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

Defense and Civil Institute of Environmental Medicine, Toronto, Ontario, Canada.
Aviation Space and Environmental Medicine (Impact Factor: 0.88). 07/1999; 70(6):583-8.
Source: PubMed


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.
Ten, healthy, non heat acclimated, males exercised at 50% VO2peak 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 Cont 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), Vo2, Vco2, VE, sweat rate (SR), HR, and sensation of thermal comfort were measured.
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 TrC, delta Tre, SR or the sensation of thermal comfort. VO2 and VF, 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.
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.

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    • "They were 21 males (age: 19.7 ± 0.4 00 yrs, mean ± SD; height: 165 ± 1.5 cm; body mass: 55.1 ± 1.1 kg) and 19 females (age: 19.7 ± 0.2 yrs; height: 155.6 ± 1.7 cm; body mass: 45.6 ± 1.3 kg). All of them were free from taking drugs or medications, and the females were in the same phase of their menstrual cycles (follicular phase) to avoid any effects of reproductive hormones on body temperature (Bell et al., 1999). All experimental procedures were fully explained to them before the experiments and their informed consent was given. "
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    Toxicological Sciences 05/2000; 55(1):133-42. · 3.85 Impact Factor
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