Factors affecting fermentation reactions in the large bowel.

Quantitatively, the main sources of C and energy for bacteria growing in the human large intestine are starches and plant cell wall polysaccharides that escape digestion in the small bowel (Macfarlane & Cummings, 1991). These substances are depolymerized by hydrolytic enzymes produced by colonic bacteria, and their component sugars are fermented to a range of products, including organic acids, alcohols and gases (Cummings et al. 1989; Macfarlane et al. 1992). The principal products are the short-chain fatty acids (SCFA) acetate, propionate and butyrate (Cummings, 1981). The physiological significance to the host of SCFA production in the colon is that the vast majority (about 90%) are absorbed from the gut and subsequently undergo a variety of metabolic fates within the body (for review, see Cummings & Macfarlane, 1991). Because of the interactions between bacterial fermentation products and host metabolism, it is important to understand the physical, chemical and biological factors that govern the degradation of carbohydrates by intestinal micro-organisms. The present paper describes in vivo and in vitro studies on carbohydrate fermentation using pure and mixed cultures of colonic bacteria to determine the role of substrate availability on endproduct formation.