Fermentation is the enzymatic decomposition and utililization of foodstuffs, particularly carbohydrates, by microbes. Fermentation takes place throughout the gastrointestinal tract of all animals, but the intensity of fermentation depends on microbe numbers, which are generally highest in the large bowel. Thus, the large intestine is quantitatively the most important site of fermention, except for species with forestomachs (ruminants). Further, there are major differences in the contribution of fermentation to energy production of different species. In carnivores like dogs and cats, and even in omnivores like humans, fermentation generates rather few calories, but in herbivores, fermentation is a way of life.
Large intestinal epithelial cells do not produce digestive enzymes, but contain 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 and metabolism of carbohydrates not digested in the small intestine (e.g. cellulose, residual starch)
- 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 or short-chain fatty acids (especially acetic, propionic and butyric acids) generated from fermentation are not only metabolized within intestinal epithelial cells, but can be absorbed by diffusion and thereby contribute fuel to systemic energy metabolism. The concentration of volatile fatty acids in the large gut is similar among mammals, but because of the enormous differences in the relative size of the large gut, the importance of microbial fermentation to energy production varies considerably among species. As examples, it has been estimated that the contribution to maintenance energy of volatile fatty acids produced in the hindgut is 6-9% in humans, 10-30% in pigs and only 2% in dogs, reflecting the relative size of their fermentation vats.
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.
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