AIMS/HYPOTHESIS: We tested the hypothesis that the reversibility of insulin resistance and diabetes observed after biliopancreatic diversion (BPD) is related to changes in circadian rhythms of gastrointestinal hormones. METHODS: Ten morbidly obese participants, five with normal glucose tolerance (NGT) and five with type 2 diabetes, were studied before and within 2 weeks after BPD. Within-day variations in glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP1) levels were assessed using a single cosinor model. Insulin sensitivity was assessed by euglycaemic-hyperinsulinaemic clamp. RESULTS: Basal GLP1 relative amplitude (amplitude/mesor x 100) was 25.82-4.06% in NGT; it increased to 41.38-4.32% after BPD but was unchanged in diabetic patients. GLP1 and GIP mesor were shifted in time after surgery in diabetic patients but not in NGT participants. After BPD, the GLP1 AUC significantly increased from 775 +/- 94 to 846 +/- 161 pmol l(-1) min in NGT, whereas GIP AUC decreased significantly from 1,373 +/- 565 to 513 +/- 186 pmol l(-1) min in diabetic patients. Two-way ANOVA showed a strong influence of BPD on both GIP (p = 0.010) and GLP1 AUCs (p = 0.033), which was potentiated by the presence of diabetes, particularly for GIP (BPD x diabetes, p = 0.003). Insulin sensitivity was markedly improved (p < 0.01) in NGT (from 9.14 +/- 3.63 to 36.04 +/- 8.55 micromol [kg fat-free mass](-1) min(-1)) and diabetic patients (from 9.49 +/- 3.56 to 38.57 +/- 4.62 micromol [kg fat-free mass](-1) min(-1)). CONCLUSIONS/INTERPRETATION: An incretin circadian rhythm was shown for the first time in morbid obesity. The effect of BPD on the 24 h pattern of incretin differed between NGT and diabetic patients. GLP1 secretion impairment was reversed in NGT and could not be overcome by surgery in diabetes. On the other hand, GIP secretion was blunted after the operation only in diabetic patients, suggesting a role in insulin resistance and diabetes.