Differential islet and incretin hormone responses in morning versus afternoon after standardized meal in healthy men.

Abstract

The insulin response to meal ingestion is more rapid in the morning than in the afternoon. Whether this is explained by a corresponding variation in the incretin hormones is not known. Our objective was to assess islet and incretin hormones after meal ingestion in the morning vs. afternoon. Ingestion at 0800 and 1700 h of a standardized meal (524 kcal) in healthy lean males (n = 12) at a University Clinical Research Unit. We assessed early (30-min) area under the curve (AUC30) of plasma levels of insulin and intact (i) and total (t) glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) after meal ingestion and made an estimation of beta-cell function by model analysis of glucose and C-peptide. Peak glucose was lower in the morning than in the afternoon (6.1 +/- 0.2 vs. 7.4 +/- 0.3 mmol/liter, P = 0.001). AUC30(insulin) (4.9 +/- 0.6 vs. 2.8 +/- 0.4 nmol/liter . 30 min; P = 0.012), AUC30(tGLP-1) (300 +/- 40 vs. 160 +/- 30 pmol/liter . 30 min, P = 0.002), AUC30(iGIP) (0.7 +/- 0.1 vs. 0.3 +/- 0.1 nmol/liter . 30 min, P = 0.002), and AUC30(tGIP) (1.1 +/- 0.1 vs. 0.6 +/- 0.1 nmol/liter . min, P = 0.007) were all higher in the morning. AUC30(iGLP-1) (r = 0.68; P = 0.021) and AUC30(iGIP) (r = 0.78; P = 0.001) both correlated to AUC30(insulin). Model analysis of beta-cell function showed a higher first-hour potentiation factor in the morning (P = 0.009). This correlated negatively with the 60-min glucose level (r = -0.63; P < 0.001). The early release of GLP-1 and GIP are more pronounced in the morning than in the afternoon. This may contribute to the more rapid early insulin response, more pronounced potentiation of beta-cell function, and lower glucose after the morning meal.