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

Abstract

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±8.5kg; VO_2peak: 55.9±5.8mLkg^-1min^-1; W_max: 318±37W) completed one VO_2peak test, one familiarisation trial and two experimental trials. After an overnight fast, participants ingested a placebo or a 6mgkg^-1 caffeine dose 60min before exercise. The exercise protocol consisted of 60min of cycle exercise at 55% W_max, followed by a 30min performance task (total kJ produced) in 30°C and 50% RH.

Results: Performance was enhanced (Cohen's d effect size=0.22) in the caffeine trial (363.8±47.6kJ) compared with placebo (353.0±49.0kJ; 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 60min of exercise (p=0.033), with no difference in thermal stress across trials (p=0.911).

Conclusions: Supplementation with 6mgkg^-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.

Keywords: Core temperature; Exercise; Fatigue; Stimulants; Substrate oxidation; Supplements.