Abstract
Colonic bacteria ferment nutrients escaped digestion and absorption in the small intestine to get energy for their proliferation, and result in luminal accumulation of their fermentation products. Colonic fermentation and their products are influenced by available substrates which vary chemical compositions and physical states. Plant fibers, resistant starch, nondigestible oligosaccharides and resistant proteins are principal substrates for colonic bacteria, and are anaerobically metabolized to various organic acids, mainly short—chain fatty acids (acetate, propionate and butyrate), ammonia and amines. Short—chain fatty acids are rapidly absorbed across Colonic mucosa and used as an energy nutrient. Short—chain fatty acids, besides their energy supplier, play a role as luminal chemical stimuli that can modify epithelial proliferation, intestinal blood flow, motility and secretion. The effects vary among short—chain fatty acids, and are dose—dependent. The sensory mechanism, therefore, of the gastrointestinal tracts must be able to detect not only the presence of individual short—chain fatty acids, but also their concentration.
