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Faculty of Agricultural and Nutritional Sciences

Factors influencing the Factors influencing the bioavailability of flavonoidsbioavailability of flavonoids

Siegfried Siegfried WolfframWolffram

InsituteInsitute of Animal Nutrition and Physiologyof Animal Nutrition and Physiology

CONTENTS

Theoretical aspectsTheoretical aspects-- Definition of bioavailabilityDefinition of bioavailability-- Factors influencingFactors influencing bioavailabilitybioavailability-- CalculationCalculation of bioavailabilityof bioavailability-- Pharmacokinetic Pharmacokinetic parametersparameters

From theory From theory to to practicepractice-- InfluenceInfluence of of the sugar moietythe sugar moiety-- First pass First pass effects effects –– role role of of intestine intestine and and liverliver-- RoleRole of of microbial metabolismmicrobial metabolism-- InfluenceInfluence of of dietary fatdietary fat-- PostPost--absorptive absorptive tissue distributiontissue distribution

Open Open questionsquestions

BackgroundBackground

Profound knowledge about the bioavailability of flavonoids including factors affecting their bioavailabilty is of crucial importance for a proper interpretation of in vitro and ex vivo data regarding biological effects of flavonoids.

Flavonoids are a large Flavonoids are a large heterogenousheterogenous group of plant group of plant polyphenolspolyphenols that that comprise more than 6000 different formscomprise more than 6000 different forms

Flavonoids are usually divided into subgroups, including Flavonoids are usually divided into subgroups, including flavones, flavones, flavonolsflavonols, , flavanonesflavanones, , flavanonolesflavanonoles, , flavanolsflavanols ((catechinscatechins), ), anthocyanidinesanthocyanidines, isoflavones and , isoflavones and chalconeschalcones

Total dietary Total dietary flavonoidflavonoid intake in humans might be less than 1 mg/ kg intake in humans might be less than 1 mg/ kg body mass and daybody mass and day

Flavonoids show numerous biological effects, though the majorityFlavonoids show numerous biological effects, though the majority of of findings still stems from in vitro/ex vivo investigations.findings still stems from in vitro/ex vivo investigations.

Definitions (1)Definitions (1)

In a nutritional sense, In a nutritional sense, bioavailabilitybioavailability not only describes the not only describes the amount of a dietary substance including effective amount of a dietary substance including effective metabolites, reaching the systemic circulation and body metabolites, reaching the systemic circulation and body tissues but also comprises their biological effects.tissues but also comprises their biological effects.

However, in the case of flavonoids the exact chemical However, in the case of flavonoids the exact chemical nature of the active nature of the active metabolite(smetabolite(s) and their mode of action ) and their mode of action are still largely unknown.are still largely unknown.

What doesWhat does bioavailability bioavailability meanmean??

Definitions (2)Definitions (2)

In vitro experiments can only address partial aspects of bioavailability, e.g., cellular mechanisms of absorption, biological effects on certain cell types, etc.

Often, bioavailability is incorrectly described as:

The apparent digestibility (difference between the amount ingested with food and the amount excreted via faeces)does not consider endogenous secretion and microbial degradation, missing link to biological effects

The amount truly absorbed missing link to biological effects

The tissue accumulation missing link to biological effects

Biological FactorsBiological FactorsChemical andChemical andPhysical FactorsPhysical Factors

Absorptive LevelAbsorptive Level

PostabsorptivePostabsorptive LevelLevel

General AspectsGeneral Aspects

Chemical form State of development

State of nutrition

State of healthAdaptive mechanisms, e.g. metabolism, excretion, etc.

State of development

State of nutritionRegulatory mechanisms on the intestinal levelMicrobial activities within the gastrointestinal tract

Amount utilized by the organism for specific function(s) = bioavailable amount

Endogenous secretion(saliva, bile, gastric and

pancreatic juice, etc.)Other dietary factors (inhibitory and/or

stimulatory)

Chemical form and food matrix

truly absorbed amount

absorbable amount

Ingested amount

Determination of bioavailability (1)Determination of bioavailability (1)

This procedure, however, does not consider organ or tissue-specific distribution and accumulation as well as metabolism after tissue uptake.

Normally, the bioavailabilitybioavailability of a certain compound is determined by the plasma concentrationplasma concentration--timetime--profileprofile in the systemic circulation (easily accessible, concentrations at the final site of action normally cannot be obtained).

Fraction of an orally applied effective compound reaching the syFraction of an orally applied effective compound reaching the systemic circulation stemic circulation in relation to intravenous injection of the substance.in relation to intravenous injection of the substance.(Bioavailability after (Bioavailability after i.vi.v. application is defined as 100%). application is defined as 100%)

Absolute (oral) bioavailability:


0 4 8 12 16 20 240

100

200

300

400

500

600

i.v. applicationp.o. application

Time, h

Plas

ma

cone

ntra

tion

BVabs =AUCp.o. • Dose i.v.

AUCi.v. • Dose p.o.

Amount of an effective compound reaching the systemic circulation as compared to another oral formulation (reference formulation = 100%)

Relative (oral) bioavailability:

0 240 480 720 960 1200 1440time (min)

0.00.20.40.60.81.01.21.41.61.82.02.2

plas

ma

conc

entr a

ti on

(µm

ol·L

-1)

Q3GQuercetinRutin

BVref (%) =AUCtest

AUCref• 100 %


Pharmacokinetic Parameters:

The AUC only describes the amount of a substance reaching the systemic circulation. The AUC does not include information about maximum plasma concentrations, the velocity of absorption, the elimination kinetics or the presence of effective concentrations.

These information can only be obtained by calculating various pharmacokinetic parameters from the plasma concentration-time curve.

In principle, plasma concentration-time profiles are governed by absorption (A), distribution (D) and elimination (E) of a drug.

0 4 8 12 16 20 240

100

200

300

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500

600

Time, h

Plas

ma

Con

cent

ratio

nAUC

A A+D D+E E

A = AbsorptionD = DistributionE = Excretion

Pharmacokinetics (1)Pharmacokinetics (1)

cmax = maximum plasma concentration

tmax = time point when cmax is reached

t1/2 = terminal elimination half-life

cmax and tmax are used to describe the velocity of absorption whereas t1/2 is a measure for the elimination from the body

Cave: calculated parameters strongly depends on the frequency of blood sampling

Pharmacokinetics (2)Pharmacokinetics (2)

Phase of Absorption (2):

0 4 8 12 16 20 240

100

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600

Time, h

Plas

ma

Con

cent

ratio

n

Cmax

tmax

AUC

t1/2

Often, a substance is subjected to more or less intensive metabolic transformations during passage across the intestinal wall and subsequent liver passage, reducing the amount reaching the systemic circulation.

In this context, so-called phase 1 (activation) and phase 2 (conjugation) reactions are of special importance

Phase 1 reactions are catalysed by members of the CYP familiy (cytochromeP450 oxidoreductases).

Conjugation (phase 2) reactions mainly comprise glucuronidation, sulfation, and/or coupling to glutathion.

Induction/suppression and/or stimulation/inhibition of such enzymes not only influences the bioavailability of the substance itself but also of many other substances including xenobiotics (drugs)

The principle aim of phase 1 and 2 reactions are to enhance elimination of a substance from the body via bile and urine thus reducing the amount bioavailable.

Intestinal and hepatic first pass effects

First pass effects (1)First pass effects (1)

Post-absorptive tissue/organ distribution:

Distribution (1)Distribution (1)

After absorption, a substance is quickly distributed in the body via circulation

The substance is (reversibly) transferred into the extravasal fluid compartment mainly by passive diffusion and filtration

Distribution also includes tissue/organ specific accumulation of the substance under steady state conditions

It is important to keep in mind, that in most cases the blood can only be considered a transiently used compartment from where uptake into tissues does (or does not) occur.

For an active compound, not only the total concentration within a tissue but more importantly the concentration at the molecular target is of relevance for its effects.

Distribution (2)Distribution (2)

Factors influencing tissue/organ distribution:

Binding to plasma/tissue proteins

Intensity of blood supply of a specific organ/tissue

Permeability of capillaries for the specific substance

Presence of specific target molecules (e.g. receptors, transporters etc.)

Storage within adipose tissue dependent on the lipid solubility of the substance

From theory to practice …From theory to practice …

Pharmakokinetic parameters for selected flavonoids1):

0.2 – 0.90.03 – 0.381.6 – 3.2EGCG

1.0 – 3.60.30 – 2.700.5 – 2.0EGC

1.6 – 5.50.09 – 0.524.3 – 9.3Rutin

5.7 – 16.00.51 – 3.800.5 – 2.9Quercetin-monoglucosides

10.4 – 32.21.26 – 4.503.0 – 5.2Genstein

6.2 – 45.10.46 – 4.044.4 – 9.3Genistin

AUC [µmol h/L]Cmax [µmol/L]Tmax [h]Flavonoid

- Only studies using well-described polyphenol sources and accurate analytical methods were considered

- Polyphenols were applied as a single dose either of the pure substance, a plant extract or whole food/beverage

- All values were normalized to an intake of 50 mg of the aglycone, assuming a linear dose response

1)Manach et al. (2005): Am. J. Clin. Nutr. 81, 230S-42S

FlavanolFlavanol analysis:analysis: QuercetinQuercetin and and QuercetinQuercetin metabolites with an intact metabolites with an intact flavonolflavonol structure by HPLCstructure by HPLC

Pigs are very similar to man regarding anatomy and physiology ofPigs are very similar to man regarding anatomy and physiology of the the gastrointestinal tractgastrointestinal tract

...... Permanent catheter placed inPermanent catheter placed in the the jugular veinjugular vein

...... Various doses of Various doses of quercetinquercetin from different from different sources applied in a test meal:sources applied in a test meal:

QuercetinQuercetin, , IsoquercitrinIsoquercitrinRutinRutin

... ... Frequent blood sampling over a period Frequent blood sampling over a period of 24 hof 24 h

Bioavailability Bioavailability –– thethe pigpig asas a a modelmodel animalanimal

Influence of the sugar moiety (1)Influence of the sugar moiety (1)

RutinRutinO

OOH

HO

OHOH

O –Glucose—Rhamnose

QuercetinQuercetin--33--OO--GlucosideGlucosideO

OOH

HO

OHOH

O –Glucose

00 240240 480480 720720 960960 12001200 14401440Time, minTime, min

00

100100

200200

300300

400400

500500

600600

700700

800800

Plas

ma

Plas

ma

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rcet

inQ

uerc

etin

Con

cent

ratio

n,

Con

cent

ratio

n, n

gng/m

l/m

l QuercetinQuercetinO

OOH

HO

OHOH

OH

Study design:Study design:

Single test meal containing 50 mg Single test meal containing 50 mg quercetinquercetin/kg /kg bwbw either as either as quercetinquercetinaglykonaglykon, quercetin, quercetin--33--OO--glucoside or glucoside or rutinrutin; n = 3; n = 3

Cermak, R. et al. (2003): J. Nutr. 133, 2802

Why is Why is quercetinquercetin better available from better available from quercetinquercetin monoglucosidesmonoglucosides as as compared to compared to rutinrutin??

There is some evidence for the involvement of the intestinal gluThere is some evidence for the involvement of the intestinal glucose cose transportertransporter SGLT1 in cellular uptake of SGLT1 in cellular uptake of quercetinquercetin monoglucosidesmonoglucosideswith with susequentsusequent hydolysishydolysis by intracellular by intracellular ßß--monoglucosidasesmonoglucosidases

More importantly More importantly quercetinquercetin--monoglucosidesmonoglucosides are substrates of the are substrates of the brushbrush--border membrane enzyme lactase/border membrane enzyme lactase/phloridzinphloridzin hydrolasehydrolase(lactase).(lactase).

QuercetinQuercetin glycosides not interacting with SGLT1 and/or lactase are glycosides not interacting with SGLT1 and/or lactase are deglycosylateddeglycosylated by microbial enzymes mainly in the large intestine.by microbial enzymes mainly in the large intestine.


Role of SGLT1 in cellular uptake of quercetin monoglucosides:Cermak, R. et al. (2004): Br. J. Nutr. 91, 849

0 1 2 3 4 50

500

1000

1500

2000

2500

+ IsoquercitrinControl

60

****

***

Time, min

Gluc

ose U

ptak

e, nm

ol/m

g Pr

otein

means ± SEM (n=6)

Phlo

ridzin

0

20

40

60

80

100

120

Initi

al D

-Glu

cose

Upt

ake,

% o

f Con

trol

means ± SEM (n=6-9)

***

**

***

***

Cont

rol

αMD

G

Quer

cetin

Q-3-

Glc

Q-4‘-

Glc

Q-3-

Gal

Q-3-

Glc-

Rha

Gen-

7-Gl

cNa

r-7-G

lcCy

a-3,5

-Di-G

lc


Specific inhibition of glucose transport across the brushSpecific inhibition of glucose transport across the brush--border membrane,border membrane,

CisCis--inhibition and transinhibition and trans--stimulation of stimulation of glcuoseglcuose transport across the brushtransport across the brush--border membrane, undborder membrane, und

Competitive type of inhibition (reduction of the apparent KCompetitive type of inhibition (reduction of the apparent Kmm--value)value)

Influence Influence of of the sugar moietythe sugar moiety –– General General rulesrules (4)(4)

With the only exception of With the only exception of anthocyaninanthocyanin glycosides, direct absorption of glycosides, direct absorption of glycosylatedglycosylated flavonoids into the circulation does not occur.flavonoids into the circulation does not occur.The sugar moiety has to be removed either by endogenous or microThe sugar moiety has to be removed either by endogenous or microbial bial enzymes.enzymes.

In the small intestine absorption of In the small intestine absorption of flavonoidflavonoid aglycaaglyca and of some simple and of some simple glucosidesglucosides does occur. In the case of simple does occur. In the case of simple glucosidesglucosides, glucose is , glucose is removed either by lactase/removed either by lactase/phloridzinphloridzin hydrolasehydrolase and/or intracellular and/or intracellular ßß--glycosidasesglycosidases..

In general, flavonoids linked to a sugar moiety containing In general, flavonoids linked to a sugar moiety containing rhamnoserhamnose are are not not degycosylateddegycosylated in the small intestine. Those sugar moieties have to be in the small intestine. Those sugar moieties have to be removed by bacterial enzymes before absorption of the removed by bacterial enzymes before absorption of the aglyconaglycon from the from the large intestine.large intestine.

Absorption from the large intestine reduces bioavailability becaAbsorption from the large intestine reduces bioavailability because of use of further microbial degradation of the liberated further microbial degradation of the liberated aglycaaglyca..

Only for Only for flavonolflavonol monogluosidesmonogluosides ((isoquercitrinisoquercitrin, , spiraeosidespiraeoside) a higher ) a higher bioavailability as compared to bioavailability as compared to quercetinquercetin aglyconaglycon or more complex or more complex glycosides have been convincingly shown.glycosides have been convincingly shown.

GlycosylationGlycosylation does, however, not influence the pattern of metabolites does, however, not influence the pattern of metabolites reaching the systemic circulationreaching the systemic circulation

FirstFirst--pass effects: Circulating metabolites after oral absorption (1)pass effects: Circulating metabolites after oral absorption (1)

Plasma concentrationPlasma concentration--timetime--coursecourseof of quercetinquercetin (50 mg/kg)(50 mg/kg)

5050

100100

150150

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250250

300300

00

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ma

Plas

ma

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rcet

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uerc

etin

Con

cent

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n (

Con

cent

ratio

n ( n

gng/m

l Pla

sma)

/ml P

lasm

a)

00 200200 400400 600600 800800 10001000 12001200 14001400

Time (min)Time (min)

Enzymatic treatment withEnzymatic treatment withßß--glucurondiaseglucurondiase//sulfatasesulfatase

without enzymeswithout enzymes

00 200200 400400 600600 800800 10001000 12001200 14001400

Time (min)Time (min)

00

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800800

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etin

Con

cent

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n (

Con

cent

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n ( n

gng/m

l Pla

sma)

/ml P

lasm

a)

Enzymatic treatment withEnzymatic treatment withßß--glucurondiaseglucurondiase//sulfatasesulfatase

Without enzymesWithout enzymes

Plasma concentrationPlasma concentration--timetime--coursecourseof of quercetinquercetin (500 mg/kg)(500 mg/kg)


Single test meal containing 50 (n = 3) or 500 (n = 1) mg Single test meal containing 50 (n = 3) or 500 (n = 1) mg quercetinquercetin/kg /kg bwbw

AderAder, P. et al. (2000): Free , P. et al. (2000): Free RadicRadic. . Biol. Med. 28, 1056Biol. Med. 28, 1056

15000 250 500 750 1000 1250Time, min

0

50

100

150

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300

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ma

Con

cent

ratio

n, n

g/m

lX ±SEM, n = 4

Quercetin

O

OOH

HO

OHOH

OH

IsorhamnetinO

OOH

HO

O–CH3

OH

OH

TamarixetinO

OOH

HO

OH

OH

O–CH3

Study design:Study design:Single test meal containing 50 mg Single test meal containing 50 mg quercetinquercetin/kg /kg bwbw

AderAder, P. et al. (2000): Free , P. et al. (2000): Free RadicRadic. . Biol. Med. 28, 1056Biol. Med. 28, 1056


In general, only In general, only glucuronoglucurono-- and/or and/or sulfosulfo--conjugates of flavonoid conjugates of flavonoid aglyca and their aglyca and their methylatedmethylated derivates are found in the systemic derivates are found in the systemic circulation (only for the circulation (only for the catechin epigallocatechingallatecatechin epigallocatechingallate (EGCG) the (EGCG) the nonnon--conjugated from is present as a major fraction).conjugated from is present as a major fraction).MethylationMethylation is is catalyzedcatalyzed by by catecholcatechol--OO--methyl methyl transferasetransferase, whereas , whereas sulfotransferasessulfotransferases are are responsible for the transfer of a responsible for the transfer of a sulfatesulfate moiety from 3’moiety from 3’--phosphoadenosinephosphoadenosine--5’5’--phophosulfatephophosulfate. . UDPUDP--glucuronosyltransferases catalyzeglucuronosyltransferases catalyze the transfer of the transfer of glucuronicglucuronic acid from UDPacid from UDP--glucuronideglucuronide..

With respect to the pattern of With respect to the pattern of methylationmethylation and conjugation species and conjugation species differences have been described.differences have been described.

In general, conjugation with In general, conjugation with glucuronateglucuronate and/or and/or sulfatesulfate clearly clearly reduces (at least some) biological activities.reduces (at least some) biological activities.

Main tissues responsible for the formation of derivatives are thMain tissues responsible for the formation of derivatives are the liver e liver and the intestinal and the intestinal mucosamucosa, the latter one being of superior , the latter one being of superior importance after dietary intake of flavonoids.importance after dietary intake of flavonoids.


Role of intestinal and biliary secretion:

Active carrierActive carrier--mediated secretion of a substance and/or its mediated secretion of a substance and/or its metabolites by metabolites by enterocytes enterocytes and and hepatocyteshepatocytes result in a reduced result in a reduced bioavailability.bioavailability.

Efflux is mediated by members of the MDR(MultiEfflux is mediated by members of the MDR(Multi--DrugDrug--Resistance) Resistance) family, such as Pfamily, such as P--glycoproteinglycoprotein, MRP2 (, MRP2 (multidrugmultidrug resistanceresistance--associated associated porteinportein), and BCRP1 (breast cancer resistance protein) ), and BCRP1 (breast cancer resistance protein) which are expressed in the brushwhich are expressed in the brush--border membrane of border membrane of enterocytesenterocytesand the and the canalicularcanalicular membrane of membrane of hepatocyteshepatocytes..

ReRe--secretion back into the intestinal lumen is of special relevancesecretion back into the intestinal lumen is of special relevanceat low substrate concentrations (as in the case of dietary at low substrate concentrations (as in the case of dietary flavonoids)flavonoids)

This has been described for several flavonoid conjugates, althouThis has been described for several flavonoid conjugates, although gh differences exist between flavonoid subclasses.differences exist between flavonoid subclasses.



Crespy, V. et al. (2003): Am. J. Physiol. 284:G980

Microbial Metabolism:Microbial Metabolism:

Microbial metabolism of flavonoids (1)Microbial metabolism of flavonoids (1)

Especially the large intestine is intensively colonized by microEspecially the large intestine is intensively colonized by micro--organisms organisms (10(1088--10101212/g content)./g content).

The intestinal microflora have an immense potential to metabolisThe intestinal microflora have an immense potential to metabolise a plenty e a plenty of substrates including flavonoidsof substrates including flavonoids

Microbial metabolism can exert different effects on the bioavailMicrobial metabolism can exert different effects on the bioavailability of ability of flavonoids:flavonoids:

-- Liberation of aglyca from Liberation of aglyca from glycosidicglycosidic forms with subsequent forms with subsequent absorption of the absorption of the aglyconaglycon, (e.g. , (e.g. quercetin quercetin from from rutinrutin))

-- Reduction of the absorbable amount of flavonoids by metabolism Reduction of the absorbable amount of flavonoids by metabolism of the aglyca (e.g., COof the aglyca (e.g., CO22 was found a major metabolite of was found a major metabolite of quercetinquercetin))

-- Generation of active metabolites with a reduced or enhanced Generation of active metabolites with a reduced or enhanced biological activity compared with the parent compound, e.g. the biological activity compared with the parent compound, e.g. the isoflavone daidzeinisoflavone daidzein can be transformed to the bacterial metabolite can be transformed to the bacterial metabolite equolequol, which is a more potent , which is a more potent estrogenestrogen as compared to as compared to daidzeindaidzein))

StudyStudy design:design:

Single Single isocaloric isocaloric test test meal containing meal containing 10 mg 10 mg quercetinquercetin/kg /kg bw without orbw without or withwiththethe additionaddition of 15% of 15% lardlard (n = 7)(n = 7)

LesserLesser, L. et al. (2004): J. , L. et al. (2004): J. NutrNutr. 134, 1508. 134, 1508

35,4 35,4 ±± 5,5*5,5*))

(100 %)(100 %)

0% F0% Fatat

MMeanseans ±± SS..EE..MM..**)) AUCAUCtotaltotal,, minmin•µ•µgg••mlml--11

Composition of DietComposition of Diet

55,1 55,1 ±± 3,63,6(156 (156 ±± 11 %)11 %)

15% F15% Fatat

Relative BioRelative Bioavailability availability ((AUCAUCtotaltotal ))

Kontrolle15% Fett

0 240 480 720 960 1200 14400

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250

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Flav

onol

Con

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g/m

l

Influence Influence of of diet compositiondiet composition (2)(2)

de Boer, V.C.J., et al. (2005): J. Nutr. 135, 1718


Rats were fed a diet containing 0.1% or 1% quercetin for a period of 11 weeks.

Tissue concentration of quercetin, isorhamnetin and tamarixetin were determined without or after enzymatic treatment with ß-glucuronidase/ sulfatase

Tissue levels were corrected for residual blood content. Thus, quercetin and quercetin metabolites detected in tissues were located in the interstitial fluid or within cells.

With respect to tissue distribution as well as the pattern of quercetin metabolites detected within tissues, there were no substantial differences between the 0.1 and 1% quercetin group.In the following table only the results from the 0.1% feeding group are shown.

n.d.n.d.n.d.n.d.white fat100%0.030.02n.d.n.d.0.050.02spleen43%0.090.1457%0.000.19n.d.0.140.33brain0%0.000.0084%0.010.7816%0.260.140.260.92bone

19%0.010.2169%0.020.7711%0.040.120.051.10muscle0%0.000.0089%0.091.2211%0.060.150.111.37thymus

17%0.230.2573%0.041.1210%0.060.150.241.53brown fat5%0.020.0966%0.101.1829%0.020.520.111.79liver

14%0.030.3263%0.021.3823%0.020.500.042.20heart1%0.050.0466%0.011.8833%0.030.930.072.85kidney3%0.120.1069%0.132.0727%0.070.820.192.98testes2%0.140.1071%0.012.8426%0.081.040.163.98lung

n.d.67%1.215.733%0.47.71.2723.4plasma% of sumSDMean% of sumSDMean% of sumSDMeanMean SD

Tamarixetin[nmol/g]

Isorhamnetin[nmol/g]

Quercetin[nmol/g]

Sum of flavonols[nmol/g]

Tissue

Tissue distributionTissue distribution (1)(1)

Some tissues (lung, liver, kidney, testes) contained considerablSome tissues (lung, liver, kidney, testes) contained considerable e amounts of free aglyca (4amounts of free aglyca (4--20%), however, this might be at least 20%), however, this might be at least partially attributable to partially attributable to deconjugationdeconjugation during sample preparation.during sample preparation.

SomeSome open questionsopen questions ......

How are the circulatingHow are the circulating flavonoid flavonoid conjugates metabolized within conjugates metabolized within tissuestissues??

HowHow areare thethe circulatingcirculating flavonoid flavonoid metabolitesmetabolites takentaken up up intointo tissuestissues((cellularcellular uptakeuptake mechanismsmechanisms)?)?

Which are the effective intracellular metabolitesWhich are the effective intracellular metabolites??

Is there any metabolic adaptation Is there any metabolic adaptation to longto long--term intake term intake of flavonoids?of flavonoids?

ThankThank youyou forfor youryour attentionattention

GreetingsGreetings fromfrom somesome coco--workersworkers ......

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