title: Marginal effects of a large caffeine dose on heat balance during exercise-heat stress.
abstract: The use of caffeine supplements in athletic and military populations has increased in recent years. Excessive caffeine consumption in conjunction with exercise in a hot environment may predispose individuals to heat illness.
  
  
  OBJECTIVE
  To examine heat balance induced by a large dose of caffeine during exercise in a hot environment.
  
  
  METHODS
  Ten men, not heat acclimated and not habitual caffeine users, consumed either caffeine (CAF; 9 mg/kg) or placebo (PLA) before performing cycle-ergometer exercise for 30 min at 50% VO2peak in a 40 °C, 25% relative humidity environment while body temperature (core and skin) and ratings of thermal comfort (TC) were monitored. Heat-exchange variables were calculated using partitional calorimetry and thermometry.
  
  
  RESULTS
  Mean body temperature (Tb) was higher (p < .05) with CAF (37.18 ± 0.15 °C) than with PLA (36.93 ± 0.15 °C) at the start of exercise. Heat production was slightly higher (~8 W, p < .05) with CAF. There were no differences in heat storage, dry heat gains, TC, or Tb during exercise.
  
  
  CONCLUSIONS
  A caffeine dose of 9 mg/kg does not appreciably alter heat balance during work in a hot environment. The small increase in Tb observed with CAF was undetected by the participants and is unlikely to increase physiological strain sufficiently to affect endurance performance or risk of heat illness.
authors.affiliation: Army Research Institute of Environmental Medicine, Thermal and Mountain Medicine Division, Natick, MA, USA.
authors.forename: Brett R
authors.initials: BR
authors.lastname: Ely
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Ely
authors.forename: Samuel N
authors.initials: SN
authors.lastname: Cheuvront
mesh_terms: D000293:Adolescent; D000328:Adult; D001833:Body Temperature Regulation / Q000187:drug effects*; D002110:Caffeine / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D002491:Central Nervous System Agents / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D004311:Double-Blind Method; D015444:Exercise / Q000502:physiology*; D006358:Hot Temperature; D006801:Humans; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology; D013312:Stress, Physiological / Q000187:drug effects*; D055815:Young Adult
chemical_list: D002491:Central Nervous System Agents; D002110:Caffeine