Acclimation to humid heat lowers resting core temperature
MICHAEL J. BUONO, JAY H. HEANEY, AND KATHERINE M. CANINE
Departments of Biology and Exercise and Nutritional Sciences,
San Diego State University, San Diego 92182; and Naval Health Research Center,
Applied Physiology Division, San Diego, California 92186
heat storage; high wet-bulb temperature; temperature regulation; resting body temperature
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 (Tre ) 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 Tre (21).
In light of the above, the purpose of this study was to
test the hypothesis that a reduction in resting Tre is
associated with the attenuation in ending Tre during
heat exposure following acclimation to humid heat.
METHODS
The subjects for this study were nine male volunteers with
a mean (6SD) age, height, weight, percent body fat, and body
surface area of 23 6 2 yr, 178 6 9 cm, 78.8 6 8.1 kg, 15 6 6%,
and 1.96 6 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 heatacclimation 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 Tre. Skin thermistors (Yellow Springs Instruments
series 400) were taped to the right shoulder, chest, thigh, and
calf. Mean skin temperature (Tsk ) 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 5 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 Tre was defined as five
consecutive 1-min data points within 0.1°C. Resting Tre 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 5 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 require-
0363-6119/98 $5.00 Copyright r 1998 the American Physiological Society
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Buono, Michael J., Jay H. Heaney, and Katherine M.
Canine. Acclimation to humid heat lowers resting core
temperature. Am. J. Physiol. 274 (Regulatory Integrative
Comp. Physiol. 43): R1295–R1299, 1998.—The purpose of
this study was to test the hypothesis that a reduction in
resting rectal temperature (Tre ) is partially responsible for
the attenuation in the rise of core temperature during heat
exposure following acclimation to humid heat. Nine male
volunteers completed 7 days of acclimation, performing 2 h of
exercise per day in a hot, humid environment (35°C, 75%
relative humidity). Mean (6SD) ending Tre significantly (P ,
0.05) decreased from 38.9 6 0.5°C on day 1 to 38.3 6 0.4°C on
day 7. Likewise, mean (6SD) resting Tre significantly (P ,
0.05) decreased from 37.0 6 0.3 to 36.7 6 0.4°C. In fact, all
nine men showed a decrease in resting Tre from day 1 to day 7,
ranging from 20.1 to 20.5°C. In addition, resting Tre and
ending Tre were significantly correlated (r 5 0.68). However,
the mean increases in Tre (ending Tre minus resting Tre ) and
heat storage that occurred on each of the 7 acclimation days
were not significantly different. These results support the
hypothesis that a reduction in resting Tre is partially responsible for the attenuation in ending Tre during heat exposure
following short-term acclimation to humid heat.
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HEAT ACCLIMATION REDUCES RESTING TEMPERATURE
ment was allowed ad libitum. No subject lost more than 2% of
their body weight during any heat exposure.
During the heat-acclimation sessions, HR, Tre, and Tsk data
were collected each minute. The increase in Tre (DTre ) that
occurred during each of the seven acclimation sessions was
calculated by subtracting the resting Tre from the Tre at the
end of the final exercise bout. DS, in watts per square meter,
was calculated for each 2-h acclimation session using the
formula (3) DS 5 0.965 3 body mass (kg) 3 (0.8 · DTre 1
0.2 · DTsk )/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
Fig. 2. A: mean (6SD) 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 5 9 subjects. B: mean (6SD) heat storage on each of the 7 days of
heat acclimation. NS indicates that no significant difference was
found across the 7 values; n 5 9 subjects.
day 1 was 1.9 6 0.3°C and was 1.6 6 0.5°C on day 7, a
change not significantly (P . 0.05) different over days.
The mean DS for each acclimation day is shown in
Fig. 2B. On day 1, the mean DS was 89 6 15 W/m2,
whereas on day 7 it was 79 6 23 W/m2. The change in
DS during acclimation was not significant (P . 0.05).
Furthermore, mean (6SD) ending HR decreased significantly (P , 0.05) from 143 6 16 to 129 6 10 beats/min
on days 1 and 7, respectively.
DISCUSSION
Fig. 1. Mean (6SD) resting and ending rectal temperature on each of
the 7 days of heat acclimation; n 5 9 subjects. * Significant (P , 0.05)
difference was found compared with day 1.
The purpose of this study was to test the hypothesis
that a reduction in resting Tre is partially responsible
for the attenuation in ending Tre 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 Tre from 38.9 to 38.3°C. The change in ending Tre
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The mean (6SD) resting and ending Tre data collected on the 7 heat-acclimation days are presented in
Fig. 1. As expected, during acclimation, the ending Tre
significantly (P , 0.05) decreased from 38.9 6 0.5°C on
day 1 to 38.3 6 0.4°C on day 7. The change in ending Tre
between days 6 and 7 was not significant. Figure 1 also
shows that the mean resting Tre significantly decreased
during acclimation from 37.0 6 0.3 to 36.7 6 0.4°C. In
fact, all nine subjects showed a decrease in resting Tre
from day 1 to day 7, ranging from 20.1 to 20.5°C. In
addition, resting Tre and ending Tre were significantly
correlated (r 5 0.68).
The mean DTre that occurred on each of the 7
acclimation days is presented in Fig. 2A. The DTre on
HEAT ACCLIMATION REDUCES RESTING TEMPERATURE
men for 9 days in humid heat (37°C, 74% RH) via 2 h of
treadmill walking. The mean DTre 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 DS and ending Tre 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 Tre is partially
responsible for the attenuation in ending Tre following
acclimation to humid heat. Previous studies (2, 8, 22)
have clearly shown that acclimation to dry heat decreases DS 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 Tre following acclimation to humid heat
would be a lowering of resting Tre. The findings of the
current study suggest that 50% of the reduction in
ending Tre is the result of a lowered resting Tre, 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 Tre was attributable to a
reduction in resting Tre.
The physiological benefits of having a lower resting
Tre 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 Tre. In other words,
it could be hypothesized that an absolute increase in Tre
over the resting value is needed to initiate sweating
and cutaneous vasodilation. If so, then lowering resting
Tre 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 Tre 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 Tre and the onset of
sweating. In the first study, Fox et al. (4) heat accli-
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between days 6 and 7 was not significant. The time
course and magnitude of the decrease in ending Tre 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 Tre 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 Tre following
acclimation. Although this finding agrees with several
older studies (10, 20, 24) that have anecdotally reported
a reduction in resting Tre with heat acclimation, to our
knowledge this is the first study to statistically examine the effect of heat acclimation on resting Tre using a
controlled study design. Specifically, to obtain a stable
resting Tre 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 Tre values within 0.1°C to ensure that a
stable resting Tre was obtained on each of the acclimation days.
The agreement between the resting Tre 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 Tre 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 Tre
fell from 37.4 to 37.0°C. More recently, Shvartz et al.
(20) anecdotally reported that mean resting Tre decreased 0.4°C following 8 days of heat acclimation. It
must be remembered that in all of these studies, resting
Tre 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 Tre ,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 Tre 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 DTre or DS. These results are not without
precedent in the literature. Ladell (10) reported that on
the first day of humid heat acclimation the mean DTre
was 1.3°C whereas on the ninth, and final, day it was
1.4°C. Similarly, Garden et al. (5) heat acclimated nine
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HEAT ACCLIMATION REDUCES RESTING TEMPERATURE
The views presented in this paper are those of the authors and do
not reflect the official policy or position of the Department of the
Navy, the Department of Defense, or the US Government.
Address for reprint requests: M. J. Buono, San Diego State Univ.,
Dept. of Exercise and Nutritional Sciences, San Diego, CA 92182.
Received 22 September 1997; accepted in final form 27 January 1998.
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mated 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 Tre with the sweating threshold during heat
acclimation. More recently, Shvartz et al. (20) anecdotally reported that after 8 days of heat acclimation,
mean resting Tre 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 Tre 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 Tre 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 Tre, 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 Tre. However, both
DTre and DS were not significantly affected by acclimation to humid heat. These findings support the hypothesis that a reduction in resting Tre is partially responsible for the attenuation in ending Tre 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 Tre
that occurs on a daily basis but also the change in
resting Tre.
HEAT ACCLIMATION REDUCES RESTING TEMPERATURE
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