Absorption of short-chain fatty acids and their role in the hindgut of monogastric animals

Abstract

The short-chain fatty acids (SCFA), such as acetate, propionate and butyrate, are produced in the hindgut of mammals by microbial fermentation. SCFA constitute the major anions in the hindgut contents and are effectively absorbed in the large intestine. Total SCFA concentrations in the hindgut contents are generally around 100 mM among mammals of various food habits having wide variations in morphology and function of the hindgut. The absorption of SCFA, unexpectedly, is nearly independent from luminal pH. This is attributed to the presence of a constant-pH microclimate at the epithelial surface. In vivo experiments indicated some linkage between SCFA and Na + absorption. However, in vitro studies showed inhibition of Na + transport by SCFA. Thus, it is still an open question as to whether SCFA directly affect Na + transport mechanisms, either by providing metabolic energy or through coupling to intracellular pH regulation. The in vivo effect could also be due to bulk flow effects. The permeability of the proximal compared with the distal colon of guinea pig is higher for acetate, equal for propionate, and lower for butyrate. This difference is due to partial absorption of SCFA in the dissociated form in the proximal segments. SCFA provide an important energy source for the animal. Intestinal cells can use SCFA, particularly butyrate, as a substrate for metabolism. Besides their contribution to energy metabolism, SCFA have various physiological functions. SCFA stimulate cell proliferation in the intestinal epithelium in vivo. In vitro SCFA inhibit cell proliferation, but they are potent enhancers of gene expression in cultured cells. SCFA evoke changes in motility. Whether SCFA stimulate mucus release is obscure. Colonic blood flow is increased by luminal SCFA, however, the mechanism is not yet understood.