Effects of Short-term Exercise in the Heat on Thermoregulation, Blood Parameters, Sweat Secretion and Sweat Composition of Tropic-dwelling Subjects

Abstract

This study investigates the effects of a shortterm aerobic training program in a hot environment on thermoregulation, blood parameters, sweat secretion and composition in tropic-dwellers who have been exposed to passive heat. Sixteen healthy Malaysian-Malay male volunteers underwent heat acclimation (HA) by exercising on a bicycle ergometer at 60% ofVO 2 max for 60 min each day in a hot environment (Ta: 31.1Ϯ0.1°C, rh: 70.0Ϯ4.4%) for 14 days. All parameters mentioned above were recorded on Day 1 and at the end of HA (Day 16). On these two days, subjects rested for 10 min, then cycled at 60% of V O 2 max for 60 min and rested again for 20 min (recovery) in an improvised heat chamber. Rectal temperature (T re ), mean skin temperature (T sk ) heart rate (HR), ratings of perceived exertion (RPE), thermal sensation (TS), local sweat rate and percent dehydration were recorded during the test. Sweat concentration was analysed for sodium [Na ϩ ] sweat and potassium. Blood samples were analysed for biochemical changes, electrolytes and hematologic indices. Urine samples were collected before and after each test and analysed for electrolytes.After the period of acclimation the percent dehydration during exercise significantly increased from 1.77Ϯ0.09% (Day 1) to 2.14Ϯ0.07% (Day 16). Resting levels of hemoglobin, hematocrit and red blood cells decreased significantly while [Na ϩ ] sweat increased significantly. For T re and T sk there were no differences at rest. T re , HR, RPE, TS, plasma lactate concentration, hemoglobin and hematocrit at the 40th min of exercise were significantly lower after the period of acclimation but mean corpuscular hemoglobin and serum osmolality were significantly higher while no difference was seen in [Na ϩ ] sweat and T sk . It can be concluded that tropicdwelling subjects, although exposed to prolonged passive heat exposure, were not fully heat acclimatized. To achieve further HA, they should gradually expose themselves to exercise-heat stress in a hot environment.

Thermoregulatory Efficiency is Increased after Heat Acclimation in Tropical Natives

“…In fact, they had a normal diet sodium content (ϳ2000 mg/day) and this may be the reason for the nonsignificant change in sweat sodium concentration in the present study. Studying HA on Malaysian subjects, Saat et al (2005b) observed higher sweat sodium concentration at rest after the protocol and a nonsignificant difference in sweat sodium concentration during exercise. It is known that sweat sodium concentration is influenced by sweat rate because electrolyte reabsorption is diminished due to augmented sweat rate (Saat et al, 2005b).…”

“…Studying HA on Malaysian subjects, Saat et al (2005b) observed higher sweat sodium concentration at rest after the protocol and a nonsignificant difference in sweat sodium concentration during exercise. It is known that sweat sodium concentration is influenced by sweat rate because electrolyte reabsorption is diminished due to augmented sweat rate (Saat et al, 2005b). To take this effect into account, we divided local sweat sodium concentration by local sweat rate.…”

“…On days 1, 6, and 11 of HA we measured local sweat rates at 5 skin sites: forehead (middle of the forehead), chest (right side, between the nipple line and the clavicle), arm (upper right arm), forearm (proximal third of the anterior right forearm) and thigh (right mid-thigh) using the absorbent filter paper patches technique Saat et al, 2005b) during the entire second half of the exercise protocol (for 30 min). On adjacent skin regions to where we measured local sweat rate, we also measured local skin temperature.…”

The effects of single versus twice daily short term heat acclimation on heat strain and 3000m running performance in hot, humid conditions

“…Participants cycled at a fixed mechanical intensity to elicit 50% 2peak for 45 minutes (Nielsen et al, 1997;Saat, et al, 2005) once a day for 4 consecutive days within 35.2±0.5°C and 60±2% RH (SDHA), within 21.7±0.6°C and 39±5% (CON), or twice daily for two consecutive days within 35.4±0.8°C and 61±3% RH (TDHA). Cycling exercise was chosen as it is non-weight-bearing and practical, thus reducing injury risk and enabling 4 to 6 participants to train simultaneously.…”

Profiling of Body Hydration and Related Blood Parameters under Hot-Arid Conditions among Indian Population doing Graded Physical Activity

“…The threshold for impaired exercise thermoregulation is 1% dehydration, and at 4%, decrements of 20 -30% are seen in work capacity (8). Hypohydration decreases cutaneous vasodilation and sweating during heat stress (30,31). Hypovolemia and hyperosmolality due to thermal dehydration also suppress thermoregulatory responses of sweating and cutaneous vasodilation, resulting in increased risk of heat illness (18).…”