Effect of a moderate caffeine dose on endurance cycle performance and thermoregulation during prolonged exercise in the heat

Objectives: This study investigated the influence of a moderate caffeine dose on endurance cycle performance and thermoregulation during prolonged exercise in high ambient temperature. Design: Double-blind cross-over study. Methods: Eight healthy, recreationally active males (mean +/- SD; age: 22 +/- 1 years; body mass: 71.1 +/- 8.5 kg: VO2peak: 55.9 +/- 5.8 mL kg(-1) min(-1) ; W-max : 318 +/- 37 W) completed one VO2peak test, one familiarisation trial and two experimental trials. After an overnight fast, participants ingested a placebo or a 6 mg kg(-1) caffeine dose 60 min before exercise. The exercise protocol consisted of 60 min of cycle exercise at 55% W-max, followed by a 30 min performance task (total kJ produced) in 30 degrees C and 50% RH. Results: Performance was enhanced (Cohen's d effect size = 0.22) in the caffeine trial (363.8 +/- 47.6 kJ) compared with placebo (353.0 +/- 49.0 kJ; p = 0.004). Caffeine did not influence core (p = 0.188) or skin temperature (p = 0.577) during exercise. Circulating prolactin (p = 0.572), cortisol (p = 0.842) and the estimated rates of fat (p = 0.722) and carbohydrate oxidation (p = 0.454) were also similar between trial conditions. Caffeine attenuated perceived exertion during the initial 60 min of exercise (p = 0.033), with no difference in thermal stress across trials (p = 0.911). Conclusions: Supplementation with 6 mg kg(-1) caffeine improved endurance cycle performance in a warm environment, without differentially influencing thermoregulation during prolonged exercise at a fixed work-rate versus placebo. Therefore, moderate caffeine doses which typically enhance performance in temperate environmental conditions also appear to benefit endurance performance in the heat. (C) 2017 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.