Br J Sports Med doi:10.1136/bjsports-2013-092417
  • Original article

Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat

  1. Paul B Laursen4,5
  1. 1School of Exercise Biomedical and Health Science, Edith Cowan University, Joondalup, Western Australia, Australia
  2. 2School of Chiropractic and Sports Science, Murdoch University, Murdoch, Western Australia, Australia
  3. 3Faculty of Health Sciences, School of Human Life Sciences, University of Tasmania, Tasmania, Australia
  4. 4High Performance Sport New Zealand, Auckland, New Zealand
  5. 5Sport Performance Research Institute New Zealand (SPRINZ), School of Sport and Recreation, AUT University, Auckland, New Zealand
  1. Correspondence to Professor Paul Laursen, High Performance Sport New Zealand, PO Box 18444, Glen Innes, Auckland 1743, New Zealand; paul.laursen{at}
  • Accepted 29 August 2013
  • Published Online First 20 September 2013


Background Laboratory studies that support the hydration guidelines of leading governing bodies have shown that dehydration to only −2% of body mass can lead to increase in body temperature and heart rate during exercise, and decrease in performance. These studies, however, have been conducted in relatively windless environments (ie, wind speed <12.9 km/h), without participants being blinded to their hydration status.

Aim To investigate the effect of blinded hydration status on cycling time-trial performance in the heat with ecologically valid facing wind speed conditions.

Methods During three experimental trials, 10 cyclists were dehydrated to −3% body mass by performing 2 h of submaximal exercise (walking and cycling) in the heat, before being reinfused with saline to replace 100%, 33% or 0% of fluid losses, leaving them 0%, −2% or −3% hypohydrated, respectively. Participants then completed a 25 km time trial in the heat (33°C, 40% relative humidity; wind speed 32 km/h) during which their starting hydration status was maintained by infusing saline at a rate equal to their sweat rate. The treatment was participant-blinded and the order was randomised. Completion time, power output, heart rate, rectal temperature and perceptual variables were measured.

Results While rectal temperature was higher beyond 17 km of the time trial in the −3% vs 0% conditions (38.9±0.3°C vs 38.6±0.3°C; p<0.05), no other differences between trials were shown.

Conclusion When well-trained cyclists performed a 25 km cycling time trial under ecologically valid conditions and were blinded to their hydration status, performance, physiological and perceptual variables were not different between trials. These data do not support the residing basis behind many of the current hydration guidelines.

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