Figure 3.

The fermentation of complex carbohydrates mainly in the colon by lactic acid bacteria produces short chain fatty acids (SCFAs), with about 95 percent of the species produced being acetate, propionate and butyrate. Propionate and butyrate mainly influence their effects on physiology in the colon and liver, with smaller amounts entering general circulation. In the colon, SCFA bind to the G-couples receptors GPR41 and GPR43, eventuating in and increase in the production of glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) which control glucose metabolism in tissues, including intestinal tissue. High levels of acetate entering general circulation triggers reactions in adipose tissue, liver and muscle, brain, and influences on immunity. The molecular mechanisms of how SCFAs induce their responses is currently the subject of intense research focus. Investigations are being undertaken to deduce the extent to which SCFAs act directly on tissue remote from the intestine, or whether they act indirectly by inducing the expression of signaling factors produced in gut tissue which then circulate systemically.