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There is a rich scientific literature regarding hydration status and physical function that began in the late 1800s, although the relationship was likely apparent centuries before that. A decrease in body water from normal levels (often referred to as dehydration or hypohydration) provokes changes in cardiovascular, thermoregulatory, metabolic, and central nervous function that become increasingly greater as dehydration worsens. Similarly, performance impairment often reported with modest dehydration (e.g., -2% body mass) is also exacerbated by greater fluid loss. Dehydration during physical activity in the heat provokes greater performance decrements than similar activity in cooler conditions, a difference thought to be due, at least in part, to greater cardiovascular and thermoregulatory strain associated with heat exposure. There is little doubt that performance during prolonged, continuous exercise in the heat is impaired by levels of dehydration >or= -2% body mass, and there is some evidence that lower levels of dehydration can also impair performance even during relatively short-duration, intermittent exercise. Although additional research is needed to more fully understand low-level dehydration's effects on physical performance, one can generalize that when performance is at stake, it is better to be well-hydrated than dehydrated. This generalization holds true in the occupational, military, and sports settings.
Hydration and Physical Performance
Bob Murray, PhD
Gatorade Sports Science Institute, Barrington, Illinois
Key words: hydration, dehydration, hyperhydration, performance, physical activity
There is a rich scientific literature regarding hydration status and physical function that began in the late
1800s, although the relationship was likely apparent centuries before that. A decrease in body water from normal
levels (often referred to as dehydration or hypohydration) provokes changes in cardiovascular, thermoregulatory,
metabolic, and central nervous function that become increasingly greater as dehydration worsens. Similarly,
performance impairment often reported with modest dehydration (e.g., 2% body mass) is also exacerbated by
greater fluid loss. Dehydration during physical activity in the heat provokes greater performance decrements than
similar activity in cooler conditions, a difference thought to be due, at least in part, to greater cardiovascular and
thermoregulatory strain associated with heat exposure. There is little doubt that performance during prolonged,
continuous exercise in the heat is impaired by levels of dehydration 2% body mass, and there is some
evidence that lower levels of dehydration can also impair performance even during relatively short-duration,
intermittent exercise. Although additional research is needed to more fully understand low-level dehydration’s
effects on physical performance, one can generalize that when performance is at stake, it is better to be
well-hydrated than dehydrated. This generalization holds true in the occupational, military, and sports settings.
Key teaching points:
Dehydration during physical activity is commonplace because voluntary fluid intake is often less than fluid loss through sweating.
Dehydration results in unavoidable, negative effects on physiological function and exercise performance. The magnitude of the
effects of dehydration is exacerbated by increases in heat stress, exercise duration, and exercise intensity.
Adequate drinking during exercise helps attenuate the reductions in blood volume, cardiac output, muscle blood flow, skin blood
flow, the rise in core temperature, and the impairment in exercise performance that accompany dehydration.
A number of consensus papers from scientific organizations
[1–3] and numerous review articles [4 6] have comprehen-
sively summarized dehydration’s impact (a decrease in body
fluid from the normal-euhydrated-state) on physical perfor-
mance, with the consistent conclusion that dehydration can
significantly impair performance, particularly during sustained
physical activity in warm/hot conditions. For that reason, fluid-
replacement guidelines [e.g., 1,3,7] have been established for
athletes, soldiers, and workers to help encourage sufficient
fluid intake to minimize the extent of dehydration during phys-
ical activity.
The purpose of this review article is to first summarize how
dehydration affects physical performance. That summary will
serve as a prelude to consideration of the extent of dehydration
required to impair performance. The current scientific consen-
sus is that dehydration equal to or exceeding a loss of 2% of
euhydrated body mass (e.g., a loss of 1.4 kg in a 70-kg athlete;
3 lb in a 150-lb athlete) can negatively impact physical perfor-
mance. In fact, it is not uncommon for athletes, soldiers, and
workers to finish exercise dehydrated by considerably more
than 2% of body mass, even when fluid is freely available
[8]. There are two primary reasons for this “voluntary” dehy-
dration: 1) sweat loss during physical activity can be quite
large, ranging from as little at 400 ml/h to well over 2 L/h [9],
Address reprint requests to: Bob Murray, PhD, Gatorade Sports Science Institute, 617 West Main Street, Barrington, IL 60021. E-mail:
Presented at the ILSI North America Conference on Hydration and Health Promotion, November 29 –30, 2006 in Washington, DC
Conflict of interest disclosure: Dr. Murray is the director of the Gatorade Sports Science Institute and is an employee of The Gatorade Company, a wholly owned subsidiary
of PepsiCo. Gatorade produces and markets hydration and sports nutrition products to athletes and physically active consumers.
Journal of the American College of Nutrition, Vol. 26, No. 5, 542S–548S (2007)
Published by the American College of Nutrition
and 2) the voluntary desire to ingest fluid during physical
activity often fails to keep pace with the rate at which sweat is
lost [10,11]. Regardless of the reasons for dehydration, the
existing scientific data allow for this generalization: during
physical activity, it is better to be well-hydrated than to be
dehydrated [12,13]. Just how close to euhydrated one must
remain to avoid a performance decrement remains a matter of
continuing scientific interest and is the focus of the latter part
of this paper.
There is substantial variation in the sweating rates among
individuals, even under like conditions of environment, exer-
cise intensity, fitness level, and the extent of heat acclimation
[3–5]. Similarly, the rate of sweating, and therefore the total
amount of sweat lost, differs markedly from day to day in the
same individual, owing largely to differences in environment,
exercise intensity, exercise duration, and the type of clothing
and equipment worn. Because sweat loss and fluid intake
during physical activity vary so widely, some individuals will
encounter significant dehydration, while others will finish the
same activity with minimal dehydration.
Water Loss during Physical Activity
During light exercise in cool or moderate environments,
sweating rates might be as little as 100 ml/h (3 oz), yet during
vigorous exercise in a hot environment, some individuals are
capable of sweating at over 3,000 ml/h (roughly 100 oz) [9].
High sweating rates (e.g., 1.5 L/h) makes significant dehy-
dration and impaired performance more likely because even
highly motivated individuals find it difficult to drink large
volumes during exercise and because maximal gastric empty-
ing rate has an upper limit. This limit has yet to be well defined,
is likely to vary widely among individuals, but could be in
excess of 1.3 L/h [5]. Because of the substantial differences in
sweating rates among individuals, there is no one-size-fits-all
recommendation for fluid intake during physical activity. Re-
maining well-hydrated requires athletes, soldiers, and workers
to continually adjust their fluid intake patterns to reflect the
extent of sweat loss during exercise and thereby minimize
Salt Loss during Physical Activity
During prolonged physical activity, large amounts of salt
(sodium chloride) can be lost in sweat [8,14]. On average,
human sweat contains 40–50 mmol of sodium per liter (920 to
1,150 mg/L.) Large sweat losses promote large salt losses; for
example, an athlete with an average sweat sodium concentra-
tion who loses5Lofsweat in a day, will lose between 4,600
and 5,750 mg of sodium (the equivalent of 11.5 to 14.4 g of
salt.) Individuals with higher fluid losses and/or saltier sweat
can lose substantially more sodium on a daily basis. It should
be noted that athletes, workers, and soldiers who are fit and
acclimated to the heat typically benefit from enhanced sodium
reabsorption in the sweat-gland tubule, an adaptive response
that helps protect plasma volume by lowering sodium loss. In
fact, some individuals are capable of producing sweat with less
than 20 mmol of sodium per liter [12,14]. Salt loss in itself does
not have a direct impact on physical performance, but adequate
replacement of sodium chloride during physical activity can
help encourage voluntary fluid intake [15], protect plasma
volume [16], and reduce urine production [17]—all responses
that promote hydration.
For experimental purposes, dehydration can be induced by
fluid restriction before or during exercise, heat exposure before
exercise, or diuretic use before exercise. In general, studies
assessing physiological and performance responses typically
used dehydration levels of 2% body mass or more to produce
enough fluid loss to ensure measurable changes and because
such levels of dehydration are common among athletes, sol-
diers, and workers [6,8]. Sweating during physical activity can
quickly result in dehydration, particularly in profuse sweaters
in warm environments. Depending upon the adequacy of fluid
intake, dehydration of 1 to 8% of body mass can occur during
physical activity [5,8].
Dehydration and Physiological Function
Dehydration’s impact on a variety of cardiovascular and
thermoregulatory functions is measurable early in exercise
(e.g., within 30 min) at a body mass loss of approximately 1%;
as the level of dehydration increases, deterioration in physio-
logical function progressively rises [3,13,18]. For example,
progressive dehydration to 4.9% of body mass during two
hours of cycling in the heat (65% VO
max; 35°C; 95°F) caused
heart rate, core temperature, and perceived exertion ratings to
continually increase over time, while blood volume, stroke
volume, cardiac output, and skin blood flow all decreased [18].
Similar results were reported by Montain and Coyle who dem-
onstrated that levels of dehydration of 1%, 2%, 3%, and
4% of body mass were increasingly detrimental to cardio-
vascular and thermoregulatory function [13].
These and other data led Cheuvront et al. [19] to propose
that dehydration induces premature fatigue during sustained
exercise by increasing thermoregulatory stress, cardiovascular
strain, negative changes in muscle metabolism (e.g., acceler-
ated rate of glycogen depletion), and alterations in central
nervous system function (e.g., reduced motivation and effort).
Hydration and Physical Performance
In short, dehydration negatively affects a wide range of phys-
iological functions, the combined effect of which is impaired
performance. Cheuvront et al. [19] point out that for many
physically active people, a fluid intake of approximately 1 L/h
will usually provide sufficient water, carbohydrate, and elec-
trolytes to limit dehydration to less than 2% body mass and
supply enough exogenous carbohydrate to provide an addi-
tional performance benefit [16]. Fortuitously, an intake approx-
imating 1 L/h is very manageable for most adults [15] whose
gastric emptying rates exceed 1 L/h.
Not surprisingly, dehydration’s impact on physiological
function is greater in warm environments than in cold
[4,20,21]. When even modest heat stress (e.g., °16C; 60°F)
is present, cardiovascular strain rises and makes maintaining
blood volume more critical to sustain blood flow to active
muscles, skin, and the brain [21].
Dehydration and Performance
A variety of performance tasks have been studied in hydra-
tion-related experiments, including measurements of time to
exhaustion, time-trial type protocols, sprint-to-the-finish proto-
cols, and designs employing tests of sports-specific skills,
muscular strength, muscular endurance, and anaerobic perfor-
mance. Three studies spanning 1944 to 2007 are summarized
below. The experimental designs and performance tasks used in
these studies are markedly different, but the uniformity of the
results reflects the current consensus of scientific literature:
dehydration incurred before or during sustained exercise often
impairs physical performance. Although research on dehydra-
tion’s effect on muscle strength and anaerobic performance is
equivocal and more work is needed, the current scientific
consensus is that dehydration has little impact on such mea-
sures [3].
In 1944, Pitts et al. [11] published a study designed to
determine how water, salt, and glucose intake affected physical
performance in the heat. Six heat-acclimated male subjects
completed a series of walks at 3.5 mph (2.2 km/h) in either
hot/dry (100°F; 38°C with 30%RH) or hot/humid environments
(95°F; 35°C with 83%RH). The walks lasted between one and
six hours, with a 10-min rest following each hour. The authors
detail the results of one subject who completed six treadmill
walks (at a 2.5% grade), all in hot/dry conditions. On two
occasions, the subject drank nothing. During another two trials,
the subject drank according to thirst. Dehydration was pre-
vented in the other two trials by having the subject drink water
every 15 minutes in volumes that equaled his sweat loss. Based
on measurements of rectal temperature, heart rate, sweating
rate, oxygen consumption, and exercise duration, Pitts et al.
“First, without water the rectal temperature rose steadily to
high levels and showed no signs of reaching a steady state.
Although the subject, being of far better than average stam-
ina, was able to march sixteen miles, he was very tired and
inefficient at the end. Second, without water for the first
hour, but thereafter enough to keep thirst quenched at all
times, the rectal temperature finally started to rise after
remaining constantly low for thirteen miles. The subject was
able to finish nineteen miles in fairly fresh condition. Fi-
nally, when he forced himself to drink water at the same rate
as he lost sweat, the body temperature remained very low
for this type of work, and the subject after sixteen miles said
that he could easily go on all day.”
It is not apparent from Pitts et al. [11] if steps were taken to
standardize the pre-exercise hydration status of their subjects.
The authors did not report changes in their subjects’ body mass
from which percent dehydration could be calculated. However,
it is clear from their data that the best physiological and
performance responses occurred when dehydration was kept to
a minimum.
The effect of modest dehydration incurred during relatively
short-duration exercise was studied by Below et al. [16]. Their
research design consisted of 50 min of cycling at 85%
max, followed by a performance task requiring subjects to
complete a set amount of work as fast as possible (requiring
10 to 12 min). The protocol was designed to mimic the
demands of a sprint to the finish at the end of a 40 km cycling
time trial. The experiment consisted of four counterbalanced
trials conducted in a warm environment (31°C; 88°F), includ-
ing a trial in which about 80% of sweat loss was replaced
(average water intake of 1.33 L) and a trial in which the
subjects ingested an average of 200 ml of water during the
exercise, enough to replace 13% of sweat loss. The researchers
standardized pre-exercise hydration status by requiring subjects
to consume a standard diet and refrain from exercise the day
before each trial. In the 2 h prior to exercise, the subjects
ingested 5 ml of water per kilogram of body mass. As a result,
there were no differences in pre-exercise measures of body
mass or hemoglobin concentration.
When the subjects ingested the small volume of water, they
dehydrated by 2% of body mass and their performance was
impaired by an average of 6.5% compared to the trial in which
they replaced 80% of sweat loss. The authors noted that better
hydration during exercise was associated with lower esopha-
geal temperature, heart rate, and ratings of perceived exertion,
all factors that could have influenced the subjects’ self-selected
work rates during the performance task.
In 2007, Ebert et al. [22] published a study designed to
determine if the body mass loss associated with dehydration
would actually benefit performance in uphill cycling by “light-
ening the load,” thereby reducing the energy cost of cycling or
increasing the power-to-mass ratio, either of which would
result in better cycling performance. Following preliminary
testing and familiarization sessions, eight trained cyclists par-
ticipated in two experimental trials: low fluid intake or high
fluid intake. Each trial consisted of2hofsteady-state cycling
at 53% of each subject’s predetermined maximal aerobic power
Hydration and Physical Performance
544S VOL. 26, NO. 5
output, followed by a cycling bout to exhaustion. The steady-
exercise took place on a cycle ergometer. After2hofexercise,
the subjects were reweighed within 3 min and thereafter
mounted their own bicycles and began to cycle on a treadmill
set at an 8% grade and a speed approximating 88% of each
subject’s maximal aerobic power output. Time to voluntary
exhaustion was recorded as the performance measure.
Euhydration prior to each trial was ensured by having the
subjects ingest a standard diet for the preceding 24 h, restricting
exercise and caffeine intake before each trial, and having the
subjects ingest an average of 350 ml of water during the 2 h
before testing. Subsequent measures of body mass, urine spe-
cific gravity, hemoglobin, and hematocrit did not differ be-
tween trials, indicating that the subjects began each trial sim-
ilarly hydrated. Both trials were conducted in a warm
environment (29.3°C; 85°F). During the 2-h submaximal exer-
cise, the subjects ingested either a small volume (total 0.4 L)
or a larger volume (total 2.4 L) of a carbohydrate-electrolyte
beverage; each volume was divided into either 50 ml or 300 ml
aliquots ingested at 15-min intervals. Carbohydrate intake was
standardized by having subjects ingest carbohydrate gels at
regular intervals during the submaximal exercise bout in the
low-fluid trial. The fluid-intake regimens were purposefully
designed to reflect common practices among many competitive
cyclists (low intake) and the upper level of fluid intake based
on previous American College of Sports Medicine recommen-
dations [23].
At the end of the 2-h submaximal exercise bout, the subjects
had dehydrated by an average of 2.5% of starting body mass
on the low-fluid-intake trial; dehydration was prevented by the
high-fluid-intake regimen (average 0.3% change in body
mass.) Following the performance task, the subjects had dehy-
drated by an average of 3.6% body mass on the low-intake
trial and 1.3% body mass on the high-intake trial.
Every subject performed worse on the low-intake trial.
Average time to exhaustion was 5.6 min slower, a relative
performance impairment of 28.6%. The authors concluded that,
“. . . dehydration-induced hyperthermia outweighed the theo-
retical benefit of a reduction in body mass on the power-to-
mass ratio and energy cost during cycling.”
Under many circumstances, dehydration impairs physical
performance, but it should be noted that may not always be the
case. A few studies have reported no performance impairment
at dehydration levels of 2% to 3% body mass [24 –27].
However, new research continues to illustrate the detrimental
effect that modest dehydration can have on various types of
performance tasks. For example, Dougherty et al. [28] reported
significant impairment in basketball skill performance at dehy-
dration of 2% body mass in adolescent subjects exercising in
a temperate environment. Future research is needed to help
further our understanding of the complex relationship among
hydration status, exercise intensity, exercise duration, exercise
task, and thermal stress, the combined effects of which
determine whether modest dehydration will negatively impact
Hydration Status in Cold Environments
Although the majority of scientific evidence illustrates that
dehydration impairs physical performance, there are circum-
stances in which modest dehydration (e.g., 2% to 3% body
mass) does not significantly alter performance. One of those
circumstances is exercise in cold weather. Cheuvront et al. [21]
showed that dehydration (3% body mass) impaired exercise
performance in a temperate environment (20°C; 68°F), but not
in a cold environment (2°C; 36°F). The performance task
consisted of 30 min of steady-state cycling at 50% VO
followed by 30 min in which the subjects (n 8) attempted to
complete as much work as possible. The authors reasoned that
higher cardiac output and lower core temperature during exer-
cise in the cold may explain why dehydration has less of an
effect on performance. However, additional research needs to
be accomplished to confirm these suspicions, in part because
other investigators have reported performance impairment as-
sociated with dehydration in cold environments [29].
Hyperhydration and Performance
Hyperhydration refers to an increase in body fluid above the
euhydrated state. This can be achieved by ingesting excess
water, often combined with a molecule such as glycerol to
create an osmotic imperative for temporary water retention.
Within a few hours, the excess fluid is excreted as urine, but
within that timeframe, hyperhydration could have performance
advantages by delaying dehydration and blunting the rise in
core temperature. A few studies reported small performance
improvements associated with hyperhydration [30 –32]. How-
ever, the current prevailing scientific consensus is that hyper-
hydration does not provide a meaningful physiological or per-
formance advantage compared to remaining well-hydrated
during exercise [3,33–36].
Hydration Status and Performance in the
Occupational Setting
In addition to the sports setting, there are numerous other
occasions when hydration can be a critical determinant of
physical performance. Jobs in mining, agriculture, logging,
firefighting, and construction often expose workers to hot en-
vironments and steady physical work that may require protec-
tive clothing and job-related equipment. Similar conditions
hold true for soldiers, from basic training to some day-to-day
duties and wartime.
Research demonstrates that, as in the athletic setting, pro-
tecting hydration status improves performance in the occupa-
tional setting [3]. Dehydration has been shown to reduce phys-
ical work capacity and lower heat tolerance, particularly in
settings of uncompensable heat stress (i.e., environments that
Hydration and Physical Performance
do not allow sufficient heat loss to enable core temperature to
plateau at safe levels). When hydration status is protected by
adequate fluid intake during physical activity, heat tolerance
and performance are improved. [37–39].
Current practical guidelines identify a loss of 2% of body
mass as the level of dehydration at which performance impair-
ment is likely to occur. This guidance is reflected in a 2005
scientific consensus statement issued by the American College
of Sports Medicine: “Dehydration of more than 2% of body
mass can compromise physiological function and impair exer-
cise performance capacity. Greater levels of dehydration fur-
ther exacerbate the negative responses” [2]. There is ample
scientific evidence to corroborate that performance impairment
often occurs when 2% or more of body mass is lost before or
during exercise. However, it is unlikely that a loss of 2% of
body mass represents a set physiological threshold above which
performance is compromised. There may be exercise and en-
vironmental conditions where less than a 2% loss in body mass
results in a performance decrement. In fact, a handful of studies
suggest this may indeed be the case. Although considerably
more research is needed to confidently conclude that slight
dehydration may impair performance, results of the following
studies suggest such a possibility.
One of the most frequently cited studies that illustrates
performance decrement associated with dehydration is that of
Armstrong et al. [40] This group studied eight experienced
runners who were required to complete runs of 1.5 km, 5.0 km,
and 10.0 km on an outdoor, 400 m track in cool conditions
(15.7°C; 60°F.) The authors relied on measuring plasma so-
dium and potassium concentrations to verify similar hydration
status among the subjects prior to the diuretic and control trials.
A unique aspect of this study is that dehydration was accom-
plished by ingesting 40 mg of furosemide diuretic 5 h before
exercise. The experimental protocol was intended to reduce the
subjects’ plasma volumes by 2%. This led to reductions in body
mass of 1.9% (1.5 km), 1.6% (5.0 km), and 2.1% (10.0 km).
Compared to runs in the euhydrated state, dehydration in-
creased run times by 3.1% (1.5 km), 6.7% (5.0 km), 6.3% (10
km); dehydration significantly increased the time required to
complete the 5- and 10-km runs. From these results, the authors
recognized that a small amount of dehydration could negatively
impact performance: “Linear regression analysis indicated that
a 1% change in body mass corresponded to running time
increases of 0.17, 0.39, and 1.57 min, respectively for
the 1,500 m, 5,000 m, and 10,000 m trials.”
Walsh et al. [41] reported that dehydration of 1.8% of
body mass significantly impaired performance following 60
min of cycling exercise at 70% VO
peak. Subjects (n 6)
exercised in the heat (32°C; 90°F) on two occasions, one where
they did not ingest any fluid and the other where they drank 400
ml of a dilute saline solution (20 mmol/L) 2 to 3 min before
exercise and 120 ml every 10 min for the first 50 min of
exercise. There were no differences in plasma osmolality or
body mass prior to the trials, indicating that the subjects were
similarly hydrated for each trial. The performance task required
the subjects to cycle to exhaustion at 90%VO
peak. When the
subjects remained well-hydrated, they exercised for an average
of 9.8 min, about 3 min longer than when dehydrated, and
performance improved by 44%. These results further demon-
strate the performance-related importance of staying well-hy-
drated during exercise in the heat, even during relatively short-
duration exercise, and also provide a directional indication
that low levels of dehydration (i.e., ⬍⫺2%) can impair
Maxwell et al. [42] reported that low-level dehydration
impaired performing a high-intensity, short-duration running
task conducted in a temperate environment. The investigators
used a maximal intermittent running test to measure perfor-
mance. Subjects (n 11) were required to complete repeated
20-second treadmill runs of increasing intensity at a 10.5%
grade, interspersed with 100 seconds of standing rest. The tests
were conducted in a cool environment (21°C; 70°F) and per-
formance was determined by the cumulative time of the high-
intensity runs. Pre-exercise hydration was standardized by hav-
ing the subjects consume 500 ml of water 90 min before
arriving at the lab; pre-exercise urine-specific gravity and body
mass measures did not differ between trials. Subjects undertook
approximately an hour of passive and active heat stress that
included 10 min in a hot bath, followed by intermittent walking
and jogging until dehydration had reached 1.5% body mass.
This was followed by2hofrecovery in ambient laboratory
conditions. For the euhydration trial, subjects ingested a sugar-
free, lemon-flavored saline solution (80 mmol/L) while walk-
ing/jogging and during the first 40 min of the 2-h recovery
period. The total volume ingested was 155% of predicted sweat
loss to account for obligatory urine excretion during the recov-
ery session. Sweat loss during the performance test averaged
500 ml for both trials, resulting in final dehydration values of
2.0% and 0.6% body mass for the dehydrated and euhy-
drated trials, respectively. The 2% level of dehydration sig-
nificantly impaired running performance by about 4% (148
9 s vs. 154 9 s). Heart rate and rectal temperature were both
significantly elevated during the dehydration trial, indicating
greater cardiovascular and thermoregulatory strain.
The data of Armstrong et al., Walsh et al., and Maxwell et
al. [40 42] show performance decrements at low levels of
dehydration and lend credence to the generalization that when
performance is at stake, it is better to be well-hydrated than
dehydrated. However, additional research is required to further
investigate the effects of low-level dehydration on physiolog-
ical and performance responses in cold, temperate, and hot
environments. Determining how low-level dehydration might
Hydration and Physical Performance
546S VOL. 26, NO. 5
affect performance in short- and long-duration steady-state and
intermittent exercise tasks will require additional measures of
cardiovascular and thermoregulatory function, muscle temper-
ature, muscle/blood metabolites, perceived exertion, and other
parameters. It is also important from both a practical and
scientific perspective to further characterize the types of per-
formance tasks (e.g., aerobic, anaerobic, strength, sports-spe-
cific, etc.) that are sensitive to changes in hydration status.
The results of many decades of research make it clear that
dehydration often impairs physical performance. For that rea-
son, current practical guidelines [3] encourage consumption of
sufficient fluid volumes during exercise to minimize dehydra-
tion. This can often be accomplished by ingesting about1Lof
fluid per h (or whatever level approximates but does not exceed
sweat loss). Under circumstances where significant dehydration
cannot be avoided (e.g., in the case of profuse sweaters or when
fluid availability is limited), the goal should be to take steps to
reduce dehydration severity by ingesting as much fluid as
comfortably tolerated (without over-drinking) and by taking
advantage of any opportunity to reduce sweating rate (e.g.,
minimizing the intensity of warm-up exercise, removing cloth-
ing and equipment, taking more frequent rest breaks, decreas-
ing exercise intensity, and taking advantage of the increased
convective heat loss associated with breezes and the increased
radiative heat loss experienced in the shade).
In summary, drinking sufficient volumes of fluid during
physical activity to minimize dehydration is arguably the sim-
plest and most effective means of sustaining physiological
function and improving physical performance.
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Received July 16, 2007
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... During endurance exercise at lower intensities for <90 min in moderate environments (20e21 C), sweat rates are typically low, often resulting in no more than a 1e2% loss of body mass and mild or insignificant reductions of physical performance (Maughan, Fenn, & Leiper, 1989). Greater total sweat loss occurring during longer exercise (>90 min) (Cheuvront, Carter, & Sawka, 2003), more-intense exercise, or exercise in hotter environment conditions (31e32 C) (Below, Mora-Rodriguez, Gonzalez-Alonso, & Coyle, 1995), stimulate greater degrees of dehydration (!2% loss of body mass), and are more likely to induce decrements in physical performance (Murray, 2007). Although some studies have shown no impairments in performance during dehydration with losses of body mass as high as 3% (Greenleaf, Prange, & Averkin, 1967), the current consensus is that dehydration resulting in !2% body mass loss significantly and consistently impairs physical performance (Cheuvront et al., 2003;Coyle, 2004;Murray, 2007), a decrement that is most apparent during physical activity in warm environments. ...
... Greater total sweat loss occurring during longer exercise (>90 min) (Cheuvront, Carter, & Sawka, 2003), more-intense exercise, or exercise in hotter environment conditions (31e32 C) (Below, Mora-Rodriguez, Gonzalez-Alonso, & Coyle, 1995), stimulate greater degrees of dehydration (!2% loss of body mass), and are more likely to induce decrements in physical performance (Murray, 2007). Although some studies have shown no impairments in performance during dehydration with losses of body mass as high as 3% (Greenleaf, Prange, & Averkin, 1967), the current consensus is that dehydration resulting in !2% body mass loss significantly and consistently impairs physical performance (Cheuvront et al., 2003;Coyle, 2004;Murray, 2007), a decrement that is most apparent during physical activity in warm environments. Further, muscle glycogen and glycogen stores in the liver may be substantially reduced during endurance exercise >90 min, significantly reducing CHO availability and subsequently impairing performance. ...
Fluid homeostasis is challenged during exercise when fluid availability is limited or when fluid loss is not properly replaced. Sports drinks are designed to improve hydration by stimulating fluid ingestion, reabsorption, and retention. Milk has been suggested to be an alternative hydration source to sports drinks due to its higher electrolyte concentrations and similar carbohydrate content. As milk has a high energy density and viscosity that may present gastric discomfort, attempts have been made to develop beverages from milk byproducts, such as milk permeate, that may be more efficacious for consumption during exercise. However, there is limited literature on milk permeate-based beverages for hydration or performance purposes. This review aims to identify the properties of milk that promote fluid retention, discuss how milk consumption both during or following exercise may impact performance or rehydration, respectively, and explore the scientific evidence regarding the use of milk permeate for sports drink production.
... Similar results have been reported in a previous study [28,29]. Shillington (2017) found that there were no significant differences observed in some of the cardiorespiratory functions such as VE body mass) may impair endurance performance even during relatively short duration such as taekwondo games [5,34]. ...
... Nevertheless, dehydration during exercise in the heat impaired greater performance decrement than similar exercise in cooler conditions. This has been explained by greater cardiovascular and thermoregulatory strain associated with heat exposure [34]. In the present study, exercise testing was relatively performed in cool conditions (21±1 ℃). ...
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Background: the present study aimed to examine the effect of rehydration with mineral water on cardiorespiratory fitness in athletes. Methods: Twenty athletes (21.7±3 years) randomly underwent a crossover design experimental trial. Three visits were arranged. The first visit was for baseline measurement. The second visit included three phases (pre-dehydration, post-dehydration, and post-rehydration), and either Zamzam (mineral water) or bottled water (control water) was used. The third visit was similar to the second visit with an exchange for the type of water. Cardiorespiratory fitness and blood parameters have been evaluated. Data were analyzed to compare the results of Zamzam water with bottled water and to compare the phases for each type of water. Results: although there was no significant difference found between Zamzam and bottled water in the cardiorespiratory fitness markers, Zamzam water maintained cardiorespiratory function including VO2peak, VT1, VT2, and VEpeak even with rehydration equal to 100% of losing body mass following exercise-induced dehydration (>-2% body mass), where rehydration with bottle water reported a significant reduction in both VO2peak and VEpeak. Conclusion: rehydration with mineral water such as Zamzam water may not impair cardiorespiratory fitness even with an amount equal to 100% of losing body mass.
... The human body cannot survive long without fluids because 5560% of body weight is fluid. The balance of the body's metabolism is important when exercising, and water can ensure the body's own metabolic condition is good [29]. Hydration problems will be related to the possibility of cases of dehydration resulting in decreased physical activity. ...
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Introduction. Having good healthy habits and making physical activity a lifestyle is everyone’s dream. Along with improving the quality of life and physical activity, it is expected to affect a person’s body age, hydration level, and fat percentage. Methods. This research is an analytic observational study conducted with a cross sectional approach. Observational analytical research is done by observing, without any action from the researcher. The research subjects were 35 students. Students with cardiovascular and respiratory disorders, who were on an exercise program, and had a history of fractures in the past 3 months or deformities of the arms, were excluded from this study. Students who were 18-21 years old, had a BMI below 30, and did not smoke or drink alcohol were involved in this study. Results. Significant correlation with p-value<0.01 in causality relationship between body age and hydration with Pearson value −0.751, body age and fat percentage 0.773, and Pearson value hydration and fat percentage −0.990. In addition, a significant correlation between quality of life and hydration has a Pearson value of −0.338. Quality of life and physical activity are not related and have a p-value of 0.39. Conclusion. The relationship between body age and hydration, and hydration with fat percentage has a very significant positive relationship. However, quality of life has no correlation with physical activity.
... Indeed, large sweat losses induce significant salt elimination; for example, an athlete who sweats about 5 L will eliminate between 4600 and 5750 milligrams (mg) of sodium. Therefore, an adequate rehydration programme must restore electrolyte losses [8]. Stasiule et al. reported that ingestion of deep mineral water could accelerate recovery of aerobic capacity and leg muscle power compared with ingestion of tap water alone [9]. ...
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A triathlon is an endurance event in which athletes need an efficient hydration strategy since hydration is restricted at different stages. However, it seems that seawater intake can be a suitable hydration alternative for this type of endurance event. Therefore, the aim of this study was to evaluate the efficacy of seawater hydration during a triathlon on cytokine production. Fifteen trained male triathletes (age = 38.8 ± 5.62 years old; BMI = 22.58 ± 2.51 kg/m2) randomly performed three triathlons, one of them consuming seawater (Totum SPORT, Laboratories Quinton International, S.L., Valencia, Spain), the other one consuming tap water ad libitum, and the last a physiologic saline solution as placebo. The triathlon consisted of an 800 m swim, a 90 km bike ride, and a 10 km run. Blood samples were taken at rest and after training, where markers of inflammation, hemoglobin, and hematocrit concentration were assessed. While the seawater was not ergogenic, it significantly increased the release of IL-6 and apelin post-exercise. However, no differences were found between the fractalkine, IL-15, EPO, osteonectin, myostatin, oncostatin, irisin, FSTL1, osteocrin, BDNF, and FGF-21 values over those of the placebo group. The present study demonstrates that hydration with seawater stimulates myokine production, which could lead to improved performance recovery after exercise.
... Dehydration is an adverse consequence of inadequate water intake, fluid loss, or both 6 . The impact of dehydration on health has been researched with regard to physical and cognitive functions 7,8 , with a particular focus on the role of dehydration in the physical actiity of athletes and military personnel 9 . Under mild levels of dehydration, individuals engaging in rigorous physical activity experience decreases in performance related to reduced endurance, increased fatigue, and altered thermoregulatory capability 10 . ...
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This study sought to characterize the impact of long-term dehydration in terms of physiological and biochemical parameters, as well as renal transcriptomes. Furthermore, we assessed whether consumption of specific types of water elicit more beneficial effects on these health parameters. To this end, C57BL/6 mice were either provided water for 15 min/day over 2 and 4 weeks (water restricted; RES), or ad libitum access to distilled (CON), tap, spring, or purified water. Results show that water restriction decreases urine output and hematocrit levels while increasing brain vasopressin mRNA levels in RES mice compared to control mice (CON). Meanwhile, blood urea nitrogen and creatinine levels were higher in the RES group compared to the CON group. Kidney transcriptome analysis further identified kidney damage as the most significant biological process modulated by dehydration. Mechanistically, prolonged dehydration induces kidney damage by suppressing the NRF2-signaling pathway, which targets the cytoprotective defense system. However, type of drinking water does not appear to impact physiological or blood biochemical parameters, nor the renal transcriptome profile, suggesting that sufficient water consumption is critical, irrespective of the water type. Importantly, these findings also inform practical action for environmental sustainability by providing a theoretical basis for reducing bottled water consumption.
... While water does not represent energy reserves per se, it is an essential nutrient (Jéquier and Constant 2010). Dehydration levels corresponding to as little as a 2% decrease of BCI results in marked declines in athletic performance in humans (Murray 2007). Thus, BCI is a meaningful index of condition associated with performance, even if it does not always directly reflect energy stores. ...
The hydration state of animals vying for reproductive success may have implications for the tempo and mode of sexual selection, which may be salient in populations that experience increasing environmental fluctuations in water availability. Using red-sided garter snakes as a model system, we tested the effect of water supplementation on courtship, mating behavior, and copulatory plug (CP) production during a drought year. Over 3 days of mating trials, water-supplemented males (WET males, n = 45) outperformed a control group that was not supplemented with water (DRY males, n = 45). Over 70% of WET males mated but just 33% of DRY males did so. As a group, WET males mated 79 times versus 28 times by DRY males. On the last day of mating trials, over 70% of WET males were still courting, with 19 of them mating, whereas less than 25% of DRY males were courting and only one mated. CP deposition accounted for 4–6% of the mass lost by mating males, but hydration did not affect CP mass or water content. These findings suggest that, in years of low water availability, the number of courting males and the intensity of their courtship declines, thereby affecting sexual selection and conflict, at least within that year.
... Dehydration effects have been well described in athletes (Maughan et al 2007a, Murray 2007, Sawka & Noakes 2007. Under mild levels of dehydration, athletes experience decrements in performance related to a reduction of endurance, increased fatigue, altered thermoregulatory capability, reduced motivation, and increased perceived effort (Cheuvront et al 2003, Montain & Coyle 1992, Paik et al 2009. ...
Water intake is crucial for maintaining body fluid homeostasis and animals’ survival. Complex brain processes trigger thirst, which arises upon losing blood volume (i.e. extracellular dehydration) or increasing blood osmolality (i.e. intracellular dehydration), to replenish water for fluid balance. The brain plays a key role in modulating these processes, but the central mechanisms regulating water intake are not fully understood. Type-1 cannabinoid receptors (CB1) are widely and abundantly expressed in the central nervous system where they modulate a variety of functions, such as memory, anxiety and feeding behavior. However, the role of CB1 receptors in the control of water intake is still a matter of debate, since pharmacological activation or blockade of CB1 receptors produced contradictory results in drinking behavior experiments.My thesis work focuses on the role of CB1 receptors in the control of water intake. By using genetic, pharmacological, anatomical, imaging, and behavioral approaches, I examined the involvement of CB1 receptors in the control of water intake induced by different physiological conditions of extracellular or intracellular dehydration. The results showed that CB1 receptor signaling is required to promote water intake. In particular, global deletion of CB1 receptors does not change plasma osmolality and body water composition, but it decreases water intake induced by water deprivation, systemic or intracerebroventricular (ICV) administration of sodium chloride, or ICV injection of the peptide hormone angiotensin II. In the attempt to better detail the neuronal mechanisms of this function, I discovered that the presence of CB1 receptors in cortical glutamatergic neurons, particularly the ones located in the anterior cingulate cortex (ACC) glutamatergic neurons promote drinking behavior. CB1 receptors are abundantly expressed in axon terminal of ACC glutamatergic neurons projecting to the basolateral amygdala (BLA) and selective expression of CB1 receptors in this circuit is sufficient to guarantee proper drinking behavior in mice. Altogether, these data reveal that CB1 receptors are necessary to promote water intake, and that their presence in the ACC-BLA circuit is sufficient for the top-down control of drinking behavior.Furthermore, I also provided evidence that CB1 controls water intake in different conditions at other levels, e.g. insular cortex, cholinergic cells, and mitochondria.In summary, my thesis work analyzed the role of CB1 receptors in distinct cell populations/neuronal circuits for the control of water intake. These results will help further understanding the functions of the ECS and the brain regulation of thirst.
... High-intensity physical activity in the heat elicits a robust sweating response in order to maintain thermal balance which may result in significant loss of body water content if fluid is not replaced. Although this sweating response is widely variable between individuals and depends on a number of factors, including physical fitness, heat acclimation, and exercise intensity (Coyle, 2004;American College of Sports et al., 2007;Murray, 2007), it is not uncommon to observe sweat rates exceeding 1 liter per hour (Marriott, 1993). Dehydration resulting in a loss of body mass >2% has been associated with a decline in performance (Pitts et al., 1944;Adolph, 1947;Maughan and Noakes, 1991;Below et al., 1995;Sawka et al., 2001; American College of Sports et al., 2007;Maughan and Shirreffs, 2010). ...
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Dehydration ≥2% loss of body mass is associated with reductions in performance capacity, and carbohydrate (CHO)-electrolyte solutions (CES) are often recommended to prevent dehydration and provide a source of exogenous carbohydrate during exercise. It is also well established that performance capacity in the heat is diminished compared to cooler conditions, a response attributable to greater cardiovascular strain caused by high skin and core temperatures. Because hydration status, environmental conditions, and carbohydrate availability interact to influence performance capacity, we sought to determine how these factors affect push-to-the-finish cycling performance. Ten young trained cyclists exercised at a moderate intensity (2.5 W·kg−1) in a hot-dry condition [40°C, 20% relative humidity (RH)] until dehydration of ~2% body mass. Subjects then consumed either no fluid (NF) or enough fluid (water, WAT; Gatorade®, GAT; or GoodSport™, GS) to replace 75% of lost body mass over 30 min. After a 30-min light-intensity warm-up (1.5 W·kg−1) in a 35°C, 20% RH environment, subjects then completed a 120-kJ time trial (TT). TT time-to-completion, absolute power, and relative power were significantly improved in WAT (535 ± 214 s, 259 ± 99 W, 3.3 ± 0.9 W·kg−1), GAT (539 ± 226 s, 260 ± 110 W, 3.3 ± 1.0 W·kg−1), and GS (534 ± 238 s, 262 ± 105 W, 3.4 ± 1.0 W·kg−1) compared to NF (631 ± 310 s, 229 ± 96 W, 3.0 ± 0.9 W·kg−1) all (p < 0.01) with no differences between WAT, GAT, and GS, suggesting that hydration is more important than carbohydrate availability during exercise in the heat. A subset of four subjects returned to the laboratory to repeat the WAT, GAT, and GS treatments to determine if between-beverage differences in time-trial performance were evident with a longer TT in thermoneutral conditions. Following dehydration, the ambient conditions in the environmental chamber were reduced to 21°C and 20% RH and subjects completed a 250-kJ TT. All four subjects improved TT performance in the GS trial (919 ± 353 s, 300 ± 100 W, 3.61 ± 0.86 W·kg−1) compared to WAT (960 ± 376 s, 283 ± 91 W, 3.43 ± 0.83 W·kg−1), while three subjects improved TT performance in the GAT trial (946 ± 365 s, 293 ± 103 W, 3.60 ± 0.97 W·kg−1) compared to WAT, highlighting the importance of carbohydrate availability in cooler conditions as the length of a push-to-the-finish cycling task increases.
Purpose: The purpose of this study is to test the hypothesis that greater habitual water intake is associated with lower risk of dry eye disease (DED). Methods: We included 51 551 participants from the population-based Lifelines cohort (mean age = 51.2 years) in this cross-sectional association study. DED was assessed using the Women's Health Study (WHS) dry eye questionnaire. Water intake was calculated from food frequency questionnaires. Logistic regressions were used to analyse the relationship between DED and water intake or 24-h urine volume, corrected for age, sex, body mass index, physical activity, smoking status, education, income, 48 comorbidities, and 15 medication groups. The main outcome measure was WHS-defined DED. Highly symptomatic dry eye and clinical diagnosis of DED were secondary outcomes. Results: In total, 9.1% of the population had WHS-defined DED. Higher water intake was associated with increased prevalence of WHS-defined DED (OR: 1.011 per 100 ml/day, 95% CI: 1.004-1.017, p = 0.003). After excluding those with a clinical diagnosis, greater water intake was still tied to increased risk of having DED symptoms (OR: 1.010 per 100 ml/day, 95% CI: 1.006-1.015, p < 0.001). Higher 24-h urine volumes were also associated with higher risk of WHS-defined DED (OR: 1.010 per 100 ml/day, 95% CI: 1.005-1.015, p < 0.001). Conclusions: In this large, population-based study, higher water intake was not tied to reduced risk of DED. Rather, it was associated with a modest increased risk of DED. Interventional studies are needed to fully understand the effect of water intake on DED, but this study found no evidence that greater water intake is beneficial for DED.
Introduction One of the challenges confronting the coaches and sport scientist is to understand the physical factors contributing to successful performance. One common method to identify the appropriate training program for improving fitness level is the analysis of the effect of these practices on various factors of training exercises. The initial testing session can give the athletes and coach an information of current functional capability at the start of a program and can allow them to compare that capacity with reference values from appropriate peer group, so that future testing can be compared to this and any changes can be noted .Also the assessment of current status reveals strengths and relative weaknesses and can be the basis for development of an optimal training program(1) .The purpose of this study was to investigate the effect of an eight-weeks general preparation exercise on some selected biomechanical, anthropometrical and physiological parameters of the Iranian national women Taekwondo team. Methodology Nine elite member of Iranian national women Taekwondo team (age 23.22±1.98 years old, and weight 61.88±8.44 kg) participated in this study. All participants read and signed an approved informed consent form. The exercise program consist of 72 sessions (3 session reviewing and practicing techniques ,2 sessions of combat (live) taekwondo ,2 sessions strength training, ,1 session interval running and 1 session of endurance training each week). The testing was conducted twice, before and after the 8-week training period. The biomechanical, anthropometrical and physiological parameters included; body weight, cardiovascular endurance, muscular endurance, speed, agility, visual reaction time, anaerobic power and body composition. body composition analyzer (in body 220) was used to assess the body fat percentage, reaction time was measured with visual reaction time apparatus(Satrap company, Iran), Bruce protocol was used to estimate vo2 max, a 40-yard sprint test was used to assess the speed, 4×9 m shuttle run test was required to assess agility , 1-min bilateral jump was used for assess the lower extremities endurance and 15 second ergo- jump and Sargent test was required to assess the anaerobic power of the subjects. And inferential statistics Kolmogorov-Smirnov test was used to check the normality distribution, using a paired t- test tests to compare variables before and after training, with a significant level of (p≤0.05) Result The results of functional test are presented in table 1. The bf % of the subjects significantly decreased .The result of body weight, agility, visual reaction time and anaerobic power (ergo jump test) test were slightly lower in after training. The result of anaerobic power, aerobic test and muscle endurance significantly increased. Table 1. P-value t(8) post-test pre-test Variable 0.347 1 61.55±8.30 81.88±8.44 Weight (kg) *0.000 5.888 15.05±5.04 16.78±5.38 BF (%) 0.128 -1.7 6.29±0.255 6.19±0.288 Speed (m/s) *0.001 -5.406 146.44±8.38 141.55±9.51 Muscle endurance( rep∙min-1) 0.176 1.486 9.24±0.241 9.35±0.352 Agility (s) 0.127 1.705 0.416±0.048 0.438±0.352 Visual reaction time(ms) *0.01 -3.344 909.01±136.46 878.66±121.40 Aneorobic power sargent (w) 0.787 0.279 35.33±5.97 35.55±7.95 aneorobic power ergo-jump (w∙kg-1) *0.000 -8.083 55.55±5.57 48.55±5.12 VO2max (ml∙kg-1∙min-1) * Differences are significant at the 0.05 level. Discussion and Conclusion In previous studies cited that having good anaerobic and aerobic capacity, power, agility are most important factors needed to achieve good result in taekwondo(2, 3). In this regard, the main emphasize of general preparation phase is enhance the cardiovascular endurance and muscular strength, significant reduction in bf% and significant increase in aerobic and anaerobic factors following exercise was similar to the other investigation(4, 5). Finally, these results can be use as a feedback to the coaches to review the applied training protocols.
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Latzka, William A., Michael N. Sawka, Scott J. Montain, Gary S. Skrinar, Roger A. Fielding, Ralph P. Matott, and Kent B. Pandolf.Hyperhydration: thermoregulatory effects during compensable exercise-heat stress. J. Appl. Physiol. 83(3): 860–866, 1997.—This study examined the effects of hyperhydration on thermoregulatory responses during compensable exercise-heat stress. The general approach was to determine whether 1-h preexercise hyperhydration [29.1 ml/kg lean body mass; with or without glycerol (1.2 g/kg lean body mass)] would improve sweating responses and reduce core temperature during exercise. During these experiments, the evaporative heat loss required (E req = 293 W/m ² ) to maintain steady-state core temperature was less than the maximal capacity (E max = 462 W/m ² ) of the climate for evaporative heat loss (E req /E max = 63%). Eight heat-acclimated men completed five trials: euhydration, glycerol hyperhydration, and water hyperhydration both with and without rehydration (replace sweat loss during exercise). During exercise in the heat (35°C, 45% relative humidity), there was no difference between hyperhydration methods for increasing total body water (∼1.5 liters). Compared with euhydration, hyperhydration did not alter core temperature, skin temperature, whole body sweating rate, local sweating rate, sweating threshold temperature, sweating sensitivity, or heart rate responses. Similarly, no difference was found between water and glycerol hyperhydration for these physiological responses. These data demonstrate that hyperhydration provides no thermoregulatory advantage over the maintenance of euhydration during compensable exercise-heat stress.
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It is the position of the American College of Sports Medicine that adequate fluid replacement helps maintain hydration and, therefore, promotes the health, safety, and optimal physical performance of individuals participating in regular physical activity. This position statement is based on a comprehensive review and interpretation of scientific literature concerning the influence of fluid replacement on exercise performance and the risk of thermal injury associated with dehydration and hyperthermia. Based on available evidence, the American College of Sports Medicine makes the following general recommendations on the amount and composition of fluid that should be ingested in preparation for, during, and after exercise or athletic competition: 1. It is recommended that individuals consume a nutritionally balanced diet and drink adequate fluids during the 24-h period before an event, especially during the period that includes the meal prior to exercise, to promote proper hydration before exercise or competition. 2. It is recommended that individuals drink about 500 ml (about 17 ounces) of fluid about 2 h before exercise to promote adequate hydration and allow time for excretion of excess ingested water. 3. During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated. 4. It is recommended that ingested fluids be cooler than ambient temperature[between 15° and 22°C (59° and 72°F)] and flavored to enhance palatability and promote fluid replacement. Fluids should be readily available and served in containers that allow adequate volumes to be ingested with ease and with minimal interruption of exercise. 5. Addition of proper amounts of carbohydrates and/or electrolytes to a fluid replacement solution is recommended for exercise events of duration greater than 1 h since it does not significantly impair water delivery to the body and may enhance performance. During exercise lasting less than 1 h, there is little evidence of physiological or physical performance differences between consuming a carbohydrate-electrolyte drink and plain water. 6. During intense exercise lasting longer than 1 h, it is recommended that carbohydrates be ingested at a rate of 30-60 g · h-1 to maintain oxidation of carbohydrates and delay fatigue. This rate of carbohydrate intake can be achieved without compromising fluid delivery by drinking 600-1200 ml· h-1 of solutions containing 4%-8% carbohydrates (g · 100 ml-1). The carbohydrates can be sugars (glucose or sucrose) or starch (e.g., maltodextrin). 7. Inclusion of sodium (0.5-0.7 g · 1-1 of water) in the rehydration solution ingested during exercise lasting longer than 1 h is recommended since it may be advantageous in enhancing palatability, promoting fluid retention, and possibly preventing hyponatremia in certain individuals who drink excessive quantities of fluid. There is little physiological basis for the presence of sodium in an oral rehydration solution for enhancing intestinal water absorption as long as sodium is sufficiently available from the previous meal.
Fluid intake became a focus of systematic research in the early part of this century in military investigations of dehydration in soldiers marching or working under hot conditions. Subsequently, the concept of voluntary dehydration was introduced.
It is well recognized that fluid ingestion can benefit performance in many exercise situations, and an extensive literature is devoted to the various performance-enhancing effects of different beverage formulations. The interest of the athletic community in sports drinks is largely confined to their potential for improved performance. To the scientist, however, the administration of drinks of varying composition during exercise offers a tool for the study of the normal physiological response to exercise. Indeed, exercise itself is often used as a model for the investigation of normal physiological function, a scientific approach that also benefits the athlete. If the normal responses to exercise are understood, and if the sequelae of fluid ingestion are also known, then predictions can be made that will allow optimizsation of the formulation of drinks intended to improve performance.
This study assessed whether replacing sweat losses with sodium-free fluid can lower the plasma sodium concentration and thereby precipitate the development of hyponatremia. Ten male endurance athletes participated in one 1-h exercise pretrial to estimate fluid needs and two 3-h experimental trials on a cycle ergometer at 55% of maximum O 2 consumption at 34°C and 65% relative humidity. In the experimental trials, fluid loss was replaced by distilled water (W) or a sodium-containing (18 mmol/l) sports drink, Gatorade (G). Six subjects did not complete 3 h in trial W, and four did not complete 3 h in trial G. The rate of change in plasma sodium concentration in all subjects, regardless of exercise time completed, was greater with W than with G (−2.48 ± 2.25 vs. −0.86 ± 1.61 mmol ⋅ l ⁻¹ ⋅ h ⁻¹ , P = 0.0198). One subject developed hyponatremia (plasma sodium 128 mmol/l) at exhaustion (2.5 h) in the W trial. A decrease in sodium concentration was correlated with decreased exercise time ( R = 0.674; P = 0.022). A lower rate of urine production correlated with a greater rate of sodium decrease ( R = −0.478; P = 0.0447). Sweat production was not significantly correlated with plasma sodium reduction. The results show that decreased plasma sodium concentration can result from replacement of sweat losses with plain W, when sweat losses are large, and can precipitate the development of hyponatremia, particularly in individuals who have a decreased urine production during exercise. Exercise performance is also reduced with a decrease in plasma sodium concentration. We, therefore, recommend consumption of a sodium-containing beverage to compensate for large sweat losses incurred during exercise.
Eight endurance-trained cyclists rode as far as possible in 1 h on a stationary cycle simulator in a moderate environment (20C, 60% relative humidity, 3 ms–1 wind speed) while randomly receiving either no fluid (NF) or attempting to replace their approximate 1.71 sweat loss measured in a previous 1-h familiarisation performance ride at approximately 85% of peak oxygen uptake with artificially sweetened, coloured water (F). During F, the cyclists drank mean 1.49 (SEM 0.14)1 of which mean 0.27 (SEM 0.08)1 remained in the stomach at the end of exercise and mean 0.20 (SEM 0.05) 1 was urinated after the trial. Thus, only mean 1.02 (SEM 0.12)1 of the ingested fluid was available to replace sweat losses during the 1-h performance ride. That fluid decreased the mean average heart rate from 166 (SEM 3) to 157 (SEM 5) beatsmin–1 (P < 0.0001) and reduced the final mean serum [Na–] and osmolalities from 143 (SEM 0.6) to 139 (SEM 0.6) matom1–1(P < 0.005) and from 294 (SEM 1.7) to 290 (SEM 1.9) mosmol1–1 (P = 0.05), respectively. Fluid ingestion did not significantly attenuate rises in plasma anti-diuretic hormone and angiotensin concentrations, or decrease the approximate-15% falls in estimated plasma volume in the F and NF trials. Nor did fluid ingestion significantly affect the approximate 1.71 h–1 sweat rates, the rises in rectal temperature (from 36.6 to 38.3C) or the ratings of perceived exertion in the two trials. Ingestion of approximately 1.51 of fluid produced an uncomfortable feeling of stomach fullness and reduced the mean distance covered in 1 h from 43.1 (SEM 0.7) to 42.3 (SEM 0.6) km (P < 0.05). Thus, trying to replace more than 1.01h–1 sweat losses during high-intensity, short duration exercise in a moderate environment would not appear to induce beneficial physiological effects, and may impair exercise performance.