Context: Insulin resistance ameliorates after bariatric surgery, yet there is still a need for data on the acute effect of Roux-en-Y gastric bypass (RYGBP) on insulin sensitivity.

Objective: The objective of the study was to describe the acute effect of RYGBP on insulin sensitivity, measured by both the euglycemic-hyperinsulinemic clamp and homeostasis model assessment insulin resistance index (HOMA-IR).

Design and Setting: Evaluations were conducted before and 1 month after RYGBP at State University of Campinas (São Paulo, Brazil).

Patients: Patients included 19 premenopausal women with metabolic syndrome aged 35.3 (6.7) yr, body mass index 45.50 (3.74) kg/m² [mean (sd)]. Six had mild type 2 diabetes, seven impaired glucose tolerance, and six normal glucose tolerance.

Interventions and Main Outcome Measures: The volunteers underwent RYGBP either alone or combined with omentectomy. Euglycemic-hyperinsulinemic clamp, HOMA-IR, nonesterified fatty acids, leptin, ultrasensitive C-reactive protein, adiponectin, and IL-6 were assessed at baseline and 4.5 (0.9) wk postoperatively.

Results: Fasting glucose decreased [99.2 (13.1) to 83.6 (8.1) mg/dl, P < 0.01] along with a reduction in fasting insulin [30.4 (17.0) to 11.4 (6.3) mU/liter, P < 0.01]. M value did not improve postoperatively [25.82 (6.30) to 22.02 (6.05) μmol/kg_FFM · min] despite of a decrease in body weight [114.8 (14.5) to 102.3 (14.5) kg, P < 0.001]. This finding was discordant to the observation of an improvement in HOMA-IR [3.85 (2.10) to 1.42 (0.76), P < 0.01]. Nonesterified fatty acids increased. Leptin and C-reactive protein decreased. IL-6 and adiponectin remained unchanged.

Conclusions: A month after RYGBP, fasting glucose metabolism improves independent of a change in peripheral insulin sensitivity.