Digestion and Metabolism

carbohydrates can be classified upon analysis into two areas

crude fiber

nitrogen-free extract (NFE)

amount of starch in feed

%CF

low in crude fiber

high starch

nitrogen-free extract 

non-structural carbs 

soluble sugars and starches 

readily digested by mammals and microbes

crude fiber

beta bonds

poorly digested by mammalian enzymes

carbohydrates must be broken down into

monosaccharides for absorption from the digestive system

this requires digestive enzymes

enzymes (non-ruminants)

bacteria (ruminants)

carbohydrates splitting enzymes are effective in

breaking down most complex carbohydrates to monosaccharides (simple sugars)

EXCEPT for those with beta linkages (cellulose)

bacteria of the

rumen of ruminants

cecum/colon of non-ruminants

produce cellulase which breaks down cellulose

anaerobic fermentation of these carbohydrates by the bacteria result in 

production of large quantities of volatile fatty acids

acetic, butyric, and propionic 

everything makes this

amylase

salivary amylase 

deactivated by HCl and pepsin 

pancreatic amylase is

very active

does most of the “work”

enzymatic activity primarily in

duodenum

with absorption occurring throughout S.I.

ranking of efficiency of carbohydrates 

glucose- close to 100%

starches- depends on % amylose

fiber fractions- lignin- % indigestible

acetic

not metabolized by rumen mucosa or liver but by peripheral tissues (muscle & adipose tissue; mammary glands)

propionic

metabolized almost completely by the liver to glucose

butyric

energy source for the rumen mucosa 

glucose catabolism yields

ATP

during normal conditions

red blood cells and the CNS use glucose as exclusive fuel source (don’t require insulin to gain access to cell)

short term energy storage 

glycogen 

long term energy source

fat

Glycolysis

anaerobic (no oxygen)

takes place in the cell cytoplasm

TCA cycle and oxidative phosphorylation are

aerobic (oxygen)

takes place in mitochondria

carbohydrates in the feed are 

broken down to glucose by rumen microorganisms 

then converted by glycolysis to

pyruvic acid

then pyruvate quickly converted to

volatile fatty acids

propionic acid is 

most efficient energy conversion from feed to animal use 

acetic/butyric acid is

less efficient

represent a significant loss of dietary energy

what are crude fibers

cellulose

hemicellulose

lignin