Synthetic GLP-1 lowers postprandial (pp) glycemia by stimulating insulin, inhibiting glucagon, and delaying gastric emptying. However, the effects of the endogenous peptide are largely unknown. Using the specific GLP-1 receptor antagonist exendin(9-39)amide (Ex(9-39)) the investigators recently showed that GLP-1 released during intestinal meal perfusion acts as an incretin hormone and as an enterogastrone. As the relative contributions of these effects to controll postprandial glycemia are unclear, the investigators used Ex(9-39) to investigate the mechanisms of action of GLP-1 after an oral meal in humans.