The causal links of these metabolic outcomes to GLP-1 and PYY signalling have been further validated in T2DM patients undergoing Roux-en-Y gastric bypass, in whom glucose tolerance is attenuated by the GLP-1 receptor antagonist exendin9-39) [213,214], while energy intake is increased with either GLP-1 receptor antagonism, or inhibition of PYY activation using a DPP-4 inhibitor [215]. This evidence concerns the gene GLP1R and type 2 diabetes mellitus.