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Does Dehydration Impair Exercise Performance?
Dehydration (water deficits of >2% body mass, BM)
degrades aerobic exercise performance in temperate
and warm-hot environments. This prevailing view is
supported by:
1. observations made on individuals performing
arduous work, with limited fluids, in warm-hot
2. the vast majority of experimental data from
laboratory and field studies in temperate and
warm-hot environments;
3. paucity of experimental data indicating otherwise;
4. ‘expert’’ panels performing comprehensive litera-
ture reviews.
Total body water (TBW) fluctuates within ~0.5% of
BM or more, thus ‘‘euhydration’’ represents a continu-
um for which no one absolute or ‘‘true’’ TBW value can
be assigned (31). A determination must therefore be
made of body water fluctuations beyond the ‘‘normal’
range that have a consistent deleterious functional
outcome (1). Cheuvront and colleagues (16) proposed
that a water deficit of >2% of BM falls beyond the
normal TBW fluctuation and consistently demonstrates
a reduction in endurance exercise performance. Bio-
logical variation will allow some individuals to be more
or less tolerant to the adverse consequences of
dehydration; however, this is not the topic of debate.
Review of Literature
Experimental Studies—Support
Twenty-eight studies support the prevailing view
that dehydration degrades aerobic exercise perfor-
mance. These consistent findings were observed despite
the studies employing a broad range of experimental
It is definitively established that some humans who
either (i) begin an exercise test with unreplaced acute
fluid losses induced, for example, by prolonged (6 h)
exposure to either a hot environment (19) or to
prolonged (2 h) prior exercise in uncomfortable heat
during which they did not ingest any fluid (21) or after
pretreatment with a diuretic that increases urinary water
and electrolyte losses (2) by inducing the uncomfort-
able symptoms of polyuria; or who (ii) drink either
nothing or little during exercise [for example, (10,
33,59,75)], will show a measurably impaired exercise
performance. Since I am a coauthor of two studies
(23,78) showing this effect under more realistic
experimental conditions, it would be disingenuous to
deny that it exists. Indeed I have recently written that:
‘these data clearly support the conclusion that not
drinking at all during exercise is likely to impair
exercise performance especially during prolonged
exercise in the heat’ (50).
Thus there is no need to debate whether or not the
exercise performance of some individuals will be impaired
if they either begin exercise in a state in which their total
body water content (TBW) has been artificially reduced or
if they are not allowed to drink during exercise. Rather my
argument is that the results of these experiments have been
interpreted according to a simplistic model of human
physiology. Here I argue that it is the development of thirst
that impairs exercise performance so that athletes who
drink sufficiently to prevent thirst during exercise will
optimize their performance regardless of the extent to
which their TBW is reduced or the extent to which they
become ‘‘dehydrated.’’ I present my argument by address-
ing four fundamental questions:
i. Of what relevance are most of these ‘‘classically
cited studies’’ to the real world of competitive sport
and recreational exercise?
ii. What is the optimal experimental design to study
the effects of ‘‘dehydration’’ on exercise perfor-
iii. What biological mechanism(s) likely explain this
iv. On the basis of this information, how should we
advise athletes to drink during exercise?
Contrasting Perspectives in Exercise Science & Sports Medicine
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DOI: 10.1249/mss.0b013e318124a664
Copyright @ 200 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.
methods regarding dehydration/rehydration procedures,
aerobic exercise tasks and environmental conditions.
During World War II, U.S. military forces fought in hot-
tropic and hot-desert environments and it was observed
that fluid consumption influenced their operational
effectiveness. Such observations stimulated an initial
series of controlled laboratory (6,59) and field (10)
studies that demonstrated that dehydration degrades
aerobic exercise performance (outdoor hikes, treadmill
walks) in warm-hot environments.
During the next five decades, other investigators,
employing more sophisticated laboratory exercise/per-
formance tests, consistently reported that dehydration
degrades aerobic exercise performance in temperate
environments (5,7,12–14,17,25,29,37,41,46,61,63,70)
and warm-hot environments (19,33,57,58,70,78). In
addition, investigators employing fieldlike evaluations
reported that dehydration degrades aerobic exercise
performance while in temperate (2,79) and warm-hot
(43,72,75) environments. Of note are the findings that
the greater the dehydration level, the greater the
degradation in aerobic exercise performance (19,58).
Also larger and more consistent degradations in aerobic
exercise performance were observed in warm-hot
environments than temperate environments.
Supporting Exemplars
The following experimental exemplars employed differ-
ent methods, but reached conclusions supporting the
prevailing view. Two studies were selected employing
increasing intensity performance tests, one study was
conducted in a temperate environment (46) and the other
study was conducted in a hot environment (19). In
addition, three studies were selected employing constant
intensity exercise followed by volitional effort perform-
ance tests (at submaximal work rates), with one study
conducted in a temperate environment (17), and the other
studies conducted in warm-hot environments (7,78).
Nielsen et al. (46) had subjects perform cycle
exercise to exhaustion (by increasing power output to
105% V
) in temperate conditions when euhy-
drated and when dehydrated by ~2.5% BM. Three
dehydration trials were conducted with water deficits
achieved by prior heat exposure, prior exercise or
diuretic administration. Dehydration reduced the total
work performed by 18%–44% for the three trials. Craig
and Cummings (19) had subjects walk to exhaustion
(by increasing treadmill grade) in hot conditions when
euhydrated and when dehydrated by ~2% BM and ~4%
BM. Subjects dehydrated by prior resting heat exposure
with restricted water intake. Total exercise time was
reduced by 22% and 48%, while V
was reduced
by 10% and 22% when dehydrated by 2% and 4% BM,
There are two features common to the conduct of
many laboratory experiments performed in applied
exercise science. First, studies are usually (and cor-
rectly) designed to establish the biological consequen-
ces of a specific intervention; for example, the effects
of a preexercise reduction in TBW or a restricted fluid
intake on physiology and exercise performance.
But the design of these experiments often enforces
behaviors that athletes would never adopt during either
competitive sport or recreational exercise. Yet the
authors of these studies usually generalize the study
findings to real-world conditions that were not simu-
lated in the original experiments.
For example, no athlete other than those involved in
sports involving weight categories, would start exercise
after spending 6 h in a hot, dry environment without
drinking as was the case in the study of Craig and
Cummings (19). Nor would they ‘‘warm up’’ by
exercising for 2 h without drinking as occurred in the
study of Dougherty et al. (21). Nor, unless they were
under treatment for hypertension, would they consider
exercising after the acute administration of a diuretic as
in the study of Armstrong et al. (2). Nor would many
modern athletes exercise with enthusiasm for any
length of time if they knew beforehand that they would
be forced to drink less than their thirst dictates, as in
many of the original classical studies (2,10,33,59,75).
Second, the findings of these studies are inevitably
interpreted according to the philosophy of reductionism
(52) and according to a simplistic model of exercise
physiology (53). So it is usually assumed that any
measured performance impairment must be caused
exclusively by the variable that is most usually measured,
namely the level of ‘dehydration’’ that develops before
or during exercise. Then the conclusion is generalized
that performance in any form of exercise, regardless of
circumstance, will always be impaired whenever that
specific (critical) level of ‘‘dehydration’’ is exceeded. But
exercise performance is a complex phenomenon that is
seldom regulated by a single variable (74).
Studies that most obviously invite this criticism of
reductionism are those in which ‘‘dehydration’’ is
induced with diuretics. One frequently referenced study
(2) failed to include controls to measure any direct
effect of the drug on exercise performance. Rather it
was presumed that the drug acts exclusively by
reducing the TBW. But athletes cannot begin an
exercise bout in an ideal frame of mind if they have
spent the previous 4 h repeatedly relieving themselves
of the uncomfortable symptoms caused by polyuria.
Similarly athletes who begin an exercise session
knowing that they will not receive any fluid, will have a
different mental attitude than if they knew beforehand
that they would be allowed to drink freely. For
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Cheuvront et al. (17) had subjects perform cycle
exercise 30 min at constant intensity (~50% V
followed by a constant duration performance time trial
(total work completed in 30 min) in a temperate
environment. Subjects were dehydrated (3% BM) by
resting heat exposure with restricted fluid replacement.
Dehydration reduced the total work completed by 8%.
Below et al. (7) had subjects perform cycle exercise
for 50 min at constant intensity (~80% V
followed by a constant work performance test (time to
complete a set amount of work) in warm-hot conditions.
Subjects either replaced (0.5% BM loss) or under-
replaced (2% BM loss) their sweat losses during
these exercise sessions. Dehydration reduced the total
work completed by 7%. Walsh and colleagues (78)
had subjects perform cycle exercise (70% V
for 60 min followed by a performance test (time to
exhaustion at 90% V
) in a warm-hot environ-
ment. Subjects either replaced or underreplaced (2%
BM loss) sweat losses during these exercise sessions.
Dehydration reduced the time to exhaustion by 31%.
Experimental Studies—Nonsupport
No study disagreed with the prevailing view in
warm-hot environments. Seven studies found that
dehydration did not degrade aerobic exercise perform-
ance in temperate environments. Four of these studies
employed marginal (G2% BM) dehydration levels
(3,9,36,38). Therefore, only three studies (30,62,71)
report data that may refute the prevailing view. One of
those studies (71) employed an activity with a
substantial anaerobic component (~4 min of high-
intensity rowing) and dehydration does not degrade
anaerobic exercise performance (31). Another study
(62) reported that subjects complained of stomach
fullness during the fluid replacement trials so discom-
fort may have confounded their results.
The last study (30) not support the prevailing view
employed maximal aerobic power (V
) as their only
index of aerobic exercise performance. Dehydration
consistently degrades V
in warm-hot environments
(19,57), but in temperate environments V
either may
(9,13,14) or may not (9,13,14) be degraded. However,
investigators who observed no change in V
dehydration but who included other indices of aerobic
exercise performance, have all reported that dehydration
degraded that other index (2,14,63). Therefore, the data
from Houston and colleagues (30) does not necessarily
refute the prevailing view.
Expert Panels—Support
Three ‘‘expert’’ panels have been established to
evaluate scientific evidence regarding several hydration
example, our as yet unpublished study (22) found that
subjects aware at the start of exercise that their fluid
intakes during exercise would be restricted, began the
exercise bout at a lower exercise intensity than when
they knew they could drink without restriction. Since
this effect was present at the outset of exercise, it could
not be due to ‘‘dehydration’’ but was due to a
behavioral change ‘‘in anticipation’’ of the exercise
bout. We have indeed shown that exercise performance
in the heat is regulated ‘‘in anticipation’’ (77).
If we do not understand all the factors that alter
exercise behavior ‘‘in anticipation,’’ it is presumptuous
to assume that alterations in exercise performance
produced by interventions imposed before the exercise
bout, can all be explained by a single variable,
‘dehydration,’’ measured during exercise.
Since we cannot separate the effect on subsequent
exercise performance of the interventions that produce
the ‘‘dehydration’’ from any effects resulting directly
from that ‘‘dehydration,’’ it follows that only studies in
which the ‘‘dehydration’’ develops during exercise
provide a valid measure of the direct effects of
‘dehydration.’’ Furthermore, since the natural behavior
is to drink ad libitum during exercise, that is, in
response to the dictates of thirst, it follows that the
control drinking condition in all experiments must be
the ad libitum condition.
Whereas such experiments will establish the drinking
behavior that optimizes exercise performance, yet they
cannot easily determine the exact mechanism explaining
this effect. For it is difficult to exclude the possibility
that the impaired exercise performance is part of the
thirst mechanism regulated centrally in the brain as part
of a complex and intelligent control system (74).
For example, our model of exercise regulation
(52,74) theorizes that sensory feedback from multiple
organs and tissues regulates the extent of skeletal
muscle recruitment during exercise and in this way
the exercise intensity that can be sustained. According
to this model, thirst, like the rating of perceived
exertion (73), acts as the conscious perception of a
developing cellular homeostatic disturbance (in this
case an unacceptable increase in plasma osmolality due
to a large reduction in TBW) to induce two crucial
behavioral changes, the goal of which are to limit the
extent of this disturbance. These behavior changes are
(i) an increase in drinking and (ii) a reduction in the
exercise intensity (until the moment the athlete
addresses the cause of the homeostatic disturbance by
starting too drink appropriately).
The essential component of this model is that it
predicts behavior change (increased drinking and
altered exercise intensity) before a catastrophic failure
of homeostasis develops (53). We have shown that this
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issues. The Institute of Medicine, National Academies
of Science appointed a panel on dietary reference
intakes for water and electrolytes that concluded that
dehydration ‘‘adversely influences aerobic exercise
tasks’’ (31). The American College of Sports Medicine
appointed two panels to conduct Strength of Recom-
mendation Taxonomy analyses on hydration issues, and
those panels both concluded (with highest strength of
support rating) that dehydration degrades aerobic
exercise performance (1,15).
There is overwhelming support from experimental
studies demonstrating that dehydration (>2% BM)
degrades aerobic exercise performance in temperate
and warm-hot environments. Any effective argument
against this prevailing view must be supported by
sound experimental studies demonstrating otherwise.
Dr. Noakes makes two points: 1) he concedes that
‘some’’ humans who dehydrate before or during
exercise will suffer impaired performance and con-
cludes there is ‘‘no need to debate’’ that issue, and 2) he
then argues that dehydration experiments should not be
interpreted ‘‘according to a simplistic model of human
physiology,’’ and it is ‘‘the development of thirst that
impairs exercise performance.’
Concession that Dehydration Impairs
Exercise Performance
Regarding the first point, use of the adjective
‘some’’ subjects implies less than the vast majority
and unfairly understates the overwhelming conclusion
of experimental studies that dehydration (>2% BM)
degrades aerobic exercise performance. In addition,
performance degradation was consistently demonstrat-
ed in studies employing a broad range of fluid loss and
rehydration (including drinking during exercise) meth-
odologies, which only strengthens the prevailing view.
‘Simplistic Model of Human Physiology’’?
Physiological mechanisms explaining dehydration-
mediated performance decrements are not simplistic
and they include augmented hyperthermia (18),
increased cardiovascular strain (39,40), reduced skele-
tal muscle blood flow (27), altered skeletal muscle
metabolism (increased glycogen utilization and lactate
accumulation) (28), altered EMG patterns (8), and
increased perception of effort (39) during exercise.
Though each physiological mechanism is unique,
evidence suggests they contribute in an integrated and
multifactorial manner to degrade performance (67). The
complex, intelligent model adequately explains the brain_s
behavioral response to the stress of exercise in the heat
(77). But traditionally taught exercise physiologists who
fail to understand the anticipatory nature of these controls
will conclude incorrectly that the very endpoint that the
brain wishes to avoid—in this case a marked rise in
plasma osmolality consequent to a large reduction in
TBW (‘‘dehydration’’)—is, in their interpretation, the
singular cause of the altered exercise performance.
Presently the published literature includes six studies
[Table II in (50)] with an appropriate experimental
design (4,7,20,22,37,38) to distinguish (i) the direct
effects of dehydration from those potentially due to the
techniques inducing the ‘‘dehydration’’ and (ii) the
possible role of thirst as a regulator of the exercise
response. These studies allow a precise conclusion: In
no study did drinking more fluid than that dictated by
thirst (and which result in lower levels of ‘‘dehydra-
tion’’) produce a superior performance than did ad
libitum drinking (according to the dictates of thirst).
Hence, these studies support the predictions of the
model that holds that thirst functions also to modulate
the exercise performance as part of an anticipatory
control and that drinking ‘‘beyond thirst’’ to prevent
‘dehydration’’ will not produce a superior response.
The finding that drinking according to the dictates of
thirst optimizes exercise performance (50) suggests that
the absolute level of ‘‘dehydration’’ does not explain
why underdrinking during exercise impairs perform-
ance. Were this the case then all athletes would develop
an identical level of ‘‘dehydration’’ when ingesting
fluid ad libitum during exercise. This level would be
somewhat less than the critical value that some believe
impairs exercise performance (21). But this is clearly
not the case. Rather our study [Fig. 1 in (15)] found that
athletes competing in endurance events in which fluids
are provided ad libitum, finish with body weight (BW)
changes that vary from a loss of -11% BW to a gain of
+6% BW. We have also provided evidence that genetic
factors regulating the thirst response might explain this
highly individualistic response (64).
Thus we have proposed that correct drinking during
exercise influences performance by preventing thirst,
the development of which alters the exercise behavior
in an anticipatory manner as part of a regulatory
process. Provided the athlete is not thirsty, we speculate
that a level of ‘‘dehydration’’ even up to -11% BW
might not impair exercise performance (68,69).
This is simple: Drink according to the dictates of
thirst. If you are thirsty, drink; if not, do not. All the
rest is detail.
Imprecise experimental design and the interpretation
of research studies according to a simplistic model of
human physiological function during exercise, do not
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relative contribution of each mechanism likely differs
depending on the exercise task, environment, and
athlete_s biomedical state, but augmented hyperthermia
appears critical to performance degradation.
Dehydration augments hyperthermia (at a given
metabolic rate) during aerobic exercise in temperate
and warm-hot environments (11,65). The augmented
hyperthermia is proportionate to the magnitude of water
deficit (39,40,66). Dehydration augments hyperthermia
by reducing heat dissipation, as both sweating (42,66)
and skin blood flow (32,45) are reduced for a given
core temperature. Dehydration from sweat losses will
increase plasma osmolality and decrease plasma vol-
ume in proportion to the magnitude of body water
deficit (31). Hyperosmolality and hypovolemia both
exert separate and combined effects on reducing heat
dissipation and thus augmenting hyperthermia when
dehydrated (18).
Hyperthermia alone increases cardiovascular strain,
alters skeletal muscle metabolism (26), reduces cerebral
blood flow and alters brain metabolism (56), alters
CNS activity (55), attenuates muscular force genera-
tion (54), and increases perception of effort. These
physiological events elicited by hyperthermia alone
are similar to the physiological events elicited by
dehydration, which augments hyperthermia. Finally,
experimental evidence on hyperthermia mediated phys-
iological events and associated laboratory performance
test degradation are supported by analyses demonstrat-
ing that actual marathon race performance is progres-
sively degraded with increasing environmental heat
stress (24).
‘Thirst Impairs Exercise Performance’’?
Dr. Noakes notion that ‘‘it is the development of
thirst that impairs exercise performance—as part of an
anticipatory control’’ is not necessarily consistent with
known physiology and lacks experimental support.
First, many key physiological signals (hyperosmolality,
hypovolemia) for thirst are the same signals mediating
augmented hyperthermia and cardiovascular strain from
dehydration (34). Second, I am unaware of data
supporting that thirst is a feed-forward process that
either precedes any osmotic-volume signals or dehy-
dration, or if occurring alone will mediate physiological
events consistent with performance degradation. Third,
of the six studies claimed to support the ‘‘thirst impairs
performance’’ notion, no study measured thirst, three
studies (4,7,38) did not achieve dehydration >2% BM,
three studies (7,37,38) did not employ ad libitum
drinking trials, and two studies (4,20) had overdrinking
causing uncomfortable gastric fullness, likely impact-
ing performance. Fourth, it is difficult to be convinced
that 11% BM dehydration does not degrade aerobic
allow the conclusion that the level of ‘‘dehydration’’ is
the exclusive cause of any impaired exercise perfor-
mance measured in subjects who begin exercise with
either a reduced TBW or who drink less than their thirst
dictates during exercise. A more defendable conclusion
is that any impairment may be a component of the thirst
mechanism, regulated centrally in the brain, the goal of
which is to ensure that athletes do not damage their
health by continuing to exercise while drinking too
little during exercise. In which case the prevention of
thirst rather than ‘‘dehydration’’ will optimize exercise
The key point in my challenging perspective was
that scientists have used a simplistic (reductionistic)
model of exercise physiology to interpret the results of
studies to determine whether ‘‘dehydration’’ alters
exercise performance. But when interpreted according
to a complex model of human exercise performance
(74), their usual experimental designs cannot prove that
a specific level of ‘‘dehydration’’ is the direct cause of
impaired exercise performance in all humans. These
designs cannot disprove the alternate theory that the
sensation of thirst, acting as part of an anticipatory
regulatory system, alters the exercise behavior in those
who either begin exercise in a ‘‘dehydrated’’ state or
who drink less than their thirst dictates.
Rather, I proposed that exercise performance is
maximized by drinking according to the dictates of
thirst (ad libitum) (50). Since ad libitum drinking which
optimizes performance does not produce identical
levels of dehydration in all humans during exercise,
but can lead to changes in per cent body weight (BW)
ranging from a gain of 6% to a loss of 11% (51), then
the conclusion that a specific level of dehydration, say
a BW loss of 2%, will always cause an impaired
exercise performance in all humans cannot be correct.
Instead I argue that any impaired exercise performance
in ‘‘dehydrated’’ subjects must be causally linked to the
thirst mechanism, not to the level of ‘‘dehydration’
(since in the absence of thirst exercise performance
may not be impaired, regardless of the level of
‘dehydration’’ that is present).
I also argue that the level of ‘‘dehydration’’ (detected
as a change in plasma osmolality) will be one of the key
homeostatic variables that a complex system will actively
regulate during exercise. According to this interpretation,
‘dehydration’’ is not the direct ‘‘cause’’ of an impaired
exercise performance. Rather, exercise performance is
modified (impaired) under certain stressful conditions in
order to ensure that the osmolality of the brain remains
within the homeostatic range. To restate the argument:
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exercise when it is unclear if hydration levels were
carefully determined and when there are no experi-
mental controls to verify that performance was pre-
served (68,69).
Dehydration (water deficits of >2% BM) consis-
tently degrades aerobic exercise performance in tem-
perate and warm-hot environments. Physiological
mechanisms contributing to dehydration-mediated per-
formance degradation include augmented hyperthermia,
increased cardiovascular strain, altered metabolic and
central nervous system functions, and increased per-
ception of effort. Dr. Noakes may not agree with
conventional physiological models explaining dehydra-
tion mediated performance degradation, but his alter-
nate notion awaits supportive experimental evidence.
An unacceptable reduction in the total body water
content is the endpoint that the brain must avoid if it is
not to be damaged by a profound rise in intracellular
osmolality. Thus, the brain must regulate the athlete_s
behaviors ‘‘in anticipation’’ to ensure that only accept-
able levels of ‘‘dehydration’’ develop during exercise.
Progressively more severe thirst will increase the
athlete_s discomfort leading to an involuntary reduction
in the work rate. Ultimately the athlete will chose to
stop exercising and to seek fluids.
Finally, this interpretation explains two phenomena
that the promoters of the ‘‘dehydration hypothesis’’ (48)
chose to ignore that: 1) elite athletes appear not to drink
very much during exercise (44,47) and 2) the fastest
finishers in endurance events are often among the most
dehydrated (17,44,60,68,69,80). If these athletes are less
likely to become thirsty as a result of genetic variance
(64), then this paradox is more easily understood.
Dr. Sawka_s prevailing perspective is carefully con-
structed and accurately reviews the state of knowledge in
this field. But the data have been interpreted according
to the conventional reductionistic model of exercise
physiology that I consider simplistic. Thus, it is not
the findings that are wrong, merely how they have
been interpreted. For when interpreted according to the
predictions of a complex regulatory system, nothing in
Dr. Sawka_s review disputes my conclusions.
Thus, his first three exemplar studies (16,19,46) all
show that exercise performance is impaired in those
who begin exercise in a ‘‘dehydrated’’ state induced by
different interventions. Logically, neither these nor any
other experimental design can determine whether it is
the development of thirst or the ‘‘dehydration’’ produced
by these interventions that impairs subsequent exercise
However, the finding by Dr. Sawka and his colleagues
(16) that prior ‘‘dehydration’’ impairs subsequent exercise
performance in the heat but not in the cold, is more easily
explained by a complex intelligent system that uses the
thirst mechanism to regulate the exercise performance.
For the measured biological responses to exercise in
that study were remarkably similar in both the heat and
the cold, regardless of each subject_s level of hydration.
This proves that the impaired exercise performance in
the ‘‘dehydrated’’ condition was not caused by those
commonly measured variables, such as increased
cardiovascular ‘‘strain’’ or accelerated hyperthermia,
which are usually invoked to explain how ‘‘dehydra-
tion’’ impairs exercise performance (17).
Instead the ratings of perceived exertion (RPE) were
identical in all exercise conditions despite differences in
exercise performance. But since ‘‘dehydrated’’ subjects
exercised at a lower work rate in the heat, their RPE
must have been ‘‘upregulated’ by the presence of thirst
Michael N. Sawka
U.S. Army Research Institute of
Environmental Medicine
Natick, MA
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or by ‘‘dehydration,’’ or by both. This regulatory
mechanism insures that the exercise work rate is reduced
‘in anticipation’’ (35), presumably to ensure that the
brain is protected from a large change in its osmolality.
We have repeatedly argued that the RPE is used by
the complex intelligent system to ensure that humans
terminate exercise while still in homeostasis (49,76,77).
Thus, the study of Dr. Sawka and his colleagues (16)
confirms that ‘‘dehydration’’ modifies the RPE
response to exercise as would be expected if a complex
system regulates human exercise performance.
During the other two exemplar studies invoked by
Dr. Sawka, subjects drank either nothing (78) or less
than the usual ad libitum volumes (5) during exercise.
Thus, neither conflicts with our hypothesis that it is
thirst acting ‘‘in anticipation’’ that modifies exercise
performance specifically to limit the extent to which
the brain_s osmolality changes during exercise.
We propose that, as part of a complex system of
exercise regulation (74), the symptom of thirst acts ‘‘in
anticipation’’ to regulate the exercise performance in
those who drink less than is necessary to maintain an
acceptable level of homeostasis. According to this
interpretation, it is not the level of ‘‘dehydration’’ that
impairs exercise performance; rather, the exercise
performance is altered to limit the extent to which the
osmolality of the brain increases. As a result, damaging
levels of ‘‘dehydration’’ are not reached by those who
drink according to the dictates of their thirst during
exercise. Nor will their performance be altered.
Timothy D. Noakes
University of Cape Town
Department of Human Biology
Sports Science Institute of South Africa
South Africa
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CONTRASTING PERSPECTIVES: DEHYDRATION Medicine & Science in Sports & Exercise
... Differences in the process of dehydration may impact physiological performance in unique ways. An optimal experimental design for hydration research has yet to be establish (31). Some research has found that dehydration does not negatively impact performance, especially with less than two percent loss in body weight (31) Other research has found that dehydration has negatively impacted endurance performance, especially when hydration levels were greater than two and a half percent of body weight loss (31). ...
... An optimal experimental design for hydration research has yet to be establish (31). Some research has found that dehydration does not negatively impact performance, especially with less than two percent loss in body weight (31) Other research has found that dehydration has negatively impacted endurance performance, especially when hydration levels were greater than two and a half percent of body weight loss (31). All of these studies utilize different methods of dehydration; fluid restriction, exercise induced, diuretics, heat exposure or a combination thereof. ...
... An optimal experimental design for hydration research has yet to be establish (31). Some research has found that dehydration does not negatively impact performance, especially with less than two percent loss in body weight (31) Other research has found that dehydration has negatively impacted endurance performance, especially when hydration levels were greater than two and a half percent of body weight loss (31). All of these studies utilize different methods of dehydration; fluid restriction, exercise induced, diuretics, heat exposure or a combination thereof. ...
This study aimed to evaluate the difference in heart rate and core temperature during aerobic exercise between two forms of dehydration: exercise-induced (EI) and fluid restricted (FR). Twenty-two subjects (N = 22; 83.35 ± 13.92 kg) completed the current study, performing a familiarization session, a pre-experimental exercise session, and two exercise testing sessions. The EI exercise trial (81.52 ± 13.72 kg) was conducted after performing exercise in a hot environment to lose three to four percent of body weight and partial rehydration. The FR exercise trial (81.53 ± 14.14 kg) was completed after 12 hours of fluid restriction. During both exercise sessions, subjects pedaled against a set resistance of 130 watts for 30 minutes. The main effect of hydration on Tc was significant, F(1, 18) = 4.474, p = .049, η p 2 = .199 (Figure 2) with core temperature being greater during the FR trial compared to the EI trial (FR = 37.58 ± .06°C vs. EI = 37.31 ± .11°C). No significant interaction was found between hydration and time for HR, F(2, 42) = 0.120, p = .887, η p 2 = .006. The main effect of time on HR was significant, F(2, 42) = 119.664, p < .001, η p 2 = .851. Fluid restriction was associated with an increase in core temperature. An increased core temperature may negatively influence performance, and care should be taken to ensure proper hydration.
... De manière intéressante, des études ont récemment démontré que cette observation demeure même lorsque le statut hydrique (déshydratation de 2-3 % de la masse corporelle) des participants est manipulé à l'aveugle par l'apport d'eau directement dans l'estomac via une sonde nasogastrique . Certains auteurs mentionnent que la sensation de soif pourrait, en partie, expliquer l'impact de la déshydratation sur la performance d'endurance (Sawka & Noakes, 2007). Récemment, Adams et al., (2018) ont utilisé une méthode similaire pour manipuler le statut hydrique (sonde nasogastrique), mais les participants pouvaient consommer 25 mL d'eau toutes les 5 minutes dans les deux conditions afin de maintenir la sensation de soif à son minimum. ...
... Individuellement ou de concert, les facteurs présentés auparavant peuvent contribuer à réduire la performance d'endurance, d'autant plus si l'exercice est réalisé dans un environnement chaud ou humide de Korte et al., 2021; Galloway interindividuelle existe pour un niveau d'hypohydratation donné comme démontré dans la Figure 13 et initialement remarqué, il y a de cela 80 ans par . Autrement dit, pour un même niveau d'hypohydratation, certains individus pourraient voir leur performance largement diminuer tandis que d'autres pourraient avoir une meilleure tolérance, et ainsi voir leur performance trivialement diminuer, se maintenir (Cheuvront et al., 2003;Sawka & Noakes, 2007), voire même légèrement augmenter (voir Kenefick, 2010 à 20 °C dans la Figure 13 ci-dessous). Cette variabilité interindividuelle, probablement causée par une meilleure tolérance/résilience, pourrait également contribuer, en partie, à expliquer certains résultats divergents observés dans la littérature. ...
Full-text available
(Thesis written in French) Dehydration, especially when ≥ 2% body mass, is usually associated with an alteration of physiological functions, subjective perceptions as well as endurance performance and cognitive functions. More specifically, it is frequently mentioned that together, the cardiovascular, thermal, metabolic impacts as well as subjective effects induced by dehydration could contribute to increase the perception of effort, which in turn would act as a mediator of endurance performance. While several studies have observed an exacerbation of the perception of effort with dehydration, evidence is contradictory, and some studies do not observe such an effect. In addition, evidence suggests that some individuals may better tolerate dehydration, while others are more widely affected. In this regard, a hypothesis has recently been put forward, according to which repeated exposure to dehydration could somewhat attenuate certain effects, in particular on subjective perceptions, including the perception of effort, and therefore reduce its subsequent impact on performance. In addition to being a possible key mediator in the relationship between exercise-induced dehydration and endurance performance, perception of effort could also play a central role in the process of habituation to the dehydration. However, the idea that humans can become habituated to dehydration has been discussed superficially and anecdotally. Several questions remain unanswered and must therefore be studied.
... A growing body of research demonstrates a significant impact of the MC on thermoregulation [26], resulting in increased cardiovascular strain, core temperature, and sweat rate all in the LP [37]. Prior research has demonstrated the relationship between increased TBW and ICF with improved exercise performance and thermoregulatory outcomes [17,29,38]. Though the current study did not explore the impact of fluid markers on thermoregulation, the improvements in fluid distribution reported may counteract the suppressed thermoregulatory responses in the LP. ...
Full-text available
This study examined the effects of creatine (Cr) loading on body mass (BM) and fluid markers of total body water (TBW), extra-cellular fluid (ECF), and intra-cellular fluid (ICF) across the menstrual cycle (MC). Thirty moderately active females, either naturally-menstruating (NM) or using hormonal contraceptives (HC), were randomized to Cr (Cr; 4 × 5 g/day of creatine monohydrate for 5 days; n = 15) or a non-caloric placebo (PL; n = 15) using a double-blind, placebo-controlled design, with a menstrual phase crossover. BM, TBW, ECF, and ICF were measured at pre- and post-supplementation in randomized order of follicular phase (FP; NM: MC days 0–8, HC: inactive pill days) or luteal phase (LP; NM: ≤15 days from next projected cycle start date, HC: active pill days) using bioelectrical impedance spectroscopy. Acute hydration status and salivary estrogen were used as covariates. Change in BM was not different between groups across MC ([PL-Cr] Δ 0.40 ± 0.50 kg; p = 0.427) or between MC phase across groups ([FP-LP] Δ 0.31 ± 0.48 kg; p = 0.528). TBW (p = 0.802), ECF (p = 0.373), and ICF (p = 0.795) were not different between supplement groups at pre-supplementation/FP time points. There were no significant differences between the NM and HC subjects at any time point, for any outcome (p > 0.05). Following LP supplementation, significant changes were observed in TBW (Cr: Δ 0.83 ± 0.38 L, PL: Δ −0.62 ± 0.38 L; p = 0.021), ECF (Cr: Δ 0.46 ± 0.15 L, PL: Δ −0.19 ± 0.15 L; p = 0.013), and ICF (Cr: Δ 0.74 ± 0.23 L, PL: Δ −0.02 ± 0.23 L; p = 0.041). These data demonstrate an increase in all fluid compartments in the LP following Cr loading, without observed alterations in body weight for females.
... Antrenman veya müsabaka esnasında, sporcular genellikle yapılan spor branşının doğası ve talepleri, sıvı alımı için müsabaka veya antrenman arası vermeyi bekleme, sıvı tüketimi alışkanlığı edinmeme gibi bazı nedenlerden dolayı ilerleyici dehidrasyon geliştirir. Bazı sporcuların dehidrasyon durumu ise spor aktivitesinin öncesinde başlayabilir ve sıvı alımını artırmaya yönelik herhangi bir çaba TBW (Total Body Water) için yeterli olmayabilir (Kenefik ve Sawka, 2007;Osterberg vd., 2009;Noakes, 2007). Bu doğrultuda, sporcularda dehidrasyonun başlamasını önlemek için etkili hidrasyon stratejileri tasarlamak ve sporcuları hidrasyonun önemi hakkında bilgilendirmek önemlidir Carvalho vd. ...
Full-text available
Futbolda sporcuların optimum performans sergileyebilmesi için dehidrasyon ve hidrasyon dengesinin normal olması gerekmektedir. Dehidrasyon, sporcuların performansını düşürmenin yanı sıra ciddi sağlık problemlerini de beraberinde getirebilmektedir. Özellikle futbol gibi dayanıklılık gerektiren uzun süreli müsabaka ve antrenman ortamlarında performansın maksimum düzeye çıkarılmasında sporcuların maç öncesi, esnası ve sonrası hidrasyon seviyesinin takip altına alınması ve hidrasyon seviyesinin uygun hale getirilmesi önemlidir. Bu araştırmanın amacı; Elit futbolcularda müsabaka öncesi dehidrasyon seviyelerinin incelenmesidir. Araştırmaya Türkiye Futbol Federasyonu 1. Lig’inde yer alan (yaş 26,79±4,04 yıl, boy 180,63±5,36 cm, vücut ağırlığı 74,53±4,60 kg) toplam 19 elit futbolcu çalışmaya dahil edilmiştir. Araştırmanın analizinde SPSS 23 analiz programı kullanılarak elde edilen veriler aritmetik ortalama, standart sapma ve yüzde delta formülü ile değerlendirilmiştir. Elde edilen bulgulara göre bir futbol müsabakası öncesinde sporcuların sıvı ve elektrolit seviyeleri %21,05 ‘i hidrasyon, %68,42’ si düşük seviyede dehidrasyon, %10,52’ si orta düzeyde dehidrasyon durumundadır. Futbolcularda müsabaka öncesinde ciddi düzeyde dehidrasyon durumu görünmesede düşük seviyede hidrasyon tespit edilmiştir. İnsan vücudundaki sıvı elektrolit dengede olması hayati bir öneme sahip olduğundan vücutta eksilen sıvı ve elektrolitler müsabaka ve antrenman öncesi tekrar yerine konulmalıdır.
... For this reason, an efficient hydration strategy must be established [4]. This is to avoid a significant negative impact on both aerobic performance [5] and athlete health [6] due to progressive dehydration combined with hyperthermia [7]. In addition, the amount of fluid consumed and its composition should be carefully considered. ...
Full-text available
In different endurance events, athletes have limited access to fluid intake, such as ultra-endurance running. For this reason, it is necessary to establish an adequate hydration strategy for this type of long-duration sporting event. Indeed, it seems that the intake of seawater is a suitable hydration alternative to improve post-exercise recovery in this type of endurance event. This seawater is characterized by being a deep natural mineral water of moderate mineralization, which is usually extracted from a depth of about 700 m. Therefore, the aim of this systematic review is to evaluate the efficacy of seawater consumption in both performance and post-exercise recovery in long-duration sport events. A systematic and comprehensive literature search was performed in PubMed, Scopus, and Web of Science in September 2022. Initially, 8 out of 558 articles met the inclusion criteria. Among these eight studies, six were randomized clinical trials, and two were observational studies (one cross-sectional and one prospective study in well-conditioned student athletes). The results showed that deep sea water consumption accelerated the recovery of aerobic capacity and leg muscle capacity on running performance. In addition, the lactate production after the running exercise in seawater was significantly lower than in pure water. In conclusion, the present review demonstrates that seawater consumption could significantly improve the capacity of recovery after exercise.
... Ensuring adequate water intake among athletes is important for maintaining euhydration status. Several previous studies have shown that hypo hydration caused by insufficient water intake reduced physical activity abilities [12][13][14][15][16][17]. In addition, insufficient water intake and being in a hypohydration status during physical activity are harmful. ...
Full-text available
Studies on the water intake of athletes in daily life are insufficient. The objective was to determine the water intake and hydration status among physically active male young adults. In this cross-sectional studies study, 111 physically active male young adults were recruited. The amount of daily total drinking fluid intake (TDF) among participants was recorded and evaluated in real time over 7 days using the “7-day 24-hour fluid intake questionnaire” (liq. In 7). The daily water intake from food (WFF) was calculated using the weighing, duplicate portion, and direct-drying method over 3 days. All urine samples over 3 days were collected, and urine biomarkers were determined. According to 24 h urine osmolality, the participants were divided into three groups with euhydration status, middle hydration, and hypo hydration statuses. Finally, 109 participants completed the study. The median daily total water intake (TWI), TDF, and WFF were 2701, ik1789, and 955 mL, respectively. Among participants, 17 participants (16%) were in euhydration status, 47 participants (43%) were in hypohydration, and 45 participants (41%) were in middle hydration. There were statistical significances in the 24 h urine volume, osmolality, urine specific gravity, and concentrations of K, Na, and Cl in different hydration statuses (χ2 = 28.212, P
... Particularly in the swimming and running stages, due to their dynamic nature, fluid intake is restricted. For this reason, an efficient hydration strategy must be established to avoid a significant negative impact on both aerobic performance [1] and athlete health [2] due to progressive dehydration combined with hyperthermia [3]. In addition, the amount of fluid consumed should be carefully considered. ...
Full-text available
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.
... It is known that daily protein intake in high quantities, which increases dietary acid load, also increases urinary specific gravity, an indicator of dehydration 14,15 . Since water loss as low as 2% of the body weight adversely affects performance 16 , understanding the factors affecting hydration and monitoring hydration level in athletes is extremely important. Although there is no study showing the relationship between dietary renal acid load and hydration parameters, it is predicted that high protein intake may cause high PRAL and consequently high urine specific gravity. ...
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Purpose: Since the dietary acid load (PRAL) may affect the acid-base balance of the body, there is an increasing interest in its role in sports performance. Typical nutritional requirements of different sports, associated with its physiological demands, might be reflected in the acid load of their diet. Thus, the purpose of this study is to compare the dietary acid load between team, endurance, and strength athletes and to determine the associations between PRAL and hydration status. Methods: Fifty-one healthy recreational male athletes (age: 18-39 yrs) from team, endurance, and strength sports participated in the study. A 3-day food diary was recorded and dietary PRAL values (mEq/day) were calculated. Urine pH and specific gravity were measured. One-way ANOVA with Bonferroni post-hoc analysis and Pearson correlation coefficient (r) were used for data analysis. Results: PRAL in endurance athletes (25.34 mEq/day) was lower compared to team and strength athletes (46.12 and 46.47 mEq/day, respectively) (p= 0.023). Percentage of high PRAL diet (≥15 (mEq/day)) was highest in team sports (89.5%), followed by strength (83.3%) and endurance sports (60%). PRAL was not associated with hydration status. Conclusion: Typical nutritional requirements of sport disciplines are reflected in the PRAL, thus PRAL should be considered when preparing nutritional strategies to improve performance.
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Sweat contains electrolytes (minerals), therefore, it is necessary to consider its loss through sweat in the fluid replacement strategy in sports. The purpose of this study was to compare the concentration of components in sweat, such as electrolytes (minerals), when endurance exercise is performed in hot and neutral thermal environments. Eight men cycled for 60 min at 55% VO2peak under two envitonment conditions: a hot environment (WetBulb Globe Temperature (WBGT): 29.0±0.2℃; Heat) and a neutral thermal environment (WBGT: 20.5±0.2℃; Con). During exercise, sweat loss, core temperature, and heart rate (HR) were measured, and sweat from the chest, back, and thigh was collected. The core temperature, sweat loss, and HR increased significantly in Heat. The sweat electrolyte concentration was significantly higher in Heat than in Con only for Na. Regarding the amount of electrolyte loss from sweat, Na and K showed a significantly higher value than Con in Heat in comparison of each region, and the total loss amount of three regions in Cu also had a significantly higher value than Con in Heat. In addition, there was no difference in the concentration of Ca and Cu between the regions, but there was a significant difference in the amount of loss. It was concluded that even if there was no difference in the electrolyte concentration in sweat, the amount of electrolyte loss increased in K and Cu in a hot environment, and that there was a site difference in the amount of loss in Ca and Cu.
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Background γ-aminobutyric acid (GABA), a common ingredient in sports supplements and other health products, regulates body temperature in the preoptic area and anterior hypothalamus (PO/AH). To date, no study has examined the effect of GABA on thermoregulation during exercise in humans in a cold temperature environment (11 ± 0.3°C, 45% ± 2% relative humidity). Methods We performed a randomized, double-blind study. Ten trained male athletes consumed either a drink (3 ml/kg weight) containing GABA (1,000 mg, trial G) or an equivalent amount of placebo drink (trial C) before exercise. They rested for 20 min and then cycled at 60% of maximum output power for 40 min, pedaling at 60 rpm, and recovered for 20 min. Core temperature (T c ), skin temperature (upper arm, chest, thigh, calf), and heart rate (HR) were monitored at rest (T 0 ), exercise begins (T 20 ), 20 min of exercise (T 40 ), the exercise ends (T 60 ), and at recovery (T 80 ). Results Compared to T 0 , T c decreased significantly at T 20 and increased significantly at T 40 , T 60 and T 80 ( p < 0.01). From 35–80 min, the T c was higher in trial G (peaked at 37.96 ± 0.25°C) than in trial C (37.89 ± 0.37°C), but it failed to reach significant difference ( p > 0.05); T sk continued to increase during exercise and was significantly higher than T 0 at T 40 ( p < 0.05), T 60 and T 80 ( p < 0.01). There was no significant difference in T sk between the two trials ( p > 0.05). Conclusion Our findings provide initial evidence that oral administration of GABA does not affect thermoregulation and has no adverse effects on the body as an ergogenic exercise supplement during exercise in cold environments.
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Exercise-heat exposure results in significant sweat losses due to large biophysical requirements for evaporative heat loss. Progressive body water losses will increase plasma tonicity and decrease blood volume (hypertonic–hypovolemia). The result is reduced dry and evaporative heat exchange through alterations in the core temperature threshold for initiation of skin blood flow and sweating as well as changes in the sensitivity of these thermo-effectors. Regulation of reduced sweating conserves body water, which reduces heat loss and increases exercise hyperthermia, but the magnitude of this effect is modified by environmental heat transfer capabilities. The focus of this paper is to (1) examine the major mechanisms by which hypohydration alters thermoregulatory responses in the heat, and (2) illustrate how important differences in environmental airflow characteristics between laboratory and field settings may modify these effects.
To assess the effects of rapid weight reduction, four university wrestlers decreased their body weight by 8% over a four-day period by food and liquid intake reductions. Significant decreases in muscle glycogen concentration and dynamic strength, but not aerobic or anaerobic capacity, accompanied weight loss. A three-hour rehydration period did not improve glycogen levels or strength performance. These results suggest that rapid weight reduction may impair wrestling performance.
Physical work capacity was measured in a standard exercise test on a bicycle ergometer: 50% of VO2 max (maximal oxygen uptake) for 8 min, 70% for 5 min, and 105% until exhaustion. The amount of work performed in the last bout was used as an index of physical work capacity. The subjects performed the test under normal (control) conditions and after: prolonged (90-160 min) bicycle exercise (50% of VO2 max); passive heating in water until a rectal temperature of 38°C was achieved; sauna dehydration until body weight loss and rectal temperature were the same as after prolonged exercise; and after diuretic dehydration to the same weight loss as that obtained in the prolonged exercise. Heart rate, oxygen consumption, rectal and muscle temperatures, and weight loss were measured. Blood samples were taken for determinations of hematocrit, hemoglobin concentration, plasma levels of sodium, potassium and chloride, osmolarity, and blood lactate. It was concluded that the reduction in physical work capacity following prolonged exercise was the result of changes in many variables, partly the depletion of energy stores. However, dehydration and hyperthermia had separate effects in reducing work capacity. Electrolyte shifts and plasma volume changes were also associated with changes in work capacity. For all pretreatments taken together, the best indicator of physical work capacity was the difference between maximal heart rate and the heart rate obtained at 70% of VO2 max in the standard test.
Purpose: In this study, we examined the effects of greater than ad libitum rates of fluid intake on 2-h running performances. Methods: Eight male distance runners performed three runs on a treadmill at 65% of peak oxygen uptake ((V) over dot O-2 peak) for 90 min and then ran "as far as possible" in 30 min in an air temperature of 25 degrees C, a relative humidity of 55% and a wind speed of 13-15 km.h(-1). During the runs, the subjects drank a 6.9% carbohydrate (CHO)-electrolyre solution either ad libitum or in set volumes of 150 or 350 mL.70 kg(-1) body mass (similar to 130 or 300 mL) every 15-20 min. Results: Higher (similar to 0.9 vs 0.4 L.h(-1)) rates of fluid intake in the 350 mL.70 kg(-1) trial than in the other trials had minimal effects on the subjects' urine production (similar to 0.1 L.h(-1)), sweat rates (similar to 1.2 L.h(-1)), declines in plasma volume (similar to 8%), and rises in serum osmolality (similar to 5 mosmol.L-1) and Na+ concentrations (similar to 7 mEq.L-1). A greater (similar to 1.0 vs 0.5 g.min(-1)) rate of CHO ingestion in the 350 mL.70 kg(-1) trial than in the other trials also did not affect plasma concentrations of glucose (similar to 5 mmol.L-1) and lactate (similar to 3 mmol.L-1) during the performance runs. In all three performance runs, increases in running speeds from similar to 14 to 15-16 km.h(-1) and rises in exercise intensities from similar to 65% to 75% of (V) over dot O-2 (peak) elevated plasma lactate concentrations from similar to 1.5 to 3 mmol.L-1 and accelerated CHO oxidation from similar to 13 to 15 mmol.min(-1). The only effect of the additional intake of similar to 1.0 L of fluid in the 350 mL.70 kg(-1) trial was to produce such severe gastrointestinal discomfort that two of the eight subjects failed to complete their performance runs. Conclusion: Greater rates of fluid ingestion had no measurable effects on plasma volume and osmolality and did not improve 2-h running performances in a 25 degrees C environment.