Absorption of VFA

70% of VFA absorbed from rumen-reticulum60 to 70% of remainder absorbed from omasum

Papillae are important – provide surface area

Absorption from rumen is by passive diffusionConcentration in portal vein less than rumen

VFA concentrationsRumen 50 - 150 mMPortal blood 1 - 2 mMPeripheral 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 dissociatedAt higher pH, 1 equiv of HCO3 enters the rumen with absorption of2 equiv of VFA

VFA AbsorptionAbsorption of Ac-

Ac- Ac- PortalHAc blood

H+ Metabolism

HCO3-

H2O H2CO3

+CO2 CO2 Carbonic

Metabolism anhydrase

HAc HAc

Rumen

VFA Absorption

Rate of absorption:

Butyrate > Propionate > Acetate

Absorption greater with increasing concentrationsof acids in the rumen

Absorption increases at lower rumen pH

Absorption greater in grain fed animalsFaster fermentation – More VFA producedLower pHGrowth 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 MetabolismVFA

VFA GIT tissues Liver

Body tissuesUse of VFA

EnergyCarbon for synthesis

Long-chain fatty acidsGlucoseAmino acidsOther

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 CO2shunt Glucose

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

Utilization of Propionate in Metabolism

Propionate

Propionate Propionyl CoA Methylmalonyl CoA

CO2 Succinyl CoA

Vit B12

Glucose Krebs

cycle 2 CO2

Energy(18 ATP/mole)

Utilization of VFA in MetabolismSummary

Acetate Energy Carbon source for fatty acids

AdiposeMammary gland

Not used for net synthesis of glucosePropionate Energy Precursor of glucoseButyrate Energy Carbon source for fatty acids - mammary

Effect of VFA on Endocrine System

PropionateIncreases blood glucoseStimulates release of insulin

ButryateNot used for synthesis of glucoseStimulates release of insulinStimulates release of glucagon

Increases blood glucose

AcetateNot used for synthesis of glucoseDoes not stimulate release of insulin

GlucoseStimulates release of insulin

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 VFAFermentation and Metabolism

Cellulose

10 Glucose VFA ATP(6730 kcal) (5240 kcal (1946 kcal)

60A 28.9%Starch 30P

10B

Absorbed as glucose ATP(6730 kcal) (2888 kcal)

42.9%

Lower Energy Value of Roughage Compared with Grain

- Less digested- Lignin limits digestibility of digestible fiber

- Greater energy lost from fermentationCH4 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 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 CarbonRuminants

Ruminants dependent on gluconeogenesisfor major portion of glucose

Sources of glucose in metabolism1. Propionate2. Amino acids3. Lactic acid4. Glycerol5. Carbohydrate digestion in intestine

Absorption of glucose from intestine

Glucose Synthesis

Acetate Amino acidsKetone Acetyl CoA Bodies

FattyButyrate acids

CitrateGlycerol

Acetyl CoALactate CO2 2 CO2

Pyruvate OxaloacetatePEP

Glucose SuccinateProteins Amino acids

Propionate

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 liver3. Ruminants have low blood glucose concentrations

- Low concentrations of glucose in RBC

Consequences of Inadequate Glucose in Metabolism

1. Low blood glucose2. High blood ketones3. High blood concentrations of long-chain fatty acids (NEFA)

Causes fatty liver and/or ketosis inlactating cows and pregnancy toxemiain 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 MetabolismRelation to Glucose

Diet fat Adipose Diet CHOH CO2 Acetate

Malonyl CoA

LCFA NEFA AcetateCO2

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 acidsReduced malonyl CoA

• Reduced sensitivity of carnitine palmitoyl- transferase-1 to malonyl CoA

Increased transfer of fatty acids intomitochondria for oxidation

• Increased ketone production

Fatty Acid Oxidation

FA acyl CoAAcetyl CoA

CO2

Acetoacetyl CoA

Acetoacetate (Mitochondria)

3-OH butyrate

FA acyl carnitine

Carnitine CoA

Low Milk Fat

Cows fed high grain diets:

Reduced milk fat percentage

Early theoryLow rumen pHShift from acetate to propionate productionIncreased blood insulinDecrease in blood growth hormone

More recent theoryIncreased production of trans fatty acids inthe rumenTrans fatty acids reduce milk fat synthesis

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

Lactic acid, Propionate, Amino acidsGlucose Ruminants

limit use of glucose

Acetyl-CoA carboxylase

Acetyl CoA Fatty acids Triglycerides

NADPH NADPAcetate Glycerol-3-PGlu-6-P dehydrogenase Gly-3-P dehydrogenase

Glucose

Long-Chain Fatty Acid Synthesis

Glucose NADPH NADP

Pyruvate Malate Fatty acidsMalate dehydrogenase NADP

Pyruvate OxaloacetateNADPH

Acetyl CoA Acetyl CoAOxaloacetate Citrate lyase

Citrate Citrate Acetate

Mitocondria Cytosol

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