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Byrne, Christopher1; Lee, Jason2

Medicine & Science in Sports & Exercise: October 2007 - Volume 39 - Issue 10 - p 1884
doi: 10.1249/mss.0b013e318148bbd5
SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief

1School of Sport and Health Sciences, University of Exeter, Exeter, United Kingdom

2Defence Medical and Environmental Research Institute, Republic of Singapore

Dear Editor-in-Chief:

We thank Cheuvront et al. for acknowledging and further highlighting the contribution of our study (1) to knowledge of the temporal core temperature response to competitive outdoor distance running in heat. Cheuvront et al. bring to the fore three of our study's findings (1) that appear inconsistent with laboratory evidence.

Firstly, our study revealed that runners will voluntarily achieve a high magnitude of core temperature (Tc) elevation (e.g., peak Tc = 40.1 ± 0.7°C, range 39.3-41.7°C; 10 of 18 runners Tc ≥ 40°C, two runners Tc > 41°C) without medical consequence (1). Cheuvront et al. recognize these data are in agreement with numerous field-based studies recording postrace Tc > 40°C in asymptomatic runners (1,3). The absence of heat stroke despite Tc > 40°C does indeed suggest the etiology involves more than a high Tc alone (4). To our knowledge, the highest individual Tc observed in an asymptomatic runner is 41.9°C, and this would indeed imply a critical thermal maximum ≥ 42°C. The concept of 40°C as a critical Tc for exercise fatigue could not be directly tested in our study because of the absence of running-speed measurements during the race.

Secondly, no statistically significant (P > 0.05) relationships were observed between Tc responses and finishing time. This is unsurprising because average running speed provides no information about metabolic rate, and relative exercise intensity (i.e., %V˙O2max) is the main determinant of exercise Tc. By extrapolating laboratory-derived speed-V˙O2 relationships to running speeds observed during an outdoor marathon, Noakes et al. (7) confirmed that postrace Tc was most closely related to absolute V˙O2 (i.e., L·min−1). Interestingly, the Tc-%V˙O2max relationship has yet to be directly confirmed during outdoor distance running. The relationship is likely to be complicated by the temporal changes in oxygen consumption, reduced running economy, and reduced V˙O2max associated with prolonged exercise in heat (8).

Thirdly, no statistically significant (P > 0.05) relationships between Tc responses and percent dehydration were evident in our data. Although in contrast to the laboratory evidence base (5), our data are in agreement with a growing body of evidence from field-based studies employing body mass change (i.e., percent dehydration) as an indicator of the change in hydration status. In a comprehensive quantitative analysis of the sources of error in this approach, Maughan et al. (6) conclude that "there is not a simple relationship between the change in body mass during prolonged exercise and the change in body hydration status." For example, the approach does not account for mass losses attributable to substrate oxidation nor water gain from metabolism or water bound to glycogen (6). According to Maughan et al. (6), the majority of the body mass losses observed in our study (i.e,. 2.8 ± 1.0%, range 0.9-3.9%) could have occurred without an effective decrease in body water.

We welcome the acknowledgement that our study has made a unique contribution to the knowledge base of exercise thermoregulation. By employing ingestible temperature sensors (2) with laboratory measures of hydration status (6) and exercise metabolism (7) in a follow-up study, we endeavor to share important insights from the 2007 Singapore Army Half-Marathon.

Christopher Byrne

School of Sport and Health Sciences

University of Exeter

Exeter, United Kingdom

Jason Lee

Defence Medical and Environmental Research Institute

Republic of Singapore

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1. Byrne, C., J. K. W. Lee, S. A. N. Chew, C. L. Lim, and E. Y. M. Tan. Continuous thermoregulatory responses to mass-participation distance running in heat. Med. Sci. Sports Exerc. 38:803-810, 2006.
2. Byrne, C., and C. L. Lim. The ingestible telemetric body core temperature sensor: a review of validity and exercise applications. Br. J. Sports Med. 41:126-133, 2007.
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7. Noakes, T. D., K. H. Myburgh, J. Du Plessis, et al. Metabolic rate, not percent dehydration predicts rectal temperature in marathon runners. Med. Sci. Sports Exerc. 23:443-449, 1991.
8. Wingo, J. E., A. J. Lafrenz, M. S. Ganio, G. L. Edwards, and K. J. Cureton. Cardiovascular drift is related to reduced maximal oxygen uptake during heat stress. Med. Sci. Sports Exerc. 37:248-255, 2005.
©2007The American College of Sports Medicine