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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





CO2 CO2 Carbonic



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



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