Analysis of the Mechanisms of Heat Acclimatization : Comparison of Heat-tolerance between Japanese and Thai Subjects

In order to clarify the mechanisms of heat acclimatization to tropical climates by permanent residence, changes in oral temperature due to heat load were compared in 10 male subjects in Chiang Mai, Thailand (tropical region) and 10 male subjects in Nagasaki, Japan (temperate region). Mean annual ambient temperature is 16.6℃ in Nagasaki and 25.9℃ in Chiang Mai. The experiments for the Thai subjects were performed in Chiang Mai and those for the Japanese subjects in Nagasaki during each region\u27s hottest months. The constitutional characteristics of the Thai subjects were a little shorter and slightly leaner than the Japanese. After staying at rest in the experimental room at 32℃ and 35% of relative humidity for at least 30 min, the lower legs were immersed into a hot water bath of 43℃ for 30 min. Mean initial oral temperature was 37.06±0.07℃ in Japanese and 37.12±0.05℃ in Thai subjects (P>0.05). Oral temperature rose after heat load and reached to 37.54±0.06℃ and 37.59±0.06℃ in Japanese and Thai subjects (P>0.05), respectively. Although the inhabitants in Chiang Mai were expected to be more acclimatized to heat compared to those in Nagasaki, no significant difference in the oral temperature was found between two groups throughout the experiment. It is speculated that the same rise in oral temperature in both groups of subjects is attributed to a lower sweat rate and an increase of dry heat loss in Thai subjects. In future studies, not only core temperature but also skin temperatures (dry heat loss) and sweat rate (evaporative heat loss) should be measured and analyzed

Study on Mechanisms of Heat Acclimatization Due to Thermal Sweating : Comparison of Heat-tolerance between Japanese and Thai Subjects

Heat tolerance and sweat response to heat load of tropical subjects in Chiang Mai and temperate subjects in Nagasaki were compared under identical conditions. Male students in Chiang Mai (n=10) and in Nagasaki (n=10) volunteered for this study. The Thai subjects were a little shorter and slightly leaner than the Japanese. Heat load was applied on the legs by immersion into hot water (43℃) for 30 min in the room at 26.6℃ and 33%rh. Sublingual (oral) temperature was measured with a thermistor probe and local sweat rate was measured by the capacitance hygrometer-sweat capsule method. Change in oral temperature, sweat onset time and local sweat volume were compared between Japanese and Thai. Initial oral temperatures (36.76±0.11℃ in Japanese, 36.71±0.23℃ in Thai) were identical, and no sweat was observed before heat load. Mean sweat onset time (9.3±2.1 min chest in Japanese, 16.6±5.6 min chest in Thai) were significantly longer and local sweat volume (10.19±5.00 mg/cm^2, chest in Japanese, 1.39±0.91 mg/cm^2, chest in Thai) was significantly smaller in Thai subjects than Japanese, however, oral temperature (37.18±0.32℃) of Thai subjects was kept slightly lower than oral temperature (37.42±0.10℃) of Japanese even under a 30 min heat load. Sweat volume on the abdomen was larger than on the chest in 9 of 10 Thai subjects. On the contrary, sweat volume on the chest was larger than that on the abdomen in 7 of 10 Japanese subjects. These results suggest that heat tolerance of tropical subjects in due to a more efficient evaporative ability due to a greater heat loss brought about by their long term exposure to heat. Furthermore, the habituation phenomenon related to the reduction of thermoregulatory effector mechanisms were also considered so as to clarify the mechanisms of thermal acclimatization