VFAs

Question 1

Describe VFAs relationship with energy

Answer 1

VFAs are the predominant form of energy absorbed from dietary fiber

They are a major source of energy for ruminants and horses, being >60% of the diet digested in ruminants

Question 2

What is the microbiome and why is it significant?

Answer 2

The microbiome is all of the microbes living on and in the animal.

Recently, many diseases have been shown to be associated with changes in the microbiome and influenced by gut microbes.

Question 3

How are VFAs derived?

Answer 3

Mostly through microbial fermentation in the rumen and/or the hindgut

Question 4

What are the most important VFAs?

Answer 4

Acetic (2 carbon)
Propionic (3 carbon)
Butyric (4 carbon)

Question 5

What is the total rate of production and concentration of VFAs related to?

Answer 5

intake of fermentable feed

More fermentable diet and higher intakes = more VFAs

Question 6

What are the rates of production and concentrations of individual VFAs affected by?

Answer 6

forage to concentrate ration

rumen environment also has an impact

Question 7

What VFAs are mostly derived from fiber

Answer 7

acetate

Question 8

What VFAs are mostly derived from starch?

Answer 8

propionate

Question 9

Describe VFA absorption

Answer 9

VFAs are absorbed through the rumen epithelium and transported by passive diffusion

Question 10

What is the rate of VFA absorption determined by?

Answer 10

• VFA concentration in the rumen fluid (most important)
• rumen fluid pH
• VFA chain length
• rumen papillae size and epithelium integrity

Question 11

What is parakeratosis?

Answer 11

This is the development of scar tissue in the epithelial cells of the rumen resulting from very low rumen pH. This condition makes it harder for absorption to occur.

Question 12

Describe the path of VFA metabolism

Answer 12

VFAs are first differentially metabolized by the rumen wall before transport into the portal blood .

Next, VFAs can be taken up and metabolized by the liver, where they are affected differently depending on the VFA.

Question 13

Describe the rumen wall’s role in VFA metabolism

Answer 13

The rumen wall “burns” some of the VFAs for energy as they pass through.

Most of the acetate makes it through

Propionate is largely absorbed

Butyrate is the most highly absorbed VFA by the rumen wall

Question 14

How is acetate metabolized by the liver?

Answer 14

It remains as acetate

Question 15

How is propionate metabolized by the liver?

Answer 15

It is broken down to glucose and CO2

Question 16

How is butyrate metabolized by the liver?

Answer 16

It is converted to ketone bodies

Question 17

Why is butyrate important?

Answer 17

It is important for gut development and health (energy and stimulus for gut)

Encapsulated butyrate is fed to swine and chickens

Question 18

What is the predominant source of energy in the ruminant that is used to make ATP and fat that is stored?

Answer 18

acetate

Question 19

How do ruminants minimize glucose demand?

Answer 19

They use acetate to supply carbon and NADPH needed for fat synthesis rather than utilizing glucose for these purposes

They also utilize the isocitrate shuttle to convert citrate from TCA cycle to isocitrate, which converts to malate. This helps form the NADPH, and malate can reenter the TCA.

Question 20

What are the “metabolic fates” of each VFA?

Answer 20

acetate –> peripheral tissues

propionate –> hepatic glucose synthesis –> peripheral tissues

butyrate –> 3 hydroxybutyrate (ketone body) –> peripheral tissue

Remember, acetate, not glucose, is the major substrate for fatty acid synthesis

Question 21

Describe enterotoxemia

Answer 21

Enterotoxemia is a disease associated with ruminal fermentation that is common in lambs.

• aka “overeating disease”
• Clostridium perfringes bacteria grows rapidly with excess levels of starch
• toxin is produced by the bacteria, and it can be lethal
• can be prevented via vaccination and treated with antibiotics

Question 22

Describe rumen acidosis

Answer 22

Rumen acidosis is a disease associated with ruminal fermentation that results from a series of events

1. High fermentation diet or slug of fermentable feeds
2. Rapid fermentation and production of VFAs
3. Decrease in rumen or hindgut pH
4. Increased lactic acid because lactic acid producing bacteria grow well at low pH (lactic acid drives pH lower)
5. Production of bacterial toxins and histamine
6. Disruption of gut epithelium and absorption of bacteria, bacterial toxins, and histamine
7. High VFA and lactate cause low blood pH (acidosis) and combined bacterial toxins and histamine cause hoof problems, liver abscesses, and poor health

Question 23

What is the primary carb substrate absorbed from the GIT for a ruminant vs a nonruminant?

Answer 23

NR: glucose
R: VFA

Question 24

What is the primary substrate for fat synthesis in a ruminant vs a nonruminant?

Answer 24

NR: glucose
R: acetate

Question 25

Describe cellular demand for glucose in a ruminant vs a nonruminant?

Answer 25

NR: very high
R: high

Question 26

Describe the importance of gluconeogenesis and the precursor in ruminants vs nonruminants

Answer 26

NR: important post-absorptive phase (glycerol, AA)
R: always important (propionate, glycerol, AA)

Question 27

Describe ketone production in ruminants vs nonruminants

Answer 27

NR: abnormal (if it occurs to any extent –> ketosis)
R: normal, but if excessive –> ketosis (usually during extreme fat mobilization)