Substrate Utilization during Sub-Maximal Exercise in Lean and Overweight Children

Abstract

PURPOSE: To examine the rates of substrate utilization in lean and overweight children during sub-maximal exercise. METHODS: Seventeen (10 boys/7 girls) overweight [BMI >85th percentile] and twenty (10 boys/10 girls) lean children aged 7–10 years participated. Aerobic fitness was assessed by maximal oxygen consumption (VO2max) test using the Bruce protocol. The exercise session consisted of subjects performing 3 separate 8-minute bouts of treadmill walking: 1.7mph, 0%grade; 2.5mph, 0%grade; and 3.4mph, 0%grade. Each 8-minute, exercise workload was followed by a 5-minute rest period. Carbohydrate (CHOox) and fat (FATox) oxidization rates were calculated from oxygen uptake and carbon dioxide output for each minute during the steady-state sub-maximal exercise bouts using the stochiometric equations of Frayn (1983). RESULTS: Overweight children had significant lower VO2max values (37.8±3.8 vs. 45.9±5.1 mL. kg-1.min-1; P<0.0001) and exercise time (464±57 vs. 539±65 s; P<0.001) when compared to lean children. No differences were observed for FATox rate between groups at any sub-maximal exercise workload. Overweight children had a significantly greater CHOox rate during each exercise bout when compared to lean children (1.7mph = 0.42±0.13 vs. 0.31±0.09 g.min-1, P<0.01; 2.5mph = 0.50±0.12 vs. 0.39±0.11 g.min-1, P<0.005; 3.4mph = 0.81±0.19 vs. 0.57±0.14 g.min-1, P<0.0005). Finally, energy expenditure was greater for the overweight children at each exercise workload when compared to lean children (1.7mph = 2.47±0.50 vs. 1.92±0.22 kcal. min-1, P<0.0001; 2.5mph = 2.99±0.59 vs. 2.30±0.29 kcal.min-1, P<0.0001; 3.4mph = 4.17±0.78 vs. 3.17±0.43 kcal.min-1, P<0.0001). CONCLUSIONS: The differences for substrate utilization between lean and overweight children may be attributed to higher relative exercise intensities in the overweight children during the exercise trial. Future investigations should examine substrate utilization rate in lean and overweight children at the same percentage of aerobic fitness.