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% 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.


CONCLUSIONS
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.
Bell
D G
DG
Defense and Civil Institute of Environmental Medicine, Toronto, Ontario, Canada.
Jacobs
I
I
McLellan
T M
TM
Miyazaki
M
M
Sabiston
C M
CM
D000328:Adult; D001831:Body Temperature / Q000187:drug effects; D001833:Body Temperature Regulation / Q000187:drug effects*; D002110:Caffeine / Q000009:adverse effects*; D002170:Canada; D000697:Central Nervous System Stimulants / Q000009:adverse effects*; D004311:Double-Blind Method; D004359:Drug Therapy, Combination; D004809:Ephedrine / Q000009:adverse effects*; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006339:Heart Rate / Q000187:drug effects; D018882:Heat Stress Disorders / Q000378:metabolism* / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008889:Military Personnel; D010101:Oxygen Consumption / Q000187:drug effects; D010807:Physical Endurance / Q000187:drug effects* / Q000502:physiology; D013997:Time Factors
D000697:Central Nervous System Stimulants; D002110:Caffeine; D004809:Ephedrine