Comparison of heat dissipation response between Malaysian and Japanese males during exercise in humid heat stress

This study investigated the differences in heat dissipation response to intense heat stress during exercise in hot and humid environments between tropical and temperate indigenes with matched physical characteristics. Ten Japanese (JP) and ten Malaysian (MY) males participated in this study. Subjects performed exercise for 60 min at 55% peak oxygen uptake in 32°C air with 70% relative humidity, followed by 30 min recovery. The increase in rectal temperature (T _re) was smaller in MY during exercise compared to JP. The local sweat rate and total body mass loss were similar in both groups. Both skin blood flow and mean skin temperature was lower in MY compared to JP. A significantly greater increase in hand skin temperature was observed in MY during exercise, which is attributable to heat loss due to the greater surface area to mass ratio and large number of arteriovenous anastomoses. Also, the smaller increase in T _re in MY may be explained by the presence of a significantly greater core–skin temperature gradient in MY than JP. The thermal gradient is also a major factor in increasing the convective heat transfer from core to skin as well as skin blood flow. It is concluded that the greater core–skin temperature gradient observed in MY is responsible for the smaller increase in T _re.     

Comparison of thermoregulatory responses to heat between Malaysian and Japanese males during leg immersion

The objective of this study was to investigate thermoregulatory responses to heat in tropical (Malaysian) and temperate (Japanese) natives, during 60 min of passive heating. Ten Japanese (mean ages: 20.8 ± 0.9 years) and ten Malaysian males (mean ages: 22.3 ± 1.6 years) with matched morphological characteristics and physical fitness participated in this study. Passive heating was induced through leg immersion in hot water (42°C) for 60 min under conditions of 28°C air temperature and 50% RH. Local sweat rate on the forehead and thigh were significantly lower in Malaysians during leg immersion, but no significant differences in total sweat rate were observed between Malaysians (86.3 ± 11.8 g m⁻² h⁻¹) and Japanese (83.2 ± 6.4  g m⁻² h⁻¹) after leg immersion. In addition, Malaysians displayed a smaller rise in rectal temperature (0.3 ± 0.1°C) than Japanese (0.7 ± 0.1°C) during leg immersion, with a greater increase in hand skin temperature. Skin blood flow was significantly lower on the forehead and forearm in Malaysians during leg immersion. No significant different in mean skin temperature during leg immersion was observed between the two groups. These findings indicated that regional differences in body sweating distribution might exist between Malaysians and Japanese during heat exposure, with more uniform distribution of local sweat rate over the whole body among tropical Malaysians. Altogether, Malaysians appear to display enhanced efficiency of thermal sweating and thermoregulatory responses in dissipating heat loss during heat loading. Thermoregulatory differences between tropical and temperate natives in this study can be interpreted as a result of heat adaptations to physiological function.     

Physique and thermoregulation in prepubertal males during exercise in a warm, humid environment

The hyperthermic response to exercise in a warm (30 degrees C), humid (80% relative humidity) environment was obtained for 23 prepubertal males. After the initial increase of core temperatures (tympanic and rectal) to elevated set points, further hyperthermia was minimal and was unrelated to any physique variable except for the case of endomorphy, which was weakly correlated with hyperthermia at the rectal site. This result for boys is attributed to the combination of small body size (relative to adults) and absence of pronounced, age-dependent muscular development as evidenced by low rating and small variation of the mesomorphy component of somatotype. Both of these factors were associated with a high surface area to weight ratio compared to adults, which facilitated heat loss. It is concluded that prepubertal males thermoregulate efficiently during moderate exercise in a warm, humid environment. Other than cases of obesity, variation in physique is not an important consideration for assessing the risk of heat strain.     

A comparison of hydration effect on body fluid and temperature regulation between Malaysian and Japanese males exercising at mild dehydration in humid heat

Background

This study investigated the effect of hydration differences on body fluid and temperature regulation between tropical and temperate indigenes exercising in the heat.

Methods

Ten Japanese and ten Malaysian males with matched physical characteristics (height, body weight, and peak oxygen consumption) participated in this study. Participants performed exercise for 60 min at 55% peak oxygen uptake followed by a 30-min recovery at 32°C and 70% relative air humidity with hydration (4 times each, 3 mL per kg body weight, 37°C) or without hydration. Rectal temperature, skin temperature, heart rate, skin blood flow, and blood pressure were measured continuously. The percentage of body weight loss and total sweat loss were calculated from body weight measurements. The percentage change in plasma volume was estimated from hemoglobin concentration and hematocrit.

Results

Malaysian participants had a significantly lower rectal temperature, a smaller reduction in plasma volume, and a lower heart rate in the hydrated condition than in the non-hydrated condition at the end of exercise (P <0.05), whereas Japanese participants showed no difference between the two hydration conditions. Hydration induced a greater total sweat loss in both groups (P <0.05), and the percentage of body weight loss in hydrated Malaysians was significantly less than in hydrated Japanese (P <0.05). A significant interaction between groups and hydration conditions was observed for the percentage of mean cutaneous vascular conductance during exercise relative to baseline (P <0.05).

Conclusions

The smaller reduction in plasma volume and percentage body weight loss in hydrated Malaysians indicated an advantage in body fluid regulation. This may enable Malaysians to reserve more blood for circulation and heat dissipation and thereby maintain lower rectal temperatures in a hydrated condition.     

Effects of short-term exercise in the heat on thermoregulation, blood parameters, sweat secretion and sweat composition of tropic-dwelling subjects

This study investigates the effects of a short-term 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% of VO2max for 60 min each day in a hot environment (Ta: 31.1+/-0.1 degrees C, rh: 70.0+/-4.4%) 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 VO2max for 60 min and rested again for 20 min (recovery) in an improvised heat chamber. Rectal temperature (Tre), mean skin temperature (Tsk) 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+]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 1.77+/-0.09% (Day 1) to 2.14+/-0.07% (Day 16). Resting levels of hemoglobin, hematocrit and red blood cells decreased significantly while [Na+]sweat increased significantly. For Tre and Tsk there were no differences at rest. Tre, 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+]sweat and Tsk. It can be concluded that tropic-dwelling 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.     

Impact of muscle injury and accompanying inflammatory response on thermoregulation during exercise in the heat.

This study examined whether muscle injury and the accompanying inflammatory responses alter thermoregulation during subsequent exercise-heat stress. Sixteen subjects performed 50 min of treadmill exercise (45-50% maximal O(2) consumption) in a hot room (40 degrees C, 20% relative humidity) before and at select times after eccentric upper body (UBE) and/or eccentric lower body (LBE) exercise. In experiment 1, eight subjects performed treadmill exercise before and 6, 25, and 30 h after UBE and then 6, 25, and 30 h after LBE. In experiment 2, eight subjects performed treadmill exercise before and 2, 7, and 26 h after LBE only. UBE and LBE produced marked soreness and significantly elevated creatine kinase levels (P < 0.05), but only LBE increased (P < 0.05) interleukin-6 levels. In experiment 1, core temperatures before and during exercise-heat stress were similar for control and after UBE, but some evidence for higher core temperatures was found after LBE. In experiment 2, core temperatures during exercise-heat stress were 0.2-0.3 degrees C (P < 0.05) above control values at 2 and 7 h after LBE. The added thermal strain after LBE (P < 0.05) was associated with higher metabolic rate (r = 0.70 and 0.68 at 2 and 6-7 h, respectively) but was not related (P > 0.05) to muscle soreness (r = 0.47 at 6-7 h), plasma interleukin-6 (r = 0.35 at 6-7 h), or peak creatine kinase levels (r = 0.22). Local sweating responses (threshold core temperature and slope) were not altered by UBE or LBE. The results suggest that profuse muscle injury can increase body core temperature during exercise-heat stress and that the added heat storage cannot be attributed solely to increased heat production.     

Time-of-day effects of exposure to solar radiation on thermoregulation during outdoor exercise in the heat

High solar radiation has been recognised as a contributing factor to exertional heat-related illness in individuals exercising outdoors in the heat. Although solar radiation intensity has been known to have similar time-of-day variation as body temperature, the relationship between fluctuations in solar radiation associated with diurnal change in the angle of sunlight and thermoregulatory responses in individuals exercising outdoors in a hot environment remains largely unknown. The present study therefore investigated the time-of-day effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during moderate-intensity outdoor exercise in the heat of summer. Eight healthy, high school baseball players, heat-acclimatised male volunteers completed a 3-h outdoor baseball trainings under the clear sky in the heat. The trainings were commenced at 0900 h in AM trial and at 1600 h in PM trial each on a separate day. Solar radiation and solar elevation angle during exercise continued to increase in AM (672–1107 W/m² and 44–69°) and decrease in PM (717–0 W/m² and 34–0°) and were higher on AM than on PM (both P < 0.001). Although ambient temperature (AM 32–36°C, PM 36–30°C) and wet-bulb globe temperature (AM 31–33°C, PM 34–27°C) also continued to increase in AM and decrease in PM, there were no differences between trials in these (both P > 0.05). Tympanic temperature measured by an infrared tympanic thermometer and mean skin temperature were higher in AM than PM at 120 and 180 min (P < 0.05). Skin temperature was higher in AM than PM at the upper arm and thigh at 120 min (P < 0.05) and at the calf at 120 and 180 min (both P < 0.05). Body heat gain from the sun was greater during exercise in AM than PM (P < 0.0001), at 0–60 min in PM than AM (P < 0.0001) and at 120–180 min in AM than PM (P < 0.0001). Dry heat loss during exercise was greater at 0–60 min (P < 0.0001), and lower at 60–120 min (P < 0.05) and 120–180 min (P < 0.0001) in AM than PM. Evaporative heat loss during exercise was greater in PM than AM at 120–180 min (P < 0.0001). Total (dry + evaporation) heat loss at the skin was greater during exercise in PM than AM (P < 0.0001), at 0–60 min in AM than PM (P < 0.0001) and at 60–120 and 120–180 min in PM than AM (P < 0.05 and 0.0001). Heart rate at 120–150 min was also higher in AM than PM (P < 0.05). Neither perceived thermal sensation nor rating of perceived exertion was different between trials (both P > 0.05). The current study demonstrates a greater thermoregulatory strain in the morning than in the afternoon resulting from a higher body temperature and heart rate in relation to an increase in environmental heat stress with rising solar radiation and solar elevation angle during moderate-intensity outdoor exercise in the heat. This response is associated with a lesser net heat loss at the skin and a greater body heat gain from the sun in the morning compared with the afternoon.     

Effects of caffeine ingestion on body fluid balance and thermoregulation during exercise

This study investigated the effects of caffeine supplementation on thermoregulation and body fluid balance during prolonged exercise in a thermoneutral environment (25 degrees C, 50% RH). Seven trained male subjects exercised on a treadmill at an intensity of 70-75% of maximal oxygen consumption to self-determined exhaustion. Subjects exercised once after caffeine and once after placebo ingestion, given in a double-blind crossover design. Five milligrams per kilogram body weight of caffeine followed by 2.5 mg.kg-1 of caffeine were given 2 and 0.5 h before exercise, respectively. Rectal temperature was recorded and venous blood samples were withdrawn every 15 min. Water loss and sweat rate were calculated from the difference between pre- and post-exercise body weight, corrected for liquid intake. Following caffeine ingestion, when compared with placebo, no significant difference in final temperature or in percent change in plasma volume were found. No significant differences were observed in total water loss (1376 +/- 154 vs. 1141 +/- 158 mL, respectively), sweat rate (12.4 +/- 1.1 vs. 10.9 +/- 0.7 g.m-2.min-1, respectively), rise in rectal temperature (2.1 +/- 0.3 vs. 1.5 +/- 0.4 degrees C, respectively), nor in the calculated rate of heat storage during exercise (134.4 +/- 17.7 vs. 93.5 +/- 22.5 W, respectively). Thus, in spite of the expected rise in oxygen uptake, caffeine ingestion under the conditions of this study does not seem to disturb body fluid balance or affect thermoregulation during exercise performance.     

Hydration effects on thermoregulation and performance in the heat

During exercise, sweat output often exceeds water intake, producing a water deficit or hypohydration. The water deficit lowers both intracellular and extracellular fluid volumes, and causes a hypotonic-hypovolemia of the blood. Aerobic exercise tasks are likely to be adversely effected by hypohydration (even in the absence of heat strain), with the potential affect being greater in hot environments. Hypohydration increases heat storage by reducing sweating rate and skin blood flow responses for a given core temperature. Hypertonicity and hypovolemia both contribute to reduced heat loss and increased heat storage. In addition, hypovolemia and the displacement of blood to the skin make it difficult to maintain central venous pressure and thus cardiac output to simultaneously support metabolism and thermoregulation. Hyperhydration provides no advantages over euhydration regarding thermoregulation and exercise performance in the heat.     

Effects of heat acclimatisation on work tolerance and thermoregulation in trained tropical natives

This study aimed to investigate the effects of heat acclimatisation on thermoregulatory responses and work tolerance in trained individuals residing in the tropics. Eighteen male trained soldiers, who are native to a warm and humid climate, performed a total of four heat stress tests donning the Skeletal Battle Order (SBO, 20.5 kg) and Full Battle Order (FBO, 24.7 kg) before (PRE) and after (POST) a 10-day heat acclimatisation programme. The trials were conducted in an environmental chamber (dry bulb temperature: 32 °C, relative humidity: 70%, solar radiation: 400 W/m²). Excluding the data sets of which participants fully completed the heat stress tests (210 min) before and after heat acclimatisation, work tolerance was improved from 173±30 to 201±18 min (∼21%, p<0.05, n=9) following heat acclimatisation. Following heat acclimatisation, chest skin temperature during exercise was lowered in SBO (PRE=36.7±0.3 vs. POST=36.5±0.3 °C, p<0.01) and FBO (PRE=36.8±0.4 vs. POST=36.6±0.3 °C, p<0.01). Ratings of perceived exertion were decreased with SBO and FBO (PRE=11±2; POST=10±2; p<0.05) after heat acclimatisation. Heat acclimatisation had no effects on baseline body core temperature, heart rate and sweat rate across trials (p>0.05). A heat acclimatisation programme improves work tolerance with minimal effects on thermoregulation in trained tropical natives.     

Effects of exercise training on plasma cytokine and chemokine levels, and thermoregulation

Pyrogenic factors may include the proinflammatory cytokines such as interleukin (IL)-1β, IL-6, tumor necrosis factor-alpha (TNF-α), and IL-8 (chemokine). Exercise also causes cytokinemia that might result in pyrogenically mediated body temperature elevation. The aim of the present study was to determine the effect of exercise training on exercise-induced plasma concentrations of IL-1β, IL-6, TNF-α, and IL-8. Messenger RNA levels of these factors were also evaluated in peripheral blood leukocytes. We also observed the relationship between cykokines, chemokines, and sweating after exercise. Nine tennis athletes (n=9) and untrained sedentary control subjects (n=10) ran for 1 h at 75% intensity of VO_2max. Venous blood samples were analyzed for plasma concentrations and mRNA expression in leukocytes of cytokines and chemokine of interest. Sweat volume was calculated by measuring body weight changes. Leukocyte mRNA expression and plasma protein levels of IL-1β, IL-6, TNF-α, and IL-8 immediately increased after exercise in both groups, but to a much greater extent in the athletic group. However, mRNA expression and plasma protein level for IL-6 and TNF-α, unlike IL-1β and IL-8, decreased more quickly in the athletic group compared to the control group during the recovery period. Compared to the control group, greater sweat loss volumes, and lower body temperatures in athletic group were observed at all time points. In conclusion, exercise training improved physical capacity and sweating function so that body temperature was more easily regulated during and after exercise. This may due to improved production of specific cytokine and chemokine in sweating during exercise.     

Thermoregulatory responses to heat and exercise in Japanese and Caucasians

Responses to heat and exercise were studied in 9 male Japanese subjects who walked on a treadmill at a speed of 4.4 – 4.8 km/h at 0 grade for 2 hours in a climatic chamber in July 1973, in Nagoya Japan. The results were compared with those obtained in a similar study made in July 1966 in Cincinnati, Ohio. The following results were obtained: (1) Japanese showed a 1.8 times higher rate of sweating than Caucasians. Total sweat from the whole body during 2 hours walk was also higher in Japanese. (2) Japanese exhibited lower chloride concentration in local sweat than Caucasians in spite of their higher dietary salt intake, higher serum chloride concentration and higher rate of sweating. While in Caucasians the sweat chloride concentration showed a tendency to continue to rise during the later period of the walk in spite of decreasing sweat rate after sweat suppression occurred, in Japanese it tended to fall in parallel with the sweat rate. No difference was observed in the length of the latent time of sweat suppression. (3) There were no differences in rectal temperature or heart rate, both at the period of equilibrium rectal temperature and at the end of the walk. (4) Mean skin temperature during the walk was significantly higher in Japanese than in Caucasians. It was concluded that the Japanese group was better heat acclimatized than Caucasians, though the two groups were considered to have been naturally heat exposed by season to the same extent.     

MK801 impairs thermoregulation in the heat

The effects of MK801 (dizocilpine), a glutamate NMDA receptor antagonist, on thermoregulation in the heat were studied in awake rats exposed to 40 degrees C ambient temperature until their body core temperature reached 43 degrees C. Under these conditions, MK801-treated rats exhibited enhanced locomotor activity and a steady rise in body core temperature, which reduced the heat exposure duration required to reach 43 degrees C. Since MK801-treated rats also showed increased striatal dopaminergic metabolism at thermoneutrality, the role of dopamine in the MK801-induced impairment of thermoregulation in the heat was determined using co-treatment with SCH23390, a dopamine D1 receptor antagonist. SCH23390 normalized the locomotor activity in the heat without any effect on the heat exposure duration. These results suggest that the MK801-induced impairment of thermoregulation in the heat is related to neither a dopamine metabolism alteration nor a locomotor activity enhancement.     

Nitric oxide and thermoregulation during exercise in the horse

Mills, Paul C., David J. Marlin, Caroline M. Scott, andNicola C. Smith. Nitric oxide and thermoregulation during exercise in the horse. J. Appl. Physiol. 82(4):1035-1039, 1997.The effect of inhibition of nitric oxideproduction on sweating rate (SR) and on core, rectal, and tail skintemperatures was measured in five Thoroughbred horses during exerciseof variable intensity on a high-speed treadmill. A standard exercisetest consisting of three canters [~55% maximumO₂ uptake(O_{2 max})], withwalking (~9%O_{2 max}) and trotting(~22% O_{2 max})between each canter, was performed twice (control or test), in randomorder, by each horse.N^G-nitro-L-arginine methyl ester(L-NAME; 20 mg/kg), acompetitive inhibitor of nitric oxide synthase, was infused into thecentral circulation and induced a significant reduction in the SRmeasured on the neck (31.6 ± 6.4 vs. 9.7 ± 4.2 g · min¹ · m²;69%) and rump (14.7 ± 5.2 vs. 4.8 ± 1.6 g · min¹ · m²;67%) of the horses during canter (P < 0.05). Significant increases in core, rectal, and tailskin temperatures were also measured (P < 0.05).L-Arginine (200 mg/kg iv)partially reversed the inhibitory effects ofL-NAME on SR, but core, rectal,and tail skin temperatures continued to increase(P < 0.05), suggesting a cumulationof body heat. The results support the contention that nitric oxidesynthase inhibition diminishes SR, resulting in elevated core andperipheral temperatures leading to deranged thermoregulation duringexercise. The inhibition of sweating byL-NAME may be related toperipheral vasoconstriction but may also involve the neurogenic controlof sweating.

     

Effect of beta-adrenergic blockade on thermoregulation during exercise

To resolve conflicting reports concerning the effects of beta-blockade (BB) on thermoregulatory reflexes during exercise, we studied six fit men during 40 min of cycle ergometer exercise at 60% maximum O2 consumption at ambient temperatures of 22 and 32 degrees C. Two hours before exercise, each subject ingested a capsule containing either 80 mg of propranolol or placebo in single-blind fashion. Heart rate at 40 min of exercise was reduced (P less than 0.01) from 125 to 103 beats min at 22 degrees C and 137 to 104 beats min at 32 degrees C, demonstrating effective BB. After 40 min of exercise, esophageal temperature (Tes) was elevated with BB (P less than 0.05) from 37.66 +/- 0.04 to 38.14 +/- 0.03 and 38.13 +/- 0.04 to 38.41 +/- 0.04 degrees C at 22 and 32 degrees C, respectively. The elevated Tes resulted from a reduced core-to-skin heat flux at both temperatures, indicated by a reduction in the slope of the forearm blood flow (FBF)-Tes relationship, and a decrease in maximal FBF. Systolic blood pressure was decreased 20 mmHg with BB (P less than 0.01), whereas diastolic blood pressure was unchanged, reducing arterial pulse pressure (PP). Because PP was decreased and cardiac filling pressure was presumably not reduced (since cardiac stroke volume was elevated), we suggest that at least a part of the relative increase in peripheral vasomotor tone during BB was the consequence of reduced sinoaortic baroreceptor stimulation.     

Effect of estrogen supplementation on exercise thermoregulation in premenopausal women

This studyexamined the effects of 3 days of estrogen supplementation (ES) onthermoregulation during exercise in premenopausal (20-39 yr) adultwomen during the follicular phase of the menstrual cycle. Subjects (11 control, 10 experimental) performed upright cycle ergometer exercise at60% of maximal O₂ consumption ina neutral environment (25°C, 30% relative humidity) for 20 min. Subjects were given placebo (P) or -estradiol (2 mg/tablet, 3 tablets/day for 3 days). All experiments were conductedbetween 6:30 and 9:00 AM after ingestion of the last tablet. Heartrate, forearm blood flow (FBF), mean skin temperature, esophagealtemperature (T_es), and forearmsweat rate were measured. Blood analysis for estrogen and progesteronereflected the follicular phase of the menstrual cycle. MaximalO₂ consumption (37.1 ± 6.2 in P vs. 38.4 ± 6.3 ml · kg¹ · min¹in ES) and body weight-to-surface area ratio (35.58 ± 2.85 in P vs.37.3 ± 2.7 in ES) were similar between groups. Synthesis of 70-kDaheat shock protein was not induced by 3 days of ES. Neither thethreshold for sweating (36.97 ± 0.15 in P vs. 36.90 ± 0.22°C in ES), the threshold for an increase in FBF (37.09 ± 0.22 in P vs. 37.17 ± 0.26°C in ES), the slope ofsweat rate-T_es relationship (0.42 ± 0.16 in P vs. 0.41 ± 0.17 in ES), nor the FBF-T_es relationship (10.04 ± 4.4 in P vs. 9.61 ± 3.46 in ES) was affected(P > 0.05) by 3 days of ES. Weconclude that 3 days of ES by young adult women in the follicular phaseof their menstrual cycle have no effect on heat transfer to the skin,heat dissipation by evaporative cooling, or leukocyte synthesis of70-kDa heat shock protein.

     

Plasma volume expansion in rats: effects on thermoregulation and exercise

Administration of polyethylene glycol (PEG, intraperitoneal, 3 ml, 30% solution) to adult male rats (300 g) resulted in an approximately 20% increment in plasma volume (PV) 24 h after PEG injection. When these animals were exercised (9.14 m/min, level treadmill) in a warm (30 degrees C, 30-40% relative humidity) environment, their mean endurance was increased from 67.9 (saline-treated controls, CONT) to 93.6 min (P less than 0.01). Total water loss was increased from 12.2 (CONT) to 17.2 g (PEG, P less than 0.01). Atropine administration (ATR, 200 micrograms/kg, tail vein) significantly (P less than 0.05) reduced both the endurance and the salivary water loss of CONT and PEG-treated rats, whereas it increased the heating rate (P less than 0.01) of both groups. PEG treatment reduced (P less than 0.01) the hematocrit and circulating protein levels both before and subsequent to exercise in the warm environment. Clinical chemical indexes of heat/exercise injury were generally unaffected by pharmacological intervention, whereas clinical chemical responses to exercise were related to the endurance time of each group. We concluded that expansion of PV by PEG provided significant beneficial effects on performance and thermoregulation during exercise in a warm environment.