Energy substrate utilization with and without exogenous carbohydrate intake in boys and men exercising in the heat

Little is known about energy yield during exercise in the heat in boys compared with men. To investigate substrate utilization with and without exogenous carbohydrate (CHO_exo) intake, seven boys [11.2 ± 0.2 (SE) yr] and nine men (24.0 ± 1.1 yr) cycled (4 × 20-min bouts) at a fixed metabolic heat production (Ḣ_p) per unit body mass (6 W/kg) in a climate chamber (38°C and 50% relative humidity), on two occasions. Participants consumed a ¹³C-enriched 8% CHO beverage (CARB) or placebo beverage (CONT) in a double-blinded, counterbalanced manner. Substrate utilization was calculated for the last 60 min of exercise. CHO_exo oxidation rate (2.0 ± 0.3 vs. 2.5 ± 0.2 mg·kg fat-free mass^-1·min^-1, P = 0.02) and CHO_exo oxidation efficiency (12.8 ± 0.6 vs. 16.0 ± 0.9%, P = 0.01) were lower in boys compared with men exercising in the heat. Total carbohydrate (CHO_total), endogenous CHO (CHO_endo), and total fat (Fat_total) remained stable in boys and men (P > 0.05) during CARB, whereas CHO_total oxidation rate decreased (P < 0.001) and Fat_total oxidation rate increased over time similarly in boys and men during CONT (P < 0.001). The relative contribution of CHO_exo to total energy yield increased over time in both groups (P < 0.001). In conclusion, endogenous substrate metabolism and the relative contribution of fuels to total energy yield were not different between groups. The ingestion of a CHO beverage during exercise in the heat may be as beneficial for boys as men to spare endogenous substrate.