Physiology: Colonic

Abstract

Even though the majority of our food undergoes digestion in the stomach and small intestine, the colon still has a major role in digestion. It processes certain starches and proteins, which are resistant to digestion and absorption in the foregut. The large quantity of heterogeneous bacteria in the colon is responsible for fermentation – the process by which these starches and proteins are broken down and energy is produced. There are more than 400 different species of bacteria, the majo-rity of which are anaerobes. The bacteria feed upon mucous, residual proteins, and primarily complex carbohydrates that enter the colon. During fermentation of complex carbohydrates, short-chain fatty acids (SCFAs) are produced. More than 95% of SCFAs are produced and absorbed within the colon. The principle ones are acetate, propionate, and butyrate. This process for the most part occurs in the right and proximal transverse colon. Protein residue, which reaches the colon is also fermented by anaerobic bacteria. Proteins are fermented in the left colon. Proteins are broken down into SCFAs, branched chain fatty acids, and ammonia, amines, phenols, and indoles. Part of these metabolites become a nitrogen source for bacterial growth. Dietary fat is probably not absorbed to any degree in the colon. The colonic mucosa is unable to nourish itself from the bloodstream. Therefore, the nutrient requirements are met from the luminal contents.