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 hear stress it could prove to be detrimental. Thus, the purpose or 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% Vo(2) peak in a 40 degrees C and 30% RH environment until rectal temperature reached 39.3 degrees 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 Fam and Cent treatments were done first and sequentially while the placebo and C+E treatments were balanced and double-blind. Tolerance times, mean skin temperature (Tsk), rectal temperature (Tre), Vo(2),Vco(2),V-E 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 Tsk, initial Tre, Delta Tie, SR or the sensation of thermal comfort. Vo(2) and V-E 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.