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Fit Persons Are at Decreased (Not Increased) Risk of Exertional Heat Illness

Kuennen, Matthew; Gillum, Trevor; Dokladny, Karol; Schneider, Suzanne; Moseley, Pope

Exercise and Sport Sciences Reviews: April 2013 - Volume 41 - Issue 2 - p 134–135
doi: 10.1097/JES.0b013e3182877ced
Letters to the Editor-in-Chief
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Department of Sports and Exercise Science West Texas A&M University Canyon, TX

Department of Kinesiology California Baptist University Riverside, CA

Department of Internal Medicine University of New Mexico Albuquerque, NM

Dear Editor-in-Chief

In the review entitled “Influence of Aerobic Fitness on Thermoregulation during Exercise in the Heat,” Dr. Mora-Rodgriguez (3) stated that “When individuals can self-pace exercise… fit individuals will be producing more heat and may fall into the higher risk category [than unfit] for heat-related injuries.” We wish to challenge this statement.

McLellan et al. (2) recently levied a similar challenge, arguing that because “aerobic fitness enhances thermotolerance,” trained persons can tolerate higher core temperatures than untrained. Our previous work (1) reaffirms the challenge by McLellan et al. (2) and lends credence to our own. In that study, eight subjects exercised (in athletic shorts/socks/shoes) to a core temperature of 39°C or higher for seven consecutive days. As shown in Panel A of the Figure, plasma endotoxin, interleukin-6 (IL-6), and IL-10 were reduced and heat shock protein 70 (HSP70) content (peripheral blood mononuclear cells) was increased after 7 d of repeated exercise-heat stress. We attributed the lack of increase in urinary lactulose excretion, plasma endotoxin, and tumor necrosis factor-α (TNF-α) (indicators of gut permeability) preacclimation to the fact that this exercise was undertaken in fit subjects who had acquired some level of thermotolerance. These subjects also exhibited improvements (reduced core, skin, and mean body temperatures, heart rate, and physiologic strain) on a standardized heat tolerance test (1).

Figure

Figure

We also assessed the impact that inhibiting the thermotolerance machinery (in the same eight subjects) had on endotoxin spillover, the ensuing inflammatory response, and achievement of the “heat-acclimated” state. We inhibited thermotolerance with an oral HSP70 blocker, which subjects began ingesting immediately before the first day of exercise-heat stress. As shown in Panel B of the Figure, gut permeability markers (urinary lactulose excretion, plasma endotoxin, TNF-α) were elevated on the first day of exercise-heat stress. They remained elevated on the seventh day. Core, skin, and mean body temperatures did not improve (1). Improvements in heart rate and physiologic strain were attenuated (1). These data effectively underscore: 1) the existence of “enhanced thermotolerance in fit populations” (2); 2) the importance of thermotolerance in the heat acclimation response; and 3) the potential for thermotolerance to reduce risk for “heat-related injuries” (3) in fit populations.

We conclude our letter with three observations:

  1. The previous research by Mora-Rodriguez et al. (4) did not allow subjects to rehydrate during exercise in the heat. Fit subjects generated more metabolic heat at a given sub-maximal exercise intensity. They also sweated more. As a result, “trained subjects dehydrated more than the untrained ones during all exercise intensities” (4). Dehydration reduces sweating, which is how metabolic heat is lost. This strategy would not be practiced in a competition scenario.
  2. Individuals who succumb to exertional heat illness during competition tend to be less (not more) trained than their nonaffected counterparts (5).
  3. Heat acclimation reduces (not elevates) the risk of exertional heat illness (1).

Matthew Kuennen

Department of Sports and Exercise Science

West Texas A&M University

Canyon, TX

Trevor Gillum

Department of Kinesiology

California Baptist University

Riverside, CA

Karol Dokladny

Suzanne Schneider

Pope Moseley

Department of Internal Medicine

University of New Mexico

Albuquerque, NM

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References

1. Kuennen M, Gillum T, Dokladny K, Bedrick E, Schneider S, Moseley P. Thermotolerance and heat acclimation may share a common mechanism in humans. Am. J. Physiol. Regul. Integr. Comp. Physiol. 2011; 301: R524–33.
2. McLellan TM, Cheung SS, Selkirk GA, Wright HE. Influence of aerobic fitness on thermoregulation during exercise in the heat. Exerc. Sport Sci. Rev. 2012; 40: 218–9.
3. Mora-Rodriguez R. Influence of aerobic fitness on thermoregulation during exercise in the heat. Exerc. Sport Sci. Rev. 2012; 40: 79–87.
4. Mora-Rodriguez R, Del Coso J, Hamouti N, Estevez E, Ortega J. Aerobically trained individuals have greater increases in rectal temperature than untrained ones during exercise in the heat at similar relative intensities. Eur. J. Appl. Physiol. 2010; 109: 973–81.
5. Wallace RF, Kriebel D, Punnett L, et al.. Risk factors for recruit exertional heat illness by gender and training period. Aviat. Space Environ. Med. 2006; 77: 415–21.
©2013 The American College of Sports Medicine