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

Aviat Space Environ Med. 1999 Jun;70(6):583-8.


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% 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.

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 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.

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.

Publication types

  • Clinical Trial
  • Controlled Clinical Trial

MeSH terms

  • Adult
  • Body Temperature / drug effects
  • Body Temperature Regulation / drug effects*
  • Caffeine / adverse effects*
  • Canada
  • Central Nervous System Stimulants / adverse effects*
  • Double-Blind Method
  • Drug Therapy, Combination
  • Ephedrine / adverse effects*
  • Exercise / physiology*
  • Exercise Test
  • Heart Rate / drug effects
  • Heat Stress Disorders / metabolism*
  • Heat Stress Disorders / physiopathology*
  • Humans
  • Male
  • Military Personnel
  • Oxygen Consumption / drug effects
  • Physical Endurance / drug effects*
  • Physical Endurance / physiology
  • Time Factors


  • Central Nervous System Stimulants
  • Caffeine
  • Ephedrine