University Of Tasmania

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

journal contribution
posted on 2023-05-18, 00:54 authored by Wall, BA, Greig WatsonGreig Watson, Peiffer, JJ, Abbiss, CR, Siegel, R, Laursen, PB

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.


Publication title

British Journal of Sports Medicine






School of Health Sciences


BMJ Publishing Group

Place of publication

British Med Assoc House, Tavistock Square, London, England, Wc1H 9Jr

Rights statement

Copyright 2013 BMJ Publishing Group

Repository Status

  • Restricted

Socio-economic Objectives

Clinical health not elsewhere classified

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