VFAs Flashcards
Describe VFAs relationship with energy
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
What is the microbiome and why is it significant?
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.
How are VFAs derived?
Mostly through microbial fermentation in the rumen and/or the hindgut
What are the most important VFAs?
Acetic (2 carbon)
Propionic (3 carbon)
Butyric (4 carbon)
What is the total rate of production and concentration of VFAs related to?
intake of fermentable feed
More fermentable diet and higher intakes = more VFAs
What are the rates of production and concentrations of individual VFAs affected by?
forage to concentrate ration
rumen environment also has an impact
What VFAs are mostly derived from fiber
acetate
What VFAs are mostly derived from starch?
propionate
Describe VFA absorption
VFAs are absorbed through the rumen epithelium and transported by passive diffusion
What is the rate of VFA absorption determined by?
- VFA concentration in the rumen fluid (most important)
- rumen fluid pH
- VFA chain length
- rumen papillae size and epithelium integrity
What is parakeratosis?
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.
Describe the path of VFA metabolism
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.
Describe the rumen wall’s role in VFA metabolism
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
How is acetate metabolized by the liver?
It remains as acetate
How is propionate metabolized by the liver?
It is broken down to glucose and CO2
How is butyrate metabolized by the liver?
It is converted to ketone bodies
Why is butyrate important?
It is important for gut development and health (energy and stimulus for gut)
Encapsulated butyrate is fed to swine and chickens
What is the predominant source of energy in the ruminant that is used to make ATP and fat that is stored?
acetate
How do ruminants minimize glucose demand?
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.
What are the “metabolic fates” of each VFA?
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
Describe enterotoxemia
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
Describe rumen acidosis
Rumen acidosis is a disease associated with ruminal fermentation that results from a series of events
- High fermentation diet or slug of fermentable feeds
- Rapid fermentation and production of VFAs
- Decrease in rumen or hindgut pH
- Increased lactic acid because lactic acid producing bacteria grow well at low pH (lactic acid drives pH lower)
- Production of bacterial toxins and histamine
- Disruption of gut epithelium and absorption of bacteria, bacterial toxins, and histamine
- High VFA and lactate cause low blood pH (acidosis) and combined bacterial toxins and histamine cause hoof problems, liver abscesses, and poor health
What is the primary carb substrate absorbed from the GIT for a ruminant vs a nonruminant?
NR: glucose
R: VFA
What is the primary substrate for fat synthesis in a ruminant vs a nonruminant?
NR: glucose
R: acetate
Describe cellular demand for glucose in a ruminant vs a nonruminant?
NR: very high
R: high
Describe the importance of gluconeogenesis and the precursor in ruminants vs nonruminants
NR: important post-absorptive phase (glycerol, AA)
R: always important (propionate, glycerol, AA)
Describe ketone production in ruminants vs nonruminants
NR: abnormal (if it occurs to any extent –> ketosis)
R: normal, but if excessive –> ketosis (usually during extreme fat mobilization)
