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's 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