1. 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 mM
Portal blood mM
Peripheral blood 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

2. VFA Absorption Absorption of Ac-
Rumen
Ac- Ac- Portal
HAc blood
H+ Metabolism
HCO3-
H2O H2CO3 +
CO2 CO2 Carbonic
Metabolism anhydrase
HAc HAc

3. 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

4. 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.

5. Tissue Metabolism VFA VFA GIT tissues Liver Body tissues Use of VFA
Energy
Carbon for synthesis
Long-chain fatty acids
Glucose
Amino acids
Other

6. 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
H+NADPH NADP+
Glycerol
Pentose PO4 CO2
shunt Glucose

7. Utilization of Butryate in Metabolism
Butyrate (As energy)
Butyrate Butyrl CoA B-hydroxybutyrate Acetyl CoA
Krebs
cycle 2 CO2
Energy
(27 ATP/mole)
Some butyrate also used as a primer for short-chain fatty acids

8. Utilization of Propionate in Metabolism
Propionate Propionyl CoA Methylmalonyl CoA
CO Succinyl CoA
Vit B12
Glucose Krebs
cycle 2 CO2
Energy
(18 ATP/mole)

9. Utilization of VFA in Metabolism Summary
Acetate
Energy
Carbon source for fatty acids
Adipose
Mammary gland
Not used for net synthesis of glucose
Propionate
Precursor of glucose
Butyrate
Carbon source for fatty acids - mammary

10. Effect of VFA on Endocrine System
Propionate
Increases blood glucose
Stimulates release of insulin
Butryate
Not used for synthesis of glucose
Stimulates release of glucagon
Acetate
Does not stimulate release of insulin
Glucose

11. Energetic Efficiency of VFA in Metabolism
ATP/mole Energy in ATP % Heat of
(kcal/mole) combustion
Acetate
Propionate
Butyrate
Glucose

12. Energetic Efficiency of VFA Fermentation and Metabolism
Cellulose
10 Glucose VFA ATP
(6730 kcal) (5240 kcal (1946 kcal)
60A 28.9%
Starch P
10B
Absorbed as glucose ATP
(6730 kcal) (2888 kcal)
42.9%

13. Lower Energy Value of Roughage Compared with Grain
Less digested
Lignin limits digestibility of digestible fiber
- Greater energy lost from fermentation
CH Heat
- Increased rumination
Rumen contractions
Chewing
- More bulk in digestive tract

14. Comparative Prices of Corn and Alfalfa Hay
NEg
Mcal/kg
$/ton DM
$/Mcal
Corn
1.55
121.75
0.0864
Alfalfa hay
0.68
75.00
0.1213

15. Requirements for Glucose Ruminants
1. Nervous system
Energy and source of carbon
2. Fat synthesis
NADPH
Glycerol
3.Pregnancy
Fetal energy requirement
4. Lactation
Milk sugar - lactose

16. Sources of Glucose Carbon Ruminants
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

17. Glucose Synthesis Acetate Amino acids Ketone Acetyl CoA Bodies Fatty
Butyrate acids
Citrate
Glycerol
Acetyl CoA
Lactate CO CO2
Pyruvate Oxaloacetate
PEP
Glucose Succinate
Proteins Amino acids
Propionate

18. Conservation of Glucose Ruminants
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

19. 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

20. Pregnancy Toxemia Pregnant 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

21. Fatty Acid Metabolism Relation to Glucose
Diet fat Adipose Diet CHOH
CO2 Acetate
Malonyl CoA
LCFA NEFA Acetate
CO2
Glycerol LCFA acyl CoA
2 CO2
Triglycerides
Carnitine
FA acyl carnitine
inhibits CO (Mitochondria)
Ketones

22. 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

23. Fatty Acid Oxidation FA acyl carnitine Carnitine CoA FA acyl CoA
Acetyl CoA
CO2
Acetoacetyl CoA
Acetoacetate
(Mitochondria)
3-OH butyrate

24. 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

25. Long-Chain Fatty Acid Synthesis Ruminants
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

26. Long-Chain Fatty Acid Synthesis
Lactic acid, Propionate, Amino acids
Glucose Ruminants
limit use of glucose
Acetyl-CoA carboxylase
Acetyl CoA Fatty acids Triglycerides
NADPH NADP
Acetate Glycerol-3-P
Glu-6-P dehydrogenase Gly-3-P dehydrogenase
Glucose

27. Long-Chain Fatty Acid Synthesis
Glucose
NADPH NADP
Pyruvate Malate Fatty acids
Malate dehydrogenase NADP
Pyruvate Oxaloacetate
NADPH
Acetyl CoA Acetyl CoA
Oxaloacetate Citrate lyase
Citrate Citrate Acetate
Mitocondria Cytosol

28. Long-Chain Fatty Acid Synthesis
Citrate Citrate
Isocitrate
NADP Isocitrate
NADPH dehydrogenase
a-Ketoglutarate
Mitochondria Cytosol
Supplies about half of NADPH for fatty acid synthesis

29. 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