In-Exercise Caffeine Improves Exercise Performance in the Heat without Exacerbating Hyperventilation and Brain Hypoperfusion

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INTRODUCTION: Pre-exercise caffeine consumption can enhance endurance performance. However, this ergogenic effect may not be seen in hot conditions, possibly due to increased physiological strain associated with very high concentrations of caffeine in the blood, including hyperthermia, exacerbated hyperventilation, and cerebral hypoperfusion. In the present study, we hypothesized that in-exercise caffeine intake, which delays elevations in blood caffeine concentrations, improves performance during the late stages of prolonged exercise in the heat. In addition, we evaluated whether in-exercise caffeine intake affects respiratory and cerebral blood flow responses, as well as body temperatures. METHODS: Twelve healthy young adults (5 females) cycled for 30 min at 55% of peak oxygen uptake in the heat (35°C and 50% relative humidity), followed by high-intensity cycling at 90% of peak oxygen uptake until exhaustion. Using a randomized, double-blind, crossover design, they consumed a moderate dose of caffeine (5 mg·kg -1 body weight) or placebo ~5 min into the exercise. RESULTS: Serum caffeine concentrations gradually increased during exercise in the caffeine trial. High-intensity cycling duration was longer in the caffeine versus placebo trials (218 ± 75 vs 165 ± 82 s, P = 0.024). Rating of perceived exertion at the end of moderate-intensity exercise was lower in the caffeine than the placebo trial ( P < 0.001). At the matched time points between trials, minute ventilation, end-tidal CO 2 pressure, middle cerebral artery mean blood velocity (an index of cerebral blood flow), and esophageal temperature were similar between the two trials. CONCLUSIONS: Caffeine intake during exercise may be effective in improving late-stage performance during prolonged exercise in the heat, without exacerbating hyperventilation, cerebral hypoperfusion, and hyperthermia at corresponding time points observed under a no-caffeine condition.

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