Following intraduodenal glucose infusion, early glucagon and GIP increments (0-15 min) were closely related in T2D, but not in healthy individuals.<h4>Conclusions</h4>These observations indicate a glycaemia-dependent shift in endogenous GIP action in T2D, characterised by attenuation of insulinotropic effects and relative amplification of glucagonotropic effects with deteriorating glycaemic control.<h4>Trial registration</h4>ACTRN12614001131640, ACTRN12614001117606 and ACTRN12615001240538. This evidence concerns the gene GCG and type 2 diabetes mellitus.