Effects of Different Doses of Caffeine on Endurance Exercise Performance in the Heat

This study investigated the effects of different doses of caffeine (3 mg/kg BW and 6 mg/kg BW) on endurance exercise performance in the heat. Seventeen participants completed four randomized, double-blind trials: one in a normal environment (24.6 ± 1.2 °C) and three in a hot environment (33.2 ± 1.4 °C), with placebo, 3 mg/kg BW, and 6 mg/kg BW caffeine interventions. Endurance exercise time, cardiorespiratory function, and subjective fatigue perception were measured during incremental cycling tests. The results showed that high temperatures significantly reduced endurance exercise performance in the placebo (<i>p</i> < 0.001) and 3 mg/kg BW (<i>p</i> = 0.003) groups compared to the normal environment, but not in the 6 mg/kg BW group (<i>p</i> = 1.000). Both caffeine doses improved exercise time compared to placebo (3 mg/kg BW, <i>p</i> = 0.005; 6 mg/kg BW, <i>p</i> < 0.001). Caffeine ingestion enhanced pulmonary ventilation (VE), with significant increases in VE_peak (3 mg/kg BW, <i>p</i> = 0.032; 6 mg/kg BW, <i>p</i> = 0.006). Aerobic capacity improved, as evidenced by elevated VO₂peak (3 mg/kg BW, <i>p</i> = 0.010; 6 mg/kg BW, <i>p</i> = 0.001) and PetO₂ (3 mg/kg BW, <i>p</i> = 0.000; 6 mg/kg BW, <i>p</i> = 0.001). Subjective fatigue perception was significantly reduced only with 6 mg/kg BW caffeine (<i>p</i> = 0.020). In conclusion, caffeine ingestion at 3 mg/kg BW and 6 mg/kg BW effectively counteracts the negative effects of heat stress on endurance exercise performance by improving respiratory function, enhancing aerobic capacity, and reducing subjective fatigue. The 6 mg/kg BW dose demonstrated superior effects, making it a potential ergogenic aid for athletes training or competing in the heat.