Acclimation to humid heat lowers resting core temperature

Abstract

The purpose of this study was to test the hypothesis that a reduction in resting rectal temperature (Tre) is partially responsible for the attenuation in the rise of core temperature during heat exposure following acclimation to humid heat. Nine male volunteers completed 7 days of acclimation, performing 2 h of exercise per day in a hot, humid environment (35°C, 75% relative humidity). Mean (±SD) ending Tre significantly ( P < 0.05) decreased from 38.9 ± 0.5°C on day 1 to 38.3 ± 0.4°C on day 7. Likewise, mean (±SD) resting Tre significantly ( P < 0.05) decreased from 37.0 ± 0.3 to 36.7 ± 0.4°C. In fact, all nine men showed a decrease in resting Tre from day 1to day 7, ranging from −0.1 to −0.5°C. In addition, resting Tre and ending Tre were significantly correlated ( r = 0.68). However, the mean increases in Tre (ending Tre minus resting Tre) and heat storage that occurred on each of the 7 acclimation days were not significantly different. These results support the hypothesis that a reduction in resting Tre is partially responsible for the attenuation in ending Tre during heat exposure following short-term acclimation to humid heat.

Influence of Heat Exposure on Serum Lipid and Lipoprotein Cholesterol in Young Male Subjects.

“…All the subjects were exposed to the same conditions. The heat load and duration of heat exposure could have been determined according to the increase in core temperature, body weight loss and metabolic rate (such as oxygen consumption) in each subject 30,31) . WBGT Index is a simple expression of the degree of heat stress.…”

Global Warming and Heat Stress Among Western Australian Mine, Oil and Gas Workers

“…Usual physiological adaptations during heat acclimation, that occur irrespective of the acclimation modality, include: a reduction in resting heart rate in the heat [27], decreased resting core temperature [8], increase in plasma volume [28], decrease in rectal and skin temperature [29], change in sweat composition [30], reduction in the sweating threshold [31] and an increase in sweating efficiency [29].…”

“…For example, Yamazaki [27] used a 6 day acclimation protocol with participants exercising at 50% V  O 2max in ambient conditions of 36°C and 50% RH. Buono et al [8] had a protocol which required their participants to exercise for 7 consecutive days for four bouts of 25 min with a 5 min rest while treadmill walking (1.34 m·s -1 at a 3% grade) and cycling (75 W at 35°C at 75% RH). Shvartz and colleagues [29] used a bench step protocol which equated to a load equal to 85% V  O 2max during ambient conditions of 21.5°C DB, 17.5°C WB, for 12 days.…”

“…Furthermore it has been demonstrated among heat acclimatised workers that core body temperature decreases by 0.1 -0.5 °C during resting periods [8].…”

Mechanisms of heat acclimation and tolerance induced by exercise training and heat exposure

“…Therefore, an individual acclimated to heat and exercise is able to dissipate a given thermal load at an even lower level of central drive than when merely exercise-acclimated. The lowering of the zero point may be caused by a reduction in resting core temperature after heat acclimation 12) . The hypothesis of Nadel et al 11) provides an ingenious explanation for the relationship between central and peripheral mechanisms and an operational definition for heat acclimation; however, the precise mechanisms for the enhancement of sweating and central shift remain unresolved.…”