Absorption of vfa
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Absorption of VFA. 70% of VFA absorbed from rumen-reticulum 60 to 70% of remainder absorbed from omasum Papillae are important – provide surface area Absorption from rumen is by passive diffusion Concentration in portal vein less than rumen VFA concentrations Rumen 50 - 150 mM

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Absorption of vfa
Absorption of VFA

  • 70% of VFA absorbed from rumen-reticulum

  • 60 to 70% of remainder absorbed from omasum

    • Papillae are important – provide surface area

  • Absorption from rumen is by passive diffusion

    • Concentration in portal vein less than rumen

      • VFA concentrations

        • Rumen 50 - 150 mM

        • Portal blood 1 - 2 mM

        • Peripheral blood 0.5 - 1 mM

  • Absorption increases at lower pH

    • H+ + Ac- HAc

  • Undissociated acids diffuse more readily

  • At pH 5.7 to 6.7 both forms are present, however most is dissociated

    • At higher pH, 1 equiv of HCO3 enters the rumen with absorption of

    • 2 equiv of VFA

  • Vfa absorption absorption of ac
    VFA AbsorptionAbsorption of Ac-


    Ac- Ac-Portal

    HAc blood



    H2O H2CO3


    CO2 CO2 Carbonic


    HAc HAc

    Vfa absorption
    VFA Absorption

    • Rate of absorption:

      • Butyrate > Propionate > Acetate

    • Absorption greater with increasing concentrations

    • of acids in the rumen

    • Absorption increases at lower rumen pH

    • Absorption greater in grain fed animals

      • Faster fermentation – More VFA produced

      • Lower pH

      • Growth of papillae

    Metabolism of vfa by git
    Metabolism of VFA by GIT

    Half or more of butyrate converted to

    - hydroxybutyric acid in rumen epithelium.

    5% of propionate converted to lactic acid by

    rumen epithelium.

    Some acetate is used as energy by tissues of gut.

    VFA and metabolites carried by portal vein to liver.

    Tissue metabolism vfa
    Tissue MetabolismVFA

    • VFA GIT tissues Liver

    • Body tissues

    • Use of VFA

      • Energy

      • Carbon for synthesis

        • Long-chain fatty acids

        • Glucose

        • Amino acids

        • Other

    Utilization of acetate in metabolism
    Utilization of Acetate in Metabolism

    1. Acetate (As energy) Energy

    Acetate Acetyl CoA Krebs cycle 2 CO2

    2 carbons (10 ATP/mole)

    2. Acetate (Carbon for synthesis of fatty acids – in adipose)

    Acetate Acetyl CoA Fatty acids Lipids



    Pentose PO4 CO2

    shunt Glucose

    Utilization of butryate in metabolism
    Utilization of Butryate in Metabolism

    Butyrate (As energy)

    Butyrate Butyrl CoA B-hydroxybutyrate Acetyl CoA


    cycle 2 CO2


    (27 ATP/mole)

    Some butyrate also used as a primer for short-chain fatty acids

    Utilization of propionate in metabolism
    Utilization of Propionate in Metabolism


    Propionate Propionyl CoA Methylmalonyl CoA

    CO2 Succinyl CoA

    Vit B12

    Glucose Krebs

    cycle 2 CO2


    (18 ATP/mole)

    Utilization of vfa in metabolism summary
    Utilization of VFA in MetabolismSummary



    Carbon source for fatty acids


    Mammary gland

    Not used for net synthesis of glucose



    Precursor of glucose



    Carbon source for fatty acids - mammary

    Effect of vfa on endocrine system
    Effect of VFA on Endocrine System

    • Propionate

      • Increases blood glucose

      • Stimulates release of insulin

    • Butryate

      • Not used for synthesis of glucose

      • Stimulates release of insulin

      • Stimulates release of glucagon

        • Increases blood glucose

    • Acetate

      • Not used for synthesis of glucose

      • Does not stimulate release of insulin

    • Glucose

      • Stimulates release of insulin

    Energetic efficiency of vfa in metabolism
    Energetic Efficiency of VFA in Metabolism

    ATP/mole Energy in ATP % Heat of

    (kcal/mole) combustion

    Acetate 10 76.0 36.3

    Propionate 18 136.8 37.2

    Butyrate 27 205.2 39.1

    Glucose 38 288.8 42.9

    Energetic efficiency of vfa fermentation and metabolism
    Energetic Efficiency of VFAFermentation and Metabolism


    10 Glucose VFA ATP

    (6730 kcal) (5240 kcal (1946 kcal)

    60A 28.9%

    Starch 30P


    Absorbed as glucose ATP

    (6730 kcal) (2888 kcal)


    Lower energy value of roughage compared with grain
    Lower Energy Value of Roughage Compared with Grain

    • Less digested

      • Lignin limits digestibility of digestible fiber

    • - Greater energy lost from fermentation

      • CH4 Heat

    • - Increased rumination

    • Rumen contractions

    • Chewing

    • - More bulk in digestive tract

    Requirements for glucose ruminants
    Requirements for GlucoseRuminants

    • 1. Nervous system

      • Energy and source of carbon

    • 2. Fat synthesis

      • NADPH

      • Glycerol

    • 3.Pregnancy

      • Fetal energy requirement

    • 4. Lactation

      • Milk sugar - lactose

    Sources of glucose carbon ruminants
    Sources of Glucose CarbonRuminants

    • Ruminants dependent on gluconeogenesis

    • for major portion of glucose

    • Sources of glucose in metabolism

      • 1. Propionate

      • 2. Amino acids

      • 3. Lactic acid

      • 4. Glycerol

      • 5. Carbohydrate digestion in intestine

        • Absorption of glucose from intestine

    Glucose synthesis
    Glucose Synthesis

    Acetate Amino acids

    Ketone Acetyl CoA



    Butyrate acids



    Acetyl CoA

    Lactate CO2 2 CO2

    Pyruvate Oxaloacetate


    Glucose Succinate

    Proteins Amino acids


    Conservation of glucose ruminants
    Conservation of GlucoseRuminants

    • 1. Glucose not extensively used for synthesis

    • of long-chain fatty acids in adipose of ruminants

      • - Not clear why glucose carbon is not used

      • Glycerol is needed for synthesis of triglycerides

      • - Comes from glucose

      • Acetate supplies carbon for fatty acid synthesis

    • 2. Low hexokinase activity in the liver

    • 3. Ruminants have low blood glucose concentrations

      • - Low concentrations of glucose in RBC

    Consequences of inadequate glucose in metabolism
    Consequences of Inadequate Glucose in Metabolism

    1. Low blood glucose

    2. High blood ketones

    3. High blood concentrations of

    long-chain fatty acids (NEFA)

    Causes fatty liver and/or ketosis in

    lactating cows and pregnancy toxemia

    in pregnant ewes

    Pregnancy toxemia pregnant ewes
    Pregnancy ToxemiaPregnant Ewes

    • During the last month of pregnancy

    • Ewes with multiple fetuses

    • Inadequate nutrition of ewe

    • High demands for glucose by fetuses

    • Low blood glucose and insulin

    • Mobilization of body fat

    • Increase in nonesterified fatty acids in blood

    • Increased ketone production by liver

    Fatty acid metabolism relation to glucose
    Fatty Acid MetabolismRelation to Glucose

    Diet fat Adipose Diet CHOH

    CO2 Acetate

    Malonyl CoA

    LCFA NEFA Acetate


    Glycerol LCFA acyl CoA

    2 CO2



    FA acyl carnitine

    Malonyl CoA

    inhibits CO2(Mitochondria)


    Low blood glucose and insulin
    Low Blood Glucose and Insulin

    • Increased release of nonesterified fatty acids

    • from adipose.

    • Less synthesis of fatty acids

      • Reduced malonyl CoA

    • Reduced sensitivity of carnitine palmitoyl-

    • transferase-1 to malonyl CoA

      • Increased transfer of fatty acids into

      • mitochondria for oxidation

    • Increased ketone production

    Fatty acid oxidation
    Fatty Acid Oxidation

    FA acyl carnitine



    FA acyl CoA

    Acetyl CoA


    Acetoacetyl CoA



    3-OH butyrate

    Low milk fat
    Low Milk Fat

    • Cows fed high grain diets:

      • Reduced milk fat percentage

      • Early theory

        • Low rumen pH

        • Shift from acetate to propionate production

        • Increased blood insulin

        • Decrease in blood growth hormone

      • More recent theory

        • Increased production of trans fatty acids in

        • the rumen

        • Trans fatty acids reduce milk fat synthesis

    Long chain fatty acid synthesis ruminants
    Long-Chain Fatty Acid SynthesisRuminants

    Synthesis is primarily in adipose or mammary gland

    – Limited synthesis in the liver

    Ruminants conserve glucose supply

    – Glucose not extensively used for long chain

    fatty acid synthesis

    Most of carbon is supplied by acetate

    Some butyrate used in mammary gland

    Glucose metabolism supplies some of NADPH

    needed for fatty acid synthesis

    Long chain fatty acid synthesis
    Long-Chain Fatty Acid Synthesis

    Lactic acid, Propionate, Amino acids

    Glucose Ruminants

    limit use of glucose

    Acetyl-CoA carboxylase

    Acetyl CoA Fatty acids Triglycerides


    Acetate Glycerol-3-P

    Glu-6-P dehydrogenaseGly-3-P dehydrogenase


    Long chain fatty acid synthesis1
    Long-Chain Fatty Acid Synthesis



    Pyruvate Malate Fatty acids

    Malate dehydrogenaseNADP

    Pyruvate Oxaloacetate


    Acetyl CoA Acetyl CoA

    Oxaloacetate Citrate lyase

    Citrate Citrate Acetate

    Mitocondria Cytosol

    Long chain fatty acid synthesis2
    Long-Chain Fatty Acid Synthesis

    Citrate Citrate


    NADP Isocitrate




    Supplies about half of NADPH for fatty acid synthesis

    Long chain fatty acid synthesis3
    Long-Chain Fatty Acid Synthesis

    • Butyrate

      • Can be used in mammary gland as primer for

      • synthesis of fatty acids

      • Shorter chain acids

    • Methylmalonyl (propionate)

      • Is used as primer for synthesis of fatty acids

      • in sheep fed high-grain diets

      • Branched-chain acids