Effects of Dehydration and Rehydration on Performance

Chapter 16
Effects of Dehydration and Rehydration
on Performance*
MICHAEL N. SAWKA, WILLIAM A. LATZKA
AND SCOTT J. MONTAIN
Introduction
Athletes encounter heat stress from climatic conditions (e.g. temperature, humidity, solar load)
and body heat production. Depending on the
climatic conditions, the relative contributions of
evaporative and dry (radiative and conductive)
heat exchange to the total heat loss will vary. The
hotter the climate, the greater the dependence
on evaporative heat loss and, thus, on sweating.
Therefore, a substantial volume of body water
may be lost via sweating to enable evaporative
cooling in hot environments. In addition, physical exercise will elevate metabolic rate above
resting levels, and thus increase the rate at which
heat must be dissipated to the environment to
keep core temperature from rising to dangerous
levels. Environmental heat stress and physical
exercise interact synergistically and may push
physiological systems to their limits (Sawka et al.
1996b).
Climatic heat stress and physical exercise will
cause both fluid and electrolyte imbalances that
need to be re-established (Marriott 1993, 1994;
Convertino et al. 1996). Athletes performing
exercise in the heat often incur body water
deficits. Generally, athletes dehydrate during
exercise because of fluid non-availability or a
mismatch between thirst and body water
* The views, opinions and/or findings contained in this
chapter are those of the authors and should not be construed as an official Department of Army position or
decision, unless so designated by other official
documentation.
216
requirements (Greenleaf 1992). In these
instances, the athlete starts exercise euhydrated,
but incurs an exercise-heat-mediated dehydration over a prolonged period. This scenario is
common for many athletic and occupational settings; however, there are several sports (e.g.
boxing, power lifting, wrestling) where athletes
will purposely achieve hypohydration prior to
competition.
This chapter reviews fluid balance in the heat
and the effects of hydration status on temperature regulation and physical exercise performance. Throughout this chapter, euhydration
refers to normal body water content, hypohydration refers to body water deficit, and hyperhydration refers to increased body water content.
Fluid and electrolyte balance
An athlete’s sweating rate is dependent upon the
climatic conditions, clothing worn and exercise
intensity (Molnar et al. 1946; Shapiro et al. 1982).
Figure 16.1 provides a range of sweating rates
expected from running in different climatic conditions (Sawka & Pandolf 1990). Athletes performing high-intensity exercise commonly have
sweating rates of 1.0–2.5 l · h–1 while in the heat.
These high sweating rates, however, are not
maintained continuously and are dependent
upon the person’s need to dissipate body heat.
Daily fluid requirements range (for sedentary to
active persons) from 2 to 4 l · day–1 in temperate
climates and from 4 to 10 l · day–1 in hot climates
(Greenleaf 1994). Clearly, hot weather and
effects of dehydration and rehydration
217
3.0
2.5
d
Sweating rate (l.h–1)
mi
2.0
t
Ho
d
an
hu
1.5
1.0
nd
dry
a
ol
Co
0.5
0
160
Fig. 16.1 An approximation of
hourly sweating rates as a
function of climate and running
speed. From Sawka and Pandolf
(1990).
200
240
280
320
Running speed (m.min–1)
16
15
intense training can greatly increase daily fluid
requirements.
Electrolytes, primarily sodium chloride and, to
a lesser extent, potassium, calcium and magnesium, are contained in sweat. Sweat sodium concentration averages approximately 35 mmol · l–1
(range, 10–70 mmol · l–1) and varies depending
upon diet, sweating rate, hydration and heat
acclimation level (Allan & Wilson 1971; Brouns
1991). Sweat glands reabsorb sodium by active
transport, and the ability to reabsorb sodium
does not increase with the sweating rate, so at
high sweating rates the concentration of sweat
sodium increases. Heat acclimation improves the
ability to reabsorb sodium so acclimated persons
have lower sweat sodium concentrations (> 50%
reduction) for any sweating rate (Dill et al. 1933;
Bass et al. 1955; Allan & Wilson 1971). Sweat
potassium concentration averages 5 mmol · l–1
(range, 3–15 mmol · l–1), calcium averages
1 mmol · l–1 (range, 0.3–2 mmol · l–1) and magnesium averages 0.8 mmol · l–1 (range, 0.2–
1.5 mmol · l–1) (Brouns 1991). Electrolyte supplementation is not necessary, except occasionally
for their first several days of heat exposure where
14
13
12
11
10
9
8
Running speed (min. km –1 run)
evidence indicates that this is warranted (Marriott 1994; Convertino et al. 1996), as normal
dietary sodium intake will replenish sweat electrolyte losses (Marriott 1994; Convertino et al.
1996).
During exercise in the heat, a principal
problem is to avoid hypohydration by matching
fluid consumption to sweat loss. This is a difficult
problem because thirst does not provide a good
index of body water requirements (Adolph &
Associates 1947; Hubbard et al. 1984; Engell et al.
1987). Thirst is probably not perceived until an
individual has incurred a water deficit of approximately 2% body weight loss (BWL) (Adolph &
Associates 1947; Hubbard et al. 1984; Armstrong
et al. 1985b). In addition, ad libitum water intake
during exercise in the heat results in an incomplete replacement of body water losses (Adolph
& Associates 1947; Hubbard et al. 1984). Heatacclimated persons will usually only replace less
than one half of their fluid deficit when replacing
fluid ad libitum (Adolph & Associates 1947). As
a result, it is likely that unless forced hydration
is stressed, some dehydration will occur during
exercise in the heat. Humans will usually fully
218
nutrition and exercise
rehydrate at mealtime, when fluid consumption
is stimulated by consuming food (Adolph &
Associates 1947; Marriott 1993). Therefore, active
persons need to stress drinking at mealtime in
order to avoid persistent hypohydration.
Persons will hypohydrate by 2–6% BWL
during situations of stress and prolonged high
sweat loss. Water is the largest component of the
human body, comprising 45–70% of body weight
(Sawka 1988). The average male (75 kg) is composed of about 45 l of water, which corresponds
to about 60% of body weight. Since adipose
tissue is about 10% water and muscle tissue is
about 75% water, a person’s total body water
depends upon their body composition. In addition, muscle water and glycogen content parallel
each other probably because of the osmotic pressure exerted by glycogen granules within the
muscle’s sarcoplasm (Neufer et al. 1991). As a
result, trained athletes have a relatively greater
total body water than their sedentary counterparts, by virtue of a smaller percentage body fat
and perhaps a higher skeletal muscle glycogen
concentration.
The water contained in body tissues is distributed between the intracellular and extracellular
fluid spaces. Hypohydration mediated by sweating will influence each fluid space as a consequence of free fluid exchange. Nose and
colleagues (1983) determined the distribution of
BWL among the fluid spaces as well as among
different body organs. They thermally dehydrated rats by 10% BWL, and after the animals
regained their normal core temperature, the
body water measurements were obtained. The
water deficit was apportioned between the intracellular (41%) and extracellular (59%) spaces;
and among the organs: 40% from muscle, 30%
from skin, 14% from viscera and 14% from bone.
Neither the brain nor liver lost significant water
content. They concluded that hypohydration
results in water redistribution largely from the
intra- and extracellular spaces of muscle and skin
in order to defend blood volume.
Sweat-induced hypohydration will decrease
plasma volume and increase plasma osmotic
pressure in proportion to the level of fluid loss
(Sawka et al. 1996a). Plasma volume decreases
because it provides the precursor fluid for sweat,
and osmolality increases because sweat is ordinarily hypotonic relative to plasma. Sodium and
chloride are primarily responsible for the elevated plasma osmolality (Senay 1968; Kubica
et al. 1983). It is the plasma hyperosmolality
which mobilizes fluid from the intracellular to
the extracellular space to enable plasma volume
defence in hypohydrated subjects. This concept
is demonstrated by heat-acclimated persons
who, compared with unacclimated persons,
have a smaller plasma volume reduction for a
given body water deficit (Sawka 1992). By virtue
of having a more dilute sweat, heat-acclimated
persons retain additional solutes within the
extracellular space to exert an osmotic pressure
and redistribute fluid from the intracellular
space (Mack & Nadel 1996).
Some persons use diuretics for medical purposes or to reduce their body weight. Diuretics
increase urine formation and often result in the
loss of solutes. Commonly used diuretics include
thiazide (e.g. Diuril), carbonic anhydrase
inhibitors (e.g. Diamox) and furosemide (e.g.
Lasix). Diuretic-induced hypohydration often
results in an iso-osmotic hypovolaemia, with a
much greater ratio of plasma loss to body water
loss than either exercise or heat-induced hypohydration. Relatively less intracellular fluid is lost
after diuretic administration, since there is not an
extracellular solute excess to stimulate redistribution of body water.
Exercise performance and
temperature regulation
Numerous studies have examined the influence
of hypohydration on maximal aerobic power and
physical exercise capacity. In temperate climates,
a body water deficit of less than 3% BWL does not
alter maximal aerobic power (Sawka et al. 1996a).
Maximal aerobic power has been reported as
being decreased (Buskirk et al. 1958; Caldwell et
al. 1984; Webster et al. 1990) when hypohydration
equalled or exceeded 3% BWL. Therefore, a critical water deficit (3% BWL) might exist before
effects of dehydration and rehydration
.
50
Exercise capacity (%)
Fig. 16.2 Relationship between
hypohydration
level and (a)
.
Vo2max. decrement, and (b)
physical exercise capacity
decrement during heat exposure.
䊏, from Craig and Cummings
(1966); 䊉, from Pinchan et al.
(1988).
VO2 max decrease (%)
50
40
30
20
10
0
0
(a)
219
1
2
3
Body water loss (%)
hypohydration reduces maximal aerobic power
in temperate climates. In hot climates, Craig
and Cummings (1966) demonstrated that small
(2% BWL) to moderate (4% BWL) water deficits
resulted in a large reduction of maximal
aerobic power. Likewise, their data indicate a
disproportionately larger decrease in maximal
aerobic power with an increased magnitude of
body water deficit. It seems environmental
heat stress has a potentiating effect on the reduction of maximal aerobic power elicited by
hypohydration.
The physical exercise capacity (exercise to
fatigue) for progressive intensity exercise is
decreased when hypohydrated. Physical exercise capacity is decreased by marginal (1–2%
.
BWL) water deficits that do not alter Vo2max.
(Caldwell et al. 1984; Armstrong et al. 1985a), and
the decreases are larger with increasing water
deficits. Clearly, hypohydration results in larger
decrements of physical exercise capacity in hot
than in temperate climates (Armstrong et al.
1985a). It appears that the thermoregulatory
system, perhaps via increased body temperatures, has an important role in the reduced exercise performance mediated by a body water
deficit. Figure 16.2 presents the relationship
.
between hypohydration level and Vo2max. decrement or physical exercise capacity decrement
during heat exposure (Craig & Cummings 1966;
Pinchan et al. 1988). Note that for a given hypohydration level, greater decrements are observed
.
for physical exercise capacity than Vo2max..
Studies have demonstrated that hypohydration can impair athletic endurance exercise
40
30
20
10
0
4
(b)
1
2
3
4
Body water loss (%)
Fig. 16.3 With many major competitions held in hot
environments, the outcome of races may depend
on maintaining hydration status. Photo ©
Allsport / Martin.
performance. Armstrong and colleagues (1985a)
studied the effects of a body water deficit on
competitive distance running performance. They
had athletes compete in 1500-, 5000- and 10 000m races when euhydrated and when hypohy-
nutrition and exercise
drated. Hypohydration was achieved by diuretic
administration (furosemide), which decreased
body weight by 2% and plasma volume by 11%.
Running performance was impaired at all race
distances, but to a greater extent in the longer
races (ª 5% for the 5000 and 10 000 m) than the
shorter race (3% for the 1500 m). Burge et al.
(1993) recently examined whether hypohydration (3% BWL) affected simulated 2000 m rowing
performance. They found that, on average, it
took 22 s longer to complete the task when hypohydrated than when euhydrated. Average power
was reduced by 5% in the hypohydrated state.
Two studies have examined the adverse effects
of hypohydration on moderate to intense cycle
ergometer performance. In both studies, highintensity performance tests were conducted
immediately after 55–60 min of cycling during
which volunteers either drank nothing or drank
sufficient fluid to replace sweat losses. Walsh
et al. (1994) reported that time to fatigue when
.
cycling at 90% Vo2max. was 51% longer (6.5 vs.
9.8 min) when subjects drank sufficient fluids to
prevent hypohydration. Below et al. (1995) found
that cyclists completed a performance ride 6.5%
faster if they drank fluids during exercise. The
results of these studies clearly demonstrate the
detrimental effects of hypohydration in submaximal exercise performance.
Investigators have documented the effects of
hypohydration on a person’s ability to tolerate
heat strain during submaximal intensity exercise. These studies demonstrate that persons
who drink can continue to exercise in the heat for
many hours, whereas those who under-drink
discontinue because of exhaustion (Adolph &
Associates 1947; Ladell & Shephard 1961; Sawka
et al. 1992). To address whether hypohydration
alters heat tolerance, Sawka and colleagues
(1992) had subjects walk to voluntary exhaustion
when either euhydrated or hypohydrated (by
8% of total body water). The experiments
were designed so that the combined environment (Ta, 49°C; rh, 20%) and exercise intensity
.
(47% Vo2max.) would not allow thermal equilibrium and heat exhaustion would eventually
occur. Hypohydration reduced tolerance time
(121–55 min), but more important, hypohydration reduced the core temperature that a person
could tolerate. Heat exhaustion occurred at a
core temperature approximately 0.4°C lower
when hypohydrated than when euhydrated.
These findings indicate that hypohydration not
only impairs exercise performance, but also
reduces tolerance to heat strain.
Hypohydration increases core temperature
responses during exercise in temperate (Grande
et al. 1959; Cadarette et al. 1984) and hot (Sawka
et al. 1983, 1985) climates. A critical water deficit
of 1% body weight elevates core temperature
during exercise (Ekblom et al. 1970). As the magnitude of water deficit increases, there is a concomitant graded elevation of core temperature
during exercise heat stress (Sawka et al. 1985;
Montain & Coyle 1992). Figure 16.4 illustrates
relationships between BWL and core temperature elevations reported by studies (Adolph &
Associates 1947; Strydom & Holdsworth 1968;
Sawka et al. 1985; Montain & Coyle 1992) which
examined several hypohydration levels (Sawka
et al. 1996a). The magnitude of core temperature
elevation ranges from 0.1 to 0.23°C for every percentage body weight lost. Hypohydration not
1.6
Increase in core
temperature ( C)
220
1.4
A
1.2
B
1.0
C
0.8
D
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
Body water loss (%)
Fig. 16.4 Relationship between the elevation in core
temperature (above euhydration) at a given
hypohydration level during exercise with
. heat stress,
according to different studies: A, 65% Vo2max., 33°C db
(Montain & Coyle 1992); B, marching. in the desert
(Adolph & Associates 1947); C, 25% Vo2max., 49°C db
(Sawka et al. 1985); D, 45 W, 34°C db (Strydom &
Holdsworth 1968). From Sawka et al. (1996a).
effects of dehydration and rehydration
only elevates core temperature responses, but
it negates the core temperature advantages conferred by high aerobic fitness and heat acclimation (Buskirk et al. 1958; Sawka et al. 1983;
Cadarette et al. 1984).
Hypohydration impairs both dry and evaporative heat loss (or, if the air is warmer than the
skin, dehydration aggravates dry heat gain)
(Sawka et al. 1985, 1989; Kenney et al. 1990;
Montain et al. 1995). Figure 16.5 presents local
sweating responses (Sawka et al. 1989) and skin
blood flow responses (Kenney et al. 1990) to
hypohydration (5% BWL) during exercise in the
heat. This figure indicates that hypohydration
reduced both effector heat loss responses for a
given core temperature level (Sawka 1992).
Hypohydration is usually associated with either
reduced or unchanged whole-body sweating
rates at a given metabolic rate in the heat (Sawka
et al. 1984). However, even when hypohydration
is associated with no change in sweating rate,
core temperature is usually elevated, so that
sweating rate for a given core temperature is
lower when hypohydrated.
Hyperhydration
Hyperhydration, increased total body water, has
been suggested to improve thermoregulation
1.0
0.6
0.2
36
(a)
during exercise-heat stress above euhydration
levels (Sawka et al. 1996a). The concept that
hyperhydration might be beneficial for exercise
performance arose from the adverse consequences of hypohydration. Studies examining
thermoregulatory effects of hyperhydration
during exercise-heat stress have reported disparate results. Some investigators report lower
core temperatures during exercise after hyperhydration (Moroff & Bass 1965; Nielsen et al. 1971;
Gisolfi & Copping 1974; Nielsen 1974; Grucza
et al. 1987), while other studies do not (Greenleaf
& Castle 1971; Nadel et al. 1980; Candas et al.
1988). Also, several studies (Moroff & Bass 1965;
Nielsen 1974; Lyons et al. 1990) report higher
sweating rates with hyperhydration. In most
studies, heart rate was lower during exercise
with hyperhydration (Sawka et al. 1996a).
We believe that these conflicting results are
due to differences in experimental design and
not hyperhydration per se. For example, studies
(Moroff & Bass 1965; Nielsen et al. 1971; Lyons
et al. 1990) reporting that hyperhydration
reduces thermal strain have not had subjects
fully replace fluid lost during exercise; therefore,
the differences reported may be due to dehydration causing increased thermal strain during
‘control’ conditions. Maintaining euhydration
during exercise is essential to determine the effi-
Blood flow (ml.100 ml–1. min–1)
Sweating rate (l.h–1)
1.4
37
38
Core temperature ( C)
221
20
16
12
8
4
36
39
(b)
37
38
39
Core temperature ( C)
Fig. 16.5 (a) Local sweating rate (Sawka et al. 1989), and (b) forearm skin blood flow (Kenney et al. 1990) responses
for euhydrated (—) and hypohydrated (5% body water loss) (---) persons during exercise with heat stress. From
Sawka (1992).
222
nutrition and exercise
cacy of hyperhydration on thermoregulation
during exercise-heat stress. In addition, some
studies (Moroff & Bass 1965; Nielsen et al. 1971)
report that overdrinking before exercise lowered
body core temperature prior to exercise. This was
likely due to the caloric cost of warming the
ingested fluid. Exercise per se did not exacerbate
the difference that existed prior to exercise.
Hyperhydration in these studies therefore did
not improve heat dissipation during the exercise
period.
While many studies have attempted to induce
hyperhydration by overdrinking water or waterelectrolyte solutions, these approaches have produced only transient expansions of body water.
One problem often encountered is that much of
the fluid overload is rapidly excreted (Freund
et al. 1995). Some evidence has been accrued that
greater fluid retention can be achieved by drinking an aqueous solution containing glycerol
while resting in temperate conditions (Riedesel
et al. 1987; Freund et al. 1995). Riedesel et al. (1987)
first reported that following hyperhydration
with a glycerol solution, rather than with water
alone, subjects excreted significantly less of the
water load. They found that subjects drinking
glycerol solutions achieved greater hyperhydration than subjects drinking water while resting
in temperate conditions. Freund et al. (1995)
reported that glycerol increased fluid retention
by reducing free water clearance. Exercise and
heat stress decrease renal blood flow and free
water clearance and therefore both stressors
might reduce glycerol’s effectiveness as a hyperhydrating agent.
Lyons et al. (1990) reported that glycerol/water
hyperhydration had dramatic effects on improving a person’s ability to thermoregulate during
exercise-heat stress. Subjects completed three
trials in which they exercised in a hot (42°C)
climate. For one trial, fluid ingestion was
restricted to 5.4 ml · kg–1 body weight, and in
the other two trials subjects ingested water
(21.4 ml · kg–1) with or without a bolus of glycerol
(1 g · kg–1). Subjects began exercise 90 min after
this hyperhydration period. Glycerol ingestion
increased fluid retention by 30% compared to
drinking water alone. During exercise, glycerol
hyperhydration produced a higher sweating rate
(300–400 ml · h–1) and substantially lower core
temperatures (0.7°C) than those produced in
control conditions with water hyperhydration.
These thermoregulatory benefits during exercise-heat stress have not been confirmed. Other
studies report similar core temperatures and
sweating rates between glycerol and water
hyperhydration fluids before exercise (Montner
et al. 1996) in a temperate climate, or as rehydration solutions during exercise in a warm climate
(Murray et al. 1991).
Recently, Latzka and colleagues (1997) examined the effects of hyperhydration on fluid
balance and thermoregulation during exerciseheat stress. Their approach was to determine if
pre-exercise hyperhydration with and without
glycerol would improve sweating responses
and reduce core temperature. The glycerol and
water dosages were similar to those employed by
Riedesel et al. (1987) and Lyons et al. (1990).
Latzka and colleagues (1997) found that during
.
exercise (45% Vo2max.) in the heat (35°C, 45% rh),
there was no difference between hyperhydration
methods for increasing total body water (ª 1.5 l).
In addition, unlike euhydration, hyperhydration did not alter core temperature (rectal or
eosophageal), skin temperature, local sweating
rate, sweating threshold, sweating sensitivity or
heart rate responses. Likewise, no differences
were found between water and glycerol hyperhydration methods for these physiological
responses. Latzka and colleagues (1997) concluded that hyperhydration provides no thermoregulatory advantage over the maintenance
of euhydration.
Conclusion
During exercise, sweat output often exceeds
water intake, producing a body water deficit or
hypohydration. The water deficit lowers both
intracellular and extracellular volume. It also
results in plasma hypertonicity and hypovolaemia. Aerobic exercise tasks are likely to be
adversely affected by hypohydration, with the
effects of dehydration and rehydration
potential affect being greater in warm environments. Hypohydration increases heat storage
and reduces one’s ability to tolerate heat strain.
The increased heat storage is mediated by
reduced sweating rate and reduced skin blood
flow for a given core temperature. Hyperhydration has been suggested to reduce thermal strain
during exercise in the heat; however, data supporting that notion are not robust.
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