Objective: To evaluate insulin action on substrate use and insulinemia in nondiabetic class III obese patients before and after weight loss induced by bariatric surgery.
Research methods and procedures: Thirteen obese patients (four men/nine women; BMI = 56.3 +/- 2.7 kg/m2) and 13 lean subjects (five men/eight women; BMI = 22.4 +/- 0.5 kg/m2) underwent euglycemic clamp, oral glucose tolerance test, and indirect calorimetry. The study was carried out before (Study I) and after ( approximately 40% relative to initial body weight; Study II) weight loss induced by Roux-en-Y Gastric bypass with silastic ring surgery.
Results: The obese patients were insulin resistant (whole-body glucose use = 19.7 +/- 1.5 vs. 51.5 +/- 2.4 micromol/min per kilogram fat-free mass, p < 0.0001) and hyperinsulinemic in the fasting state (332 +/- 86 vs. 85 +/- 5 pM, p < 0.0001) and during the oral glucose tolerance test compared with the lean subjects. Fasting plasma insulin normalized after weight loss, whereas whole-body glucose use increased (35.5 +/- 3.7 micromol/min per kilogram fat-free mass, p < 0.05 vs. Study I). The higher insulin clearance of obese did not change during the follow-up period. Insulin-induced glucose oxidation and nonoxidative glucose disposal were lower in the obese compared with the lean group (all p < 0.05). In Study II, the former increased slightly, whereas nonoxidative glucose disposal reached values similar to those of the control group. Fasting lipid oxidation was higher in the obese than in the control group and did not change significantly in Study II. The insulin effect on lipid oxidation was slightly improved (p = 0.01 vs. Study I).
Discussion: The rapid weight loss after surgery in obese class III patients normalized insulinemia and improved insulin sensitivity almost entirely due to glucose storage, whereas fasting lipid oxidation remained high.