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Fermentation takes place in the large bowel of all animals, but
there are major differences in its contribution to the nutrition of
different species. In carnivores like dogs and cats, and even in omnivores
like humans, fermentation generates very few calories. In herbivores,
however, fermentation is a way of life.
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The large intestine does not produce its own digestive enzymes, but
contains huge numbers of bacteria which have the enzymes to digest
and utilize many substrates. In all animals, two processes are attributed
to the microbial flora of the large intestine:
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Cellulose is common constituent in the diet of many animals, including
man, but no mammalian cell is known to produce a cellulase. Several
species of bacteria in the large bowel synthesize cellulases and digest
cellulose. Importantly, the major end products of microbial digestion
of cellulose and other carbohydrates are volatile fatty acids, lactic
acid, methane, hydrogen and carbon dioxide. Fermentation is thus the
major source of intestinal gas . Volatile fatty acids (acetic, proprionic
and butyric acids) generated from fermentation can be absorbed by
diffusion in the colon.
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Though debatable, synthesis of vitamin K by colonic bacteria can
provides a valuable supplement to dietary sources and could account
for the fact that clinical vitamin K deficiency is rare
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