Role of intestinal bacteria in nutrient metabolism

The human large intestine contains a microbiota, the components of which are generically complex and metabolically diverse. Its primary function 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 the available energy of the carbohydrate. The principal SCFA, acetate, propionate and butyrate, are metabolized by the colonic epithelium (butyrate), liver (propionate) and muscle (acetate). Intestinal bacteria also have a role in the synthesis of vitamins B and K and the metabolism of bile acids, other sterols and xenobiotics. The colonic microflora are also responsive to diet. In the presence of fermentable carbohydrate substrates such as non-starch polysaccharides, resistant starch and oligosaccharides, bacteria grow and actively synthesize protein. The amount of protein synthesis and turnover within the large intestine is difficult to determine, but around 15 g biomass is excreted in faeces each day containing 1 g bacterial-N. Whether bacterially synthesized amino acids are ever absorbed from the colon remains unclear. Finally, individual colonic micro-organisms such as sulphate-reducing bacteria, bifidobacteria and clostridia, respond selectively to specific dietary components in a way that may be important to health.