecules) per volume of plasma (i.e., per dL). Core
cholesterol mass is related to both LDL particle size (the volume
of a sphere is a third power of the radius — it can take 40-70%
more small particles than large LDL particles to traffic a given
cholesterol mass) and the number of TG molecules per LDL
particle.
TG molecules are larger than cholesterol ester molecules, so as the
number of TG molecules per particle increases, the number of
cholesterol molecules will be less – in a very non-linear manner.
Regardless of size it takes many more TG-rich LDL particles (which
are necessarily cholesterol-depleted) to traffic a given cholesterol
mass than TG-poor LDL particles. The persons with the highest
LDL particles typically (though not always) have small LDL particles
that are TG-rich. These are incredibly cholesterol-depleted LDL
particles.
Summary
Take a look at this figure below from the 2011 Otvos et al. paper I
referenced above. It’s a scatterplot of each data point (i.e., patient)
in the study. The solid red line shows perfect concordance between
LDL-P and LDL-C. The dashed red lines show a +/- 12% margin
The straight dope on cholesterol – Part VI
15 of 17
on each side. Look at how many dots (remember: each dot
represents a person) lie OUTSIDE of the dashed red lines. Now
look again.
When people argue with me about why it’s unnecessary to check
LDL-P or apoB because it’s much easier and cheaper to check
LDL-C, I like to remind them of what Clint Eastwood would probably
say in such a situation: “You’ve got to ask yourself one
question: Do I feel lucky? Well, do ya, punk?”
With respect to laboratory medicine, two markers that have a
high correlation with a given outcome are concordant – they
equally predict the same outcome. However, when the two tests
do not correlate with each other they are said to be discordant.
1.
LDL-P (or apoB) is the best predictor of adverse cardiac events,
which has been documented repeatedly in every major
cardiovascular risk study.
2.
The straight dope on cholesterol – Part VI
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LDL-C is only a good predictor of adverse cardiac events when
it is concordant with LDL-P; otherwise it is a poor predictor of
risk.
3.
There is no way of determining which individual patient may
have discordant LDL-C and LDL-P without measuring both
markers.
4.
Discordance between LDL-C and LDL-P is even greater in
populations with metabolic syndrome, including patients with
diabetes. Given the ubiquity of these conditions in the U.S.
population, and the special risk such patients carry for
cardiovascular disease, it is difficult to justify use of LDL-C,
HDL-C, and TG alone for risk stratification in all but the most
select patients.
5.
This raises the question: if indeed LDL-P is always as good and
in most cases better than LDL-C at predicting cardiovascular
risk, why do we continue to measure (or calculate) LDL-C at all?
6.
(To Part VII »)
30
MAY
The straight dope on cholesterol – Part VI
17 of 17
eatingacademy.com
The straight dope on cholesterol – Part
VII
Previously, in Part I, Part II, Part III, Part IV, Part V ,and Part VI of
this series, we addressed these 8 concepts:
#1 — What is cholesterol?
#2 — What is the relationship between the cholesterol we eat
and the cholesterol in our body?
#3 — Is cholesterol bad?
#4 — How does cholesterol move around our body?
#5 – How do we measure cholesterol?
#6 – How does cholesterol actually cause problems?
#7 – Does the size of an LDL particle matter?
#8 – Why is it necessary to measure LDL-P, instead of just
LDL-C?
In this post we’ll continue to build out the story with the next
concept:
#9 – Does “HDL” matter after all?
(No so) Quick refresher on take-away points from previous
posts, should you need it:
Cholesterol is “just” another fancy organic molecule in our body
but with an interesting distinction: we eat it, we make it, we
1.
The straight dope on cholesterol – Part VII
1 of 17
store it, and we excrete it – all in different amounts.
The pool of cholesterol in our body is essential for life. No
cholesterol = no life.
2.
Cholesterol exists in 2 forms – unesterified or “free” (UC) and
esterified (CE) – and the form determines if we can absorb it
or not, or store it or not (among other things).
3.
Much of the cholesterol we eat is in the form of CE. It is not
absorbed and is excreted by our gut (i.e., leaves our body in
stool). The reason this occurs is that CE not only has to be
de-esterified, but it competes for absorption with the vastly
larger amounts of UC supplied by the biliary route.
4.
Re-absorption of the cholesterol we synthesize in our body
(i.e., endogenous produced cholesterol) is the dominant source
of the cholesterol in our body. That is, most of the cholesterol
in our body was made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will discover that synthesis
and absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
up. To see an important reference on this topic, please look
here.
7.
Cholesterol and triglycerides are not soluble in plasma (i.e.,
they can’t dissolve in water) and are therefore said to be
8.
The straight dope on cholesterol – Part VII
2 of 17
hydrophobic.
To be carried anywhere in our body, say from your liver to your
coronary artery, they need to be carried by a special protein-
wrapped transport vessel called a lipoprotein.
9.
As these “ships” called lipoproteins leave the liver they undergo
a process of maturation where they shed much of their
triglyceride “cargo” in the form of free fatty acid, and doing so
makes them smaller and richer in cholesterol.
10.
Special proteins, apoproteins, play an important role in moving
lipoproteins around the body and facilitating their interactions
with other cells. The most important of these are the apoB
class, residing on VLDL, IDL, and LDL particles, and the apoA-I
class, residing for the most part on the HDL particles.
11.
Cholesterol transport in plasma occurs in both directions,
from the liver and small intestine towards the periphery and
back to the liver and small intestine (the “gut”).
12.
The major function of the apoB-containing particles is to traffic
energy (triglycerides) to muscles and phospholipids to all
cells. Their cholesterol is trafficked back to the liver. The apoA-I
containing particles traffic cholesterol to steroidogenic tissues,
adipocytes (a storage organ for cholesterol ester) and
ultimately back to the liver, gut, or steroidogenic tissue.
13.
All lipoproteins are part of the human lipid transportation system
and work harmoniously together to efficiently traffic lipids. As
you are probably starting to appreciate, the trafficking pattern is
highly complex and the lipoproteins constantly exchange their
core and surface lipids.
14.
The measurement of cholesterol has undergone a dramatic
evolution over the past 70 years with technology at the heart of
the advance.
15.
The straight dope on cholesterol – Part VII
3 of 17
Currently, most people in the United States (and the world for
that matter) undergo a “standard” lipid panel, which only
directly measures TC, TG, and HDL-C. LDL-C is measured or
most often estimated.
16.
More advanced cholesterol measuring tests do exist to directly
measure LDL-C (though none are standardized), along with the
cholesterol content of other lipoproteins (e.g., VLDL, IDL) or
lipoprotein subparticles.
17.
The most frequently used and guideline-recommended test that
can count the number of LDL particles is either
apolipoprotein B or LDL-P NMR, which is part of the NMR
LipoProfile. NMR can also measure the size of LDL and other
lipoprotein particles, which is valuable for predicting insulin
resistance in drug naïve patients, before changes are noted in
glucose or insulin levels.
18.
The progression from a completely normal artery to a “clogged”
or atherosclerotic one follows a very clear path: an apoB
containing particle gets past the endothelial layer into the
subendothelial space, the particle and its cholesterol content is
retained, immune cells arrive, an inflammatory response ensues
“fixing” the apoB containing particles in place AND making more
space for more of them.
19.
While inflammation plays a key role in this process, it’s the
penetration of the endothelium and retention within the
endothelium that drive the process.
20.
The most common apoB containing lipoprotein in this process is
certainly the LDL particle. However, Lp(a) and apoB containing
lipoproteins play a role also, especially in the insulin resistant
person.
21.
If you want to stop atherosclerosis, you must lower the LDL
particle number. Period.
22.
The straight dope on cholesterol – Part VII
4 of 17
At first glance it would seem that patients with smaller LDL
particles are at greater risk for atherosclerosis than patients with
large LDL particles, all things equal.
23.
“A particle is a particle is a particle.” If you don’t know the
number, you don’t know the risk.
24.
With respect to laboratory medicine, two markers that have a
high correlation with a given outcome are concordant – they
equally predict the same outcome. However, when the two tests
do not correlate with each other they are said to be discordant.
25.
LDL-P (or apoB) is the best predictor of adverse cardiac events,
which has been documented repeatedly in every major
cardiovascular risk study.
26.
LDL-C is only a good predictor of adverse cardiac events when
it is concordant with LDL-P; otherwise it is a poor predictor of
risk.
27.
There is no way of determining which individual patient may
have discordant LDL-C and LDL-P without measuring both
markers.
28.
Discordance between LDL-C and LDL-P is even greater in
populations with metabolic syndrome, including patients with
diabetes. Given the ubiquity of these conditions in the U.S.
population, and the special risk such patients carry for
cardiovascular disease, it is difficult to justify use of LDL-C,
HDL-C, and TG alone for risk stratification in all but the most
select patients.
29.
To address this question, however, one must look at changes in
cardiovascular events or direct markers of atherosclerosis (e.g.,
IMT) while holding LDL-P constant and then again holding
LDL size constant. Only when you do this can you see that
the relationship between size and event vanishes. The only
30.
The straight dope on cholesterol – Part VII
5 of 17
thing that matters is the number of LDL particles – large, small,
or mixed.
Concept #9 – Does “HDL” matter after all?
Last week was the largest annual meeting of the National Lipid
Association (NLA) in Phoenix, AZ. The timing of the meeting could
not have been better, given the huge buzz going around on the
topic of “HDL.” (If you’re wondering why I’m putting HDL in quotes,
I’ll address it shortly.)
What buzz, you ask? Many folks, including our beloved health
columnists at The New York Times, are talking about the death of
the HDL hypothesis – namely, the notion that HDL is the “good
cholesterol.”
Technically, this “buzz” started about 6 years ago when Pfizer made
headlines with a drug in their pipeline called torcetrapib.
Torcetrapib was one of the most eagerly anticipated drugs ever,
certainly in my lifetime, as it had been shown to significantly raise
plasma levels of HDL-C. You’ll recall from part II of this series,
HDL particles play an important role in carrying cholesterol from the
subendothelial space back to the liver via a process called reverse
cholesterol transport (RCT). Furthermore, many studies and
epidemiologic analyses have shown that people with high plasma
levels of HDL-C have a lower incidence of coronary artery disease.
In the case of torcetrapib, there was an even more compelling
reason to be optimistic. Torcetrapib blocked the protein
cholesterylester transfer protein, or CETP, which facilitates the
collection and one-to-one exchange of triglycerides and cholesterol
esters between lipoproteins. Most (but not all) people with a
mutation or dysfunction of this protein were known to have high
levels of HDL-C and lower risk of heart disease. Optimism was very
high that a drug like torcetrapib, which could mimic this effect and
create a state of more HDL-C and less LDL-C, would be the biggest
blockbuster drug ever.
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The past month or so has seen this discussion intensify, which I’ll
quickly try to cover below.
The data
Torcetrapib
After several smaller clinical trials showed that patients taking
torcetrapib experienced both an increase in HDL-C and a reduction
in LDL-C, a large clinical trial pitting atorvastatin (Lipitor) against
atorvastatin + torcetrapib was underway. This trial was to be the
jewel in the crown of Pfizer. It was already known that Lipitor
reduced coronary artery disease (and reduced LDL-C, though this
may have been a bystander effect and real reduction in mortality
may be better attributed to the reduction in LDL-P).
I still remember exactly where I was standing, on the corner of
Kerney St. and California St. in the heart of San Francisco’s
financial district, on that December day back in 2006 when it was
announced the trial had been halted because of increased mortality
in the group receiving torcetrapib. In other words, adding
torcetrapib actually made things worse. I was shocked.
Many reasons were offered for this, including the notion that
torcetrapib was, indeed, helpful, but because of unanticipated
side-effects, (raising blood pressure in some patients and altering
electrolyte balance in others), the net impact was harmful. Some
even suggested that the drug could be useful in the “right” patients
(e.g., those with low HDL-C, but normal blood pressure).
Furthermore, in two subsequent studies looking at carotid IMT
(thickening of the carotid arteries) and intravascular ultrasound,
there was no reduction in atherosclerosis.
This was a big strike against the HDL hypothesis and work on
torcetrapib was immediately halted.
Niacin
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Niacin has long been known to raise HDL-C and has actually been
used therapeutically for this reason for many years. The AIM-HIGH
trial (Atherothrombosis Intervention in Metabolic Syndrome with
Low HDL/High Triglycerides – you can’t have trials in medicine
without catchy names!) sought to test this. The trial randomly
assigned over 3,000 patients with known and persistent, but stable
and well treated cardiovascular risk, to one of two treatments:
Simvastatin (40-80 mg/day), +/- ezetimibe (10 mg/day) as
necessary to maintain LDL-C below 70 mg/dL + placebo (a tiny
dose of crystalline niacin to cause flushing);
1.
As above, but instead of a placebo, patients were given 1,500 to
2,000 mg/day of extended-release niacin.
2.
Both arms of the study had their LDL-C < 70 mg/dL, non-HDL-C <
100 md/dL and apoB < 80 mg/dL, but despite the statin or statin +
ezetimibe treatment still had low HDL-C. So, if niacin raised HDL-C
and reduced events, the HDL raising hypothesis would be proven.
Simvastatin, as its name suggests, is a statin which primarily works
by blocking HMG-CoA reductuse, an enzyme necessary to
synthesize endogenous cholesterol. Ezetimibe works on the other
end of problem, by blocking the NPC1L1 transporter on gut
enterocytes and hepatocytes at the hepatobiliary junction (for a
quick refresher, go back to part I of this series and look at the
second figure – ezetimibe blocks the “ticket taker” in the bar).
After two years the niacin group, as expected, had experienced a
significant increase in plasma HDL-C (along with some other
benefits like a greater reduction in plasma triglycerides). However,
there was no improvement in patient survival. The trial was futile
and the data and safety board halted the trial. In other words, for
patients with cardiac risk and LDL-C levels at goal with medication
niacin, despite raising HDL-C and lowering TG, did nothing to
improve survival. This was another strike against the HDL
hypothesis.
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Dalcetrapib
By 2008, as the AIM-HIGH trial was well under way, another
pharma giant, Roche, was well into clinical trials with another drug
that blocked CETP. This drug, a cousin of torcetrapib called
dalcetrapib, albeit a weaker CETP-inhibitor, appeared to do all the
“right” stuff (i.e., it increased HDL-C) without the “wrong” stuff (i.e., it
did not appear to adversely affect blood pressure). It did nothing to
LDL-C or apoB.
This study, called dal-OUTCOMES, was similar to the other trials in
that patients were randomized to either standard of care plus
placebo or standard of care plus escalating doses of dalcetrapib.
A report of smaller safety studies (called dal-Vessel and
Dal-Plaque) was published a few months ago in the American
Heart Journal, and shortly after Roche halted the phase 3 clinical
trial. Once again, patients on the treatment arm did experience a
significant increase in HDL-C, but failed to appreciate any clinical
benefit. Another futile trial.
Currently, two additional CETP inhibitors, evacetrapib
(manufactured by Lilly) and anacetrapib (manufactured by Merck)
are being evaluated. They are much more potent CETP inhibitors
and, unlike dalcetrapib, also reduce apoB and LDL-C and Lp(a).
Both Lilly and Merck are very optimistic that their variants will be
successful where Pfizer’s and Roche’s were not, for a number of
reasons including greater anti-CETP potency.
Nevertheless, this was yet another strike against the HDL
hypothesis because the drug only raised HDL-C and did nothing to
apoB. If simply raising HDL-C without attacking apoB is a viable
therapeutic strategy, the trial should have worked. We have been
told for years (by erroneous extrapolation from epidemiologic data)
that a 1% rise in HDL-C would translate into a 3% reduction in
coronary artery disease. These trials would suggest otherwise.
Mendelian randomization
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On May 17 of this year a large group in Europe (hence the spelling
of randomization) published a paper in The Lancet, titled, “Plasma
HDL cholesterol and risk of myocardial infarction: a mendelian
randomisation study.” Mendelian randomization, as its name sort of
suggests, is a method of using known genetic differences in large
populations to try to “sort out” large pools of epidemiologic data.
In the case of this study, pooled data from tens of studies where
patients were known to have myocardial infarction (heart attacks)
were mapped against known genetic alterations called SNPs
(single nucleotide polymorphisms, pronounced “snips”). I’m not
going to go into detail about the methodology because it would take
3 more blog posts., But, the reason for doing this analysis was to
ferret out if having a high HDL-C was (only) correlated with better
cardiovascular outcome, which has been the classic teaching, or if
there was any causal relationship. In other words, does having a
high HDL-C cause you to have a lower risk of heart disease or is it
a marker for something else?
This study found, consistent with the trials I’ve discussed above,
that any genetic polymorphism that seems to raise HDL-C does not
seem to protect from heart disease. That is, patients with higher
HDL-C due to a known genetic alteration did not seem to have
protection from heart disease as a result of that gene. This
suggests that people with high or low HDL-C who get coronary
artery disease may well have something else at play.
Oh boy. This seems like the last nail in the casket of the entire
“HDL” hypothesis, as evidenced by all of the front page stories
worldwide.
The rub: the difference between HDL-C and HDL-P
The reason I’ve been referring to high density lipoprotein as “HDL,”
unless specifically referring to HDL-C, is that HDL-P and HDL-C are
not the same thing. Just as you are now intimately familiar with the
notion that LDL-C and LDL-P are not the same thing, the same is
true for “HDL” which simply stands for high density lipoprotein, and
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like LDL is not a lab assay. In fact, unpublished data from the
MESA trial found that the correlation between HDL-C and HDL-P
was only 0.73, which is far from “good enough” to say HDL-C is a
perfect proxy for HDL-P.
HDL-C, measured in mg/dL (or mmol/L outside of the U.S.), is the
mass of cholesterol carried by HDL particles in a specified volume
(typically measured as X mg of cholesterol per dL of plasma).
HDL-P is something entirely different. It’s the number of HDL
particles (minus unlipidated apoA-I and prebeta-HDLs: at most 5%
of HDL particles) contained in a specified volume (typically
measured as Y micromole of particles per liter).
As you can see in the figure below (courtesy of Jim Otvos’
presentation at the NLA meeting 2 weeks ago), the larger an HDL
particle, the more cholesterol it carries. So, an equal number of
large versus small HDL particles (equal HDL-P) can carry very
different amounts of cholesterol (different HDL-C). Of course, it’s
never this simple because HDL particles, like their LDL
counterparts, don’t just carry cholesterol. They carry triglycerides,
too. Keep in mind, HDL core CE/TG ratio is about 10:1 or greater –
if the large HDL carries TG, it will not be carrying very much
cholesterol.
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So, the important point is that HDL-C is not the same as HDL-P
(which is also not the same as apoAI, as HDL particles can carry
more than one apoAI).
But there’s something else going on here. If you look at the figure
below, from the Framingham cohort, you’ll note something
interesting. As HDL-C rises, it does so not in a uniform or “across
the board” fashion. A rise in HDL-C seems to disproportionately
result from an increase in large HDL particles. In other words, as
HDL-C rises, it doesn’t necessarily mean HDL-P is rising at all, and
certainly not as much.
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As you can see, for increases in HDL-C at low levels (i.e., below 40
mg/dL) the increase in small particles seems to account for much of
the increase in total HDL-P, While for increases over 40 mg/dL, the
increase in large particles seems to account for the increase in
HDL-C. Also note that as HDL-C rises above 45 mg/dL, there is
almost no further increase in total HDL-P – the rise in HDL-C is
driven by enlargement of the HDL particle – more cholesterol per
particle – not the drop in small HDL-P. This reveals to us that the
small HDL particles are being lipidated.
Is there a reason to favor small HDL particles over large ones?
In the 2011 article, “Biological activities of HDL subpopulations and
their relevance to cardiovascular disease,” published in Trends in
Molecular Medicine, the authors describe in great detail some of
protective mechanisms imparted by HDL particles.
Large HDL particles may be less protective and even dysfunctional
in certain pathological states, whereas small to medium-sized HDL
particles seem to confer greater protection through the following
mechanisms:
Greater antioxidant activity
Greater anti-inflammatory activity
Greater cholesterol efflux capacity
Greater anti-thrombotic properties
In other words, particle for particle, it seems a small HDL particle
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may be better at transporting cholesterol from the subendothelial
space (technically, they acquire cholesterol from cholesterol-laden
macrophages or foam cells in the subedothelial space) elsewhere,
better at reducing inflammation, better at preventing clotting, and
better at mitigating the problems caused by oxidative free radicals.
Of course, reality is complicated. If there was no maturation from
small to large HDL particles (i.e., the dynamic remodeling of HDL),
the system would be faulty. So, the truth is that all HDL sizes are
required and that HDL particles are in a constant dynamic state (or
“flux”) of lipidating and delipidating, and the real truth is no
particular HDL size can be said to be the best. If the little HDLs do
not enlarge, the ApoA-I mediated lipid trafficking system is broken.
The truth about the old (and overly simplistic) term called
reverse cholesterol transport (RCT)
HDL particles traffic cholesterol and proteins and last in plasma on
average for 5 days. They are in a constant state of acquiring
cholesterol (lipidation) and delivering cholesterol (delipidation).
There are membrane receptors on cells that can export cholesterol
to HDL particles (sterol efflux transporters) or extract cholesterol
or cholesterol ester from HDL particles (sterol influx transporters).
The vast majority of lipidation occurs (in order): 1) at the liver, 2) the
small intestine, 3) adipocytes and 4) peripheral cells, including
plaque if present. The liver and intestine account for 95% of this
process. The amount of cholesterol pulled out of arteries (called
macrophage reverse cholesterol transport) is critical to disease
prevention but is so small it has no effect on serum HDL levels.
Even in patients with extensive plaque, the cholesterol in that
plaque is about 0.5% of total body cholesterol. HDL particles
circulate for several days as a ready reserve of cholesterol: almost
no cell in humans require a delivery of cholesterol as cells
synthesize all they need. However, steroidogenic hormone
producing tissues (e.g., adrenal cortex and gonads) do require
cholesterol and the HDL particle is the primary delivery truck.
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If, as is the case in a medical emergency, the adrenal gland must
rapidly make a lot of cortisone, the HDL particles are there with the
needed cholesterol. This explains the low HDL-C typically seen in
patients with severe infections (e.g., sepsis) and severe
inflammatory conditions (e.g., Rheumatoid Arthritis).
Sooner or later HDL particles must be delipidated, and this takes
place at: 1) the adrenal cortex or gonads 2) the liver, 3) adipocytes,
4) the small intestine (TICE or transintestinal cholesterol efflux) or
give its cholesterol to an apoB particle (90% of which are LDLs) to
return to the liver. A HDL particle delivering cholesterol to the liver
or intestine is called direct reverse cholesterol transport (RCT),
whereas a HDL particle transferring its cholesterol to an apoB
particle which returns it to the liver is indirect RCT. Hence, total
RCT = direct RCT + indirect RCT.
The punch line: a serum HDL-C level has no known relationship to
this complex process of RCT. The last thing a HDL does is lose its
cholesterol. The old concept that a drug or lifestyle that raises
HDL-C is improving the RCT process is wrong; it may or may not
be affecting that dynamic process. Instead of calling this RCT, it
would be more appropriately called apoA-I trafficking of cholesterol.
Why do drugs that specifically raise HDL-C seem to be of little
value?
As I’ve argued before, while statins are efficacious at preventing
heart disease, it’s sort of by “luck” as far as most prescribing
physicians are concerned. Most doctors use cholesterol lowing
medication to lower LDL-C, not LDL-P. Since there is an overlap
(i.e., since the levels of LDL-P and LDL-C are concordant) in many
patients, this misplaced use of statins seems to work “ok.” I, and
many others far more knowledgeable, would argue that if statins
and other drugs were used to lower LDL-P (and apoB), instead of
LDL-C, their efficacy would be even greater. The same is true for
dietary intervention.
Interestingly, (and I would have never known this had Jim Otvos not
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graciously spent a hour on the phone with me two weeks ago giving
me a nuanced HDL tutorial), a study that went completely
unnoticed by the press in 2010, published in Circulation, actually
did a similar analysis to the Lancet paper, except that the authors
looked at HDL-P instead of HDL-C as the biomarker and looked at
the impact of phospholipid transfer protein (PLTP) on HDL
metabolism. In this study, though not the explicit goal, the authors
found that an increase in the number of HDL particles and smaller
HDL particles decreased the risk of cardiovascular disease. The
key point, of course, is that the total number of HDL particles rose,
and it was driven by increased small HDL-P. The exact same thing
was seen in the VA-HIT trial: the cardiovascular benefit of the
treatment (fibrate) was related to the rise in total HDL-P which was
driven by the fibrates’ ability to raise small HDL-P.
It seems the problem with the “HDL hypothesis” is that it’s using the
wrong marker of HDL. By looking at HDL-C instead of HDL-P,
these investigators may have missed the point. Just like LDL, it’s
all about the particles.
Summary
HDL-C and HDL-P are not measuring the same thing, just as
LDL-C and LDL-P are not.
1.
Secondary to the total HDL-P, all things equal it seems smaller
HDL particles are more protective than large ones.
2.
As HDL-C levels rise, most often it is driven by a
disproportionate rise in HDL size, not HDL-P.
3.
In the trials which were designed to prove that a drug that raised
HDL-C would provide a reduction in cardiovascular events, no
benefit occurred: estrogen studies (HERS, WHI), fibrate studies
(FIELD, ACCORD), niacin studies, and CETP inhibition studies
(dalcetrapib and torcetrapib). But, this says nothing of what
happens when you raise HDL-P.
4.
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Don’t believe the hype: HDL is important, and more HDL
particles are better than few. But, raising HDL-C with a drug isn’t
going to fix the problem. Making this even more complex is that
HDL functionality is likely as important, or even more important,
than HDL-P, but no such tests exist to “measure” this.
5.
One last thing for San Diego residents…
On Wednesday June 20 I’ll be giving a talk at the UCSD School of
Medicine titled: The limits of scientific evidence and the ethics of
dietary guidelines.
The talk will be given at the UCSD School of Medicine in the
Medical Teaching Facility, Room 175 (map: http://maps.ucsd.edu
/mapping/viewer/default.htm) [It’s in the blue section of the map,
numbered building 830]. I’m told folks should show up 15 minutes
early to find parking and to get a seat.
The seminar runs from 4:30-6:30 pm and includes lots of time for
Q&A. Unfortunately, it won’t be filmed, so I hope you can make it.
I am actually really looking forward to this talk, as it covers material
I have not spoken on publicly before. I hope some of you can make
it.
13
JUN
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eatingacademy.com
The straight dope on cholesterol – Part
VIII
Last week the Journal of the American Medical Association (JAMA)
published an article titled Lipid-Related Markers and Cardiovascular
Disease Prediction, which you can download here. This is quite
timely as we are in the midst of our series on cholesterol and heart
disease risk factors.
I was planning to write a post on my interpretation of this report, as
I know many of you have questions about it, when I was reminded
of one of my favorite principles in life: never be afraid to outsource
to those more qualified.
While there are many folks more qualified than me to address this
entire topic of cholesterol and heart disease risk, there are a
handful who have always been very generous with their time and
insights on this subject and who I consider mentors on this topic.
This list includes Drs. Tom Dayspring, Tara Dall, Allan Sniderman,
and Jim Otvos.
Below are excerpts of comments from Drs. Dayspring and Dall,
followed by the comments of Dr. Sniderman, with my comments
interspersed for clarification.
Initial response by Drs. Dayspring and Dall
The authors from the The Emerging Risk Factors Collaboration
(ERFC) conclude:
In a study of individuals without known cardiovascular disease
(CVD), the addition of information on the combination of
apolipoprotein B and A-I, lipoprotein(a), or lipoprotein associated
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phospholipase A2 (Lp-PLA2) mass to risk scores containing total
cholesterol and HDL-C led to slight improvement in CVD
prediction.
In other words, the authors concluded that advanced lipid testing,
beyond “just” LDL-C, HDL-C, TG, and total cholesterol did little to
help predict heart disease in people without a known history of
heart disease.
The accompanying editorial by Dr. Scott Grundy (the former NCEP
chairman) raises several flaws of the analysis including old apoB
data where studies used primitive and non-standardized apoB
assays as well as the use of out-dated older risk assessment tools
established 20-30 years ago when cardiac disease manifestation
and presentation were very different than today.
These analyses are flawed with respect to examining the
atherogenic lipoprotein variables in patients in which cholesterol
measurements and lipoprotein concentration measurements are
not also examined in the patients where the variables are
discordant. These measures (cholesterol concentrations and apoB)
are correlated. However, in the many patients where the measures
are discordant, apoB and LDL-P are the proven better variables to
measure both risk prediction and therapeutic goals. It is also
unfortunate that this study provided no LDL-P analysis. Thus,
these analyses might be of some interest to epidemiologists who
look at entire populations, yet have little value to practicing
clinicians who treat people one at a time.
It is difficult to make the case for apoA-I by itself in routine
screening as it is not the most accurate way of quantifying total
HDL-P. However both AMORIS (which somehow was not included
in this analysis) and INTERHEART — two very large trials —
revealed that the best risk predictor was the apoB/apoA-I ratio. So,
in drug naive patients the ratio (which requires apoA-I
measurement) is validated. No ratio is likely valid in patients on lipid
modulating medications as drugs do not effect apoB and apoA-I
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equally nor do apoB and apoA-I have equal predictive abilities.
With respect to inflammation markers, such as highly sensitive
C-reactive protein (hs-CRP), their appropriate use (as discussed in
the recent NLA biomarker statement) is to be used not in place of
lipid or lipoprotein concentrations but afterwards to better fine tune
risk which several studies have shown they do. Their elevation,
based on current knowledge, should lead the clinician to obtain
more resolute lipid and lipoprotein goals of therapy, not per se any
(still nonexistent) inflammatory goals of therapy. However, current
studies do suggest further studies will be needed to show if it is
important to also normalize at least some inflammatory markers.
The JAMA study states:
The addition of the combination apolipoprotein B and A-I,
lipoprotein(a), or lipoprotein-associated phospholipaseA2
(Lp-PLA2) to risk scores containing total cholesterol and HDL-C
provided slight improvement in CVD prediction.
When you apply that slight improvement to 300 million Americans
you are talking about millions of persons who would indeed benefit.
Interestingly, last year the NLA reviewed all of these data, and
much more, and came to the conclusion that apoB, LDL-P, Lp-PLA2
and Lp(a) were indeed useful in almost all folks who have greater
than a 5% ten-year Framingham Risk score (most adults over 40
years of age).
Subsequent response by Drs. Dayspring and Dall
This study combined data from 37 prospective cohort studies where
plasma apolipoprotein levels were measured at baseline in patients
followed for an average of 10 years. They conducted 2 analyses:
One which used apolipoprotein B (apoB), apolipoprotein A-I
(apoA-I), lipoprotein(a) (Lp(a)), or Lp-PLA2 instead of total
cholesterol (TC) and HDL cholesterol (HDL-C), and
1.
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One using the alternative biomarkers in addition to TC and
HDL-C.
2.
They concluded that replacement of TC and HDL-C with
apolipoproteins or their ratios was not associated with improved
cardiovascular risk prediction, whereas adding lipoprotein factors to
TC and HDL-C was associated with slight improvement in risk
prediction.
Several previous epidemiologic studies have demonstrated that
apolipoproteins, including apoB, may be as good as, and often
better than, LDL-C , non-HDL-C and cholesterol ratios for
estimating coronary heart disease (CHD) risk. In a previous
meta-analysis assessing the association between baseline apoB
levels and CHD risk from 19 prospective studies with follow-up of 9
years, apoB was a significant predictor of CHD, with an overall
relative risk of about 2 (i.e., double the risk) for the upper tertile
(i.e., upper third of the population) compared with the lower tertile.
Non-HDL-C has been suggested as a potential surrogate for apoB.
However, while non-HDL-C and apoB are highly correlated they
can also be discordant in many patients, including those with and
without metabolic syndrome, as shown here and here.
Clinical trials showing that apoB was superior, even to non-HDL-C,
in predicting risk for CHD are numerous, including AMORIS, Leiden
Heart Study, AFCAPS/TexCAPS, LIPID, Health Professional
Follow-up Study, NHANES, The Chinese Heart Study, Framingham
Offspring Study, Cardiovascular Risk in Young Finns,
INTERHEART, and IDEAL (summarized here).
Strong evidence now also exists that cardiovascular disease risk
tracks with LDL-P/apoB (not LDL-C) in patients with discordant
levels of these markers. Discordance analyses in the MESA study
show that LDL-C over- or underestimated LDL-related risk in many
patients, leading to suboptimal LDL management. This recently
published study in JAMA did not account for specific groups that
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were discordant but looked only at the population as a whole.
Remember physicians do not treat populations; they treat
individual patients, one by one.
How can a physician know if a patient is discordant if they do not
measure apoB or LDL-P? To restate the point, another limitation of
this study is that it did not include studies that used LDL-P analysis.
Most physicians view it as their goal not to miss one patient who
could benefit from preventive therapies through lifestyle and
counseling interventions or medications proven to reduce
cardiovascular (CV) risk in the primary prevention setting.
Multiple organizations support the use of apoB level as both marker
of CV risk and treatment goal. Current Canadian lipid guidelines
have incorporated apoB as an alternate primary target of therapy
due to the wealth of data supporting apoB in CV risk prediction. The
American Diabetes Association and American College of
Cardiology consensus statement in 2008 also recommended apoB
as a target of therapy in those with high cardiometabolic risk.
Furthermore, as part of the comprehensive diabetes care treatment
goals, the American Association of Clinical Endocrinology
published recommendations for apoB as another target of therapy
in addition to LDL-C, Non HDL-C, HDL-C and triglycerides. The
recommendations from AACC Lipoproteins and Vascular Diseases
Division Working Group on Best Practices also list goals of therapy
for apoB and LDL-P.
The JAMA study in question included studies from 1968 to 2007.
ApoB assays have improved significantly over the years, as early
assays were more primitive, non-standardized, and therefore less
reliable. The study authors recognize this limitation in their
comment section and it was also addressed in the accompanying
editorial by Dr. Scott Grundy, the former NCEP chairman. He
highlighted several other flaws in the analysis including the use of
out-dated risk assessment tools established 20-30 years ago when
coronary artery disease presentation was very different to today. In
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addition, correct interpretation of the study findings is difficult
without consideration of treatment differences among the patients
included in the study (e.g., patients with multiple high risk markers
may have been treated more aggressively, resulting in fewer
events).
As a practicing physician, I have used apoB/LDL-P for more than a
decade in order not to miss any patient that could be at risk and
might benefit from preventive therapy. I do not want any of my
patients to become part of the national statistic:
50% of people with heart disease have normal traditional lipid
values.
Population studies have diluted relevant clinical meaning to
physicians treating individual patients. Clearly a better measure is
needed to understand risk in individual patients. Randomized
controlled clinical outcomes trials in children are rare, but does that
mean we don’t treat children or young women? ApoB and/or
LDL–P can help physicians target which of those primary
prevention patients need more aggressive lifestyle or medical
therapy.
Conversely, it is also important not to over-treat patients with high
cholesterol who in fact may not have apoB, LDL-P or lipoprotein
(a)-related risk. It may also not be cost-effective or even
reasonable to treat such patients based on cholesterol levels. The
key is early detection for effective prevention. After very careful
review of all the published studies to date, the National Lipid
Association’s published consensus on advanced biomarker testing
in 2011 recommends that, except in the lowest risk patients, apoB
and LDL-P should be considered in most patients for both risk
assessment as well as ongoing clinical management.
On the other hand, apoA-I is minimally useful as a test in isolation
as there is not a one-to-one relationship between each HDL particle
and apoA-1 (as there is for an LDL particle and apoB). As there
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may be several apoA-I apolipoproteins on each HDL particle,
measuring apoA-I alone will not accurately quantify HDL-P. HDL is
incredibly complex and the functionality of the HDL particle will
likely be the focus of future assays and studies. For example, the
use of apoA-I as a tool to diagnosis familial
hypoalphalipoproteinemia is very helpful. This condition is very
difficult to treat clinically but is an important secondary cause of low
HDL-C that should be ruled out. Additionally, in both the
INTERHEART and AMORIS studies the best predictor of
cardiovascular risk was the apoB/apoA-I ratio.
Lipoprotein associated phospholipase A2 (Lp-PLA2) is an
inflammatory marker, not intended for use as a “stand-alone”
marker to assess cardiovascular risk, but in combination with other
lipoprotein-based tools (e.g., apoB and LDL-P). It is well
recognized that inflammation plays a role in atherosclerosis.
Currently accepted methods of assessing inflammation such as
high sensitivity C reactive protein (hs-CRP) may be elevated in
many disease states including, but not limited to, vascular disease.
Furthermore, hs-CRP levels may also fluctuate greatly so multiple
measurements are typically required. I have always considered
Lp-PLA2 to be a superior marker of vascular disease or what I
consider “angry arteries.” When Lp-PLA2 is elevated, treatments
aimed at reducing inflammation (e.g., dietary modification, omega-3
fatty acid supplementation, smoking cessation) become important.
Clinically, high levels of Lp-PLA2 indicate that the disease process
has not been effectively halted — arterial plaque may still be
actively forming — and more aggressive treatment is required as
unstable plaque may be present. Lp-PLA2 is not meant to be used
as a marker in isolation or to replace other traditional methods of
risk assessment. However, it greatly augments the utility of the
latter, and is a very useful tool to guide us in ongoing treatment
decisions.
Until levels of a patient’s biomarkers lie within the optimal range, it
is not clear that their risk has been eliminated. If our goal is to
reduce the epidemics of cardiovascular disease and diabetes, we
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need to be aware of the role lipoproteins play in cardiovascular and
diabetes disease prediction and continue to carry out research to
find better ways of detecting disease at earlier and earlier stages.
Additional commentary by Dr. Sniderman
In the JAMA study the average non-HDL-C was 175 mg/dl, which is
the 82nd percentile of the U.S. population whereas the average
apoB was 110 mg/dl, which is the 67th percentile. They should
match, but they do not. Obviously, some populations have higher
non-HDL-C than Americans. The Swedes, for example, certainly
do. But the Swedes have higher apoB levels to match. No
population that I have ever seen has values this discordant, which
means their lipoprotein composition is different from any I have ever
seen. This raises the question as to how accurately apoB was
measured. From Table 1, of the 26 studies with data on apoB,
blood was collected in 1 starting in 1968, in 1 in 1970, in 9 in the
1980’s, in 8 in the early 1990’s.
When were the apoB’s measured in relation to when they were
obtained? We don’t know. Reading the original papers, in the
great majority, measuring apoB was not part of the protocol. For 13
of the studies, no methods are listed and another 5 are listed as
in-house assays (i.e., non-standardized). None of these can have
standardized results. Nor are they necessarily accurate. Even the
studies employing commercial assays were not necessarily
standardized. The actual average values for apoB are listed in
Table 1 and, not surprisingly, they are extraordinarily variable.
These are mainly European studies and the average apoB ranges
from 0.86 to 1.33 with many of values in the 1.0 to 1.2 range. This
variance exceeds anything I have ever seen and anything that I
think is epidemiologically possible.
The trends can be compared within studies but the problem is that
this is a patient level study, which means all of these different
results from all of these different assays are mixed together. How
do you mix apples and oranges and nuts and pineapples and
pretend they are all cherries? Obviously, you can’t. If you did not
The straight dope on cholesterol – Part VIII
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measure something accurately, if the results from the different
studies differ so radically, how can they be lumped together?
What ERFC claims is the strength of their study is actually its
weakness. Our meta-analysis was done at the study level. All the
studies in our meta-analysis were published and therefore all the
methods to measure apoB are listed. There are two sources of
assay error: imprecision (lack of reproducibility) and inaccuracy
(lack of standardization). Study level analyses are certainly affected
by imprecision but not so much by inaccuracy since the trends in
each study are what is quantitated. This means our design, in this
instance, is stronger than their design.
The irony of this analysis is that the assays for apoB have been
standardized and are precise accurate but require the use of a
standardized assay. LDL-C has not been standardized and the
errors in measuring LDL-C are much more substantial than the
errors in measuring apoB. ApoB is measured much more
accurately and precisely in clinical practice today than it was
measured in the research studies in ERFC. ApoB was evaluated in
this study based on methods that no one would use today.
If one accepts ERFC, then total cholesterol is just as good as
LDL-C, non-HDL-C and apoB. This is not a reasonable conclusion.
What does this imply about the studies that showed LDL-C was
better than total cholesterol (TC) and the studies that showed
non-HDL-C was better than LDL-C and the studies that showed
apoB was better than LDL-C and non-HDL-C? It’s hard to imagine,
based on the conclusions of the ERFC, that we should go back to
using TC as the screening tool for CV risk.
On page 2501, ERFC writes: “replacement of information on total
cholesterol (TC) and HDL-C with apolipoprotein B and A-I
significantly worsened risk discrimination and risk classification.”
However, look at Figure 1. What happens when TC and HDL-C are
replaced by the TC/HDL-C ratio? Taking out the numerator (TC)
and the denominator (HDL-C) and putting in the ratio (TC/HDL-C) is
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the worst thing one can do. The c-index change is -0.0098 — more
than three times worse than with apoB, and net reclassification is
much worse also. How can the way the same numbers are entered
make such a difference?
The current study published in JAMA does not create a
compelling case to abandon the use of advanced lipid testing
in favor of standard testing. It suffers from many
methodological flaws and, upon careful examination in the
context of the entire body of literature, actually reinforces the
need for lipoprotein testing in all but a select few patients.
26
JUN
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eatingacademy.com
The straight dope on cholesterol – Part
IX
Previously, across 8 parts of this series we’ve laid the groundwork
to ask perhaps the most important question of all:
What should you eat to have the greatest chance of delaying
the arrival of cardiovascular disease?
Before we get there, since this series has been longer and more
detailed than any of us may have wanted, it is probably worth
reviewing the summary points from the previous posts in this series
(or you can just skip this and jump to the meat of this post).
What we’ve learned so far
Cholesterol is “just” another fancy organic molecule in our body
but with an interesting distinction: we eat it, we make it, we
store it, and we excrete it – all in different amounts.
1.
The pool of cholesterol in our body is essential for life. No
cholesterol = no life.
2.
Cholesterol exists in 2 forms – unesterified or “free” (UC) and
esterified (CE) – and the form determines if we can absorb it
or not, or store it or not (among other things).
3.
Much of the cholesterol we eat is in the form of CE. It is not
absorbed and is excreted by our gut (i.e., leaves our body in
stool). The reason this occurs is that CE not only has to be
de-esterified, but it competes for absorption with the vastly
larger amounts of UC supplied by the biliary route.
4.
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Re-absorption of the cholesterol we synthesize in our body
(i.e., endogenous produced cholesterol) is the dominant source
of the cholesterol in our body. That is, most of the cholesterol
in our body was made by our body.
5.
The process of regulating cholesterol is very complex and
multifaceted with multiple layers of control. I’ve only
touched on the absorption side, but the synthesis side is also
complex and highly regulated. You will discover that synthesis
and absorption are very interrelated.
6.
Eating cholesterol has very little impact on the cholesterol
levels in your body. This is a fact, not my opinion. Anyone
who tells you different is, at best, ignorant of this topic. At worst,
they are a deliberate charlatan. Years ago the Canadian
Guidelines removed the limitation of dietary cholesterol. The
rest of the world, especially the United States, needs to catch
up. To see an important reference on this topic, please look
here.
7.
Cholesterol and triglycerides are not soluble in plasma (i.e.,
they can’t dissolve in water) and are therefore said to be
hydrophobic.
8.
To be carried anywhere in our body, say from your liver to your
coronary artery, they need to be carried by a special protein-
wrapped transport vessel called a lipoprotein.
9.
As these “ships” called lipoproteins leave the liver they undergo
a process of maturation where they shed much of their
triglyceride “cargo” in the form of free fatty acid, and doing so
makes them smaller and richer in cholesterol.
10.
Special proteins, apoproteins, play an important role in moving
lipoproteins around the body and facilitating their interactions
with other cells. The most important of these are the apoB
class, residing on VLDL, IDL, and LDL particles, and the apoA-I
11.
The straight dope on cholesterol – Part IX
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class, residing for the most part on the HDL particles.
Cholesterol transport in plasma occurs in both directions,
from the liver and small intestine towards the periphery and
back to the liver and small intestine (the “gut”).
12.
The major function of the apoB-containing particles is to traffic
energy (triglycerides) to muscles and phospholipids to all
cells. Their cholesterol is trafficked back to the liver. The apoA-I
containing particles traffic cholesterol to steroidogenic tissues,
adipocytes (a storage organ for cholesterol ester) and
ultimately back to the liver, gut, or steroidogenic tissue.
13.
All lipoproteins are part of the human lipid transportation system
and work harmoniously together to efficiently traffic lipids. As
you are probably starting to appreciate, the trafficking pattern is
highly complex and the lipoproteins constantly exchange their
core and surface lipids.
14.
The measurement of cholesterol has undergone a dramatic
evolution over the past 70 years with technology at the heart of
the advance.
15.
Currently, most people in the United States (and the world for
that matter) undergo a “standard” lipid panel, which only
directly measures TC, TG, and HDL-C. LDL-C is measured or
most often estimated.
16.
More advanced cholesterol measuring tests do exist to directly
measure LDL-C (though none are standardized), along with the
cholesterol content of other lipoproteins (e.g., VLDL, IDL) or
lipoprotein subparticles.
17.
The most frequently used and guideline-recommended test that
can count the number of LDL particles is either
apolipoprotein B or LDL-P NMR, which is part of the NMR
LipoProfile. NMR can also measure the size of LDL and other
18.
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lipoprotein particles, which is valuable for predicting insulin
resistance in drug naïve patients, before changes are noted in
glucose or insulin levels.
The progression from a completely normal artery to a “clogged”
or atherosclerotic one follows a very clear path: an apoB
containing particle gets past the endothelial layer into the
subendothelial space, the particle and its cholesterol content is
retained, immune cells arrive, an inflammatory response ensues
“fixing” the apoB containing particles in place AND making more
space for more of them.
19.
While inflammation plays a key role in this process, it’s the
penetration of the endothelium and retention within the
endothelium that drive the process.
20.
The most common apoB containing lipoprotein in this process is
certainly the LDL particle. However, Lp(a) and apoB containing
lipoproteins play a role also, especially in the insulin resistant
person.
21.
If you want to stop atherosclerosis, you must lower the LDL
particle number. Period.
22.
At first glance it would seem that patients with smaller LDL
particles are at greater risk for atherosclerosis than patients with
large LDL particles, all things equal.
23.
“A particle is a particle is a particle.” If you don’t know the
number, you don’t know the risk.
24.
With respect to laboratory medicine, two markers that have a
high correlation with a given outcome are concordant – they
equally predict the same outcome. However, when the two tests
do not correlate with each other they are said to be discordant.
25.
LDL-P (or apoB) is the best predictor of adverse cardiac events,
which has been documented repeatedly in every major
26.
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cardiovascular risk study.
LDL-C is only a good predictor of adverse cardiac events when
it is concordant with LDL-P; otherwise it is a poor predictor of
risk.
27.
There is no way of determining which individual patient may
have discordant LDL-C and LDL-P without measuring both
markers.
28.
Discordance between LDL-C and LDL-P is even greater in
populations with metabolic syndrome, including patients with
diabetes. Given the ubiquity of these conditions in the U.S.
population, and the special risk such patients carry for
cardiovascular disease, it is difficult to justify use of LDL-C,
HDL-C, and TG alone for risk stratification in all but the most
select patients.
29.
To address this question, however, one must look at changes in
cardiovascular events or direct markers of atherosclerosis (e.g.,
IMT) while holding LDL-P constant and then again holding
LDL size constant. Only when you do this can you see that
the relationship between size and event vanishes. The only
thing that matters is the number of LDL particles – large, small,
or mixed.
30.
HDL-C and HDL-P are not measuring the same thing, just as
LDL-C and LDL-P are not.
31.
Secondary to the total HDL-P, all things equal it seems smaller
HDL particles are more protective than large ones.
32.
As HDL-C levels rise, most often it is driven by a
disproportionate rise in HDL size, not HDL-P.
33.
In the trials which were designed to prove that a drug that raised
HDL-C would provide a reduction in cardiovascular events, no
benefit occurred: estrogen studies (HERS, WHI), fibrate studies
34.
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(FIELD, ACCORD), niacin studies, and CETP inhibition studies
(dalcetrapib and torcetrapib). But, this says nothing of what
happens when you raise HDL-P.
Don’t believe the hype: HDL is important, and more HDL
particles are better than few. But, raising HDL-C with a drug isn’t
going to fix the problem. Making this even more complex is that
HDL functionality is likely as important, or even more important,
than HDL-P, but no such tests exist to “measure” this.
35.
Did you say “delay?”
That’s right. The question posed above did not ask how one could
“prevent” or eliminate the risk cardiovascular disease, it asked how
one could “delay” it. There is a difference. To appreciate this
distinction, it’s worth reading this recent publication by Allan
Sniderman and colleagues. Allan sent me a copy of this paper
ahead of publication a few months ago in response to a question I
had posed to him over lunch one day. I asked,
“Allan, who has a greater 5-year risk for cardiovascular disease, a
25 year-old with a LDL-P/apoB in the 99th percentile or a
75-year-old with a LDL-P/apoB in the 5th percentile?”
The paper Allan wrote is noteworthy for at least 2 reasons:
It’s an excellent reminder that age is a paramount risk factor for
cardiovascular disease.
1.
It provides a much better (causal) model for atherosclerosis
than the typical age-driven models, and explains why age is an
important risk factor.
2.
What do I mean by this? Most risk calculators (e.g., Framingham)
take their inputs (e.g., age, gender, LDL-C, HDL-C, smoking,
diabetes, blood pressure) and calculate a 10-year risk score. If
you’ve ever played with these models you’ll quickly see that age
drives risk more than any other input. But why? Is there something
The straight dope on cholesterol – Part IX
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inherently “risky” about being older?
Sniderman and many others would argue (and I agree) that the
reason age is a strong predictor of risk has to do with exposure to
apoB particles — LDL, Lp(a), and apoB-carrying remnants. Maybe
it’s because I’m a math geek, but such models just seem intuitive to
me because I think of most things in life in terms of calculus,
especially integrals, the “area under a curve.”
[I once tried to explain to a girlfriend who thought I wasn’t spending
enough time with her that my interest in her should be thought of in
terms of the area under the curve, rather than any single point in
time. That is, think in terms of the integral function, not the point-
in-time function. Needless to say, she broke up with me on the spot
(in the middle of a parking lot!), despite me drawing a very cool
picture illustrating the difference, which I’ve re-created, below.]
The reason age is such a big driver of risk is that the longer your
artery walls are exposed to the insult of apoB particles, the more
likely they are to be damaged, for all the reasons we covered in
Part IV of this series. [This paper also reviews the clinical situation
of PCSK9 mutations which builds a very compelling case for the
causal model of apoB particles in the development of
atherosclerosis].
The straight dope on cholesterol – Part IX
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What does eating have to do with cardiovascular risk?
So now that everyone is on the edge of their seat in anticipation of
this punch-line, let me provide two important caveats.
First, there are no long-term studies – either in primary or
secondary prevention – examining the exact question we all want
to know the answer to with respect to the role of dietary
intervention on cardiovascular disease. There are short-term
studies, some of which I will highlight, which look at proxies for
cardiovascular disease, but all of the long-term studies (looking at
secondary prevention), are either drug studies or multiple
intervention studies (e.g., cholesterol-lowering drug(s) + blood
pressure reducing drug(s) + dietary intervention + exercise + …).
In other words, the “dream” study has not been done and won’t be
done for a long time. The “dream” study would follow 2 randomized
groups for many years and only make one change between the
groups. Group 1 would consume a standard American diet and
group 2 would consume a very-low carbohydrate diet.
Furthermore, compliance within each group would be excellent
(many ways to ensure this, but none of them are inexpensive – part
of why this has not been done) and the study would be powered to
detect “hard outcomes” (e.g., death), instead of just “soft outcomes”
(e.g., changes in apoB, LDL-C, LDL-P, TG).
Second, everything we have learned to date on the risk
relationship between cardiovascular disease and risk markers is
predicated on the assumption that a risk maker of level X in a
person on diet A is the same as it would be for a person on diet
B.
Since virtually all of the thousands of subjects who have made up
the dozens of studies that form the basis for our understanding on
this topic were consuming some variant of the “standard American
diet” (i.e., high-carb), it is quite possible that what we know about
risk stratification is that this population is not entirely fit for
extrapolation to a population on a radically different diet (e.g., a
The straight dope on cholesterol – Part IX
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very-low carbohydrate diet or a ketogenic diet). Many of you have
asked about this, and my comments have always been the same.
It is entirely plausible that an elevated level of LDL-P or apoB in
someone consuming a high-carb diet portends a greater risk than
someone on a ketogenic or low-carb diet. There are many reasons
why this might be the case, and there are many folks who have
made compelling arguments for this hypothesis.
But we can’t forget the words of Thomas Henry Huxley, who said,
“The great tragedy of science is the slaying of a beautiful
hypothesis by an ugly fact.” Science is full of beautiful hypothesis
slayed by ugly facts. Only time will tell if this hypothesis ends up in
that same graveyard, or changes the way we think about
lipoproteins and atherosclerosis.
The role of sugar in cardiovascular disease
Let’s start with what we know, then fill in the connections, with the
goal of creating an eating strategy for those most interested in
delaying the onset of cardiovascular disease.
There are several short-term studies that have carefully examined
the impact of sugar, specifically, on cardiovascular risk markers.
Let’s examine one of them closely. In 2011 Peter Havel and
colleagues published a study titled Consumption of fructose and
HFCS increases postprandial triglycerides, LDL-C, and apoB in
young men and women. If you don’t have access to this journal,
you can read the study here in pre-publication form. This was a
randomized trial with 3 parallel arms (no cross-over). The 3 groups
consumed an isocaloric diet (to individual baseline characteristics)
consisting of 55% carbohydrate, 15% protein, and 30% fat. The
difference between the 3 groups was in the form of their
carbohydrates.
Group 1: received 25% of their total energy in the form of glucose
Group 2: received 25% of their total energy in the form of fructose
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Group 3: received 25% of their total energy in the form of high
fructose corn syrup (55% fructose, 45% glucose)
The intervention was relatively short, consisting of both an inpatient
and outpatient period, and is described in the methodology section.
Keep in mind, 25% of total energy in the form of sugar is not as
extreme as you might think. For a person consuming 2,400
kcal/day this amounts to about 120 pounds/year of sugar, which is
slightly below the average consumption of annual sugar in the
United States. In that sense, the subjects in Group 3 can be
viewed as the “control” for the U.S. population, and Group 1 can be
viewed as an intervention group for what happens when you do
nothing more in your diet than remove sugar, which was the first
dietary intervention I made in 2009.
Despite the short duration of this study and the relatively small
number of subjects (16 per group), the differences brought on by
the interventions were significant. The figure below shows the
changes in serum triglycerides via 3 different ways of measuring
them. Figure A shows the difference in 24-hour total levels (i.e., the
area under the curve for serial measurements – hey, there’s our
integral function again!). Figure B shows late evening (post-
prandial) differences. Figure C shows the overall change in fasting
triglyceride level from baseline (where sugar intake was limited for
2 weeks and carbohydrate consumption consisted only of complex
carbohydrates).
The straight dope on cholesterol – Part IX
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The differences were striking. The group that had all fructose and
HFCS removed from their diet, despite still ingesting 55% of their
total intake in the form of non-sugar carbohydrates, experienced a
decline in total TG (Figure A, which represents the daily integral of
plasma TG levels, or AUC). However, that same group
experienced the greatest increase in fasting TG levels (Figure C).
Post-prandial TG levels were elevated in all groups, but significantly
higher in the fructose and HFCS groups (Figure B). The question
this begs, of course, is which of these measurements is most
predictive of risk?
Historically, fasting levels of TG are used as the basis of risk
profiling (Figure C), and according to this metric glucose
consumption appears even worse than fructose or HFCS.
However, recent evidence suggests that post-prandial levels of TG
(Figure B) are a more accurate way to assess atherosclerotic risk,
as seen here, here, and here. One question I have is why did the
AUC calculations in Figure A show a reduction in plasma TG level
for the glucose group?
The figure below summarizes the differences in LDL-C, non-HDL-C,
apoB, and apoB/apoA-I.
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Again, the results were unmistakable with respect to the impact of
fructose and HFCS on lipoproteins, and by extension, the relative
lack of harm brought on by glucose in isolation. [Of course,
removal of glucose and fructose/HFCS would have been a very
interesting control group.]
One of the simultaneous strengths and weaknesses of this study
was the heterogeneity of its subjects, who ranged in BMI from 18 to
35, in age from18 to 40, and in gender. While this provided at least
one interesting example of age-related differences in carbohydrate
metabolism (older subjects had a greater increase in triglycerides in
response to glucose than younger subjects), it may have actually
diluted the results. There were also significant differences between
genders in the glucose group.
What was most interesting about this study was the clear difference
between the 3 groups that was not solely a function of fructose
load. In other words, the best outcome from a disease risk
standpoint was in the glucose group, while the worst outcome was
not in the all-fructose group, but in the 50/50 (technically 55/45)
mixed group. This is a very powerful indication that while glucose
and fructose alone can be deleterious in excess, their combination
seems synergistically bad.
The role of saturated fat in cardiovascular disease
In the next week or two I’ll be posting an hour-long comprehensive
lecture I gave at UCSD a few weeks ago on this exact topic. Rather
than repeat any of it here, I’ll highlight one study that I did not
include in that lecture. The study, Effect of a high saturated fat and
no-starch diet on serum lipid subfractions in patients with
documented atherosclerotic cardiovascular disease, published in
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2003, treated 23 obese patients (average BMI 39) with known
cardiovascular disease (status post coronary artery bypass surgery
and/or stent placement) with a high-fat ketogenic diet. Because the
study was free-living and relied on self-reporting, not all subjects
had documented levels of elevated serum B-OHB. However, the
subjects were instructed to avoid starch and consume 50% of their
caloric intake via saturated fat, primarily in the form of red meat and
cheese. There were no restrictions on fruits and vegetables, which
may have accounted for the observation that not all subjects were
ketotic during the 6-week intervention. In total, only 5 of the 23
patients achieved documented ketosis.
All of the subjects were on statins and entered the study at a goal
LDL-C level target of 100 mg/dL, which may have been the only
way the authors could get the IRB to approve such a study.
The table below shows the changes in lipoprotein fractions
following the intervention (there was no control group):
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This study was conducted during the height of the “outcry” over the
Atkins diet. While most doctors reluctantly agreed that Dr. Atkins’
diet could reduce body fat, most believed it was still very
dangerous. In the words of Dean Ornish, “Sure you can lose
weight on a low-carb diet, but you can also lose weight on heroin
and no one would recommend that!”
Fair point. In fact, the authors of this study acknowledged that they
“strongly expected” this dietary intervention to increase risk for
cardiovascular disease, which is why they only included subjects on
statins with low LDL-C. However, as you can see from the table
above, the authors were startled by the results. The subjects
experienced a significant reduction in plasma triglycerides and
VLDL triglycerides, without an increase in LDL-C or LDL-P. In fact,
LDL size and HDL size increased and VLDL size decreased – all
signs of improved insulin resistance. Furthermore, fasting glucose
and insulin levels also decreased significantly. The mean
HOMA-IR was reduced from 5.6 to 3.6 (normal is 1.0) and
TG/HDL-C from 3.3 to 2.0 (normal is considered below 3, but
“ideal” is probably below 1.0) in just 6 weeks. Taken together,
these changes, combined with the dramatic change in VLDL size,
suggest insulin resistance was dramatically improved while
consuming a diet of 50% saturated fat!
As all of these patients were taking statins, we’re really robbed of
seeing the impact of this diet on LDL-P, which did not change.
Also, CRP levels rose (though not clinically or statistically
significantly).
Putting it all together
It is very difficult to make the case that when carbohydrates in
general, and sugars in particular, are removed or greatly reduced in
the diet, insulin resistance is not improved, even in the presence of
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high amounts of saturated fats. When insulin resistance improves
(i.e., as we become more insulin sensitive), we are less likely to
have the signs and symptoms of metabolic syndrome. As we meet
fewer criteria of metabolic syndrome, our risk of not only heart
disease, but also stroke, cancer, diabetes, and Alzheimer’s disease
goes down.
Furthermore, as this study on the Framingham cohort showed us,
the more criteria you have along the spectrum of metabolic
syndrome, the more difficult it becomes to predict your risk, due to a
widening gap in discordant risk markers, as shown in this figure.
As I noted at the outset, the “dream” trial has not yet been done,
though we (NuSI) plan to change that. Until then each of us has to
make a decision several times every day about what we will and
won’t put in our mouths. Much of this blog is dedicated to
underscoring the impact of carbohydrate reduction on insulin
resistance and metabolic syndrome.
The results of the trials to date, combined with a nuanced
understanding of the lipoprotein physiology and their role on the
atherosclerotic disease process, bring us to the following
conclusions:
The consumption of sugar (sucrose, high fructose corn syrup)1.
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increases plasma levels of triglycerides, VLDL and apoB, and
reduces plasma levels of HDL-C and apoA-I.
The removal of sugar reverses each of these.2.
The consumption of fructose alone, though likely in
dose-dependent fashion, has a similar, though perhaps less
harmful, impact as that of fructose and glucose combined (i.e.,
sugar).
3.
The addition of fat, in the absence of sugar and starch, does not
raise serum triglycerides or other biomarkers of cardiovascular
disease.
4.
The higher the level of serum triglycerides, the greater the
likelihood of discordance between LDL-C and LDL-P (and
apoB).
5.
The greater the number (from 0 to 5) of inclusion criteria for
metabolic syndrome, the greater the likelihood of discordance
between LDL-C and LDL-P (and apoB).
6.
I would like to address one additional topic in this series before
wrapping it up – the role of pharmacologic intervention in the
treatment and prevention of atherosclerotic disease, so please hold
off on questions pertaining to this topic for now.
12
JUL
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