Exogenous Carbohydrate Oxidation During Exercise In Boys

Abstract

Our previous research has shown that the relative contribution of exogenous carbohydrate (CHOexo) to exercise energy expenditure is higher in young boys compared with adult men. We hypothesized that this age difference in substrate utilization may be related to puberty. PURPOSE To determine the influence of pubertal status on CHOexo oxidation during high-intensity exercise in boys. METHODS Nineteen 12-yr-old boys (YOUNG) and thirteen 14-yr-old boys (OLD) were grouped according to pubertal stage by self-assessed Tanner (T) staging for pubic hair (stages 1–5). Subjects cycled for 60 min at 70 % VO2max while intermittently drinking a 13C-enriched 6 % CHOexo (4 % sucrose, 2 % glucose) solution (∼1.5g/kg). RER and 13C/12C ratio in expired CO2 were determined during the last 15 min of exercise. RESULTS Compared with OLD, YOUNG tended to have a lower RER with CHOexo ingestion (RER: 0.95 ± 0.05 vs. 0.98 ± 0.03; p=0.08) and the % contribution from CHOexo to total energy yield during exercise tended (p=0.08) to be higher in YOUNG (21.1 ± 4.5 %) than in OLD (18.6 ± 2.9 %). The oxidation ef.ciency of CHOexo (oxidation/intake) was identical between YOUNG and OLD (averaging ∼35%). When YOUNG were divided into T stages and compared with OLD, only T 1–2 boys had a lower (p=0.02) RER (0.95 ± 0.05 vs. 0.98 ± 0.03) and a higher (p=0.02) % contribution from CHOexo (22.6 ± 3.1 vs. 18.6 ± 2.9). T1-2 had a higher (p < 0.001) % contribution from CHOexo (22.6 ± 3.1 vs. 16.9 ± 3.4) than T5. T3-4 also tended (p=0.09) to have a higher % contribution from CHOexo (20.3 ± 3.4 vs. 16.9 ± 3.4) than T5. The oxidation ef.ciency of CHOexo did not differ statistically (p=0.18), but was highest in T1-2 (∼37%) and lowest in T5 (∼33%). Correlation analyses in OLD and YOUNG groups combined showed that CHOexo oxidation effciency was related to % body fat (r=−0.48, p=0.01) and VO2max expressed in mL·kg fat free mass-1·min-1 (r=0.35, p=0.06). CONCLUSIONS These results suggest that puberty is an important determinant in fuel selection during high-intensity exercise inasmuch as CHOexo appears to be a preferred fuel in pre and early pubertal boys. Thus, CHOexo intake may be particularly beneficial for performance of young boys by providing additional fuel. Supported, in part, by the Gatorade Sports Science Institute.