INTRODUCTION

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ONE OF THE TRADITIONAL indexes of successful heat acclimation is a lower core temperature during heat exposure compared with preacclimation values (2, 8, 10, 12, 16, 24). It is widely assumed that the primary physiological mechanism responsible for the attenuation in core temperature is a reduction in heat storage (S) via improved heat loss mechanisms. Although this is clearly true for acclimation to dry heat (2, 8, 22), the potential for improved evaporative cooling in hot, humid environments is substantially reduced (6, 8, 10, 22). Interestingly, even with these limitations, most studies (5, 10, 12, 20, 22, 24) that have acclimated humans in hot, humid conditions still report reductions in core temperature during heat exposure compared with preacclimation data.

One hypothesis not thoroughly explored is that a reduction in resting core temperature is partially responsible for the attenuation of core temperature during heat exposure following acclimation to humid heat. Three avenues of support for such a hypothesis can be found in the literature. First, several older studies (4, 5, 10) have anecdotally reported that a reduction in resting rectal temperature (T_re) occurs following heat acclimation. Second, more recent studies (13, 18) have shown that other thermoregulatory set points, such as the thresholds for the onset of sweating and cutaneous vasodilation, are reduced significantly following heat acclimation. Third, endurance training in a temperate climate, which induces some degree of heat acclimation, has been shown to decrease resting T_re (21).

In light of the above, the purpose of this study was to test the hypothesis that a reduction in resting T_re is associated with the attenuation in ending T_re during heat exposure following acclimation to humid heat.

	METHODS

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The subjects for this study were nine male volunteers with a mean (±SD) age, height, weight, percent body fat, and body surface area of 23 ± 2 yr, 178 ± 9 cm, 78.8 ± 8.1 kg, 15 ± 6%, and 1.96 ± 0.11 m², respectively. Signed informed consent was obtained before the start of data collection.

The subjects were a subsample of a larger study that examined heat tolerance in men vs. women. All of the women in the larger study were excluded from the subsample because it is well known that various phases of the menstrual cycle can significantly alter resting core temperature. Additionally, some of the men in the larger study were excluded because they missed one or more of the heat-acclimation sessions. Last, because of the known circadian rhythm in resting core temperature, men with both morning and afternoon heat-acclimation sessions were excluded.

The subjects reported to the laboratory for seven heat-acclimation sessions (within an 8-consecutive-day period) in the morning, after having refrained from exercise for at least 12 h. Furthermore, in addition to normal ad libitum daily fluid intake, they were instructed to drink one liter of water each night of the study before going to bed and another liter on awakening each morning. They were also given 0.25 liter of an electrolyte-glucose drink (Gatorade) on arrival to the laboratory each morning. The subjects gave a urine sample, and if they were dehydrated (specific gravity >1.028) they were required to drink additional fluid. Next, a thermistor (Sheridan) was inserted 15 cm beyond the anal sphincter to measure T_re. Skin thermistors (Yellow Springs Instruments series 400) were taped to the right shoulder, chest, thigh, and calf. Mean skin temperature (T_sk) was calculated using the Ramanathan (17) formula. Finally, a Polar heart watch was used to measure heart rate (HR).

After being instrumented, the subjects rested in a seated position for ~30 min in a temperate environment (air temperature = 21-23°C), during which time data were collected on a computerized system which printed the values to the nearest 0.1°C each minute. A stable resting T_re was defined as five consecutive 1-min data points within 0.1°C. Resting T_re was calculated as the mean of the five consecutive values.

After resting data collection, each subject completed four 25-min exercise bouts with 5 min of seated rest between each in a hot, humid environment [35°C, 75% relative humidity (RH), wind speed = 0 miles/h]. The exercise bouts consisted of treadmill walking at 1.34 m/s at a 3% grade or stationary cycling on a Monark ergometer at 75 W. Both modes of exercise produced absolute oxygen uptakes of ~1.2 l/min. Each subject completed two bouts of each mode of exercise, always finishing with treadmill walking. The subjects had a mandatory water intake of 0.25 liter every 30 min during each heat exposure. Water consumption over this requirement was allowed ad libitum. No subject lost more than 2% of their body weight during any heat exposure.

During the heat-acclimation sessions, HR, T_re, and T_sk data were collected each minute. The increase in T_re (T_re) that occurred during each of the seven acclimation sessions was calculated by subtracting the resting T_re from the T_re at the end of the final exercise bout. S, in watts per square meter, was calculated for each 2-h acclimation session using the formula (3) S = 0.965 × body mass (kg) × (0.8 · T_re + 0.2 · T_sk)/body surface area (m²).

Data obtained across the 7-day heat-acclimation period were statistically analyzed using repeated-measures ANOVA procedures. When significance was found with the ANOVA, a post hoc Tukey's test was used to determine which means were different. The alpha level was set at P < 0.05.

	RESULTS

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The mean (±SD) resting and ending T_re data collected on the 7 heat-acclimation days are presented in Fig. 1. As expected, during acclimation, the ending T_re significantly (P < 0.05) decreased from 38.9 ± 0.5°C on day 1 to 38.3 ± 0.4°C on day 7. The change in ending T_re between days 6 and 7 was not significant. Figure 1 also shows that the mean resting T_re significantly decreased during acclimation from 37.0 ± 0.3 to 36.7 ± 0.4°C. In fact, all nine subjects showed a decrease in resting T_re from day 1 to day 7, ranging from 0.1 to 0.5°C. In addition, resting T_re and ending T_re were significantly correlated (r = 0.68).

View larger version (20K): [in this window] [in a new window]   Fig. 1.   Mean (±SD) resting and ending rectal temperature on each of the 7 days of heat acclimation; n = 9 subjects. * Significant (P < 0.05) difference was found compared with day 1.

The mean T_re that occurred on each of the 7 acclimation days is presented in Fig. 2A. The T_re on day 1 was 1.9 ± 0.3°C and was 1.6 ± 0.5°C on day 7, a change not significantly (P > 0.05) different over days.

View larger version (46K): [in this window] [in a new window]   Fig. 2.   A: mean (±SD) change in rectal temperature (ending value minus resting value) on each of the 7 days of heat acclimation. NS indicates that no significant difference was found across the 7 values; n = 9 subjects. B: mean (±SD) heat storage on each of the 7 days of heat acclimation. NS indicates that no significant difference was found across the 7 values; n = 9 subjects.

The mean S for each acclimation day is shown in Fig. 2B. On day 1, the mean S was 89 ± 15 W/m², whereas on day 7 it was 79 ± 23 W/m². The change in S during acclimation was not significant (P > 0.05). Furthermore, mean (±SD) ending HR decreased significantly (P < 0.05) from 143 ± 16 to 129 ± 10 beats/min on days 1 and 7, respectively.

	DISCUSSION

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The purpose of this study was to test the hypothesis that a reduction in resting T_re is partially responsible for the attenuation in ending T_re during heat exposure following acclimation to humid heat. As can be seen in Fig. 1, the 7-day heat-acclimation protocol was successful, as evidenced by a significant 0.6°C decrease in ending T_re from 38.9 to 38.3°C. The change in ending T_re between days 6 and 7 was not significant. The time course and magnitude of the decrease in ending T_re and HR with heat acclimation are consistent with previous studies (5, 16, 20, 24). For example, Shvartz et al. (20) reported a 0.6°C decrease in ending T_re following 8 days of heat acclimation. Thus the above findings suggest that our subjects successfully acclimated to the heat.

Interestingly, Fig. 1 also shows that our subjects had a significant 0.3°C decrease in resting T_re following acclimation. Although this finding agrees with several older studies (10, 20, 24) that have anecdotally reported a reduction in resting T_re with heat acclimation, to our knowledge this is the first study to statistically examine the effect of heat acclimation on resting T_re using a controlled study design. Specifically, to obtain a stable resting T_re during the acclimation period we controlled the following variables: 1) hydration level, 2) 12-h exercise abstinence, 3) gender of subjects, 4) time of day for data collection, and 5) room temperature. Furthermore, we instituted a criterion of requiring five consecutive 1-min T_re values within 0.1°C to ensure that a stable resting T_re was obtained on each of the acclimation days.

The agreement between the resting T_re results of the current study and those of several older reports is remarkable. For example, over 40 years ago, Ladell (10) heat acclimated 17 men for 9 days in a hot, humid (38°C, 80% RH) environment. Mean resting T_re decreased 0.3°C over the course of the study. Approximately 25 years ago, Wyndham et al. (24) heat acclimated men for nine successive days in warm air (32°C) that was fully saturated with water vapor (100% RH). Although not specifically addressed in their original manuscript, tabular data show that mean resting T_re fell from 37.4 to 37.0°C. More recently, Shvartz et al. (20) anecdotally reported that mean resting T_re decreased 0.4°C following 8 days of heat acclimation. It must be remembered that in all of these studies, resting T_re was not a primary dependent variable and thus control of potential confounding factors and statistical analysis was not performed. Even with these limitations, it is our opinion that the current results, combined with past findings, strongly suggest that heat acclimation has the potential to significantly reduce resting T_re ~0.3-0.5°C. Such a conclusion supports Kenney's observation that, "after acclimation, people seem to defend and maintain core temperature around a lower setpoint temperature" (23). Interestingly, this phenomenon does not appear to be exclusive to humans. Sato et al. (19) reported that resting T_re decreased ~0.3°C following heat acclimation in male patas monkeys.

The most surprising finding of the current study was that humid heat acclimation did not significantly decrease either T_re or S. These results are not without precedent in the literature. Ladell (10) reported that on the first day of humid heat acclimation the mean T_re was 1.3°C whereas on the ninth, and final, day it was 1.4°C. Similarly, Garden et al. (5) heat acclimated nine men for 9 days in humid heat (37°C, 74% RH) via 2 h of treadmill walking. The mean T_re on day 2 (day 1 data were not presented) was ~1.8°C, whereas it was 1.9°C on day 9. Finally, Shvartz et al. (22) heat acclimated two groups of subjects for 6 consecutive days. One group was exposed to humid (90% RH) heat while the other acclimated to dry (20% RH) heat. After heat acclimation, the physiological parameters obtained from both groups during a standardized heat-tolerance test were compared with a nonacclimated control group. The mean S and ending T_re were significantly (P < 0.05) reduced in the hot-dry acclimation group compared with the control group; however, they were not significantly different between the hot-humid acclimation group and the control group.

It is our opinion that the above findings support the hypothesis that a reduction in resting T_re is partially responsible for the attenuation in ending T_re following acclimation to humid heat. Previous studies (2, 8, 22) have clearly shown that acclimation to dry heat decreases S via increased evaporative heat loss. However, the potential for improved evaporative cooling is significantly reduced in humid conditions (6, 8, 10, 22), particularly when the required evaporative cooling exceeds the maximal evaporative cooling capacity of the environment. For example, it has been shown that changes in sweat rate account for only 10% of the variability in mean body temperature during heat acclimation (20). Furthermore, convective and radiative heat loss is unchanged following acclimation to humid heat (6). Thus, by process of elimination, the only compensatory mechanism left to produce an attenuation of ending T_re following acclimation to humid heat would be a lowering of resting T_re. The findings of the current study suggest that 50% of the reduction in ending T_re is the result of a lowered resting T_re, and 50% is due to increased heat loss (i.e., decreased S). This agrees with the results of Shvartz et al. (22), who found that after endurance training approximately one-half of the decrease in exercise T_re was attributable to a reduction in resting T_re.

The physiological benefits of having a lower resting T_re may be twofold in nature. First, it has been shown (4, 6, 13, 18) that the thermoregulatory thresholds for the onset of sweating and cutaneous vasodilation are typically reduced by ~0.3-0.5°C following heat acclimation. Coincidentally, this magnitude of reduction is similar to that reported for resting T_re. In other words, it could be hypothesized that an absolute increase in T_re over the resting value is needed to initiate sweating and cutaneous vasodilation. If so, then lowering resting T_re should reduce the thresholds for these thermoregulatory effectors by a similar magnitude. Support for this hypothesis is found in the two studies (4, 20) that measured both resting T_re and the threshold for the onset of sweating during heat acclimation. Neither of these studies, however, addresses the potential linkage between the reduction in resting T_re and the onset of sweating. In the first study, Fox et al. (4) heat acclimated 20 young men under extremely humid conditions (using a hot water bath and vapor-barrier jacket). They found that mean resting core temperature significantly (P < 0.001) decreased by 0.19°C following acclimation. Similarly, the mean threshold for the onset of sweating was significantly (P < 0.001) reduced by 0.18°C. From their original data, we have calculated the correlation between the change in resting core temperature and the change in threshold for the onset of sweating. We obtained an r of 0.67 (P < 0.002), which supports the hypothesis that there is a coupling of resting T_re with the sweating threshold during heat acclimation. More recently, Shvartz et al. (20) anecdotally reported that after 8 days of heat acclimation, mean resting T_re fell by 0.40°C while the mean threshold for sweating decreased by 0.49°C. Last, Olschewski and Bruck (15) have shown that precooling subjects before exercise reduces both resting core temperature and the threshold for both sweating and cutaneous vasodilation by ~0.2-0.3°C. Taken together, all of the above data support the concept that one of the benefits of a lower resting T_re following heat acclimation may be lower thermoregulatory thresholds for sweating and cutaneous vasodilation. It is our opinion that further work on this interesting topic is certainly warranted in the future.

Second, the lower resting T_re may simply allow an acclimated individual to exercise for a longer period of time in the heat before a critical temperature is reached (5). Specifically, precooling studies (1, 7, 11, 15) that lowered resting core temperature ~0.2-0.3°C have shown this to have a beneficial effect on exercise performance. It is believed that the beneficial effects may be the result of a reduced rate of anaerobic glycolysis, which may subsequently reduce the rate of glycogen depletion and lactic acid formation (9). Additionally, it recently has been suggested that the ultimate cause for exhaustion during exercise in the heat may be related to central nervous system dysfunction that reduces the mental drive for motor performance (1, 14). Specifically, core temperatures greater than 39°C may reduce the function of motor centers and the ability to recruit motor units (14). Thus it may be that a reduction in resting T_re, produced either by precooling or heat acclimation, simply increases the time until an absolute critical core temperature is reached and the central drive to exercise in the heat is reduced.

In conclusion, the current study found that 7 days of humid heat acclimation resulted in significant decreases in both resting and ending T_re. However, both T_re and S were not significantly affected by acclimation to humid heat. These findings support the hypothesis that a reduction in resting T_re is partially responsible for the attenuation in ending T_re during heat exposure following acclimation to humid heat. Therefore, future studies that examine heat acclimation in humans should report not only the change in ending T_re that occurs on a daily basis but also the change in resting T_re.