Hypohydration impairs endurance exercise performance in temperate but not cold air
Samuel N. Cheuvront1, Robert Carter1, III, John W. Castellani1, Michael N. Sawka1
1U.S. Army Research Institute of Environmental Medicine, Natick, MA
Hypohydration and performance in temperate and cold air
Samuel N. Cheuvront, PhD
U.S. Army Research Institute of Environmental Medicine
Thermal and Mountain Medicine Division
Natick, MA 01760-5007
Articles in PresS. J Appl Physiol (July 14, 2005). doi:10.1152/japplphysiol.00329.2005
Copyright © 2005 by the American Physiological Society.
This study compared the effects of hypohydration on endurance exercise performance
in temperate and cold air environments. On four occasions, 6 men and 2 women (age =
24 ± 6 yr, ht = 170 ± 6 cm, wt = 72.9 ± 11.1 kg, VO2peak = 48 ± 9 ml/kg/min) were
exposed to 3-hr of passive heat stress (45°C) in the early morning with (euhydration,
EUH) or without (hypohydration, HYP, – 3% body mass) fluid replacement. Later in the
day, subjects sat in a cold (2°C) or temperate (20°C) environment with minimal clothing
for 1-hr before performing 30-min of cycle ergometry at 50%VO2peak followed
immediately by a 30-min performance time trial. Rectal (Tre) and mean skin
temperatures (Tsk), heart rate (HR), and RPE measurements were made at regular
intervals. Performance was assessed by the total amount of work (kJ) completed in the
30 min time trial. Tsk was significantly lower in the cold compared with the temperate
trial, but there was no independent effect of hydration. Tre in both HYP trials was higher
than EUH after 60-min of exercise, but the difference was only significant within the
temperate trials (P<0.05). HR was significantly higher at 30-min within the temperate
trial (HYP > EUH) and at 60-min within the cold trial (HYP > EUH) (P<0.05). RPE
increased over time with no differences among trials. Total work performed during the
30-min time trial was not influenced by environment, but was less (P<0.05) for HYP than
EUH in the temperate trials. The corresponding change in performance (EUH – HYP)
was greater for temperate (-8%) than for cold (-3%) (P<0.05). These data demonstrate
that 1) hypohydration impairs endurance exercise performance in temperate but not
cold air, but 2) cold stress per se does not.
Keywords: dehydration, thermoregulation, environment
Persons exercising in cold weather can incur substantial fluid losses (6) and are
commonly advised to maintain hydration to avoid cold injury and sustain performance.
Despite these assertions, recent research shows that hypohydration (reduced body
water) does not increase the risk of hypothermia or peripheral cold injury (23, 24).
Similarly, hypohydration in excess of 2% body mass impairs endurance exercise
performance in hot and temperate environments with the magnitude of effect largest in
the heat (4, 30). The extent to which this is true in cooler environments is unknown, but
there is evidence that the mechanisms for hypohydration-mediated fatigue in warmer
environments are blunted in the cold and may therefore have less impact.
Hyperthermia and cardiovascular strain are two major factors implicated in the genesis
of hypohydration-mediated endurance exercise fatigue in hot and temperate
environments (4). Both the independent and combined effects of hyperthermia and
hypovolemia on cardiovascular strain dynamics in the heat have been elegantly
described (9 – 11, 28). Recent examination of the same parameters during exercise in
cooler environments (3 – 8°C) indicates that core temperature elevations associated
with hypohydration are significantly reduced (9, 17). Tachycardia is also attenuated and
stroke volume and cardiac output better preserved during progressive dehydration up to
– 4% of body mass during both moderate (50%VO2max) (17) and more intense
(72%VO2max) (9) exercise-cold stress. Taken together, cardiovascular strain attributed
to hyperthermia and hypovolemia in warm and hot climates is blunted in cooler
conditions, which may preserve endurance exercise performance.
Although multiple meteorological variables can influence endurance exercise success
(32), performances typically improve as air temperatures decline (32, 33). Laboratory
(7) and field data (32, 33) support an “optimal” air temperature threshold near 12°C;
above or below this temperature, performance is relatively impaired (7). Endurance
performance limitations in hot environments are well documented (21, 31), but evidence
during exercise-cold stress is complicated by the comparison reference temperature (7),
wearing heavily insulated clothing (25), and possibly the choice of an open-ended
endurance exercise task (1, 7, 25). The best explanation for fatigue offered by cold
performance studies also implicates factors other than cardiovascular strain or oxygen
uptake as performance limiting. Competitive endurance athletes appear to perform at a
high level coincident with significant body water losses in cool environments (4), but few
studies have manipulated hydration state to experimentally compare the impact of
hypohydration on performance in cold versus more temperate conditions. Those that
have are difficult to interpret due to the absence of true control conditions (18) or
wearing heavily insulated clothing (27).
The purpose of this study was to compare the effects of hypohydration on endurance
exercise performance in temperate and cold air. Our hypothesis was that cold air would
mitigate the decrement in performance attributable to hypohydration in a temperate
environment. A combination of air motion and low air temperatures was used to induce
cold stress beyond previous studies, but without exceeding cold injury thresholds. In
addition, a close-ended exercise task was selected to reduce the influence of cold
stress tolerance on performance.
Eight healthy volunteers (age = 24 ± 6 yr, ht = 170 ± 6 cm, wt = 72.9 ± 11.1 kg, body fat
22 ± 6 %) participated in this study and completed all phases of experimentation.
Subjects (6 men, 2 women) were physically active and moderately fit (VO2peak = 48 ± 9
ml/kg/min). Subjects were provided informational briefings and gave voluntary and
informed written consent to participate. Investigators adhered to AR 70-25 and
USAMRMC Regulation 70-25 on the use of volunteers in research and the appropriate
Institutional Review Boards approved this study.
Each subject’s VO2peak was measured using an incremental cycle ergometer protocol
with continuous gas exchange measurements (TrueMax, ParvoMedics, Sandy, Utah).
The calculated workload at 50% VO2peak was validated during 30-min of steady-state
cycling one day later. The ergometer used (Lode Excalibur Sport, Lode, Groningen,
The Netherlands) allows pedal-rate independent (hyperbolic) and dependent (linear)
modes of cycling. Individual linear factors (LF) were calculated [W = LF × (rpm)2] for
each subject to reflect a 50% VO2peak exercise intensity at a pedal cadence of 60 rpm.
The linear factor setting provided room to increase work output during the time trial
before reaching maximal sustainable workloads, which were estimated from VO2peak
testing at ~100 rpm. Practice trials included 30-min of steady-state cycling (50%
VO2peak), followed immediately by a 30-min performance time trial. Three practice
sessions were used to reduce training and learning effects (8, 13). Elapsed time was
displayed and the total work (kJ) completed in 30-min was given as feedback for
motivation to improve with each subsequent practice ride. Experimental test scenarios
were the same as those used in practice except that subjects were blinded to all test
parameters but elapsed time. Semi-nude body mass (shorts only) was measured after
voiding and before breakfast each morning for 10-days to establish a normal individual
baseline body mass for euhydration assessment on test days. All experimentation
began within 3-days of completing preliminary procedures.
A counterbalanced 2 x 2 (hydration x environment) experimental design was employed.
Each was separated by at least 48-hrs. Experiments were conducted at the same time
of day and women were tested in the follicular phase of their menstrual cycle to control
for circadian and ovulatory fluctuations in body temperature. On the morning of each
trial, body mass was measured with an electronic precision balance scale (Toledo 1D1
accuracy ± 20 g, Worthington, OH) for comparison against within subject 10-day
averages, and a 10-ml venous blood sample was collected for serum osmolality
determination. A standardized breakfast was provided, after which subjects rested in a
seated position for ~1-hr before moving to a hot room (45°C, 50%rh, 1 m/s air speed)
for 3-hrs of passive heat exposure with (euhydration, EUH) or without (hypohydration,
HYP) fluid replacement. A 2-hr recovery period followed in which a shower was
permitted and a small snack was provided (200 ml water and 250 kcal). The precise
fluid deficit incurred was calculated from the acute change in body mass from pre to
post heat exposure, corrected for snack, and expressed as a percentage of pre-
exercise body mass.
In the afternoon, subjects sat in a cold (2°C, 50%rh, 2.2 m/s air speed) or temperate
(20°C, 50%rh, 1 m/s air speed) environment with minimal clothing (t-shirt, shorts, socks,
shoes, cotton gloves and head band) for 1-hr before performing 30-min of cycle
ergometry at 50%VO2peak followed immediately by a 30-min performance time trial. No
motivation was provided during the time trial and subjects performed without distraction
from any data collection measurements. Time trial performance was assessed by the
total amount of work (kJ) completed in 30-min. Rectal (Tre) and mean skin temperatures
(Tsk) (26) and heart rate (HR) were collected remotely at regular intervals throughout
testing. RPE was assessed at 30-min and again immediately following the completion
of exercise. Gas exchange measurements were made once in the initial 10-min of
exercise using an automated system and workloads adjusted to reflect a 50%VO2peak
Following tests for normality of distribution and equality of variances, treatment effects
were analyzed using a paired t-test, one-way or two-way ANOVA for repeated
measurements. A one-sample t-test was also used to compare performance effects
against a hypothetical value of importance (20). When appropriate, Tukey’s HSD
procedure was used to identify pairwise differences among means following significant
main and/or interaction effects. The primary outcome variable of interest in this
experiment was time trial performance. An analysis selecting conventional α (0.05) and
β (0.20) parameters showed that 8 subjects would provide sufficient power to detect a
5% change in time trial performance (~15 kJ) using the mean total work (295 kJ) and
coefficient of variation (CV = 2.5%) calculated from trials of negligible difference (i.e.,
practice trials 2 and 3, P>0.05) during two weeks of time trial practice. The desire to
detect a two-fold change from the %CV was chosen based on the likelihood of
experimental perturbations producing unique performance infidelity (12, 13, 15), thus
increasing variability. A sample size of 8 allows detection of said differences with a CV
up to 4.5%. The practical importance of hydration effects on performance within
environments was also interpreted using 95% confidence limits of the true effect for %
change in performance to include comparison against an a priori zone of indifference
(2.5% coefficient of variation) using a one-sample t-test, which affords evaluation
against an evidentiary standard other than zero (5, 14, 20). The spirit of this approach,
most closely related to equivalence testing in the clinical sciences (5), has recently been
championed as a performance interpretation tool for the exercise sciences (14).
Graphical data are presented with unidirectional error bars and may be slightly
juxtaposed for presentation clarity. All data are presented as means ± SD except where
Hydration. Euhydration was estimated on the morning of each trial by a body mass
within 1% of the average 10-day baseline (3). Two subjects > 1% lower than 10-day
baseline were given additional water with breakfast. Serum osmolality (289 ± 1
mOsmol/kg) confirmed euhydration (16). The fluid deficit achieved before the start of
each HYP trial was – 2.9 ± 0.7% and – 3.0 ± 0.8% of body mass for Cold and Temp,
respectively. Values for EUH trials were – 0.3 ± 0.6 (Cold) and – 0.4 ± 0.7% (Temp) of
starting baseline. Differences were significant (P<0.05) between hydration levels (HYP
vs. EUH), but not between environments (Cold vs. Temp). Thus, subjects were
adequately matched for pre-exercise hydration status in EUH and HYP trials.
Table 1 presents individual and mean time trial performance data. Total work in Temp
HYP was lower than Temp EUH (P = 0.012). There was no effect of hydration in the
Cold (Cold EUH vs. Cold HYP) and no independent effect of environment on
performance (Cold EUH vs. Temp EUH). Associated mean power outputs from Table 1
were 140 ± 30 (Temp HYP), 152 ± 30 (Temp EUH), 154 ± 36 (Cold EUH), and 150 ± 35
(Cold HYP) W. Viewed individually, all 8 subjects performed worse when hypohydrated
in temperate air, while only 5 of 8 experienced the same from hypohydration in the cold.
Figure 1 presents the % change in performance from EUH to HYP in Temp and Cold
environments. The change was significantly larger for Temp (-7.6 ± 5.9%) than Cold (-
2.7 ± 4.9%) (P = 0.021). The means and 95% confidence limits for performance (-12.6
to -2.7% Temp; -6.8 to 1.4% Cold) provide the likely range of the true change effects
and illustrate why there is a difference between HYP and EUH within Temp, but not
within Cold (i.e., confidence interval crosses zero for Cold). In addition, only the range
of the confidence interval for Temp falls entirely outside the a priori zone of indifference
(P = 0.04, one-sample t-test), which provides evidence that the negative effect of
hypohydration on performance in temperate air is also of practical importance.
Metabolic Rate. Metabolic rates during the initial 30-min of cycling were calculated from
a 2-min gas sample made 5-min into exercise and adjusted to reflect ~50% of VO2peak.
All measurements were similar (P>0.05) at 49 ± 6 (Cold EUH), 51 ± 5 (Cold HYP), 47 ±
2 (Temp EUH) and 48 ± 3% (Temp HYP) VO2peak. Subjects were therefore matched
among trials for exercise intensity preceding the cycling time trial.
Cardiovascular Strain and Thermoregulatory Strain. Figure 2 A and B represent HR
and perceived exertion responses to exercise at 30 and 60-min of exercise. Data
collected at rest were unreliable due to extreme shivering in the cold trials and were
therefore excluded from the analysis. All 60-min HR exceeded 30-min values (P <
0.05). HR for Temp HYP at 30-min was higher than for Temp EUH and Cold HYP.
Both Temp HYP and Cold HYP were higher than Temp EUH (~5 b/min, P>0.05) and
Cold EUH (~11 b/min, P<0.05) at 60-min. RPE increased over time with no differences
among trials (60-min > 30-min, P<0.05). Tre increased significantly over time in all trials
(Figure 3A). At rest, Tre was higher in both Cold compared to Temp trials due to
rigorous shivering. No differences among trials were seen at 30-min, but Temp HYP
was higher than Cold EUH (0.4°C, P<0.05) and Temp EUH (0.3°C, P<0.05) at exercise
cessation. Tsk was significantly lower in the Cold (Fig 3B) and was independent of
This study determined the effects of hypohydration on endurance exercise performance
in temperate and cold air. In accordance with our hypothesis, the principal finding of
this study is that hypohydration by – 3% body mass impaired cycling time trial
performance in a temperate, but not a cold, environment. In addition, we found that
cold stress per se did not reduce performance.
The % change in performance (EUH – HYP) within cold (-2.7 ± 4.9%) and temperate (-
7.6 ± 5.9%) environments was statistically different (Figure 1). The 95% confidence
limits were plotted about the mean to provide insight into the likely range of the true
change value (Figure 1). These limits were also applied in the traditional sense to
examine the importance of the change relative to an evidentiary standard other than
zero (5, 14, 20). This standard is the zone of indifference selected a priori as any value
within the typical noise of the performance measurement (i.e., 2.5% coefficient of
variation) (5, 14). While the choice of 95% confidence limits for this integrated analytic
approach is admittedly conservative (14, 15), the fact that the entire Temp confidence
interval lies outside this zone (Figure 1) strongly supports the conclusion that the
performance impairment due to hypohydration in temperate environments is both
statistically significant and of practical importance (5, 14, 20). No statistical difference in
performance was observed between EUH and HYP in cold air, but since half of the Cold
interval lies outside the zone of indifference, the meaning of this effect is ambiguous at
best (5, 14, 20).
The preservation of endurance performance in cold air when hypohydrated may be
explained by differences in cardiovascular and oxygen uptake dynamics. Although the
present experiment was not designed to assess the mechanisms behind performance
changes, reasonable explanations can be gleaned from our observations when
combined with the work of others. For example, Gonzalez-Alonso et al. (9 – 11)
demonstrated that tachycardia (via hyperthermia) and hypovolemia explain most of the
reduction in cardiac output between EUH and HYP in hot environments, with similar
effects of lesser magnitude in the cold (9). In addition, hypovolemia reduces VO2max
and endurance capacity even in the presence of normothermia and cool skin (22). It is
conceivable that hypovolemia and a higher Tre (~0.30°C) and HR (~5 b/min) in Temp
HYP (Figure 2A) reduced stroke volume, cardiac output, and oxygen uptake enough to
reduce performance (253 kJ) relative to Temp EUH (273 kJ) despite similar efforts
(Figure 2B). However, the preservation of performance in Cold HYP (270 kJ) vs. Cold
EUH (277 kJ) occurred with similar Tre differences and a larger HR disparity (11 b/min)
between HYP and EUH (Figure 2A). It therefore remains possible that cold skin in Cold
HYP (Fig 3B) maintained a larger central blood volume and better preserved stroke
volume and cardiac output (9, 17, 19, 22, 29). Similar Tre and HR at exhaustion
between Cold HYP and Temp HYP also seem to support this conclusion since
performance in Temp HYP, but not Cold HYP, was less than Cold and Temp EUH
The finding that cold stress per se did not reduce performance (Temp EUH vs. Cold
EUH) (Table 1) is in opposition to others (1, 7, 25), but comparisons are made difficult
by several methodological factors. For example, Galloway and Maughan (7) found that
time to fatigue at 4°C was reduced compared with 11°C, but no different from 20°C, or
improved relative to 31°C. Patton and Vogel (25) compared -20°C and 20°C, but
wearing heavily insulated clothing at -20°C limits the interpretation of reduced
performance at the latter temperature. Both of these studies and that of Adolph and
Molnar (1) also used open-ended endurance exercise tasks. Adolph and Molnar (1)
suggested that the most important predictor of performance in the cold using this kind of
task was exposure time and cold tolerance. Indeed, Cabanac and Leblanc (2)
demonstrated that simultaneous exposure to cold and exercise fatigue produces a
sensory conflict resolved by compromise toward the least displeasing input signal, but
others (7, 25) implicate local muscle effects for accelerated fatigue under similar
circumstances. Exposure time in this experiment was fixed, which may have alleviated
motivation issues related to cold tolerance. Post-experiment interviews even suggest
that total work in Cold EUH and Cold HYP may have actually been augmented by cold
avoidance (2). It is inconclusive whether using time to exhaustion, rather than a time
trial, would (1, 25), or would not (7) have altered the performance outcomes observed
herein between 2°C and 20°C.
We conclude that moderate hypohydration impairs endurance performance in
temperate, but not cold, air. Cold stress per se had no effect. Application of these
findings to competitive endurance exercise contested in environments similar to those
described herein is logical (13, 14), but tentative given the subject population tested.
These findings are nonetheless of phenomenological importance.
The authors wish to thank Carrie Vernieuw, Lou Stephenson, Scott Robinson, Lenny
Elliott, Walida Leammukda, Leslie Levine, Kaye Brownlee, Laurie Bronson, Lenny
Souza, Eric Lammi, Mike Durkot, and Erik Lloyd for expert technical assistance. The
opinions or assertions contained herein are the private views of the authors and should
not to be construed as official or reflecting the views of the Army or the Department of
Defense. Approved for public release: distribution is unlimited.
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