GIP and type 2 diabetes mellitus: A blunted incretin response was observed in people with T2D.23 Subsequently, GLP-1 was recognized as a major player in the incretin system, raising the possibility of manipulating this system to treat people with T2D.24 Enteroendocrine L-cells of the small intestine secrete incretin hormones, including GLP-1 and glucose-dependent insulinotropic peptide (GIP), in response to oral glucose or fat.