Substrate utilization in boys during exercise with [C-13]-glucose ingestion

The influence of glucose ingestion on substrate utilization during prolonge d exercise in children and adolescents is currently unknown. In the present study we determined the effect of intermittent exogenous glucose (GLU(exo) ) ingestion on substrate utilization during prolonged exercise, in adolesce nt boys ages 13-17 years. Healthy untrained volunteers performed four 30-mi n exercise bouts on a cycle ergometer, separated by 5-min rest periods (con gruent to 60% maximum O-2 consumption), on two occasions spaced 1-4 weeks a part. Two trials were performed, a control trial (CT), in which subjects in gested water intermittently during the exercise, and a glucose trial (GT), in which subjects ingested a C-13-enriched GLU(exo) drink (congruent to3 g glucose . kg body mass(-1)), also intermittently during the exercise. Total mass free fatty acids (FAT(total)), glucose (GLU(total)) and carbohydrate (CHOtotal) oxidation was determined from indirect calorimetry, while GLU(ex o) oxidation was calculated from the C-13/C-12 ratio in expired air after 5 -10 min and 25-30 min of exercise in each bout. Heart rate and rating of pe rceived exertion (RPE) were determined at the same time intervals. The oxid ation of CHOtotal was 169.1 (12.9) g . 120 min(-1) and 203.1 (15.9) g . 120 min(-1) (P < 0.01) and that of FAT(total) was 31.0 (4.2) g 120 min(-1) and 17.1 (2.5) g . 120 min(-1) (P < 0.01) in CT and GT, respectively. GLU(exo) oxidation in GT was 57.8 (4.3)g . 120 min(-1), or 34.2 (2.2)% of that inge sted. Endogenous glucose oxidation was 169.1 (12.9) g . 120 min(-1) and 145 .3 (11.9) g . 120 min(-1) (P < 0.01) in CT and GT, respectively. Insulin an d glucose concentrations were higher in GT than in CT by 226% and 37%, resp ectively (both P < 0.05). Free fatty acids and glycerol concentrations were lower in GT than in CT, by 27% and 79%, respectively (both P < 0.05). Hear t rate was similar between trials, but RPE was lower in GT vs CT at both 11 5 and 135 min. Thus, under these experimental conditions, GLU(exo) intake s pares endogenous carbohydrate and fat by 16% and 45%, respectively, contrib utes to approximately 25% of the total energy demand of exercise, and lower s the RPE.