Thermal regulation in the heat during exercise after caffeine and ephedrine ingestion

Background: Ingesting a combination of caffeine and ephedrine (C+E) has been shown to raise metabolic heat production and body temperature. This side effect of C+E ingestion may be positive during a cold stress scenario, however, during heat stress it could prove to be detrimental. Thus, the purpose of this study was to clarify the effect of C+E ingestion on body temperature regulation during moderate exercise in a hot dry environment. Methods: Ten, healthy, non heat acclimated, males exercised at 50% V̇O₂peak in a 40°C and 30% RH environment until rectal temperature reached 39.3°C; heart rate (HR) remained at 95% of peak value or greater for 3 min, dizziness or nausea precluded further exercise, or 3 h had elapsed. They did this four times at weekly intervals: familiarization (Fam), control (Cont), placebo, and C+E (5 mg · kg^-1 caffeine + 1 mg · kg^-1 ephedrine) trials. The Faro and Cont treatments were done first and sequentially while the placebo and C+E treatments were balanced and double-blind. Tolerance times, mean skin temperature (T̄sk), rectal temperature (Tre), V̇O₂, V̇CO₂, V̇E, sweat rate (SR), HR, and sensation of thermal comfort were measured. Results: Tolerance times (mean ± SD in minutes) were similar for the placebo (120.0 ± 28.4) and C+E (121.3 ± 33.9) trials and both times were significantly longer than Cont (106.6 ± 24.0) trial. C+E did not affect T̄sk, initial Tre, Δ Tre, SR or the sensation of thermal comfort. V̇O₂ and V̇E, were significantly increased by C+E. HR was elevated by C+E compared with the other trials, but only during the initial 20 min of exercise. Conclusion: Although the metabolic rate was slightly increased with C+E treatment, it was sufficiently offset by increased heat loss mechanisms so that internal body temperature was not increased during moderate exercise in a hot, dry environment.