The Acute Effects of Caffeine and Normobaric Hypoxia on Cardiorespiratory Exercise Responses in Healthy Adults: A Randomized Controlled Trial.

The ergogenic effects of caffeine for endurance performance at sea level are well established. However, whether caffeine enhances exercise capacity or modulates cardiorespiratory responses in hypoxia remains unclear. Twenty-nine healthy, active volunteers (14 women) completed four tests in a pseudorandomized order, comprising 10 min of steady-state cycling (40% normoxic maximal aerobic power) followed by incremental exercise tests to exhaustion. Participants consumed 6 mg/kg body mass anhydrous caffeine, or placebo, 45 min before exercise. Tests were performed while breathing room air or a hypoxic gas mixture (simulating ∼3,500 m). At peak exercise intensity, oxygen uptake was affected by condition (hypoxia vs. normoxia; 44.8 ± 7.3 vs. 51.4 ± 7.9 ml·kg-1·min-1; p < .001) but not supplement (caffeine vs. placebo; 48.2 ± 8.2 vs. 48.0 ± 8.4 ml·kg-1·min-1; p = .501). Maximal aerobic power and pulse oxygen saturation were lower in hypoxia (both p < .001), but unaffected by caffeine (p = .061 and .898, respectively). Pulmonary ventilation was unaffected by hypoxia (p = .154) or caffeine (p = .117). Heart rate was higher in normoxia and with caffeine (both p < .001). During steady-state cycling, hypoxia (p < .001) and caffeine (p = .026) increased pulmonary ventilation. However, the hypoxia-induced pulse oxygen saturation decrease (p < .001) was not attenuated with caffeine (interaction p = .283). Perceived exertion was higher in hypoxia (p < .001) and lower with caffeine (p = .026). Caffeine increased vastus lateralis oxygenation in hypoxia (caffeine vs. placebo; 52 ± 8 vs. 50 ± 10%; p = .020) but not in normoxia (caffeine vs. placebo; 59 ± 7 vs. 59 ± 8%; p > .999). Overall, caffeine did not enhance exercise capacity in either environmental condition. However, during steady-state cycling, caffeine enhanced pulmonary ventilation, reduced perceived exertion, and mitigated hypoxia-induced muscle oxygenation reductions.