ENERGY CONTRIBUTIONS OF VOLATILE FATTY-ACIDS FROM THE GASTROINTESTINAL-TRACT IN VARIOUS SPECIES

The VFA, also known as short-chain fatty acids, are produced in the gastrointestinal tract by microbial fermentation of carbohydrates and endogenous substrates, such as mucus. This can be of great advantage to the animal, since no digestive enzymes exist for reaking down cellulose or other complex cerbohydrates. The VFA are produced in the largest amounts in herbivorous animal species and especially in the forestomach of ruminants. The VFA, however, also are produced in the lower digestive tract of humans and all animal species, and intestinal fermentation resembles that occuring in the rumen. The principal VFA in either the rumen or large intestine are acetate, propionate, and butyrate and are produced in a ratio varying from ~75:15:10 to 40:40:20. Absorption of VFA at their site of production os rapid, and large quantutues are metabolized by the ruminal or large intestinal epithelium before reaching the portal blood. Most of the butyrate is converted to ketone bodies or CO2 by the epithelial cells, and nearly all of the remainder is rmoved by the liver. Propionate is similarly removed by the liver but is largely converted to glucose. Although species differences excist, acetate is used principally by peripheral tissues, especially fat and muscle. Considerable energy is obtained from VFA inherbivorous species, and far more research has been conducted on ruminants than on other species. Significant VFA, however, are now known to be produced in omnivorous species, such as pigs and humans. Current estimates are that VFA contribute ~70% to the caloric requirements of ruminants, such as sheep and cattle, ~10% for humans, and ~20-30% for several other omnivorous or herbivorous animals. The amount of fiber in the diet undoubtedly affects the amount of VFA produced, and thus the contribution of VFA to theenergy needs of the body could become considerably greater as the dietary fiber increases. Pigs and some species of monkey most closely resemble humans, anmd current research should be directed toward examining the fermentation processes and VFA metabolism in those species. In addition to the energetic or nutritional contributions of VFA to the body, the VFA may indirectlu influence cholesterol synthesis and even help regulate insulin or glucagon secretion. In addition, VFA production and absorption have a very significant effect on epithelial cell growth, blood flow, and the normal secretory and absorptive functions of the large intestine, cecum, and rumen. The absorption of VFA and sodium for example, seem to be interdependent, and release of bicarbonate usually occurs during VFA absorption. More knowledge is yet to be gaines on the large intestine of humans and animals and much can be developed by applying information and procedures developed from studies on the ruminant forestomach.