GCG and type 2 diabetes mellitus: In T2DM, the incretin effect is markedly impaired as a result of the loss of insulinotropic effect of GIP, while GLP-1 still maintains its pleiotropic physiological functions including stimulating insulin secretion, improving hepatic insulin sensitivity and suppressing appetite (Wu et al., 2014, 2016), which might account for the partially restored incretin effect after bariatric surgery (Laferrere et al., 2007).