Cold fluid ingestion does not enhance exogenous carbohydrate oxidation during prolonged endurance exercise in the heat

This study investigated the effects of ingesting cold vs warm carbohydrate beverages on exogenous carbohydrate oxidation and time-trial performance during prolonged cycling in the heat. Eleven trained, non-heat-acclimated cyclists (21 ± 3 yr; 69.3 ± 8.3 kg; V̇O _2peak: 64.7 ± 5.0 mL/kg/min) completed two trials comprising 100 min of submaximal cycling at 50% W _peak followed by a time trial, in a hot environment (36.2 ± 0.3°C; 55 ± 3% relative humidity). Participants ingested a 15% dextrose solution enriched with [U- ¹³ C] glucose (60 g/h; bolus every 20 min) provided either cold (COLD; 4.0 ± 1.6°C) or at room temperature (WARM; 35.8 ± 0.5°C). Heart rate and rectal temperature were lower in COLD than WARM during the 100 min exercise ( P ≤ 0.005), while skin temperature did not differ between conditions. Average (40-100 min) exogenous carbohydrate oxidation (COLD: 0.50 ± 0.10 vs. WARM: 0.51 ± 0.10 g/min; P = 0.80), peak exogenous carbohydrate oxidation, total carbohydrate and fat oxidation, and blood glucose, lactate, and insulin concentrations did not differ between trials (all P ≥ 0.28). Sweat losses and body mass changes were unaffected by condition ( P ≥ 0.42). Time-trial performance was not different between COLD and WARM ( P = 0.38). Despite attenuating cardiovascular and thermal strain, ingesting a cold carbohydrate beverage during exercise in the heat did not enhance exogenous carbohydrate oxidation, substrate utilisation, or performance compared with a warm beverage.