Weight loss and changes in energy metabolism in massively obese adolescents.

OBJECTIVE: To investigate the energy metabolism modifications induced by energy restriction and weight loss in massively obese adolescents.
SUBJECTS: Ten massively obese girls (179 +/- 31% of ideal body weight; age, 13.3-16.4 y) after 2-5 weeks on a low-energy 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 +/- 904 kJ/d) 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 +/- 17% reduction in excess weight, REE decreased (6405 +/- 613 kJ/d) 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 = -0.80, P = 0.01) and was higher in the six obese adolescents with low CIT than in the four with normal CIT (396 +/- 52 vs 283 +/- 26 mmol.l-1.min-1, 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.