ABSTRACT
In humans, the colon receives dietary material, which has already been digested in the upper gut, from the ileum, and the contents are then mixed and retained for 6-12 hours in the cecum and right colon. Thereafter, digesta are ejected and pass through the transverse to the left colon for storage, absorption of water, and eventual excretion (1). The typical retention time for the colon is about 60 h in U.K. subjects (2). Thus, the large gut is a tubular system, with nutrients flowing into the cecum, and bacteria, their metabolic products, and undigested food being excreted in feces. The major end products of fermentation in humans are the SCFA (short chain fatty acids), acetate, propionate, and butyrate (3); gases H2 and CO2 (4,5); ammonia (6); amines (7); phenols (8,9); and energy, which bacteria use for growth and maintenance of cellular function. Only about 5% of total SFCA are excreted in feces. In vivo, SCFA contribute about 10% of the daily energy requirements of the host. Most SCFA produced by gut bacteria are absorbed and metabolized in the body (10).
