Butyrate, a kind of short-chain fatty acids, can stimulate the colonic L cells to secrete a large amount of GLP-1 and a minor amount of PYY by activating the colonic free fatty acid receptors FFAR2 (GPR43) and FFAR3 (GPR41), and GLP-1 and PYY have the potential anti-diabetes and anti-obesity effects (77). Here, GCG is linked to diabetes mellitus.