ROLE OF INTESTINAL BACTERIA IN NUTRIENT METABOLISM (REPRINTED FROM CLINICAL NUTRITION VOL 16, PG 3, 1997)

The human large intestine contains a microbiota, the components of whi ch are generically complex and metabolically diverse. Its primary func tion is to salvage energy from carbohydrate not digested in the upper gut. This is achieved through fermentation and absorption of the major products, short chain fatty acids (SCFA), which represent 40-50% of t he available energy of the carbohydrate. The principal SCFA, acetate, propionate and butyrate, are metabolized by the colonic epithelium (bu tyrate), liver (propionate) and muscle (acetate). Intestinal bacteria also have a role in the synthesis of vitamins B and K and the metaboli sm of bile acids, other sterols and xenobiotics. The colonic microflor a are also responsive to diet. In the presence of fermentable carbohyd rate substrates such as non-starch polysaccharides, resistant starch a nd oligosaccharides, bacteria grow and actively synthesize protein. Th e amount of protein synthesis and turnover within the large intestine is difficult to determine, but around 15 g biomass is excreted in faec es each day containing 1 g bacterial-N. Whether bacterially synthesize d amino acids are ever absorbed from the colon remains unclear. Finall y, individual colonic micro-organisms such as sulphate-reducing bacter ia, bifidobacteria and clostridia, respond selectively to specific die tary components in a way that may be important to health.