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Volatile fatty acids

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).

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Volatile fatty acids

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  1. Volatile fatty acids

  2. 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).

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

  4. 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

  5. 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

  6. Rumen Fermentation Starch Hemicellulose Sugars Cellulose Glucose Pectins Pyruvate Lactic Acetic Formic H2+CO2 Propionic Butyric Methane (CH4)

  7. Acetate • Pyruvate + Pi + ADP  Acetate + ATP + H2 + CO2 • Cellulolytic bacteria • Energy source for rumen epithelium and muscle • Not utilized by liver

  8. 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

  9. 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]

  10. Propionate • Pyruvate + CoA + 4H+ Propionate + H2O • Amylolytic bacteria • Utilized by rumen epithelium • Converted to lactate and pyruvate • Important as a precursor for gluconeogenesis

  11. 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

  12. 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

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

  14. Hepatic metabolism of VFA Peripheral blood Rumen Portal vein Liver 70 Acetate 50 Acetate Propionate 20 10 Glucose Glucose CO2 3-hydroxy Butyrate (BHB) 3-OH butyrate Butyrate 10 1 4

  15. 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

  16. Cont. • In converting acetate to pyruvate • also CO2; CH4 • Ionophore feed additives • Increases propionate • Decreases acetate

  17. 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)

  18. 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

  19. 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

  20. End products • VFAs • CO2 • CH4 • NH3 • Microbes

  21. How pH is altered • Diet • Intake • Feeding frequency • Chewing/rumination

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