BACKGROUND: Obesity is associated with poor fitness and adverse metabolic consequences in children. OBJECTIVE: To investigate how exercise and lifestyle modification may improve fitness and insulin sensitivity in this population. DESIGN AND SUBJECTS: Randomized controlled trial, 21 obese (body mass index > 95% percentile) subjects, ages 10 to 17 years. METHODS: Subjects were given standardized healthful lifestyle advice for 8 weeks. In addition, they were randomized to an in-home supervised exercise intervention (n = 10) or control group (n = 11). MEASUREMENTS: Fasting laboratory studies (insulin, glucose, lipid profile) and assessments of fitness, body composition, skeletal muscle oxidative phosphorylation and intramyocellular lipid content (IMCL), were performed at baseline and study completion. RESULTS: Subjects were 13.0 +/- 1.9 (standard deviation) years old, 72% female and 44% non-white. Exercise improved fitness (p = 0.03) and power (p = 0.01), and increased IMCL (p = 0.02). HOMA-IR decreased among all subjects in response to lifestyle modification advice (p = 0.01), regardless of exercise training assignment. In univariate analysis in all subjects, change in cardiovascular fitness was associated with change in HOMA-IR. In exploratory analyses, increased IMCL was associated with greater resting energy expenditure (r = 0.78, p = 0.005) and a decrease in fasting respiratory quotient (r = -0.70, p = 0.02) (n = 11). CONCLUSIONS: Change in fitness was found to be related to change in insulin resistance in response to lifestyle modification and exercise in obese children. IMCL increased with exercise in these obese children, which may reflect greater muscle lipid oxidative capacity.

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