Effect of Pre-Exercise Caffeine Intake on Endurance Performance and Core Temperature Regulation During Exercise in the Heat: A Systematic Review with Meta-Analysis

Abstract

Background: Heat is associated with physiological strain and endurance performance (EP) impairments. Studies have investigated the impact of caffeine intake upon EP and core temperature (CT) in the heat, but results are conflicting. There is a need to systematically determine the impact of pre-exercise caffeine intake in the heat. Objective: Use a meta-analytical approach to determine the effect of pre-exercise caffeine intake on EP and CT in the heat. Design: Systematic review with meta-analysis. Data sources: Four databases and cross-referencing. Data analysis: Weighted mean effect summaries using random-effects models for EP and CT, as well as meta regressions with robust standard errors to explore confounders. Study selection: Placebo-controlled, randomized studies in adults (≥ 18 yrs old) with caffeine intake at least 30 min before endurance exercise ≥ 30 min, performed in ambient conditions ≥ 27°C. Results: Respectively 6 and 12 studies examined caffeine's impact on EP and CT, representing 52 and 205 endurance-trained individuals. On average, 6 mg/kg body mass of caffeine were taken 1 h before exercises of 70 min conducted at 34°C and 47% relative humidity. Caffeine supplementation improved EP by 2.0 +/- 0.7% (95% CI: 0.6 to 3.5%) and increased the rate of change in CT by 0.10 +/- 0.04°C/h (95% CI: 0.03 to 0.16°C/h), compared with the ingestion of a placebo. Conclusion: Caffeine ingestion of 6 mg/kg body mass 1 h before an exercise in the heat provides a worthwhile improvement in EP of 2%, while trivially increasing the rate of change in CT by 0.10°C/h.