SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief
I read with interest the article by Byrne et al. (1), "Continuous Thermoregulatory Responses to Mass-Participation Distance Running in Heat." The message that is clearly stated in the abstract, results, and first paragraph of the discussion is that a significant portion of participants in distance races in the heat will voluntarily achieve core temperatures in excess of 40°C without medical complications. Unfortunately, I am not convinced of the validity of their method of data collection and have great concern about what has been conveyed to the readers of Medicine & Science in Sports & Exercise®. The message that should be highlighted is shown in Table 2 on page 806. In this table, the authors show that the change in core temperature from the beginning to end of the race averages 2.1°C, with only one runner showing a change in heat storage that produced changes greater than 3.0°C in core temperature.
I challenge the authors to document that resting core temperatures at 0600 h were above 37.5°C, as was shown in Figure 1 for 15 of the 18 runners. Indeed, the authors show that two of the runners who revealed core temperatures greater than 41°C had resting values that approached 38.5°C at the start of the race. We have studied the thermoregulatory responses to exercise during a period of sustained wakefulness in the heat and have reported that the circadian rhythm in core temperature is still maintained during periods of submaximal exercise (2,3). Resting core (measured by rectal thermistor or radiopill technology) temperatures for all 11 subjects were at or below 37.0°C at 0400 h and increased to approximately 37.5°C at 1600 h, either the day before or the following afternoon.
Byrne et al. should have calculated the heat stress index as the ratio of Ereq to Emax to find that their conditions represented compensable heat stress. Most, if not all, of the participants should have been able to thermoregulate at elevated but controlled body temperatures. Most of the runners in the study by Byrne et al. show this response, with the exception of three subjects in Figure 1C on page 805. Thus, the pattern of response is correct for the conditions to which the subjects were exposed, but the absolute values of core temperature are excessively high.
What would have caused the elevated values reported by Byrne et al? Clearly, the method of measuring core temperature with the radiopill technology has been shown to be valid when compared with either esophageal or rectal temperature (4). One possibility is if the pills' serial and calibration numbers are not correctly entered, or if the receiver is not correctly assigned to the proper subject, then erroneous findings will occur. Although the receiver will record a signal, it will not be correctly recording from the ingested pill.
In conclusion, until the validity of their radiopill data is substantiated, or their measurements replicated, the absolute values of core temperatures reported in this paper need to be viewed with skepticism.
Tom M. McLellan, PhD
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 the heat. Med. Sci. Sports Exerc.
2. Ducharme, M. B., T. M. McLellan, D. Moroz, A. Buguet, and M. W. Radomski. A 36-hour comparison of core temperature at rest and during exercise using rectal probe and pill telemetry. Proc. Aust. Physiol. Pharmacol. Soc.
3. McLellan, T. M., M. B. Ducharme, F. Canini, et al. Effect of modafinil on thermoregulation during sleep deprivation and exercise in a warm environment. Aviat. Space Environ. Med.
4. O'Brien, C., R. W. Hoyt, M. J. Buller, J. W. Castellani, and A. J. Young. Telemetry pill measurement of core temperature in humans during active heating and cooling. Med. Sci. Sports Exerc.