Lecture 11 - Volatile Fatty Acids

What are volatile fatty acids (VFAs)?

Short-chain fatty acids produced by microbial fermentation of dietary organic matter.

Where are VFAs produced in ruminants?

In the rumen by microbial fermentation.

Where are VFAs produced in non-ruminants?

In the hindgut (cecum and colon).

Main VFAs produced?

Acetate (CH₃COOH), propionate (CH₃CH₂COOH), and butyrate (CH₃CH₂CH₂COOH).

Are all VFAs produced regardless of diet?

Yes, but proportions vary by diet.

Acetate:propionate ratio in forage diet?

~3:1 (more acetate).

Acetate:propionate ratio in high-grain diet?

~1:1 (more propionate).

What happens when animals switch to high-grain diets?

More VFAs produced, lower pH, faster fermentation, longer rumen papillae.

What is endogenous acetate?

Acetate produced by animal tissues (not microbes).

Where are most VFAs absorbed in ruminants?

Across the rumen wall (rumen papillae).

what % of VFAs are absorbed from rumen-reticulum?

~70%.

% absorbed from omasum?

60–70% of remaining VFAs.

In what form are VFAs absorbed?

Acid form.

Factors affecting VFA absorption?

Concentration, pH, and chain length.

How does VFA concentration affect absorption?

Higher concentration → faster absorption.

How does pH affect VFA absorption?

Lower pH → greater absorption.

Which VFA is absorbed fastest?

Butyrate (Butyrate > Propionate > Acetate).

Are all absorbed VFAs transferred to blood?

No, some are metabolized by rumen wall cells.

Where do absorbed VFAs go first?

Into the portal vein → liver.

In blood, what form do VFAs take?

Anionic form (acetate⁻, propionate⁻, butyrate⁻).

What happens to acetate in the liver?

Most passes through unchanged to peripheral blood.

What happens to propionate in the liver?

90% converted to glucose (gluconeogenesis).

What happens to butyrate in the liver?

> 90% converted to 3-hydroxybutyrate (a ketone body).

Why is hepatic glucose production important in ruminants?

Most dietary glucose is fermented; propionate → glucose is main blood glucose source.

Which VFA is found most in peripheral blood?

Acetate.

Which tissues oxidize acetate for ATP?

Skeletal muscle, heart, and kidneys.

Which tissues use acetate for fatty acid synthesis?

Adipose tissue and mammary glands.

What does adipose tissue do with acetate?

Converts it to fatty acids and stores as triglycerides.

What does the mammary gland do with acetate?

Converts it to milk fat.

What is the main carbon source for fatty acid synthesis in ruminants?

Acetate (not glucose)..

Major uses of acetate?

Energy, fatty acid synthesis (adipose/mammary), not for glucose synthesis.

Major uses of propionate?

Energy and glucose precursor (gluconeogenesis).

Major uses of butyrate?

Energy and fatty acid synthesis (mammary tissue).

What must happen before acetate can be metabolized?

It must be activated to acetyl-CoA.

How do tissues take up acetate?

By passive diffusion (depends on arterial acetate concentration).

What determines blood acetate levels?

Rate of rumen fermentation and VFA absorption.

Faster rumen fermentation means what for tissues?

More acetate available for energy and fat synthesis.

Where does acetyl-CoA in cytoplasm go?

Used for long-chain fatty acid synthesis (adipose, mammary).

Where does acetyl-CoA in mitochondria go?

Oxidized in TCA cycle for energy (muscle, heart, kidneys).

% of CO₂ carbon from acetate in a well-fed ruminant?

30–35%..

% of ATP requirements met by acetate oxidation in well-fed ruminants?

30–35%.

When animal is at maintenance, what happens to acetate?

Oxidized for energy; little stored as fat.

When animal is in positive energy balance, what happens to acetate?

Extra acetate used for fatty acid synthesis and stored in adipose tissue.

Where does fermentation occur in non-ruminants?

Cecum and colon (hindgut fermentation).

Species with important colonic fermentation?

Horses.

When do pigs rely more on fermentation?

When raised on pasture (forage diets).

Where are VFAs absorbed in non-ruminants?

Colonic mucosa.

What happens to VFAs in colonic mucosa?

Some are used locally for energy before entering blood.

What happens to propionate in non-ruminants?

Converted to glucose in the liver.

How much glucose does fermentation supply in non-ruminants?

A small amount compared to intestinal absorption.

% of ATP from acetate oxidation in non-ruminants vs ruminants?

10–15% in non-ruminants vs 30–35% in ruminants.

Why is fermentation less important in non-ruminants?

Most nutrients are digested and absorbed before reaching the hindgut.

% of digestible feed fermented to VFAs, methane, CO₂, and ammonia?

Over 60%.

Where else does microbial fermentation occur besides rumen?

Hindgut of non-ruminants (less extensive).

What happens to VFAs and ammonia after absorption?

Enter portal blood → liver metabolism.

What happens to CO₂ and methane?

Released as gases (eructation).

Postabsorptive fate of acetate?

Used for energy or fat synthesis.

Postabsorptive fate of propionate?

Converted to glucose in the liver → used for ATP, lactate, or glycogen.

Postabsorptive fate of butyrate?

Converted to 3-hydroxybutyrate → oxidized by tissues.

What happens to ammonia in the liver?

Converted to urea → excreted or recycled via saliva.

Do rumen wall cells use VFAs?

Yes — especially butyrate for their own energy.

Major substrate for fat synthesis in ruminants?

Acetate (not glucose).

Primary site of VFA production in ruminants?

Rumen.

Main VFAs and their roles?

Acetate (fat synthesis), Propionate (glucose), Butyrate (energy).

Which VFA provides most glucose in ruminants?

Propionate.

Which VFA provides most fat in ruminants?

Acetate

Which VFA is converted to 3-hydroxybutyrate?

Butyrate.

Major difference between ruminants and non-ruminants?

Amount and importance of microbial fermentation for energy.