Resting metabolic rate and postabsorptive substrate oxidation in morbidly obese subjects before and after massive weight loss

OBJECTIVE: To investigate the changes of resting metabolic rate (RMR) and postabsorptive oxidation of carbohydrates (CHO), lipids (LIP) and proteins (PT) in morbidly obese subjects after long-term stable massive weight reduction. DESIGN: Longitudinal, intervention study of a bilio-pancreatic by-pass. SUBJECTS: Ten (Four men, six women) obese subjects (BMI: 53.7 ± 2.1 kg/m2, men, nine women) control subjects (BMI: 21.4 ± 1.0 kg/m2). MEASUREMENTS: RMR and substrates oxidation (indirect calorimetry), body composition (bioelectrical impedance), plasma concentrations of glucose, FFA, insulin and thyroid hormones before (T0, prior to surgery), during (T1: 1-3 months, and T2: 9-16 months following surgery) and after (T3: 36-42 months following surgery) massive weight loss. RESULTS: At T3 BMI proved to be stably reduced to 32.9 ± 1.3 (P < 0.0001). The RMR progressively reduced both in absolute value (T3: -2566 kJ/24 h; P < 0.0001 vs T0) and normalized for fat-free mass (FFM; T0: 123 ± 3.8, T1: 109 ± 3.3, T2: 112 ± 7.1, T3: 105 ± 4.6 kJ/kg-FFM·24 h; P < 0.05). At T3 the normalized RMR became significantly lower than the corresponding value of a control group (118 ± 2.5 kJ/kg-FFM.24 h; P < 0.01). The long term trend in substrates oxidation showed that CHO oxidation increased while LIP oxidation decreased; PT oxidation did not significantly change for the whole of the study. CONCLUSION: Previously morbid obese subjects have an energy sparing metabolism and a normal postabsorptive oxidation of LIP and CHO.