The influence of hydration state on thermoregulation during a 161-km ultramarathon

ABSTRACT

It is advised that individuals should avoid losing >2% of their body mass during exercise in order to prevent hyperthermia. This study sought to assess whether a loss of >2% body mass leads to elevations in core temperature during an ultramarathon. Thirty runners agreed to take part in the study. Body mass and core temperature were measured at the start, at three locations during the race and the finish. Core temperature was not correlated with percent body mass change (p = 0.19) or finish time (p = 0.11). Percent body mass change was directly associated with finish time (r = 0.58, p < 0.01), such that the fastest runners lost the most mass (~3.5–4.0%). It appears that a loss of >3% body mass does not contribute to rises in core temperature. An emphasis on fluid replacement for body mass losses of this magnitude during prolonged exercise is not justified as a preventative measure for heat-related illnesses.

KEYWORDS:

• Endurance exercise
• ultramarathon
• thermoregulation
• core temperature
• dehydration

Acknowledgements

This material is the result of work supported with resources and the use of facilities at the VA Northern California Health Care System, Oakland University, Gettysburg University and San Francisco State University. The work was also supported by the Western States Endurance Run Foundation. We thank the following individuals for assistance with data collection: Dr Jim Agnew, Maddison Bowles, Dr Jeffrey A. Chan, Colleen Conners-Pace, Ali Etemady-Deylamy, Dr Keith George, Dr Steve Hammer, Dr Tamara Hew-Butler, Dr Tracy Beth Høeg, Samuel Jurek, Rachel Lord, Dr Christine Mathiesen, Dr David Oxborough, Dr David Paris, Emily Pearce, Peter K. Raisanen, Professor John Somauroo, Lisa Weiss and Casey Westbrooke. The contents reported here do not represent the views of the Department of Veterans Affairs or the United States Government.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Western States Endurance Run Foundation.