Studies on physiological responses of residents in Okinawa to a hot environment

In an attempt to reconfirm Kuno's observation that changes in sweating reaction during long-term heat acclimatization differ from those during short-term heat acclimatization, Ohara's sweating test was performed in summer in Okinawa on 37 male subjects, including 19 residents born and raised in Okinawa (group O) and 18 residents born and raised on one of the main Japanese islands (group M). Seasonal variation of adaptability to heat was also studied in some subjects of both groups. Group O showed significantly less sweat loss, lower Na concentration in sweat and a longer latent period for onset of sweating than group M. Group O showed no seasonal variation in sweat loss, while group M showed considerably greater sweat loss in summer than in winter. In both groups, lower Na concentration in sweat, lower rise in rectal temperature and lesser increase in heart rate in summer than in winter were observed. Seasonal difference in physiological responses of group O to heat exposure was less than that of group M. In conclusion, it was assumed that acclimatization to heat of group O had advanced further than that of group M and this was the reason for longer latent period of sweating and lesser sweat loss in group O in spite of the same rise in rectal temperature in both groups. Discussions were carried out to explain how the sweating pattern and mechanism of acclimatization in group O were different from those in group M.