Fermentation is the enzymatic decomposition and utilization of foodstuffs, particularly carbohydrates, by microbes. 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.

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:

• Digestion of carbohydrates not digested in the small intestine.
  
  
• Synthesis of vitamin K and certain B vitamins.

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.

You obtain a few calories from eating a salad. A rabbit, on the other hand, has a relatively huge fermentation vat (cecum), and obtains much of its energy from the plants it consumes.

Synthesis of vitamin K by colonic bacteria provides a valuable supplement to dietary sources and makes clinical vitamin K deficiency rare. Similarly, formation of B vitamins by the microbial flora in the large intestine is useful to many animals. They are not absorbed in the large intestine, but are present in feces. The behavior of coprophagy or eating feces seen particularly in rodents, rabbits and other animals is thought to be a behavioral adaption to recovery of these valuable resources.

A more comprehensive description of fermentation is presented in the section on digestive physiology of herbivores.

Next page > Gross and Microscopic Anatomy > Absorption, Secretion and Formation of Feces > Large Intestinal Motility > Microbial Fermentation