Dipeptidyl peptidase-4 (DPP-4) inhibitors prevent degradation of incretin hormones (glucagon-like peptide 1 [GLP-1] and glucose-dependent insulinotropic polypeptide [GIP]), whereas metformin may increase GLP-1 levels. We examined, in a four-period crossover trial, the influence of metformin (2,000 mg/day), sitagliptin (100 mg/day), or their combination, on GLP-1 responses and on the incretin effect in 20 patients with type 2 diabetes, comparing an oral glucose challenge (75 g, day 5) and an "isoglycemic" intravenous glucose infusion (day 6). Fasting total GLP-1 was significantly increased by metformin and not changed by sitagliptin.
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