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. 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 .
You obtain a few calories from eating a salad. 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.