VOLATILE FATTY ACIDS Volatile Fatty Acids Major VFA

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 goes towards fermentation (methane).

Cont. Regulation of microbial growth/function � Bacteria vs. protozoa � Competition � Environmental p. H � Alteration with diet � Alteration with intake

Cont. Acetate: mostly from cellulose � Important Propionate: mostly from starch � Important for milk fat synthesis 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 � Propionic acid � Butyric acid CH 3 COOH CH 3 CH 2 CH 2 COOH

Rumen Fermentation Starch Hemicellulose Cellulose Sugars Glucose Pectins Pyruvate Lactic Acetic Propionic Butyric Formic H 2+CO 2 Methane (CH 4)

Acetate Pyruvate + Pi + ADP Acetate + ATP + H 2 + CO 2 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 Co. A � 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 CO 2 and H 2 O (i. e. , ATP) when in low energy balance Used for fatty acid synthesis when animal is in high energy balance � Arterial Tissue concentration uptake is directly related to rate of rumen fermentation [blood concentration]

Propionate Pyruvate + Co. A + 4 H+ Propionate + H 2 O Amylolytic bacteria Utilized by rumen epithelium � Converted to lactate and pyruvate Important as a precursor for gluconeogenesis

Hepatic propionate metabolism Glucose TCA Cycle OAA Succinyl Co. A Coenzyme B 12 Methylmalonyl Co. A ADP + Pi ATP Biotin, Mg ++ Propionyl Co. A AMP + 2 Pi ATP Propionate Co. A

Butyrate Pyruvate + Co. A Acetyl-Co. A + H 2 + CO 2 2 Acetyl-Co. A + 4 H+ Butyrate + H 2 O + Co. A Metabolized by rumen epithelium to ketone bodies (acetoacetate, -hydroxybutyrate) � Later metabolized in liver Net ATP production is 25 per mole

Ruminal VFA absorption Rumen lumen Acetate Rumen wall 70 Portal vein 50 20 Propionate 20 10 10 Butyrate 10 1 9 Values are relative flux rates

Hepatic metabolism of VFA Rumen Portal vein Acetate 70 50 Propionate 20 10 Liver Peripheral blood Acetate Glucose CO 2 Butyrate 10 1 4 3 -OH butyrate 3 -hydroxy Butyrate (BHB)

Absorption to portal blood Passively absorbed by rumen epithelium Rate: � Concentration � p. H � Chain length Tissue uptake related to rate of fermentation Absorbed in undissociated acid form � CH 3 COOH � p. K ~4. 75 (acetic acid) vs CH 3 COO- (acetate)

Cont. In converting acetate to pyruvate � also CO 2; CH 4 Ionophore feed additives � Increases propionate � Decreases acetate

Normal process Propionate to lactate (normal process) Causes lowering p. H Lactate to pyruvate Requires lactate fermenters (altering p. H) this pyruvate is mainly used to synthesize glucose (hepatic tissues)

Sudden dietary changes Propionate to lactate; reduced p. H Lactate needs to be converted to pyruvate Microbes converting lactate grow slow !!!!! p. H 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 CO 2 CH 4 NH 3 Microbes

How p. H is altered Diet Intake Feeding frequency Chewing/rumination