EXAM 2: Ruminant Carbohydrate digestion & metabolism

T or F: Ruminants don’t directly utilize most of the carbohydrates they eat

True

GIT bacteria ferment them to end products - VFAs

• 70-80% of energy comes from VFA

Plant carbohydrates

Fiber

Primary and secondary cell wall

Cell wall components

Cellulose

• Crystalline, resistant to hydrolysis

• Must be moistened in rumen…can cause a lag time before digested

Hemicellulose

• Easily hydrolyzed (in dilute acid/base); not crystalline like cellulose

Lignin

• Randomly bound throughout cell wall

• Fills space between cellulose, hemicellulose, etc

• Gives structure & support to plant

• Plant content increases with maturity Interferes with digestion of fiber

The cellulosome

Factors effecting cell wall digestion pt. 1

Grain in diet

• Increased grain → Decreased pH → Reduced cellulolytic bacteria → Reduced cell wall digestion

Fats, particularly unsaturated oils

• Toxic to cellulolytic bacteria → Reduced cell wall digestion

• Limit dietary fat to 5% of DM

Factors effecting cell wall digestion pt. 2

Bacterial nutrition

• N, and S increase fiber digestion

Increased rate of passage leads to reduced cell wall digestion

• Factors increasing rate of passage of forages from the rumen

  • Increased feed intake

  • Grinding of feedstuffs

  • Things that decrease rumen size (Pregnancy)

Significance of VFA absorption

• 95% of VFA are absorbed before the abomasum

• 15% of VFA (primarily butyrate) entering rumen epithelium do not enter portal circulation

VFA absorption is through diffusion and facilitated transport modified by metabolism

• No active transport

• Bicarb enters rumen in exchange for VFA to facilitate absorption through specific transporters

  • Increases with increasing VFA concentration and decreasing ruminal pH

VFA metabolism in the ruminal epithelium

Acetate → None

Propionate → Approximately 2-5% converted to lactate

Butyrate → Approximately 90% converted to the ketones beta-OH-butyrate or acetoacetate

Butyrate metabolism in rumen epithelium

Butyrate extensively metabolized in rumen epithelium to ketone bodies

Liver metabolism of Butyrate

• liver has low afinity for B-hydroxy-butyrate

• only 20% is metabolized in the liver

Peripheral tissues use butyrate

1. Oxidation (TCA) by muscle for ATP production

2. Long chain fatty acid synthesis (lipogenesis) in adipose and mammary tissue

Propionate metabolism

~94% of propionate entering liver converted to _________

Glucose (gluconeogenesis)

Mechanism of gluconeogenesis:

Hormones:

• glucagon & glucorticoids (cortisol) stimulates

• Insulin inhibits

Glucose conservation in ruminant

• Typically have low blood glucose

• Low glucokinase activity in the liver

  • Thus, little glycolysis occurs in liver, so net output of glucose from liver in ruminant

• Unlike monogastrics-ruminants do not use glucose to supply carbon for long chain fatty acid synthesis

  • Instead, acetate (or butyrate) is used

Propionate use

Acetate metabolism

Very little removed by liver (~80% escapes)

Peripheral tissues use acetate for:

1) Oxidation (TCA cycle) as Acetyl CoA

2) Primary precursor for de novo lipogenesis in ruminant

• Long chain fatty acid synthesis

• Acetyl-Co A Synthetase activity in ruminant adipose tissue is 2-3 times higher than in rat adipose

  • Activation of Acetate to Acetyl-CoA so it can enter metabolic pathways

VFA pathways to ATP

Energetic efficient of VFA metabolism vs Direct glucose absorption

Feed carbohydrates fermented to ________ to ______ for bacteria needs

Feed carbohydrates fermented to pyruvate to ATP for bacteria needs

VFA waste products provide…

ATP, glucose, or lipid for animal

VFA must be ________ to enter pathways (like other fatty acids)

VFA must be activated to enter pathways (like other fatty acids)

T or F: Site of VFA utilization and end product differs

true