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 pH Alteration with diet Alteration with intake

Cont. Acetate: mostly from cellulose Propionate: mostly from starch 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 Glucose Pyruvate Hemicellulose Starch Sugars Pectins Lactic Acetic Formic H2+CO2 Butyric Propionic Methane (CH4) Cellulose

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 TCA Cycle OAA Glucose Succinyl CoA Coenzyme B12 Methylmalonyl CoA ADP + Pi Biotin, Mg ++ ATP Propionyl CoA AMP + 2 Pi ATP CoA Propionate

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

Hepatic metabolism of VFA Peripheral blood Rumen Portal vein Liver Acetate 70 50 Acetate Propionate 20 10 Glucose Glucose CO2 3-hydroxy Butyrate (BHB) 3-OH butyrate Butyrate 10 1 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 Ionophore feed additives 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 pyruvate 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