The effect of low-intensity exercise in the heat on thermoregulation and certain biochemical changes in temperate and tropical subjects under poorly and well-hydrated states was examined. Two VO_2max matched groups of subjects consisting of 8 Japanese (JS) and 8 Malaysians (MS) participated in this study under two conditions: poorly-hydrated (no water was given) and well-hydrated (3 mL·Kg⁻¹ body weight of water was provided at onset of exercise, and the 15th, 35th and 55th min of exercise). The experimental room in both countries was adjusted to a constant level (Ta: 31.6±0.03°C, rh: 72.3±0.13%). Subjects spent an initial 10 min rest, 60 min of cycling at 40% VO_2max and then 40 min recovery in the experimental room. Rectal temperatures (T_re) skin temperatures (T_sk), heart rate (HR), heat-activated sweat glands density (HASG), local sweat rate (M_sw-back) and percent dehydration were recorded during the test. Blood samples were analysed for plasma glucose and lactate levels.
The extent of dehydration was significantly higher in the combined groups of JS (1.43±0.08%) compared to MS (1.15±0.05%). During exercise M_sw-back was significantly higher in JS compared to MS in the well-hydrated condition. The HASG was significantly more in JS compared to MS at rest and recovery. T_re was higher in MS during the test. T_sk was significantly higher starting at the 5th min of exercise until the end of the recovery period in MS compared to JS.
In conclusion, tropical natives have lower M_sw-back associated with higher T_sk and T_re during the rest, exercise and recovery periods. However, temperate natives have higher M_sw-back and lower T_sk and T_re during experiments in a hot environment. This phenomenon occurs in both poorly-hydrated and well-hydrated states with low intensity exercise. The differences in M_sw-back, T_sk and T_re are probably due to a setting of the core temperature at a higher level and enhancement of dry heat loss, which occurred during passive heat exposure.