VOLATILE FATTY ACIDS

Volatile Fatty Acids

Major VFA: acetic acid; propionic acid; butyric acid.

Major VFAs are absorbed and used as primary energy source by ruminants.

The tissue use of VFA is lower than tissue use of the sugars (e.g., glucose).

~10 % of energy consumed is used for fermentation (methane).

Cont.

Regulation of microbial growth/function Bacteria vs. protozoa Competition Environmental pH Alteration with diet Alteration with intake

Cont.

Acetate: mostly from cellulose Important for milk fat synthesis

Propionate: mostly from starch Important to produce glucose

Butyrate: mostly derived from acetate Important in ketones usage as an energy

source

Volatile Fatty Acids (VFA)

Produced from the fermentation of pyruvate Rumen and hind gut Types/ratios depends on diet

3 major VFAs Acetic acid CH3COOH Propionic acid CH3CH2COOH Butyric acid CH3CH2CH2COOH

Rumen Fermentation

Cellulose Glucose

Pyruvate

Hemicellulose

Starch

Sugars

Pectins

Lactic Acetic Formic H2+CO2

ButyricPropionic Methane(CH4)

Acetate

Pyruvate + Pi + ADP Acetate + ATP +

H2 + CO2

Cellulolytic bacteria Energy source for rumen epithelium and

muscle Not utilized by liver

Acetate utilization

Important as a precursor to de novo fatty acid synthesis Adipose Lactating mammary gland

Oxidized via TCA Activated to acetyl CoA Used by skeletal muscle, kidneys, and heart for

energy Net gain of 10 ATP per mole of acetate

Acetate utilization

Dependent upon Energy balance

Generates CO2 and H2O (i.e., ATP) when in low energy balance

Used for fatty acid synthesis when animal is in high energy balance

Arterial concentration Tissue uptake is directly related to rate of

rumen fermentation [blood concentration]

Propionate

Pyruvate + CoA + 4H+ Propionate + H2O

Amylolytic bacteria Utilized by rumen epithelium

Converted to lactate and pyruvate Important as a precursor for

gluconeogenesis

Hepatic propionate metabolism

Glucose

Propionate

Propionyl CoA

Methylmalonyl CoA

Succinyl CoA

OAA TCACycle

Coenzyme B12

Biotin, Mg ++

ATP

AMP + 2 Pi

ATP

ADP + Pi

CoA

Butyrate

Pyruvate + CoA Acetyl-CoA + H2 + CO2

2 Acetyl-CoA + 4H+ Butyrate + H2O + CoA

Metabolized by rumen epithelium to ketone bodies (acetoacetate, -hydroxybutyrate) Later metabolized in liver

Net ATP production is 25 per mole

Ruminal VFA absorption

Rumenlumen

Rumenwall

Portalvein

Acetate

Propionate

Butyrate

50

10

1

70

20

10

Values are relative flux rates

20

10

9

Hepatic metabolism of VFA

Rumen Portalvein

Acetate

Propionate

Butyrate

50

Glucose

3-hydroxyButyrate (BHB)

70

20

10

10

1

Liver

3-OHbutyrate

Acetate

Peripheralblood

Glucose

CO2

4

Absorption to portal blood

Passively absorbed by rumen epithelium Rate:

Concentration pH Chain length

Tissue uptake related to rate of fermentation Absorbed in undissociated acid form

CH3COOH (acetic acid) vs CH3COO- (acetate) pK ~4.75

Cont.

In converting acetate to pyruvate also CO2; CH4

Ionophore feed additives Increases propionate Decreases acetate

Normal process

Propionate to lactate (normal process) Causes lowering pH

Lactate to pyruvate Requires lactate fermenters (altering pH) this pyruvate is mainly used to

synthesize glucose (hepatic tissues)

Sudden dietary changes

Propionate to lactate; reduced pH Lactate needs to be converted to

puruvate Microbes converting lactate grow

slow !!!!! pH continues to drop Too acidic environment Lactic acidosis; can be lethal

why

Sudden changes in diet; too much concentrate

Stress + reduced feed intake Empty feed bunks Reduced feed intake; how palatable ?

IMBALANCE BETWEEN MICROBES PRODUCING LACTATE AND MICROBES CONVERTING LACTATE TO PYRUVATE

End products

VFAs CO2

CH4

NH3

Microbes

How pH is altered

Diet Intake Feeding frequency Chewing/rumination