Influence of Hydration on Physiological Function and
Performance During Trail Running in the Heat
Douglas J. Casa, PhD, ATC, FNATA, FACSM*; Rebecca L. Stearns, MA, ATC*;
Rebecca M. Lopez, MS, ATC*; Matthew S. Ganio, PhD*; Brendon P. McDermott,
PhD, ATC*; Susan Walker Yeargin, PhD, ATC; Linda M. Yamamoto, MA*;
Stephanie M. Mazerolle, PhD, ATC*; Melissa W. Roti, PhD`;
Lawrence E. Armstrong, PhD, FACSM*; Carl M. Maresh, PhD, FACSM*
*Department of Kinesiology, University of Connecticut, Storrs; 3Indiana State University, Terre Haute; 4Movement
Science, Sport and Leisure Studies, Westfield State College, Westfield, MA. Dr Ganio is now at the Texas Health
Resources Presbyterian Hospital, Dallas. Dr McDermott is now at the University of Tennessee at Chattanooga.
Context: Authors of most field studies have not observed
decrements in physiologic function and performance with
increases in dehydration, although authors of well-controlled
laboratory studies have consistently reported this relationship.
Investigators in these field studies did not control exercise
intensity, a known modulator of body core temperature.
Objective: To directly examine the effect of moderate water
deficit on the physiologic responses to various exercise
intensities in a warm outdoor setting.
Design: Semirandomized, crossover design.
Setting: Field setting.
Patients or Other Participants: Seventeen distance runners
(9 men, 8 women; age 5 27 6 7 years, height 5 171 6
9 cm, mass 5 64.2 6 9.0 kg, body fat 5 14.6% 6 5.5%).
Intervention(s): Participants completed four 12-km runs
(consisting of three 4-km loops) in the heat (average wet bulb
globe temperature 5 26.56C): (1) a hydrated, race trial (HYR),
(2) a dehydrated, race trial (DYR), (3) a hydrated, submaximal
trial (HYS), and (4) a dehydrated, submaximal trial (DYS).
Main Outcome Measure(s): For DYR and DYS trials,
dehydration was measured by body mass loss. In the
submaximal trials, participants ran at a moderate pace that
was matched by having them speed up or slow down based on
pace feedback provided by researchers. Intestinal temperature
was recorded using ingestible thermistors, and participants
wore heart rate monitors to measure heart rate.
Results: Body mass loss in relation to a 3-day baseline was
greater for the DYR (24.30% 6 1.25%) and DYS trials (24.59%
6 1.32%) than for the HYR (22.05% 6 1.09%) and HYS
(22.0% 6 1.24%) trials postrun (P , .001). Participants ran
faster for the HYR (53.15 6 6.05 minutes) than for the DYR
(55.7 6 7.45 minutes; P , .01), but speed was similar for HYS
(59.57 6 5.31 minutes) and DYS (59.44 6 5.44 minutes; P .
.05). Intestinal temperature immediately postrun was greater for
DYR than for HYR (P , .05), the only significant difference.
Intestinal temperature was greater for DYS than for HYS
postloop 2, postrun, and at 10 and 20 minutes postrun (all: P
, .001). Intestinal temperature and heart rate were 0.226C and
6 beats/min higher, respectively, for every additional 1%
body mass loss during the DYS trial compared with the HYS
trial.
Conclusions: A small decrement in hydration status impaired
physiologic function and performance while trail running
in the heat.
Key Words: environmental physiology, dehydration, rehydration,
core temperature, heart rate
Key Points
N The physiologic and performance decrements associated with dehydration that exist in laboratory settings also exist in field settings.
N Methodologic challenges in the field setting make isolating these effects difficult.
In laboratory settings, when athletes perform intense
aerobic exercise in the heat and become hypohydrated
to a level of 2% body mass loss or greater, or when they
start to exercise at this level, physiologic strain increases
predictably and performance decreases.1–4 For example,
during exercise in the heat, core body temperature and
heart rate (HR) increase by 0.126C to 0.256C and 3 to 5
beats/min, respectively, for every 1% of body mass lost.1–4
This graded core temperature increase with increasing
water deficit indicates that physiologic adjustments are
both necessary and effective for maintaining heat loss when
sweat rate and skin blood flow are reduced. Dehydration
reduces the overall water volume in the body, resulting in a
reduction in central blood volume and, therefore, skin
blood flow.1 Dehydration initiates a cascade of events in
which blood volume decreases, causing a compensatory
increase in HR, followed by a decrease in stroke volume
due to the increased heart rate and decreased filling time
for the heart.5 Evidence for greater physiologic strain and
resultant compromised performance includes increased
HR, decreased stroke volume, thermoregulatory strain
(or the physiologic response to excessive heat production
and storage in the absence of heat dissipation), stress
response, perception of effort and anticipatory regulation
Journal of Athletic Training 2010;45(2):147–156
g by the National Athletic Trainers’ Association, Inc
www.nata.org/jat
original research
Journal of Athletic Training 147
of pace, hypovolemia, hyperosmolality, and a decrease in
percentage of total work completed, among other factors.
1,6–12
Because exercise intensity is the single greatest influence on
the rate of core temperature rise in the heat,13–15 authors of
many of these laboratory studies control intensity. Also
regulated in well-controlled studies are other variables that
could compromise consistency among trials, including
ambient temperature, humidity, length of exercise, time of
day, nutritional intake, clothing, airflow, and heat acclimatization
status. Only when these variables are controlled can
the influence of hydration status be isolated.16
In recent field studies, investigators have examined the
influence of hydration status on numerous physiologic and
performance outcomes, notably core body temperature and
finishing time. Many of these researchers17–21 reported
findings inconsistent with previous laboratory studies, in
that body temperature did not increase with dehydration.
Moreover, other performance outcomes were not related to
hydration status, and in some cases, greater dehydration
was associated with better performance.21,22
A plausible cause of these different findings is variable
intensity of exercise in field studies, rather than a lack of
research quality or the inability to control or monitor
exercise intensity. Athletes who are allowed to perform an
exercise task unsupervised may manipulate intensity based
on physiologic and psychological feedback. Thus, a
dehydrated person may actually have a lower body core
temperature because intensity has decreased.23 Similarly, a
euhydrated person can perform at a higher intensity for a
longer period of time and, ironically, may have a higher
body core temperature than a dehydrated counterpart.14
Both scenarios are possible in a field setting when intensity
is not controlled. Therefore, the premise that certain
physiologic responses (eg, body core temperature) are not
influenced by hydration status may, in fact, reflect uncontrolled
intensity and not hydration status itself. Comparing
field and laboratory findings has been made even more
difficult by other methodologic considerations, such as the
lack of crossover, within-subjects comparisons, and uncontrolled
weather conditions (eg, temperature, humidity, sun,
wind). Given that a lack of controlled intensity is the only
consistent factor permeating all the field studies in which
hydration did not contribute to physiologic strain, we felt this
was a worthy factor to examine further in the field setting.
The purpose of our study was to determine the role of
hydration status with regard to physiologic function and
performance in a field setting, using both controlled and
uncontrolled exercise intensities. We controlled absolute
intensity (ie, finishing time) in 2 submaximal trials by
providing verbal feedback to the volunteers.Wehypothesized
thatwhen finishing timewas consistent, physiologic responses
wouldbeduetodifferences inhydrationstatus, similar tothose
previouslynotedin laboratory studies.Whenintensitywasnot
consistent, as in a race, we expected to observe performance
decrements, but not necessarilymore exaggerated physiologic
responses, when athletes were more dehydrated.
METHODS
Participants
Seventeen heat-acclimatized, experienced, well-trained runners
volunteered (race trials: 9 men, 8 women; submaximal
trials: 10 men, 7 women). We excluded participants for any of
the following: chronic health problems; a history of cardiovascular,
metabolic, or respiratory disease; fever or other
current illness; age outside the range of 18 to 59 years; a
condition that could cause complications from ingesting the
Cor-Temp disposable temperature sensor (HQ, Inc,Palmetto,
FL); a history of exertional heat stroke or heat exhaustion
within the last 3 years; amusculoskeletal injury during the time
of the running trials; and any woman who was pregnant,
attempting to become pregnant, or thought she might be
pregnant. We also excluded individuals whose exercise or
physical activity consisted of less than 30 minutes per day, 4
times per week, at a moderate intensity for the past 3 months.
Participants completed a running history questionnaire and a
medical history questionnaire before enrolling in the study.
Some level of heat acclimatization was assumed based on the
running history questionnaire, which indicated that the
participants were regularly engaged in training sessions
outside in warm temperatures in the weeks leading up to data
collection. The data collection took place in the middle of
June. All participants read and signed a written informed
consent form. This study was approved by the university’s
institutional review board.
Testing Protocol
Volunteers reported to a nearby state park for familiarization
and baseline measurements. To familiarize themselves
with the 4-km course, participants completed 2
practice runs on the course, 2 to 4 weeks before the first
trial. The 4-km course consisted of 75% single-track trails
typical of New England trails (occasional rocks, roots, ruts,
turns, and low branches). Twenty-five percent of the course
was run on a hard-packed dirt road. The course had many
rolling hills and 1 brief (less than 1-minute) but steep hill
climb. To gauge an individual’s running ability, we
instructed him or her that one of the familiarization runs
must consist of running 1 loop (4 km) hard. This timed
practice run was used to group participants into 3 groups
of 4 and 1 group of 5 runners with similar abilities. This
was done so that monetary incentives for race times
(combined time for the 2 races) would be partitioned out
based on performance compared with runners of similar
ability. The experimental trials consisted of completing
four 12-km timed trail runs on 4 separate days, as
described below:
Race Trials
A. Twelve-kilometer race, beginning hydrated and receiving
fluid during the race (hydrated race 5 HYR)
B. Twelve-kilometer race, beginning hypohydrated and
not receiving fluid during the race (dehydrated race 5
DYR)
Submaximal Trials
A. Twelve-kilometer, submaximal run, beginning hydrated
and receiving fluid during the run (hydrated
submaximal 5 HYS)
B. Twelve-kilometer, submaximal run, beginning hypohydrated
and not receiving fluid during the run
(dehydrated submaximal 5 DYS)
148 Volume 45 N Number 2 N April 2010
Water (400 mL) was provided at the end of the first
(4 km) and second loops (8 km) for the 2 hydrated trials.
Data collection always preceded fluid replacement for these
trials. The 4 trials took place across a 14-day period. Days
1 and 14 were ‘‘all-out’’ races (HYR and DYR). Days 4
and 7 were submaximal runs (HYS and DYS). All
participants were randomly assigned to either a hydrated
or a hypohydrated protocol for each trial; approximately
half were randomly assigned to HYR and the others to
DYR. They then followed the opposite hydration protocol
for the second race. The submaximal trial followed the
same procedure. All volunteers received a calibrated scale
(model BWB-800 A; Tanita Corporation, Tokyo, Japan) to
record body mass measurements for 17 consecutive days.
They took their own nude body mass measures each
morning for the 3 days before the first trial, which served as
a baseline weight, and every day until the last trial. For the
submaximal effort, we explained to the participants that we
wanted them to run a pace they considered ‘‘moderate’’
effort. They knew this would be at an intensity that was not
maximal and something they could match again a few days
later in the other submaximal trial. We informed participants
that the key component of the submaximal trials was
an attempt to match finishing time for both trials.
The day before each trial, all individuals were randomly
assigned to either the hydrated or hypohydrated group and
were informed of their assignment. Those in the hypohydrated
groups (DYR and DYS) were instructed to start
fluid restriction 22 hours before their individual start times
(start times were approximately 1:00 PM). Those in the
hydrated groups (HYR and HYS) were allowed to
consume fluids ad libitum. All participants performed a
typical easy training run (60-minute run or 90 minutes of
jogging and walking/hiking) after 3:00 PM the day before
the trial. All consumed the same dinner the night before
each trial and the same breakfast and snack each morning
of the 4 testing days. They ingested an ingestible thermistor
before breakfast, which was 5 hours, on average, before
testing began. Those in the hypohydrated groups (DYR
and DYS) drank 200 mL of water when consuming the
ingestible thermistor.
Before the start of each trial, participants completed the
Profile of Mood States (POMS) questionnaire24 and a 56-
question Environmental Symptoms Questionnaire (ESQ)25
while sitting in the shade. We then took the following
baseline measurements: body mass, body temperature
(TGI), HR, rating of perceived exertion (RPE), urine color,
urine specific gravity (USG), urine osmolality (Uosm),
perceived thirst,26 and perceived thermal sensations.27
After baseline measurements, volunteers began trail runs
individually, with 4-minute intervals separating start times.
Each participant’s start time was consistent for the first 3
trials. Start times for the fourth trial were advanced by 1
hour because of predicted higher temperatures for the day;
however, the separations between start times remained the
same. After running 4 km and 8 km (loops 1 and 2),
participants had a mandatory 4-minute break, during
which TGI, HR, RPE, perceived thirst, and perceived
thermal sensations were measured. To replicate the pace
performed during the first submaximal trial on the second
submaximal trial, we provided feedback at 4 evenly spaced
points (every 1 km) along each loop. If at any point during
a trial an individual needed to urinate, urine was collected
in a jug, measured, and calculated into the participant’s
body mass loss and sweat rate. At the conclusion of each
trial, immediate postrun measurements consisted of body
mass, TGI, HR, perceived thirst, perceived thermal
sensation, and blood lactate. Ten minutes after the run,
HR and TGI were measured and the POMS and ESQ were
completed. Twenty minutes after the run, TGI, HR,
perceived thirst, and perceived thermal sensation were
measured. Participants in the hypohydrated trials remained
on site until they were rehydrated to within 2% of baseline
body mass measures. All volunteers received monetary
compensation for their participation, and a portion of the
payment was an incentive based on performance, as noted
above.
Instrumentation
The wet bulb globe temperature was measured every
20 minutes. The value for each individual’s trial was
calculated using the wet bulb globe temperature measured
during that time. The average value for each person’s trial
was then compared with other trials to identify differences.
Each participant’s percentage of body fat was calculated
using 3-site skin-fold measurements (following rehydration
after the last trial).28 Duplicate measures (Uosm, blood
lactate) were averaged. Percentage of dehydration at each
time point was calculated by subtracting weight from the 3-
day average baseline weight and dividing by the 3-day
baseline and multiplying by 100. Sweat rate was calculated
by the following formula: [(pretest body mass 2 posttest
body mass) + fluid consumed 2 urine output]/time. The
HR was measured using monitors (model E40; Polar
Electro, Lake Success, NY). Urine color was determined by
a urine color chart.29 Urine osmolality was determined via
freezing-point depression using an osmometer (model
3DII; Advanced Instruments, Inc, Needham Heights,
MA). Blood lactate was measured using a lancet device
(Accu-Chek Softclix, F. Hoffmann-LaRoche Ltd, Basel,
Switzerland) and a portable lactate analyzer (Accutrend
Lactate, Sports Resource Group, Inc, Hawthorne, NY).
Profile of Mood States scores were calculated by
entering participants’ responses into an electronic POMS
scoring system (version 6.6; Education and Industrial
Testing Service, Massachusetts Institute of Technology,
Cambridge, MA). Changes in POMS scores were calculated
by subtracting the pretest value from the posttest value.
Statistical Analysis
Montain and Coyle1 demonstrated an increase of 0.156C
for every additional 1% level of dehydration. Based on our
prediction of a 4% difference in dehydration at the end of
the race, the difference would be about 0.66C. According to
our experience in previous studies, predicted mean core
temperatures of 38.66C and 39.26C, with an a level of .05
and a power of 0.8, would require a minimum sample size
of 16.
Race and submaximal trials were analyzed as separate
data sets. A 2-way, repeated-measures analysis of variance
(hydration status 3 time) was used to test for differences
between trials and across time. Greenhouse-Geisser corrections
were conducted when the assumption of sphericity
was violated. We used a Bonferroni correction with post
hoc t tests to determine pairwise differences in the event of
Journal of Athletic Training 149
a significant F ratio. Statistical analysis for pretest and
posttest values during trials was performed using a pairedsamples
t test with a Bonferroni correction. The effect of
hydration status on body core temperature and HR
immediately postrun and 10 minutes postrun was calculated
by the following formula:
[(Absolute difference in HR) or
(core temperature difference between hydrated and
dehydrated trials)]
4 (Absolute difference in dehydration level between
hydrated and dehydrated trials)
5 (Change in HR or core temperature for each 1% body
mass loss)
An a level of .05 was used to determine statistical
significance. All data analyses were performed using SPSS
(version 10.0; SPSS Inc, Chicago, IL).
RESULTS
Participant Characteristics
The 18 participants had the following characteristics: age
5 27 6 7 years, height 5 171 6 9 cm, mass 5 64.2 6 9.0 kg,
and body composition 5 14.6% 6 5.5% fat, based on 3-site
skin-fold caliper measurements. Because of external
constraints, 17 volunteers completed the submaximal
exercise trials (10 men, 7 women) and 17 completed the
race trials (9 men, 8 women). A total of 16 participants
completed both submaximal and race trials. Preliminary
statistics revealed no differences between the sexes in terms
of the main outcome variables. Therefore, the data for men
and women were combined.
Environmental Conditions
The wet bulb globe temperatures for the 2 race trials
(HYR: 25.3 6 2.16C; DYR: 27.0 6 1.56C) were similar (P
. .05). Dry bulb and wet bulb temperatures for the HYR
were 26.29 6 2.836C and 23.99 6 2.946C, and dry bulb and
wet bulb temperatures for the DYR were 28.01 6 2.836C
and 22.79 6 0.766C. The differences between the wet bulb
and dry bulb temperatures were not significant for either
trial (HYR or DYR). Similarly, the wet bulb globe
temperatures for the 2 submaximal trials (HYS: 27.1 6
1.66C; DYS: 26.9 6 1.56C) were not different (P . .05).
Dry bulb and wet bulb temperatures for the HYS were
28.25 6 2.136C and 22.50 6 1.636C, and dry bulb and wet
bulb temperatures for the DYS were 27.85 6 2.216C, and
22.23 6 1.626C (P . .05).
Body Mass Changes
Between-trials comparisons (HYR versus DYR and
HYS versus DYS) demonstrated differences in body mass
at the prerun and postrun time points (all comparisons: P
Figure 1. Body mass (mean 6 SD) throughout the race (A) and submaximal (B) trials, with associated percentage of body mass lost
compared with baseline. a P # .05 for the same time point between hydration states. Figure 1A reprinted with permission of the National
Strength and Conditioning Association, Colorado Springs, CO, from Stearns RL, Casa DJ, Lopez RM, et al.49
150 Volume 45 N Number 2 N April 2010
, .001). Dehydrated participants (DYR and DYS) had
reductions in body mass compared with their 3-day
baselines for the morning of, immediately before, and after
all dehydrated trials (all comparisons: P , .001; Figure 1).
Volunteers in the DYR lost an average of 1.5 kg before the
start and finished with an average loss of 2.8 kg, whereas
those in the HYR started at an average of 0.54 kg less than
their baselines and lost an average of 1.37 kg. The
percentage of dehydration calculated pre-exercise via body
mass losses was supported by urine hydration markers
(Table 1).
Core Body Temperature
Hydration status before all trials did not influence core
body temperature leading up to the start of each run
between HYR and DYR or HYS and DYS (P . .05).
However, postrun, core body temperature in DYR was
greater (39.49 6 0.376C) than in HYR (39.18 6 0.476C, P
5 .038; Figure 2A). Core body temperature for HYR
versus DYR at loop 2 (39.12 6 0.686C versus 39.37 6
0.456C) and at 10 minutes (38.87 6 0.596C versus 38.99 6
0.566C) and 20 minutes postrun (38.25 6 0.626C versus
38.45 6 0.536C) was not different. For DYS, core body
temperature was greater than for HYS after the second
loop (39.04 6 0.336C versus 38.75 6 0.396C, P 5 .009),
postrun (P , .001), and 10 minutes (P 5 .001) and
20 minutes postrun (P 5 .001; Figure 2B). Core body
temperature immediately postrun for HYS was 38.62 6
0.336C, whereas for DYS, it was 39.17 6 0.426C.
Comparing the submaximal trials at the immediately
postexercise time point, body core temperature increased
0.226C for every additional 1% of body mass loss
(Figure 2).
Sweat Rate
Sweat rates for the HYR and DYR were 1.57 6 0.44 L/h
and 1.45 6 0.39 L/h, respectively. The 8% difference in
sweat rates between the HYR and DYR was not significant
(P . .05). Sweat rates for the HYS and DYS were 1.41 6
0.31 L/h and 1.32 6 0.28 L/h, respectively. The 6%
difference in sweat rates between the HYS and DYS was
not significant (P . .05).
Heart Rate
Hydration status before all trials did not influence
pretrial HR between hydrated and dehydrated races or
submaximal trials (both comparisons: P . .05). The HR
for DYR was greater (P # .05) than that for HYR at
10 minutes postrun (132 6 18 beats/min versus 118 6 10
beats/min, P 5 .003) and 20 minutes postrun (114 6 16
beats/min versus 103 6 12 beats/min, P 5 .009;
Figure 2A). The HR for DYS was greater (P # .05) than
that for HYS after the second loop (175 6 10 beats/min
versus 167 6 9 beats/min, P 5 .016), postrun (179 6 11
beats/min versus 164 6 10 beats/min, P , .001), and at
Table 1. Urine Osmolality, Urine Color, Urine Specific Gravity, and Blood Lactate Level (Mean 6 SD)
Trial
Hydration
State
Urine Osmolality Urine Color Urine Specific Gravity Lactate, mmol/L
Pretrial Posttrial Pretrial Posttrial Pretrial Posttrial Posttrial
Race
Hydrated 324 6 252 410 6 244 3 62 56 2 1.009 6 0.006 1.012 6 0.006 5.0 6 1.2
Dehydrated 1002 6 106a 820 6 154a 6 6 1a 6 6 1a 1.027 6 0.004a 1.020 6 0.004a 4.6 6 1.4
Submaximal
Hydrated 296 6 254 371 6 216 2 62 56 3 1.008 6 0.007 1.013 6 0.007 2.8 6 0.7
Dehydrateda 995 6 112 929 6 103 6 61 76 1 1.026 6 0.003 1.026 6 0.003 3.9 6 2.2
a P , .05 between hydrated and dehydrated trials at the corresponding time points.
Figure 2. Heart rate and core body temperature (mean 6 SD) throughout race (A) and submaximal (B) trials. a P # .05 for the same time
point between hydration states.
Journal of Athletic Training 151
10 minutes postrun (125 6 16 beats/min versus 101 6 16
beats/min, P , .001) and 20 minutes postrun (116 6 145
beats/min versus 94 6 12 beats/min, P , .001; Figure 2B).
Comparing the submaximal trials at the immediately
postexercise time point, a 6-beats/min increase in HR
occurred for every additional 1% of body mass loss.
Additionally, HR was 10 beats/min higher at 10 minutes
postexercise for every additional 1% of body mass loss
(Figure 2).
Performance
We closely monitored HYS and DYS loop time and
provided feedback to attempt to keep finishing times
consistent. Total times to complete HYS and DYS were
not different (59.57 6 5.31 minutes and 59.44 6
5.44 minutes, respectively; P . .05). However, time for
loop 1 was faster during DYS (19.37 6 1.58 minutes) than
for HYS (19.48 6 1.65 minutes, P 5 .033). Times for loop
2 (19.85 6 1.80 minutes and 19.97 6 1.97 minutes,
respectively) and loop 3 (20.23 6 2.03 minutes and 20.25 6
1.98 minutes, respectively) were not different between HYS
and DYS (P . .05). For the race, HYR was faster than
DYR for loops 1 (17.32 6 1.94 minutes versus 17.77 6
2.06 minutes, P 5 .028), 2 (17.85 6 2.05 minutes versus
18.42 6 2.46 minutes, P 5 .010), and 3 (18.01 6 2.20
minutes versus 19.47 6 3.16 minutes, P 5 .003) and for
total time (53.15 6 6.05 minutes versus 55.70 6 7.45
minutes, P 5 .001; Figure 3).
Perceptual Responses
Perceived thirst was greater in DYS versus HYS prerun
(6.0 6 1.0 versus 2.0 6 1.0), postloop 1 (7.0 6 1.0 versus
3.0 6 1.0), postloop 2 (7.0 6 1.0 versus 4.0 6 1.0), postrun
(9.0 6 1.0 versus 6.0 6 2.0), and 20 minutes postrun (8.0 6
1.0 versus 3.0 6 1.0) (all comparisons: P , .001; Figure 4).
Thermal sensation was greater in DYS versus HYS
postloop 2 (6.5 6 1.0 versus 5.5 6 1.0, P 5 .035), postrun
(6.0 6 1.00 versus 5.5 6 1.0, P 5 .003), and 20 minutes
postrun (5.0 6 1.0 versus 4.0 6 0.5) (all comparisons: P ,
.001). Rating of perceived exertion was greater in DYS
versus HYS postloop 1 (14.0 6 2.0 versus 12.0 6 2.0, P 5
.002), postloop 2 (15.0 6 2.0 versus 12.0 6 2.0, P 5 .001),
and postrun (16.00 6 2.0 versus 13.0 6 2.0, P , .001).
Thirst was greater in DYR versus HYR prerun (6.0 6
1.0 versus 2.0 6 1.0), postloop 1 (7.0 6 1.0 versus 5.0 6
1.0), postloop 2 (8.0 6 1.0 versus 5.0 6 2.0), postrun (8.0 6
1.0 versus 3.0 6 2.0), and 20 minutes postrun (8.0 6 1.0
versus 4.0 6 2.0) (all comparisons: P , .001). Thermal
sensation was greater in DYR versus HYR postloop 2 (7.0
6 0.5 versus 6.5 6 0.5, P 5 .027) and 20 minutes postrun
(5.0 6 1.0 versus 4.5 6 1.0, P 5 .018). The RPE was
greater in DYR versus HYR postloop 2 (18.0 6 1.0 versus
17.0 6 2.0, P 5 .044), postrun (19.0 6 1.0 versus 18.0 6
1.0, P 5 .037), and 20 minutes postrun (8.0 6 2.0 versus 6.0
6 1.0, P 5 .028).
Environmental Symptoms Questionnaire. Changes in the
ESQ, measured by assessing the change from prerun to
postrun, were greater in DYR versus HYR (26 6 21 versus
12 6 12, P 5 .007). No differences were noted between
prerun and postrun scores in HYS and DHS (7 6 10 versus
15 6 19, P 5 .104).
Profile of Mood States. From prerun to postrun,
Tension/Anxiety decreased to a greater extent in DYS
versus HYS (P 5 .03). Fatigue/Inertia (P 5 .021) and Total
Mood Disturbances (P 5 .028) increased more from prerun
to postrun in DYS than in HYS (Table 2).
DISCUSSION
Our primary purpose was to evaluate the influence of
hydration status on physiologic strain and performance at
various exercise intensities in a field setting. The research
questions were as follows: (1) When finishing time remains
constant (as a result of pace feedback), how does
dehydration influence physiologic function?; and (2) How
does an athlete’s maximal-intensity exertion while hydrated
or dehydrated influence physiologic function and performance?
Submaximal Running (HYS and DYS)
When finishing time was held constant (absolute
intensity consistent between trials), physiologic responses
were notably influenced when body mass loss differences
were 2.56% (2.03% versus 4.59%). When dehydration
increased, postrun body core temperature and HR were
approximately 0.56C and 15 beats/min higher, respectively
(Figure 2), even though intensity was moderate, finishing
times were the same, environmental conditions were not
extreme, and differences in hydration status were just
2.5% (Figure 1). These results are similar to those of
previous laboratory1–3,9,30 and field31–33 studies but
contrast with those of some previous field studies17–21 in
which body core temperature was not influenced by
hydration. We examined a realistic athletic model,34 in
which a slight fluid deficit is carried forward from the
previous day and increased during an acute bout of
activity.
In previous laboratory studies,1–4,9 exercise in warm or
hot conditions resulted in 0.126C to 0.256C and 3 to 5
beats/min increases in body core temperature and HR,
respectively, for every additional 1% of body mass lost. We
Figure 3. Race trial performance times (mean 6 SD) a P # .05 for
the same time point between hydration states. Reprinted with
permission of the National Strength and Conditioning Association,
Colorado Springs, CO, from Stearns RL, Casa DJ, Lopez RM, et al.49
152 Volume 45 N Number 2 N April 2010
noted 0.226C and 6-beats/min increases in body core
temperature and HR, respectively, for every additional 1%
of body mass lost (Figure 2). Also, HR was 10 beats/min
higher at 10 minutes postrun for every additional 1% of
body mass lost (Figure 2). Thus, dehydration may exacerbate
physiologic strain even more during the recovery from
exercise, a concept with ramifications for interval training
and sports with work and rest periods.
Figure 4. Rating of perceived exertion (RPE), thermal sensation, and thirst (mean 6 SD) throughout the race (A) and submaximal (B) trials.
a P # .05 for the same time point between hydration states.
Table 2. Changes in Profile of Mood States Scores for Race and Submaximal Trials (Mean 6 SD)
Trial
Hydration
State
Changes in Profile of Mood States Scores (Postrun 2 Prerun)
Tension-
Anxiety
Depression-
Dejection
Anger-
Hostility
Vigor-
Activity
Fatigue-
Inertia
Confusion-
Bewilderment
Total Mood
Disturbance
Race
Hydrated 4 62 060 060 46 0 27 61 06 1 27 6 5
Dehydrated 23 61 16 3 22 6 6 24 63 966 163 76 3
Submaximal
Hydrated 22 62 06 1 22 6 5 23 61 262 061 06 8
Dehydrated 0 6 1a 1 61 26 3 25 61 66 2a 2 61 15 6 5a
a P , .05 between hydrated and dehydrated trials at the corresponding time points.
Journal of Athletic Training 153
Our findings likely differed from those of previous field
studies because both trials (HYS and DYS) required
participants to finish in approximately the same time.
Dehydrated runners (2.53% additional body mass loss at
the end of the run) experienced greater physiologic strain
attaining the designated finishing time, as indicated by
multiple exaggerated postrun responses (including TGI,
HR, thermal sensation, RPE, POMS, and plasma lactate
level; Figures 2 and 4; Tables 1 and 2). The effort was
perceived to be greater, indicating that the psychological
response paralleled the exaggerated physiologic responses.
8,35–38 Put simply, dehydrated runners had to work
harder (and knew it) to accomplish a task that was easier
when they were less dehydrated.
These deficits associated with running in a slightly more
dehydrated condition reflect combined psychologically and
physiologically mediated responses. But one could argue,
given the data, that the response is not entirely psychological.
Athletes would likely reduce running pace if they
sensed a fluid deficit,11,12,37,39 but in our study, pace
remained constant as part of the research design. So, while
perceptual measures (RPE, thermal sensation, thirst,
POMS) were exaggerated during and after the run,
physiologic measures clearly indicated that hydration
status altered the thermoregulatory response when finishing
time was controlled. The magnitude of the HR
differences (nearly 20 beats/min higher for DYS versus
HYS) at the end of the run and after 10 minutes of recovery
reminds us of the effects of hydration status on cardiovascular
function. Additionally, the 0.226C rise in body
temperature for every additional 1% of body mass loss
confirms that in field studies, when an athlete is not given
the opportunity to decrease pace, the difference in
hydration status affects thermoregulatory function. Although
this finding has long been shown in laboratory
studies,1–4,30,33 some authors17–22,40,41 have strongly speculated
that this physiologic response does not occur in field
studies. Our results show that hydration status and core
temperature were not related in previous studies because
participants could decrease pace as a protective or
necessary response to hyperthermia or fatigue. When a
critical factor that mediates body core temperature is
controlled (eg, finishing time), the relationship between
hydration status and body core temperature is apparent.
Additionally, wind velocity is another factor to consider.
Laboratory studies have been criticized42 as exaggerating
the physiologic strain attributed to dehydration due to a
lack of sufficient wind velocity. Differences in cooling as a
result of varying wind velocities have been demonstrated in
a laboratory setting43; however, Duglas et al44 showed that
very high wind speeds (.20 m/s) could offset dehydration
levels of approximately 4%. An examination of the effect of
wind velocity in a warm environment had never been
performed with participants who were dehydrated up to
approximately 4% in the field. Our results reveal that when
wind velocity at least equaled running velocity, it was
insufficient to counter the levels of dehydration and
resulting performance decrements that occurred.
The 6 greatest benefits of enhanced hydration status in a
submaximal scenario (body mass loss of 2.0% versus 4.59%
at the finish) were (1) lower core temperature during the
effort, upon finishing, and for the 20 minutes of recovery;
(2) decreased cardiovascular strain during the effort, upon
finishing, and for the 20 minutes of recovery; (3) decreased
perceptions of warmth and exertion; (4) decreased thirst
level, which may have psychological ramifications40,41; (5)
decreased perturbations of psychological state, as indicated
by numerous responses in the POMS; and (6) decreased
blood lactate level.
Maximal Running (HYR and DYR)
We examined the effect of hydration status (body mass
loss of 0.8% versus 2.3% immediately before the race and
of 2.05% versus 4.3% at the end of the race) on
performance and physiologic function while running in
warm conditions. Previous authors19–21 have shown that
when relationships between hydration status and some
element of performance (eg, finishing time) are assessed,
increasing dehydration often does not degrade performance.
In fact, quite the opposite was true: the fastest
runners seemed to be the most dehydrated.22,40 Reasonable
explanations for this phenomenon include less weight to
carry (a greater benefit in events such as distance running,
because less energy is required to maintain and produce
forward movement) and less time needed to stop or slow
down and rehydrate. Past examinations begged the
following question: If the same runner undertook the same
activity and maintained a better hydration status, would he
or she perform better? Without a control group, there was
no way of knowing. Therefore, we sought to examine this
question directly by asking runners to race at maximal
effort on 2 occasions at 2 hydration levels.
Finishing body core temperature was lower in the
hydrated runners, even though they were running at a
faster pace (Figure 2). This finding is surprising, because
exercise intensity plays such an important role in the rate of
body core temperature rise.13,14,23 The additional thermoregulatory
strain imposed by exercising while more
dehydrated may have exerted a greater influence than the
magnitude of temperature change imposed by the faster
pace attained when racing in a less dehydrated condition.
So each group had a physiologic explanation for a high
body temperature while racing in the heat. The hydrated
group would have a high core body temperature because of
greater intensity, as supported by the previous literature,
13,14,23 and the dehydrated group would also have a
high core temperature because of greater thermoregulatory
strain (as shown by the submaximal temperature response
when finishing times were the same). For most of the race
(postloops 1 and 2) these contributors (intensity versus
dehydration) of a rapid rise in core temperature provided
similar magnitudes of response. However, by the end of the
race, the greater level of dehydration coupled with a
slightly greater exercise duration drove temperature significantly
higher, trumping the higher intensity experienced by
the hydrated group. The self-imposed (or actual physiologic)
constraints that caused a decrease in pace, while
seemingly protective, could not completely offset rising
temperature. The dehydrated runners ran slower and at a
lower intensity but reached a core body temperature that
was higher than that of the same runner at a higher
intensity but in a less dehydrated state. This finding may
provide a clue to the cause of heat illness in athletes. An
athlete who is dehydrated but motivated to perform at a
high level may have a rise in body temperature, even with a
154 Volume 45 N Number 2 N April 2010
decrease in pace, which is relevant when athletes are forced
to perform at maximal effort as a result of external (eg,
military supervisor, coach) or internal (eg, qualifying for a
team or time) influences.
As indicated by similar HRs and higher RPEs and body
core temperatures, the dehydrated runners were providing
maximal effort. Yet performance decreased and the
perceptual environmental symptoms associated with maximal
exercise (as indicated by a greater change in postrace
ESQ scores) increased. Despite feeling as if they were
running maximally and physiologically straining their
bodies, the dehydrated runners still completed the course
in slower times. The greater thirst responses in the more
dehydrated group (as we found) may have provided a
powerful psychological cue to reduce intensity in response
to perceived compromised hydration. This response to
thirst may pose a risk if pace continues at nondehydrated
intensity.22,37,40 Additionally, the participants’ lack of
blinding regarding hydration status may have offered
psychological cues to modify pace and effort. However,
even though they ran more slowly, they still had a higher
finishing temperature and perceived more adverse signs
and symptoms during the maximal effort. So the reduced
intensity likely provided some degree of self-imposed
protection against the risks of maintaining an unrealistic
pace in the presence of the given hydration status. But the
decreased pace could not completely protect against the
increased thermoregulatory strain (demonstrated by the
increase in core body temperature due to heat production
and gain imposed by the greater dehydration in combination
with stressful environmental factors) during maximal
effort.
Dehydration linked with performance decrements occurs
during endurance activity. A meta-analysis45 of 14 such
studies showed that the decrement begins at about 2% of
body mass loss, especially in temperate or warm environments,
a finding corroborated by others.46 Sports medicine
governing bodies47,48 have promoted appropriate rehydration
practices, encouraging athletes to rehydrate at a rate
that prevents body mass loss from exceeding 2%. Such
recommendations (rehydrating to keep body mass loss to
less than 2%, never overhydrating, following thirst dictates
for slower runners, and individualizing rehydration plans
based on sweat rate for faster runners) remain wise advice.
In a race scenario, enhanced hydration status (2.05%
versus 4.3% of body mass lost at the finish) resulted in (1)
faster finishing time in the 7.5-mi (12-km) race, (2) lower
body core temperature upon finishing, (3) fewer environmental
symptom changes from prerace to postrace, (4)
enhanced recovery, given the lower HR for at least
20 minutes postrun, (5) decreased feelings of warmth and
exertion (even though running faster), as evidenced by
decreased thermal sensation and lower RPE, respectively,
(6) decreased thirst response, which may have psychological
ramifications,40,41 and (7) decreased ability to evenly
pace oneself (discussed in a recent publication).49
In conclusion, it seems likely that the physiologic and
performance decrements associated with dehydration that
have been consistently shown in a laboratory setting exist
when athletes perform athletic activities in a natural sport
setting. The differences that have been noted between
laboratory and field settings are linked to the methodologic
challenges in a field setting, which make it harder to isolate
the effects and are not due to the absence of the physiologic
effects themselves.
ACKNOWLEDGMENTS
We thank the Gatorade Sport Science Institute (Chicago, IL) for
partially funding this study. We also acknowledge the runners and
researcherswhomade this study possible.We recognize the following
people and thank them for all the time and effort they dedicated to
this study: JeffreyAnderson, PatrickAustin, PaulBoyd,TutitaCasa,
Kelli Christensen, Nora Decher, Mike Eckert, John Folsom, Kris
Friend, Robert Howard, Ben Keegan, Jennifer Klau, Elaine Lee,
Heather Mispagel, Janice Palmer, Erin Quann, Ian Scruggs, Liz
Silverberg, Kate Sanders, Ben St. Martin, Martha Stearns, Curt
Vincente, Brittanie Volk, Angie West, and Brad Yeargin.
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Address correspondence to Douglas J. Casa, PhD, ATC, FNATA, FACSM, Department of Kinesiology, University of Connecticut, 2095
Hillside Road, Unit 1110, Storrs, CT 06269-1110. Address e-mail to: Douglas.casa@uconn.edu.
Volume 45 * April 2010