The large bowel—a supplementary rumen?

Abstract

The large intestine has attracted additional interest with recent evidence that diseases such as colonic cancer, ulcerative colitis and colonic diverticulosis have a lower incidence in human populations that consume a relatively-high percentage of plant fibre. Studies conducted on a variety of species have shown that the mammalian large intestine and the ruminant forestomach show some striking similarities with respect to microbial digestion, secretion and absorption (Stevens et al. 1980). The following discussion will demonstrate some major analogies and attempt to explain their functional significance. The ruminant forestomach Results from extensive studies of the ruminant forestomach are contained in a number of reviews (Sellers & Stevens, 1966; Hungate, 1968; Stevens, 1973; Bryant, 1977; Phillipson, 1977). The forestomach consists of the reticulo-rumen (a large, blind, multicompartmental sac) and the omasum. The reticulo-rumen provides a fermentation chamber which, in adult cattle, can constitute approximately 25% of the animal's bodyweight. Its construction and motor activity delay transit of digesta, especially particulate matter, as well as the indigenous microbes. The omasum serves as a pump for the transfer of digesta to the remainder of the gastrointestinal tract. Rumen microbes ferment a wide range of carbohydrates, including sugars, starches, cellulose, hemicelluloses and pectins. Under normal conditions the principle end-products are three volatile fatty acids (VFA), acetate, propionate and butyrate, plus carbon dioxide and methane. Although the pK, of these organic acids is approximately 4.8, a pH of 5.5-7.0 is required for maintenance of microbes responsible for their production. This is accomplished by both the rapid absorption of VFA and the addition of large volumes of heavily-buffered saliva. Parotid secretions, which provide the major source, have a sodium:potassium value similar to that of plasma, high concentrations of bicarbonate and phosphate and a pH of 8.2. Approximately 60% of the Na and chloride entering the reticulo-rumen via the diet and saliva are absorbed directly from this chamber (Dobson, 1961). Na and C l are absorbed by active transport. C1 transport appears to be partially accomplished by exchange for HCO,, which provides an additional mechanism for the buffering of VFA. Ingestion of a diet abnormally high in starch results in a fulminating production of VFA within the rumen. The marked lowering of digesta pH results in more *For reprints.