Impact of incretin on early-phase insulin secretion and glucose excursion

This study investigated the impact of incretin on early-phase insulin secretion and glucose excursion. The normal glucose tolerance (NGT), impaired glucose tolerance (IGT), and type 2 diabetes mellitus (T2DM) groups included 16, 8, and 19 subjects, respectively. Subjects underwent continuous glucose monitoring for 3 days, followed by an oral glucose tolerance test. Plasma glucose, insulin, glucagon, total glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-l (GLP-1) levels were measured at 30-min increments for 2 h after glucose intake. Differences with P < 0.05 were considered statistically significant. The area under the curve (AUC) of total GIP (120-min GIP-AUC) of the T2DM group was significantly lower than those of the NGT and IGT groups. The 120-min GLP-1-AUC of the NGT group was significantly larger than those of the T2DM and IGT groups. The early-phase insulin secretion index (ΔI30/ΔG30) of the T2DM group was significantly lower than those of the NGT and IGT groups. Mean amplitudes of glycemic excursions (MAGEs) went in the order of NGT < IGT < T2DM (P < 0.01, IGT vs. NGT; P < 0.001, T2DM vs. IGT). The 120-min GIP-AUC was negatively correlated with MAGE (r = -0.464), but uncorrelated with ΔI30/ΔG30. The 120-min GLP-1-AUC was positively correlated with ΔI30/ΔG30 (r = 0.580), but negatively correlated with MAGE (r = -0.606). Incretin may ameliorate glucose excursions, and GLP-1 may exert them by promoting early-phase insulin secretion. No correlation was observed between GIP secretion and early-phase insulin secretion.