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OBJECTIVE: To investigate the energy metabolism modi®cations induced by energy restriction and weight loss in massively obese adolescents. SUBJECTS: Ten massively obese girls (179 AE 31% of ideal body weight; age, 13.3 ± 16.4 y) after 2 ± 5 weeks on a lowenergy diet and 4.5 ± 11.5 months later, that is, after a substantial weight loss, and eight controls. MEASUREMENTS: Resting energy expenditure (REE) and carbohydrate-induced thermogenesis (CIT) after a sucrose load (by indirect calorimetry), plasma glucose and insulin before and after the sucrose load. RESULTS: After 2 ± 5 weeks on a low-energy diet, REE (7415 AE 904 kJad) was lower than the expected value calculated from the regression equation of REE on fat free mass in controls (P 0.005). After a 37 AE 17% reduction in excess weight, REE decreased (6405 AE 613 kJad) and remained lower than the expected value (P 0.005). At the early stages of weight loss, the area under the plasma glucose response curve was negatively correlated with CIT (r 7 0.80, P 0.01) and was higher in the six obese adolescents with low CIT than in the four with normal CIT (396 AE 52 vs 283 AE 26 mmol.l 71 .min 71 , P 0.01). After substantial weight loss, the area under the plasma insulin response curve decreased by 32% (P 0.02), and both CIT and the area under the plasma glucose response curve became similar in obese patients with low and normal CIT prior to weight loss. CONCLUSION: These results indicate that in massively obese adolescents, REE for fat-free mass is decreased at the very beginning of the process of losing weight and remains decreased as long as energy restriction and weight reduction carry on. They also indicate that the impaired CIT sometimes observed returns to normal after weight reduction suggesting that it is secondary to a decrease in glucose uptake induced by obesity-associated insulin resistance.

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