Carbohydrate intake reduces fat oxidation during exercise in obese boys

The recent surge in childhood obesity has renewed interest in studying exercise as a therapeutic means of metabolizing fat. However, carbohydrate (CHO) intake attenuates whole body fat oxidation during exercise in healthy children and may suppress fat metabolism in obese youth. To determine the impact of CHO intake on substrate utilization during submaximal exercise in obese boys, seven obese boys (mean age: 11.4 ± 1.0 year; % body fat: 35.8 ± 3.9%) performed 60 min of exercise at an intensity that approximated maximal fat oxidation. A CHO drink (CARB) or a placebo drink (CONT) was consumed in a double-blinded, counterbalanced manner. Rates of total fat, total CHO, and exogenous CHO (CHOexo) oxidation were calculated for the last 20 min of exercise. During CONT, fat oxidation rate was 3.9 ± 2.4 mg × kg fat-free mass (FFM)−1 × min−1, representing 43.1 ± 22.9% of total energy expenditure (EE). During CARB, fat oxidation was lowered (p = 0.02) to 1.7 ± 0.6 mg × kg FFM−1 × min−1, contributing to 19.8 ± 4.9% EE. Total CHO oxidation rate was 17.2 ± 3.1 mg × kg FFM−1 × min−1 and 13.2 ± 6.1 mg × kg FFM−1 × min−1 during CARB and CONT, respectively (p = 0.06). In CARB, CHOexo oxidation contributed to 23.3 ± 4.2% of total EE. CHO intake markedly suppresses fat oxidation during exercise in obese boys.