Download Volatile Fatty Acids

VOLATILE FATTY
ACIDS
Volatile Fatty Acids
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


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 goes towards
fermentation (methane).
Cont.

Regulation of microbial growth/function
 Bacteria
vs. protozoa
 Competition
 Environmental pH
 Alteration with diet
 Alteration with intake
Cont.

Acetate: mostly from cellulose
 Important

Propionate: mostly from starch
 Important

for milk fat synthesis
to produce glucose
Butyrate: mostly derived from acetate
 Important
in ketones usage as an energy source
Volatile Fatty Acids (VFA)

Produced from the fermentation of pyruvate
 Rumen
and hind gut
 Types/ratios depends on diet

3 major VFAs
 Acetic
acid
 Propionic acid
 Butyric acid
CH3COOH
CH3CH2COOH
CH3CH2CH2COOH
Rumen Fermentation
Starch
Hemicellulose
Cellulose
Sugars
Glucose
Pectins
Pyruvate
Lactic
Acetic
Propionic
Butyric
Formic
H2+CO2
Methane
(CH4)
Acetate
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


Pyruvate + Pi + ADP 
Acetate + ATP + H2 + CO2
Cellulolytic bacteria
Energy source for rumen epithelium and muscle
Not utilized by liver
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
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
 Tissue
concentration
uptake is directly related to rate of rumen
fermentation [blood concentration]
Propionate



Pyruvate + CoA + 4H+  Propionate + H2O
Amylolytic bacteria
Utilized by rumen epithelium
 Converted

to lactate and pyruvate
Important as a precursor for gluconeogenesis
Hepatic propionate metabolism
Glucose
TCA
Cycle
OAA
Succinyl CoA
Coenzyme B12
Methylmalonyl CoA
ADP + Pi
ATP
Biotin, Mg ++
Propionyl CoA
AMP + 2 Pi
ATP
Propionate
CoA
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
Ruminal VFA absorption
Rumen
lumen
Acetate
Rumen
wall
70
Portal
vein
50
20
Propionate
20
10
10
Butyrate
10
1
9
Values are relative flux rates
Hepatic metabolism of VFA
Rumen
Portal
vein
Acetate
70
50
Propionate
20
10
Liver
Peripheral
blood
Acetate
Glucose
Glucose
CO2
Butyrate
10
1
4
3-OH
butyrate
3-hydroxy
Butyrate
(BHB)
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
 pK
~4.75
(acetic acid) vs CH3COO- (acetate)
Cont.

In converting acetate to pyruvate
 also

CO2; CH4
Ionophore feed additives
 Increases
propionate
 Decreases acetate
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)
Sudden dietary changes






Propionate to lactate; reduced pH
Lactate needs to be converted to pyruvate
Microbes converting lactate grow slow !!!!!
pH continues to drop
Too acidic environment
Lactic acidosis; can be lethal
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
End products



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
VFAs
CO2
CH4
NH3
Microbes
How pH is altered
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

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Diet
Intake
Feeding frequency
Chewing/rumination