Purpose: Although the effect of caffeine in thermoneutral or cool environmental conditions has generally shown performance benefits, its efficacy in hot, humid conditions is not as well known. The purpose of this study was to further examine the effect of caffeine ingestion on endurance running performance in the heat. Methods: Ten trained endurance runners (6 males; mean [SD] age = 26 [9] y, height = 176.7 [5.1] cm, and mass = 72.1 [8.7] kg) came to the lab for 4 visits. The first was a VO2max test to determine cardiorespiratory fitness; the final 3 visits were 10-km runs in an environmental chamber at 30.6°C and 50% relative humidity under different conditions: 3 mg/kg body mass (low caffeine dosage), 6 mg/kg (moderate caffeine dosage), and a placebo. Repeated-measures analyses of variance were used to determine the effect of condition on the 10-km time, heart rate, core temperature, rating of perceived exertion, and thermal sensation. Results: There was no difference in the 10-km time between the placebo (53.2 [8.0] min), 3 mg/kg (53.4 [8.4]), and 6 mg/kg (52.7 [8.2]) conditions (P = .575, 2 p =.060 p2=.060 ). There was not a main effect of average heart rate (P = .406, 2 p =.107 p2=.107 ), rating of perceived exertion (P = .151, 2 p =.189 p2=.189 ), or thermal sensation (P = .286, 2 p =.130 p2=.130 ). There was a significant interaction for core temperature (P = .025, 2 p =.170 p2=.170 ); the moderate-dosage caffeine condition showed a higher rate of rise in core temperature (0.26 [0.08] °C/km vs 0.20 [0.06] and 0.19 [0.10] °C(km in the low-caffeine and placebo conditions, respectively). Conclusion: The results support previous research showing a thermogenic effect of caffeine, as the moderate-dosage condition led to a greater rate of heat storage and no performance benefits.
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