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Energy substrate utilization during prolonged exercise with and without carbohydrate intake in preadolescent and adolescent girls

Brian W. Timmons,¹ Oded Bar-Or,^1, and Michael C. Riddell²

¹Children's Exercise and Nutrition Centre, McMaster University, Hamilton, Ontario; and ²School of Kinesiology and Health Science, Faculty of Science and Engineering, York University, Toronto, Ontario, Canada

Submitted 4 January 2007 ; accepted in final form 28 June 2007

Little information is available on energy metabolism during exercise in girls, particularly the contribution of exogenous carbohydrate (CHO_exo). The purpose of this study was to determine substrate utilization during exercise with and without CHO_exo intake in healthy girls. Twelve-yr-old preadolescent (YG; n = 12) and 14-yr-old adolescent (OG; n = 10) girls consumed flavored water (WT) or ¹³C-enriched 6% CHO (CT) while cycling for 60 min at 70% maximal aerobic power (O_2max). Substrate utilization was calculated for the final 15 min of exercise. CHO_exo decreased fat oxidation by 50% in YG but not in OG (P < 0.001) and decreased endogenous CHO oxidation by 15% in OG but not in YG (P = 0.006). Endogenous CHO oxidation was lower in YG than in OG regardless of trial (P 0.01), whereas fat oxidation was higher in YG only during WT (P < 0.001). CHO_exo oxidation rate was similar between YG and OG (7.1 ± 0.5 and 6.8 ± 0.4 mg·kg^–1·min^–1, respectively, P = 0.67), contributing 19% to total energy expenditure. Serum estradiol levels in all girls correlated with fat (r = –0.50 to –0.59, P = 0.03 to 0.005) and endogenous CHO oxidation (r = 0.50 to 0.63, P = 0.03 to 0.005) but not with CHO_exo oxidation (r = –0.09, P = 0.71). We conclude that CHO_exo influences endogenous substrate utilization in an age-dependent manner in healthy girls but that total CHO_exo oxidation during exercise is not different between YG and OG. Our results also point to potential sex-related differences in energy substrate utilization even during childhood.

children; cycling; energy metabolism; stable isotope tracers

This article has been cited by other articles:

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