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 AcRumen
Ac-
AcHAc
Portal
blood
H+ Metabolism
HCO3H2O
H2CO3
+
CO2
CO2
Metabolism
HAc
HAc
Carbonic
anhydrase
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
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
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
1. Acetate (As energy)
Acetate
Acetyl CoA
Energy
Krebs cycle
2 carbons
2 CO2
(10 ATP/mole)
2. Acetate (Carbon for synthesis of fatty acids – in adipose)
Acetate
Acetyl CoA
Fatty acids
H+NADPH
NADP+
Lipids
Glycerol
Pentose PO4
shunt
CO2
Glucose
Utilization of Butryate in Metabolism
Butyrate (As energy)
Butyrate
Butyrl CoA
B-hydroxybutyrate
Krebs
cycle
Energy
(27 ATP/mole)
Acetyl CoA
2 CO2
Some butyrate also used as a primer for short-chain fatty acids
Utilization of Propionate in Metabolism
Propionate
Propionate
Propionyl CoA
Methylmalonyl CoA
CO2
Glucose
Succinyl CoA
Vit B12
Krebs
cycle
Energy
(18 ATP/mole)
2 CO2
Utilization of VFA in Metabolism
Summary
Acetate
Energy
Carbon source for fatty acids
Adipose
Mammary gland
Not used for net synthesis of glucose
Propionate
Energy
Precursor of glucose
Butyrate
Energy
Carbon source for fatty acids - mammary
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
ATP/mole Energy in ATP
(kcal/mole)
% Heat of
combustion
Acetate
10
Propionate 18
Butyrate
27
76.0
136.8
205.2
36.3
37.2
39.1
Glucose
288.8
42.9
38
Energetic Efficiency of VFA
Fermentation and Metabolism
Cellulose
10 Glucose
(6730 kcal)
Starch
VFA
(5240 kcal
60A
30P
10B
Absorbed as glucose
(6730 kcal)
ATP
(1946 kcal)
28.9%
ATP
(2888 kcal)
42.9%
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
Comparative Prices of Corn and Alfalfa Hay
NEg
Mcal/kg
$/ton
DM
$/Mcal
NEg
Corn
1.55
121.75
0.0864
Alfalfa hay
0.68
75.00
0.1213
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
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
Glucose Synthesis
Acetate
Ketone
Acetyl CoA
Bodies
Fatty
Butyrate
acids
Amino acids
Citrate
Glycerol
Acetyl CoA
Lactate
CO2
2 CO2
Pyruvate
Oxaloacetate
PEP
Glucose
Succinate
Proteins
Amino acids
Propionate
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
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
•
•
•
•
•
•
•
•
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
Diet fat
Adipose
Diet CHOH
CO2 Acetate
Malonyl CoA
LCFA
NEFA
Acetate
CO2
Glycerol
LCFA acyl CoA
2 CO2
Triglycerides
Carnitine
FA acyl carnitine
Malonyl CoA
inhibits
CO2
(Mitochondria)
Ketones
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 palmitoyltransferase-1 to malonyl CoA
Increased transfer of fatty acids into
mitochondria for oxidation
• Increased ketone production
Fatty Acid Oxidation
FA acyl carnitine
Carnitine
CoA
FA acyl CoA
Acetyl CoA
CO2
Acetoacetyl CoA
Acetoacetate
(Mitochondria)
3-OH butyrate
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
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
Lactic acid, Propionate, Amino acids
Glucose
Ruminants
limit use of glucose
Acetyl-CoA carboxylase
Acetyl CoA
NADPH
Acetate
Fatty acids
Triglycerides
NADP
Glycerol-3-P
Glu-6-P dehydrogenase
Gly-3-P dehydrogenase
Glucose
Long-Chain Fatty Acid Synthesis
Glucose
NADPH
NADP
Pyruvate
Malate
Fatty acids
Malate dehydrogenase
Pyruvate
NADP
Oxaloacetate
NADPH
Acetyl CoA
Oxaloacetate
Citrate
Mitocondria
Acetyl CoA
Citrate lyase
Citrate
Cytosol
Acetate
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
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