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33
Human Water and
Electrolyte Balance
Scott J. Montain, Samuel N. Cheuvront,
Robert Carter, III, Michael N. Sawka
Introduction
Humans demonstrate a remarkable ability to regulate
daily body water and electrolyte balance so long as food
and fluid are readily available.
1
The imposition of exercise
and environmental stress can, however, challenge this
ability. Most circumstances involving physical exercise re-
quire the formation and vaporization of sweat as the prin-
ciple means of heat removal in man. Sweat losses, if not
replaced, reduce body water volume and electrolyte con-
tent. Excessive body water or electrolyte losses can disrupt
physiological homeostasis and threaten both health and
performance.
1
Persons often dehydrate during physical activity or ex-
posure to hot weather because of fluid non-availability or
because of a mismatch between thirst and body water
losses.
2
In these instances, the person begins the task with
normal total body water, and dehydrates over a prolonged
period. This scenario is common for most athletic and
occupational settings; however, in some situations the
person might begin exercise with a body water deficit.
For example, in several sports (e.g., boxing, power lifting,
wrestling) athletes frequently dehydrate to compete in
lower weight classes. Also, persons medicated with di-
uretics may be dehydrated prior to initiating exercise. If
sodium chloride deficits occur, then the extracellular fluid
volume will contract and cause “salt depletion dehydra-
tion.” A sodium chloride deficit usually occurs due to
sweat sodium losses combined with excessive water con-
sumption, but a sodium deficit can also occur without
excessive water intake owing to high sweat sodium losses.
Both of these scenarios produce sodium dilution, which
is more commonly known as hyponatremia or “water in-
toxication.”
3,4
AG-PPKN-0909 R1 CH33 04-07-06 12:03:36
This chapter reviews the physiology, needs, and assess-
ment of human water and electrolyte balance. The extent
to which water and electrolyte imbalances affect tempera-
ture regulation and exercise performance are also consid-
ered. Throughout the chapter, the term euhydration re-
fers to normal body water content, hypohydration refers
to a body water deficit, and hyperhydration refers to in-
creased body water content. Dehydration refers to the
dynamic loss of body water.
Physiology of Water and
Electrolyte Balance
Net body water balance (loss ⳱gain) is generally regu-
lated well as a result of thirst and hunger drives coupled
with free access to food and beverage.
1
This is accom-
plished by neuroendocrine and renal responses
5
to body
water volume and tonicity changes, as well as non-regula-
tory social-behavioral factors.
6
These homeostatic re-
sponses collectively ensure that small degrees of over- and
underhydration are readily compensated for in the short
term. Using water balance studies, Adolph
7,8
found that
daily body water varied narrowly between 0.22%and
0.48%in temperate and warm environments, respectively.
However, exercise and environmental insult often pose a
greater acute challenge to fluid balance homeostasis.
Water (total body water) is the principal chemical con-
stituent of the human body. For an average young adult
male, total body water is relatively constant and represents
50%to 70%of body weight.
9
Variability in total body
water is primarily due to differences in body composi-
tion.
1,9
Total body water is distributed into intracellular
fluid (ICF) and extracellular fluid (ECF) compartments.
The ICF and ECF contain about 65%and 35%of total
2Present Knowledge in Nutrition, Ninth Edition •Section VI: Minerals and Trace Elements
body water, respectively. The ECF is further divided into
the interstitial and plasma spaces. Water balance repre-
sents the net difference between water intake and loss.
When losses exceed intakes, total body water is decreased.
When body water deficits occur from sweat losses, a
hypertonic hypovolemia generally results. Plasma volume
decreases and plasma osmotic pressure increases in pro-
portion to the decrease in total body water. Plasma vol-
ume decreases because it provides the fluid for sweat, and
osmolality increases because sweat is ordinarily hypotonic
relative to plasma. Resting plasma osmolality increases in
alinear manner from about 283 mosmol/kg when euhy-
drated, to more than 300 mosmol/kg when hypohydrated
by 15%of total body water.
1
The increase in osmotic
pressure is primarily due to increased plasma sodium and
chloride. with no consistent effect on potassium concen-
trations.
10-12
Incomplete fluid replacement decreases total body
water, and as a consequence of free fluid exchange, affects
each fluid space.
13-16
For example, Nose et al.
15
deter-
mined the distribution of body water loss among fluid
spaces as well as among different body organs during hy-
pohydration. They thermally dehydrated rats by 10%of
body weight, and after the animals regained their normal
core temperature, the body water measurements were ob-
tained. The fluid deficit was apportioned between the
intracellular (41%)and extracellular (59%)spaces. Re-
garding organ fluid loss, 40%came 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 redistribu-
tion largely from the intra- and extracellular spaces of
muscle and skin in order to maintain blood volume.
Different methods of dehydration are known or sus-
pected to affect the partitioning of body water losses dif-
ferently than those just described. For example, diuretics
increase urine formation and generally result in the loss of
both solutes and water. Diuretic-induced hypohydration
generally results in an iso-osmotic hypovolemia, with a
much greater ratio of plasma loss to body water loss than
either exercise or heat-induced hypohydration.
10,17
As a
result, relatively less intracellular fluid is lost after diuretic
administration, since there is not an extracellular solute
excess to stimulate redistribution of body water. In con-
trast, several studies
13,18,19
report substantial decreases in
skeletal muscle intracellular water content following pro-
longed exercise without fluid replacement, presumably
the result of water released with the breakdown of muscle
glycogen. Exercise-induced hypohydration may therefore
result in a greater intracellular water loss than simple
sweat-induced hypohydration (passive thermal dehydra-
tion). Kozlowski and Saltin
20
reported data to support
this view, but Costill and Saltin
18
found no difference
between exercise and thermal dehydration for the parti-
tioning of water between the fluid compartments. It is
therefore clear that the ratio of intracellular to extracellu-
lar water losses that occur with dehydration from sweating
AG-PPKN-0909 R1 CH33 04-07-06 12:03:36
and diuretic use are different, but any difference between
active and passive sweating remains unresolved. Other
factors such as heat acclimatization status, posture,
climate, mode, and intensity of exercise can also pro-
duce significant variability in the responses described
above.
21,22
Water and Electrolyte Needs
Human water and electrolyte needs should not be
based on a “minimal” intake, as this might eventually lead
to a deficit and possible adverse performance and health
consequences. Instead, the Food and Nutrition Board of
the Institute of Medicine bases water needs on Adequate
Intake (AI). The AI is based on experimentally derived
intake levels that are expected to meet nutritional ade-
quacy for essentially all members of a healthy population.
The AI level for water is 2.7 to 3.7 L/d for sedentary
women and men over age 19, respectively.
1
These values
represent total water intake from all fluids (80%)and
foods (20%). The AI for sodium is 1.5 g/d or 3.8 g/d
sodium chloride.
1
The report also indicates that athletes
and workers performing stressful exercise in the heat can
exceed the AI for water and sodium.
Table 1
23
illustrates the wide variability in hourly sweat
losses observed both within and between sports and occu-
pations. Depending upon the duration of activity and heat
stress exposure, the impact of these elevated hourly sweat
rates on daily water requirements will vary. Figure 1 de-
picts generalized modeling approximations for daily water
requirements based upon calculated sweating rates as a
function of daily energy expenditure (activity level) and
air temperature.
1
Applying this prediction model, it is
clear that daily water requirements can increase two- to
six-fold from baseline by simple manipulation of either
variable. For example, daily water requirements for any
given energy expenditure in temperate climates (20⬚C)
can triple in very hot weather (40⬚C). In addition to air
temperature, other environmental factors also modify
sweat losses; these include relative humidity, air motion,
solar load, and choice of clothing for protection against
Table 1. Sweating Rates for Different Sports (Data
are from Rehrer N, Burke L. Sweat losses during
various sports. Aust J Nutr Diet. 1996;53:
S13–S16.)
Mean Range
Sport L/h
Water polo 0.55 0.30–0.80
Cycling 0.80 0.29–1.25
Running 1.10 0.54–1.83
Basketball 1.11 0.70–1.60
Soccer 1.17 0.70–2.10
Human Water and Electrolyte Balance •Montain, Cheuvront, Carter III, and Sawka 3
0
2
4
6
8
10
12
14
Avera
g
e Da
y
time Dr
y
Bulb Tem
p
erature
(
oC
)
15 20 25 30 35 40
Average Daytime Dry Bulb Temperature (oF)
59 68 77 86 95 104
3600 Kcal/day
2900 Kcal/day
2400 Kcal/day
1900 Kcal/day
Minimal Daily H
2
O needs
Figure 1. Daily water needs estimated from sweat loss predictions
due to changes in physical activity and air temperature. Daily en-
ergy expenditures of 1900, 2400, 2900 and 3600 kcal correspond
to sedentary, low activity, active, and very active, respectively.
(From Food and Nutrition Board, Institute of Medicine. Dietary
Reference Intakes for Water, Potassium, Sodium, Chloride, and
Sulfate. Washington, DC: National Academies Press; 2004. Avail-
able online at: http://www.nap.edu/books/0309091691/html.)
environmental elements.
24
Therefore, it is expected that
water losses, and therefore water needs, will vary consid-
erably among moderately active people based on changing
extraneous influences.
Sweat is hypotonic to extracellular fluid, but contains
electrolytes, primarily sodium chloride and, to a lesser
extent, potassium, calcium, and magnesium.
25-27
Sweat
sodium concentration averages 35 mEq/L (range 10–70
mEq /L) and varies depending upon diet, sweating rate,
hydration level, and heat acclimation state.
25,28,29
Sweat
potassium concentration averages 5 mEq/L (range 3–15
mEq /L), calcium 1 mEq/L (range 0.3–2 mEq/L), mag-
nesium 0.8 mEq /L (range 0.2– 1.5 mEq /L), and chloride
30 mEq/L (range 5–60 mEq/L).
29
Neither gender nor
aging seem to have marked effects on sweat electrolyte
concentrations.
30,31
Sweat glands reabsorb sodium by ac-
tive transport, but the ability to reabsorb sweat sodium
does not increase proportionally with the sweating rate.
As a result, the concentration of sweat sodium increases
at high sweating rates.
25,28
Heat acclimation improves the
ability to reabsorb sodium, so heat-acclimated persons
have lower sweat sodium concentrations (⬎50%reduc-
tion) for any given sweating rate.
28
Figure 2 depicts generalized modeling approximations
for daily sodium needs based upon calculated sweating
rates as a function of daily energy expenditure (activity
level) and air temperature.
1
This analysis assumes that
persons are heat acclimated and have a sweat sodium con-
centration of 25 mEq/L (about 0.6 g/L). The average
American diet contains about 4 g/d of sodium,
1
but this
varies greatly depending upon ethnic preferences for food.
Increases or decreases in sodium stores are usually cor-
rected by adjustments in a person’s salt appetite. In addi-
tion, when physical activity increases, the additional ca-
loric intake associated with increased activity usually
AG-PPKN-0909 R1 CH33 04-07-06 12:03:36
0
2000
4000
6000
8000
10000
Avera
g
e Da
y
time Dr
y
Bulb Tem
p
erature
(
oC
)
15 20 25 30 35 40
Average Daytime Dry Bulb Temperature (oF)
59 68 77 86 95 104
3600 Kcal/da
y
2900 Kcal/da
y
2400 Kcal/da
y
1900 Kcal/da
y
Figure 2. Daily sodium needs estimated from sweat loss predic-
tions due to changes in physical activity and air temperature. Daily
energy expenditures of 1900, 2400, 2900, and 3600 kcal corre-
spond to sedentary, low activity, active, and very active, respec-
tively. (From Food and Nutrition Board, Institute of Medicine. Di-
etary Reference Intakes for Water, Potassium, Sodium, Chloride,
and Sulfate. Washington, DC: National Academies Press; 2004.
Available online at: http://www.nap.edu/books/0309091691/html.)
covers the additional sodium required.
1
Therefore, so-
dium supplementation is generally not necessary (unless
subjects are performing very heavy activity) for the first
several days of heat exposure, as normal dietary sodium
intake appears adequate to compensate for sweat sodium
losses.
2,32
If persons need additional sodium, this can be
achieved by salting their food to taste. Another strategy
is to rehydrate with fluids containing about 20 mEq/L
of sodium. Most commercial sports beverages approxi-
mate this concentration.
2,32
Hydration Assessment
Although plasma osmolality is the criterion used as the
hydration assessment measure for large-scale fluid needs
assessment surveys,
1
the optimal choice of method for
assessing hydration, particularly in sport, is limited by the
circumstances and intent of the measurement. Popular
hydration assessment techniques vary greatly in their ap-
plicability to laboratory or field use due to methodological
limitations, which include the necessary circumstances for
accurate measurement, ease of application, and sensitivity
for detecting small, but meaningful changes in hydration
status.
33
Although there is presently no consensus for using
one assessment approach over another, in most athletic
arenas, the use of first morning body mass measurements
in combination with some measure of urine concentration
should allow ample sensitivity (low false negative) for de-
tecting deviations in fluid balance. When more precision
of acute hydration changes is desired, plasma osmolality
and isotope dilution provide for gradations in measure-
ment.
33
However, the simplest way to track acute hydra-
tion changes is to measure body mass before and after
exercise using the reasonable assumption that 1 g of lost
4Present Knowledge in Nutrition, Ninth Edition •Section VI: Minerals and Trace Elements
Table 2. Hydration Assessment Indices
Measure Practicality Validity (Acute vs. Chronic) Euhydration Cutoff
Total body water Low Acute and chronic ⬍2%
Plasma osmolality Medium Acute and chronic ⬍290 mOsmol
Urine specific gravity High Chronic ⬍1.020 g/ml
Urine osmolality High Chronic ⬍700 mOsmol
Urine color High Chronic ⬍4
Body mass High Acute and chronic* ⬍1%
*Potentially confounded by changes in body composition during very prolonged assessment periods.
mass is equivalent to 1 mL of lost fluid. In fact, if proper
controls are made, body mass changes can provide a more
sensitive estimate of acute total body water changes than
repeat measurements by dilution methods.
34
For longer
periods (1–2-weeks), body mass may even remain stable
enough to be a reliable hydration measure during periods
of hard exercise and acute fluid flux whether in temper-
ate
35,36
or hot
37
climates. Table 2 provides definable
thresholds from the literature,
1,37-43
which can be used
as a guide to detect a negative body fluid balance. Fluid
intakes should be considered adequate when any two as-
sessment outcomes are consistent with euhydration.
Fluid Balance, Temperature
Regulation, and Exercise
Performance
The difficulty encountered when trying to match fluid
consumption to sweat losses during exercise can produce
hypohydration by 2%to 6%of body weight.
44
Although
this is more common in hot environments, similar losses
are observed in cold climates when working in heavy
clothing.
45
The mismatch between intakes and losses is
due to physiological and behavioral factors.
Hypohydration
Hypohydration increases core temperature responses
during exercise in temperate and hot climates.
46
In fact,
adeficit of only 1%of body weight elevates core tempera-
ture during exercise.
47
As the magnitude of water deficit
increases, the magnitude of core temperature elevation
ranges from 0.1 to 0.23⬚Cfor every percent body weight
lost,
1,46
but the core temperature elevation may be greater
during exercise in hot compared with temperate cli-
mates.
48
In addition, altering the time of fluid ingestion
(early or late into the exercise bout) does not modify the
core temperature elevation from progressive dehydra-
tion.
49
Hypohydration not only elevates core temperature,
but also negates the core temperature advantages con-
ferred by high aerobic fitness and heat acclimation.
46
When hypohydrated, elevated core temperature re-
sponses result from a reduction in the capacity for heat
dissipation. The relative contributions of evaporative and
AG-PPKN-0909 R1 CH33 04-07-06 12:03:36
dry heat loss during exercise depend upon the specific
environmental conditions,
50
but both avenues of heat loss
are adversely affected by hypohydration.
1
Local sweating
and skin blood flow responses are both reduced for a given
core temperature,
46,48
and whole-body sweating is usually
either reduced
51
or unchanged
52
during exercise at a given
metabolic rate in the heat. However, even when hypohy-
dration is associated with no change in whole-body
sweating rate, core temperature is usually elevated, so that
whole-body sweating rate for a given core temperature is
lower. Both the singular and combined effects of plasma
hyperosmolality and hypovolemia have been suggested as
mediating the reduced heat loss response during exercise-
heat stress.
48
Figure 3 summarizes the relative contribu-
tions of hyperosmolality and hypovolemia to adverse ther-
moregulatory responses during exercise in temperate and
hot climates.
48
These effects are noticeably smaller in
cooler environments.
48,53,54
Hypohydration can decrease dynamic exercise perfor-
mance.
1
Dehydration by more than 2%of body weight
degrades endurance exercise, especially in hot environ-
ments.
1,55
The magnitude of the performance decrement
is variable, and probably depends on the individual, on
environmental conditions, and on exercise mode differ-
ences. However, for a given person and event, the greater
the dehydration level (after achieving the threshold for
performance degradation) the greater the performance
Figure 3. Effects of reducing total body water (TBW) on sweating
(S) and skin blood flow (F) thresholds and sensitivities. Substan-
tial, moderate, and limited descriptors are based on amount of
supporting data. (From Cheuvront SN, Carter III R, Montain SJ,
et al. Influence of hydration and airflow on thermoregulatory con-
trol in the heat. J Therm Biol. 2004;29:471–477.)
Human Water and Electrolyte Balance •Montain, Cheuvront, Carter III, and Sawka 5
Heat Strain
(Hyperthermia
)
Cardiovascular
Strain
Performance
Reduction
Central Nervous
S
y
stem Muscle
Metabolism
Figure 4. Physiologic factors that contribute to dehydration-me-
diated performance decrements. (From Cheuvront SN, Carter III
R, Sawka MN. Fluid balance and endurance exercise perfor-
mance. Curr Sports Med Rep. 2003;2:202–208.)
decrement. Dehydration probably does not alter muscle
strength,
56
but sometimes has been reported to reduce
dynamic small muscle endurance.
57,58
In addition, dehy-
dration of over 2%often adversely influences cognitive
function in the heat; however, this area requires more
research.
1
Physiologic factors that contribute to dehydration-
mediated performance decrements include hyperther-
mia,
59,60
increased cardiovascular strain,
61,62
altered met-
abolic function,
63
or perhaps via events originating in the
central nervous system
55,58
(Figure 4). Though each fac-
tor is unique, evidence suggests that they interact to con-
tribute in concert, rather than in isolation, to degrading
exercise performance. The relative contribution of each
factor may differ depending on the event, environmental
conditions, and athletic prowess, but elevated hyperther-
mia probably acts to accentuate the performance decre-
ment.
55,64,65
Hyperhydration
Hyperhydration is not easy to sustain, since overdrink-
ing of water or carbohydrate- electrolyte solutions pro-
duce a fluid overload that is rapidly excreted by the kid-
neys.
66
Greater fluid retention can be achieved with an
aqueous solution containing glycerol,
66,67
which increases
fluid retention by reducing free water clearance.
66
How-
ever, both exercise and heat stress decrease renal blood
flow and free water clearance and therefore negate glyc-
erol’s effectiveness as a hyperhydrating agent if ingested
during exercise.
68
Studies demonstrate that total body
water can be increased by approximately 1.5 L and sus-
tained for several hours with glycerol hyperhydration
66,68
;
however, glycerol provides no cardiovascular or thermore-
gulatory advantages over water ingestion alone when
taken during exercise or heat stress.
68-70
The effects of
glycerol hyperhydration on performance are mixed. Glyc-
erol hyperhydration may
71-73
or may not
69,70
improve ex-
ercise performance. Comparing study outcomes is com-
plicated by differences in performance measures, climate,
and the potentially confounding study design limita-
tions.
68
Hyponatremia
Symptomatic hyponatremia (typically associated with
serum sodium concentrations of less than 125–130 mEq/
AG-PPKN-0909 R1 CH33 04-07-06 12:03:36
L) has been observed during marathon and ultramarathon
competition,
74-76
military training,
77,78
and recreational
activities.
79
In athletic events, the condition is more likely
to occur in females and slower competitors. The severity
of the symptomatology is related to the magnitude that
the serum sodium concentration falls and the rapidity
with which it develops.
80
If hyponatremia develops over
many hours, it might cause less brain swelling and fewer
adverse symptoms.
80
The hyponatremia associated with
prolonged exercise develops primarily because individuals
drink excessively large quantities of hypotonic fluids (rela-
tive to sweating rate) for many hours.
3,76,77
Unreplaced
sodium losses contribute to the rate and magnitude of
sodium dilution. Additionally, nausea (which increases
vasopressin levels) and heat/exercise stress (which reduce
renal blood flow and urine output) can negatively affect
the ability of the kidney to rapidly correct the fluid-
electrolyte imbalance.
81
The syndrome can be prevented
by not drinking in excess of the sweating rate, and by
consuming salt-containing fluids or foods when partici-
pating in exercise events that produce multiple hours of
continuous or near-continuous sweating.
Summary
Among the greatest challenges to body water homeo-
stasis is exercise and environmental stress. Sweating re-
sults in water and electrolyte losses. Because sweat output
often exceeds water intake, there is an acute water deficit
that results in a hypertonic hypovolemia and intracellular
and extracellular fluid contraction. Although water and
electrolyte needs increase as a result of exercise, eloquent
physiological and behavioral adaptations allow humans to
regulate daily body water and electrolyte balance so long
as food and fluid are readily available. Although there is
presently no consensus for choosing one hydration assess-
ment approach over another, deviations in daily fluid bal-
ance can be determined with ample sensitivity using a
combination of any two common assessment measures.
Hypohydration increases heat storage by reducing sweat-
ing rate and skin blood flow responses for a given core
temperature. Aerobic exercise tasks can be adversely af-
fected if hypohydration exceeds about 2%of normal body
mass, with the potential effect greater in warm environ-
ments. Hyperhydration provides no thermoregulatory or
exercise performance advantages over euhydration in the
heat. Excessive consumption of hypotonic fluid over
many hours can lead to hyponatremia. Marked electrolyte
losses can accelerate the dilution and exacerbate the prob-
lem. Hyponatremia can be avoided by proper attention
to diet and fluid needs.
Acknowledgments
The authors would like to thank Rob Demes for tech-
nical assistance in preparing this manuscript. The views,
opinions, and/or findings contained in this report are
6Present Knowledge in Nutrition, Ninth Edition •Section VI: Minerals and Trace Elements
those of the authors and should not be construed as an
official Department of the Army position, or decision,
unless so designated by other official documentation. Ap-
proved for public release; distribution unlimited.
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