Introduction and Definition of Terms
Body fluid balance is controlled by both
physiological and behavioral actions.
However, when there is lack of fluid
availability, exposure to extreme
environments, or illness, inability to
maintain fluid balance can seriously jeopardize
health and the ability to perform. This
chapter presents an overview of topics
surrounding hydration, dehydration, and
rehydration. The terms euhydration,
hypohydration, and hyperhydration will be
used.
Euhydration defines a “normal,” narrow
fluctuation in body water content, whereas
the terms hypohydration and hyperhydration
define, respectively, a general deficit
(hypohydration) and surfeit
(hyperhydration) in body water content
beyond normal.
The term dehydration specifically defines
the condition of hypertonic hypovolemia
brought about by the net loss of hypotonic
body fluids. Isotonic or hypotonic
hypovolemia, manifest by large losses of
solute and water, is defined simply as
hypovolemia. Table 70-1 lists the two
principal forms of body water deficit and
the physiology and particular
circumstances associated with each form
of deficit.
Body Water, Fluid Turnover,
and Fluid Requirements
Total body water (TBW) is the principal
chemical component of the human body
and represents 50% to 70% of body mass
for the average young adult male. It is
regulated within ±0.2 to 0.5% of daily
body mass. Body water is required to
sustain the cardiovascular and thermo-
regulatory systems and to support cellular
homeostasis. While “normal” hydration is
achieved with a wide range of water
intakes by sedentary and active people
across the life span, homeostasis of body
water can be difficult to maintain when
challenged by strenuous physical work,
heat stress, or illness. Despite population
variability in age, body composition, and
physical fitness, it is important to note that
variability in TBW is accounted for almost
entirely by body composition, since lean
body mass contains ~73% water and fat
body mass consists of ~10% water.
Trained athletes have relatively high
TBW values by virtue of having a high
muscle mass and low body fat. In contrast,
obese individuals with the same body
mass as their lean counterparts will have
markedly smaller TBW volumes. Any
absolute fluid deficit will have more
severe consequences for individuals with
a smaller TBW.
Daily water balance depends on the net
difference between water gain and water
loss. Approximately 5% to 10% of TBW
is turned over daily via obligatory
(non-exercise) fluid loss avenues. Water
gain occurs from consumption (liquids and
food) and production (metabolic water),
while water losses occur from respiratory,
gastrointestinal, renal, and sweat losses.
Water loss in respiration is influenced by
the inspired air and pulmonary ventilation.
Of important note, the volume of
metabolic water produced during cellular
metabolism (~0.13 g/kcal) is
approximately equal to respiratory water
losses (~0.12 g/kcal), which results in
water turnover with no net change in
TBW.
Gastrointestinal tract losses tend to be
negligible (~100-200 mL/day); however,
certain illnesses, such as diarrhea, can lead
to loss of large amounts of fluid and
electrolytes. The ability to vary urine
output represents the primary means to
regulate net body water balance across a
broad range of fluid intake volumes and
losses from other avenues. Water losses in
urine approximate 1 to 2 L/ day. However,
urine output volumes may be larger or
smaller depending on daily fluid
consumption and activity. Minimum
outputs of ~20 mL/hr and maximal
volumes of ~1000 mL/hr are possible.
Net body water balance (loss = gain) is
regulated remarkably well day-to-day as
a result of thirst and hunger, coupled with
ad libitum access to food and beverages,
which offset water losses.
Although acute mismatches between fluid
gain and loss may occur due to illness,
environmental exposure, exercise, or
physical work, it is a reproducible
phenomenon that intakes are generally
adequate to offset net loss from day to
day. It is recognized, however, that after
significant body water deficits like those
associated with physical work or heat
stress, many hours of re-hydration and
electrolyte consumption may be needed
to reestablish body water balance. For
example, if hypo-hydrated by more than
about 4% of total body mass, it may take
>24 hours to fully re-hydrate via water
and electrolyte replacement. While daily
strenuous activity in a hot environment
can result in mild water balance deficits
even with unlimited access to food and
fluids, adherence to recognized water
intake guidance minimizes water deficits,
as determined by daily body mass stability.
An adequate intake (AI) for daily total
water is 3.7 L and 2.7 L for adult males
and females, respectively. Of these
prescribed volumes, 20% of the AI for
water is found in food eaten during meals
and snacks and the remaining 80%
(~3 L for males and 2.2 L for females)
can come from beverages of all types.
Daily water intake, however, varies greatly
for individuals and between groups. For
example, the daily water needs of
sedentary men are ~1.2 L or ~2.5 L,
and increase to ~3.2 L if performing
modest physical activity. Compared with
sedentary adults, active adults who live in
a warm environment are reported to have
daily water needs of ~6 L, and highly
active populations have been reported to
have markedly higher values. Data are
limited regarding fluid needs for women,
but typically they exhibit lower daily
water turnover rates than do their male
counterparts.
In general, fluid requirements vary based
on an individual’s body size(i.e.weight),
activity level, and the environment in
which they work, live, or perform activity.
Hydration Assessment
Human hydration assessment is a key component for prevention
and proper treatment of fluid and electrolyte imbalances.
When fluids are limited, illness strikes, or there is exposure to
extreme environments, cumulative fluid deficits can threaten
homeostasis, health, and performance. Health is also threatened
by fluid deficits, which can increase the risk of serious heat
illness, and by fluid surfeits, which increase the risk of hyponatremia.
In many clinical and most sports and wilderness
medicine situations, hypertonic-hypovolemia occurs when there
is net loss of hypotonic body fluids. However, substantial
solute (electrolyte) can also be lost in situations where there is
heavy work and heat stress induces profuse sweating, during
cold or high-altitude exposure, and in numerous illnesses and
disorders (e.g., gastroenteritis, hyperemesis, diuretic treatment,
dialysis) producing an isotonic or hypotonic-hypovolemia.
An appreciation for the different types of body fluid losses that
occur in response to illness, fluid restriction, or exposure to
extreme environments is fundamental to proper hydration assessment (Table 70-1).
Most circumstances involving strenuous
work in austere environments require
formation and vaporization of sweat as a
principal means of heat removal. Thus,
when sweat losses result in a body water
deficit, there is a predictable rise in
extracellular tonicity, which modulates
renal function and urine composition in
accordance with the body water deficit.
The basic principles of body fluid
regulation thus provide the framework for
using blood (osmolality, sodium, fluid
regulatory hormones) and urine
(osmolality, specific gravity, color) as
principal body fluid hydration assessment
measures. Similarly, because humans
maintain a relatively stable total body
water pool despite diverse factors that
affect water requirements (e.g., climate,
activity, dietary solute load), acute changes in body mass may be used to accurately measure
dehydration across medical disciplines.
Physical signs and symptoms (dizziness,
headache, tachycardia, capillary refill time,
sunken eyes, skin turgor*) only manifest
when fluid losses are severe and become
debilitating. *NOTE: Skin turgor is a sign
commonly used by health care workers to
assess the degree of fluid loss or
dehydration. Fluid loss can occur from
common conditions, such as diarrhea or
vomiting. Infants and young children with
vomiting, diarrhea, and decreased or no
fluid intake can rapidly lose a significant
amount of fluid. These findings are too
generalized to be useful in athletic
settings since they share symptoms
indicative of other ailments (e.g., acute
mountain sickness).and their use in
assessment could lead to an incorrect
diagnosis.
All hydration assessment methods vary
greatly in applicability because of
limitations such as the necessary
circumstances for reliable measurement,
principles of operation, cost and
complexity.
Table 70-2 provides the advantages and
disadvantages of numerous approaches
and should be consulted when deciding
on the choice of hydration marker.
Definitive hydration assessment requires
monitoring of changes in hydration state.
Although change can provide good
diagnostic accuracy, it requires a valid
baseline, control over confounding
variables, and serial measures. Large
population heterogeneity explains, in part,
why there
are presently few hydration status markers that
display potential for high nosological sensitivity from a more
practical, single measure.38,103 Although Table 70-3 provides euhydration
thresholds for the most useful of hydration assessment
measures, they too require considerable methodological control,
expense, and analytical expertise to be of practical use for dayto-day
hydration monitoring of athletic sojourners.
There is presently no scientific consensus for how to best
assess hydration status in a field setting. However, in most field
settings, the additive use of first morning body mass measurements
in combination with some measure of first morning urine
concentration and gross thirst perception provides a simple and
inexpensive way to dichotomize euhydration from gross dehydration
resulting from sweat loss and poor fluid intakes. This approach is represented using a Venn Diagram decision tool
(Figure 70-1).41 It combines three of the simplest markers of
hydration, including body mass (weight), urine, and thirst (WUT).
No marker by itself provides enough evidence of dehydration,
but the combination of any two simple self-assessment markers
means dehydration is likely. The presence of all three makes
dehydration very likely. The balance between science and simplicity
in the choice of these measures for field hydration assessment
is outlined below.
Urine Concentration. Urinalysis is a frequently used clinical
measure to distinguish between normal and pathologic conditions.
Urinary markers for dehydration include urine volume,
urine specific gravity (USG), urine osmolality (UOsm) and urine
color (UCol). Urine is a solution of water and various other substances.
Thus, its concentration varies inversely with volume,
which is reduced with dehydration. Urine output generally
approximates 1 to 2 L/day but can be increased by an order of
magnitude when consuming large volumes of fluid.84 This large
capacity to vary urine output represents the primary avenue to
regulate net body water balance across a broad range of fluid
intake volumes and losses from other avenues. Whereas the
quantification of urine volume is impractical on a daily basis, the
quantitative (USG, UOsm) or qualitative (UCol) assessment of its
concentration is far simpler. As a screening tool to dichotomize
euhydration from dehydration, urine concentration (USG, UOsm,
UCol) is a reliable assessment technique9,17,182 with reasonably
definable thresholds.
In contrast, urine measures often correlate poorly with “gold
standards” like plasma osmolality and fail to reliably track documented
changes in body mass corresponding to acute dehydration
and rehydration.97,151 It appears that changes in plasma
osmolality that stimulate endocrine regulation of renal water and
electrolyte reabsorption are delayed at the kidney when there
occur acute fluxes in body water.151 It is also likely that drink
composition influences this response. Shirreffs and Maughan186
demonstrated that drinking large volumes of hypotonic fluids
results in copious urine production long before euhydration is
achieved. Urine concentration measurements can also be confounded
by diet, which may explain large cross-cultural differences
in urine osmolality.110 However, use of the first morning
void following an overnight fast minimizes confounding influences
and maximizes measurement reliability.9,182 Urinalysis of
specific gravity, osmolality, and color can therefore be used to
assess and distinguish euhydration from dehydration so long as
the first morning void is used.
Inexpensive and easy-to-use commercial instruments are available
for assessing USG and conductivity (osmolality equivalent)17,182;
a urine color chart is also available.9
The simplest of
FIGURE 70-1 W stands for “weight.” U stands for “urine.” T stands
for “thirst.” When two or more simple markers of dehydration are
present, it is likely that you are dehydrated. If all three markers are
present, dehydration is very likely.41
Likely
Very
likely
Likely Likely
W U
T
these, color, is included in the Venn diagram. Under ideal circumstances,
the urine (first morning) should be in a clean, clear
vial or cup and the color assessed against a white background.
Urine color can be compared against a urine color chart9
or
assessed relative to the degree of darkness. Paler color urine
(similar to lemonade) indicates adequate hydration and the darker
yellow/brown the urine color (similar to apple juice), the greater
degree of dehydration. Assessing urine that has been diluted in
toilet water or while in mid-flow may alter the urine color. When
in less than ideal conditions, urine in a urinal is less dilute and
in the field, snow can provide a suitable background. Example
photos of urine color with corresponding numerical color,9
USG,
and urine osmolality values are presented in Figure 70-2.
Body Mass. Body mass is an often used measurement for rapid
assessment of athlete hydration changes in both laboratory and
field environments. Changes in acute hydration are calculated as
the difference between pre- and postexercise body mass. The
level of dehydration is best expressed as a percentage of starting
body mass rather than as a percentage of TBW, since the latter
ranges widely.84 Use of this technique implies that 1 g of lost mass
is equivalent to 1 mL of lost water. So long as total body water
loss is of interest, failure to account for carbon exchange represents
the only small error (~10%) in this assumption.39 Indeed,
acute body mass changes (water) are frequently the standard
against which the resolution of other hydration assessment
parameters are compared in the laboratory. In fact, if proper controls
are made, body mass changes can provide a more sensitive
estimate of acute total body water changes than can repeat measurements
by dilution methods.78
There is also evidence that body mass is a sufficiently stable
physiological parameter for potential daily fluid balance monitoring,
even over longer periods (1-2 weeks) that include hard
exercise and acute fluid flux.35,102 Young, healthy men undergoing
daily exercise-heat stress maintain a stable first morning body
mass so long as they make a conscious effort to replace exercise
sweat losses.35 Similarly, ad libitum intakes of food and fluid will
balance sweat losses incurred with regular exercise, resulting in
a stable daily body mass.102 Over longer time frames, changes in
body composition (fat and lean mass) that occur with chronic
energy imbalance are also reflected grossly as changes in body
mass, thus limiting this technique. Clearly, if first morning body
mass stability is used to monitor changes in hydration, it should
be used in combination with another hydration assessment technique
(urine concentration) to dissociate gross tissue losses from
water losses if long-term hydration status is of interest.
Thirst. Although genuine thirst develops only after dehydration
is present and is alleviated before euhydration is achieved,75,84
thirst is one of the only reliable subjective feelings reported by
a person in response to fluid restriction.184 Plasma osmolality near
295 mmol/kg will produce an arginine vasopressin (AVP) level
of around 5 pg/mL, which results in a maximal urine concentrating
capacity.156 The average plasma osmolality at which thirst is
stimulated above baseline is also approximately 295 mmol/kg.156
If we assume that a normal resting plasma osmolality of 285
becomes concentrated to 295, the ratio 285/295 multiplied by a
normal 42 L total body water gives an estimated 40.5 L or 1.5 L
Body fluid balance is controlled by both
physiological and behavioral actions.
However, when there is lack of fluid
availability, exposure to extreme
environments, or illness, inability to
maintain fluid balance can seriously jeopardize
health and the ability to perform. This
chapter presents an overview of topics
surrounding hydration, dehydration, and
rehydration. The terms euhydration,
hypohydration, and hyperhydration will be
used.
Euhydration defines a “normal,” narrow
fluctuation in body water content, whereas
the terms hypohydration and hyperhydration
define, respectively, a general deficit
(hypohydration) and surfeit
(hyperhydration) in body water content
beyond normal.
The term dehydration specifically defines
the condition of hypertonic hypovolemia
brought about by the net loss of hypotonic
body fluids. Isotonic or hypotonic
hypovolemia, manifest by large losses of
solute and water, is defined simply as
hypovolemia. Table 70-1 lists the two
principal forms of body water deficit and
the physiology and particular
circumstances associated with each form
of deficit.
Body Water, Fluid Turnover,
and Fluid Requirements
Total body water (TBW) is the principal
chemical component of the human body
and represents 50% to 70% of body mass
for the average young adult male. It is
regulated within ±0.2 to 0.5% of daily
body mass. Body water is required to
sustain the cardiovascular and thermo-
regulatory systems and to support cellular
homeostasis. While “normal” hydration is
achieved with a wide range of water
intakes by sedentary and active people
across the life span, homeostasis of body
water can be difficult to maintain when
challenged by strenuous physical work,
heat stress, or illness. Despite population
variability in age, body composition, and
physical fitness, it is important to note that
variability in TBW is accounted for almost
entirely by body composition, since lean
body mass contains ~73% water and fat
body mass consists of ~10% water.
Trained athletes have relatively high
TBW values by virtue of having a high
muscle mass and low body fat. In contrast,
obese individuals with the same body
mass as their lean counterparts will have
markedly smaller TBW volumes. Any
absolute fluid deficit will have more
severe consequences for individuals with
a smaller TBW.
Daily water balance depends on the net
difference between water gain and water
loss. Approximately 5% to 10% of TBW
is turned over daily via obligatory
(non-exercise) fluid loss avenues. Water
gain occurs from consumption (liquids and
food) and production (metabolic water),
while water losses occur from respiratory,
gastrointestinal, renal, and sweat losses.
Water loss in respiration is influenced by
the inspired air and pulmonary ventilation.
Of important note, the volume of
metabolic water produced during cellular
metabolism (~0.13 g/kcal) is
approximately equal to respiratory water
losses (~0.12 g/kcal), which results in
water turnover with no net change in
TBW.
Gastrointestinal tract losses tend to be
negligible (~100-200 mL/day); however,
certain illnesses, such as diarrhea, can lead
to loss of large amounts of fluid and
electrolytes. The ability to vary urine
output represents the primary means to
regulate net body water balance across a
broad range of fluid intake volumes and
losses from other avenues. Water losses in
urine approximate 1 to 2 L/ day. However,
urine output volumes may be larger or
smaller depending on daily fluid
consumption and activity. Minimum
outputs of ~20 mL/hr and maximal
volumes of ~1000 mL/hr are possible.
Net body water balance (loss = gain) is
regulated remarkably well day-to-day as
a result of thirst and hunger, coupled with
ad libitum access to food and beverages,
which offset water losses.
Although acute mismatches between fluid
gain and loss may occur due to illness,
environmental exposure, exercise, or
physical work, it is a reproducible
phenomenon that intakes are generally
adequate to offset net loss from day to
day. It is recognized, however, that after
significant body water deficits like those
associated with physical work or heat
stress, many hours of re-hydration and
electrolyte consumption may be needed
to reestablish body water balance. For
example, if hypo-hydrated by more than
about 4% of total body mass, it may take
>24 hours to fully re-hydrate via water
and electrolyte replacement. While daily
strenuous activity in a hot environment
can result in mild water balance deficits
even with unlimited access to food and
fluids, adherence to recognized water
intake guidance minimizes water deficits,
as determined by daily body mass stability.
An adequate intake (AI) for daily total
water is 3.7 L and 2.7 L for adult males
and females, respectively. Of these
prescribed volumes, 20% of the AI for
water is found in food eaten during meals
and snacks and the remaining 80%
(~3 L for males and 2.2 L for females)
can come from beverages of all types.
Daily water intake, however, varies greatly
for individuals and between groups. For
example, the daily water needs of
sedentary men are ~1.2 L or ~2.5 L,
and increase to ~3.2 L if performing
modest physical activity. Compared with
sedentary adults, active adults who live in
a warm environment are reported to have
daily water needs of ~6 L, and highly
active populations have been reported to
have markedly higher values. Data are
limited regarding fluid needs for women,
but typically they exhibit lower daily
water turnover rates than do their male
counterparts.
In general, fluid requirements vary based
on an individual’s body size(i.e.weight),
activity level, and the environment in
which they work, live, or perform activity.
Hydration Assessment
Human hydration assessment is a key component for prevention
and proper treatment of fluid and electrolyte imbalances.
When fluids are limited, illness strikes, or there is exposure to
extreme environments, cumulative fluid deficits can threaten
homeostasis, health, and performance. Health is also threatened
by fluid deficits, which can increase the risk of serious heat
illness, and by fluid surfeits, which increase the risk of hyponatremia.
In many clinical and most sports and wilderness
medicine situations, hypertonic-hypovolemia occurs when there
is net loss of hypotonic body fluids. However, substantial
solute (electrolyte) can also be lost in situations where there is
heavy work and heat stress induces profuse sweating, during
cold or high-altitude exposure, and in numerous illnesses and
disorders (e.g., gastroenteritis, hyperemesis, diuretic treatment,
dialysis) producing an isotonic or hypotonic-hypovolemia.
An appreciation for the different types of body fluid losses that
occur in response to illness, fluid restriction, or exposure to
extreme environments is fundamental to proper hydration assessment (Table 70-1).
Most circumstances involving strenuous
work in austere environments require
formation and vaporization of sweat as a
principal means of heat removal. Thus,
when sweat losses result in a body water
deficit, there is a predictable rise in
extracellular tonicity, which modulates
renal function and urine composition in
accordance with the body water deficit.
The basic principles of body fluid
regulation thus provide the framework for
using blood (osmolality, sodium, fluid
regulatory hormones) and urine
(osmolality, specific gravity, color) as
principal body fluid hydration assessment
measures. Similarly, because humans
maintain a relatively stable total body
water pool despite diverse factors that
affect water requirements (e.g., climate,
activity, dietary solute load), acute changes in body mass may be used to accurately measure
dehydration across medical disciplines.
Physical signs and symptoms (dizziness,
headache, tachycardia, capillary refill time,
sunken eyes, skin turgor*) only manifest
when fluid losses are severe and become
debilitating. *NOTE: Skin turgor is a sign
commonly used by health care workers to
assess the degree of fluid loss or
dehydration. Fluid loss can occur from
common conditions, such as diarrhea or
vomiting. Infants and young children with
vomiting, diarrhea, and decreased or no
fluid intake can rapidly lose a significant
amount of fluid. These findings are too
generalized to be useful in athletic
settings since they share symptoms
indicative of other ailments (e.g., acute
mountain sickness).and their use in
assessment could lead to an incorrect
diagnosis.
All hydration assessment methods vary
greatly in applicability because of
limitations such as the necessary
circumstances for reliable measurement,
principles of operation, cost and
complexity.
Table 70-2 provides the advantages and
disadvantages of numerous approaches
and should be consulted when deciding
on the choice of hydration marker.
Definitive hydration assessment requires
monitoring of changes in hydration state.
Although change can provide good
diagnostic accuracy, it requires a valid
baseline, control over confounding
variables, and serial measures. Large
population heterogeneity explains, in part,
why there
are presently few hydration status markers that
display potential for high nosological sensitivity from a more
practical, single measure.38,103 Although Table 70-3 provides euhydration
thresholds for the most useful of hydration assessment
measures, they too require considerable methodological control,
expense, and analytical expertise to be of practical use for dayto-day
hydration monitoring of athletic sojourners.
There is presently no scientific consensus for how to best
assess hydration status in a field setting. However, in most field
settings, the additive use of first morning body mass measurements
in combination with some measure of first morning urine
concentration and gross thirst perception provides a simple and
inexpensive way to dichotomize euhydration from gross dehydration
resulting from sweat loss and poor fluid intakes. This approach is represented using a Venn Diagram decision tool
(Figure 70-1).41 It combines three of the simplest markers of
hydration, including body mass (weight), urine, and thirst (WUT).
No marker by itself provides enough evidence of dehydration,
but the combination of any two simple self-assessment markers
means dehydration is likely. The presence of all three makes
dehydration very likely. The balance between science and simplicity
in the choice of these measures for field hydration assessment
is outlined below.
Urine Concentration. Urinalysis is a frequently used clinical
measure to distinguish between normal and pathologic conditions.
Urinary markers for dehydration include urine volume,
urine specific gravity (USG), urine osmolality (UOsm) and urine
color (UCol). Urine is a solution of water and various other substances.
Thus, its concentration varies inversely with volume,
which is reduced with dehydration. Urine output generally
approximates 1 to 2 L/day but can be increased by an order of
magnitude when consuming large volumes of fluid.84 This large
capacity to vary urine output represents the primary avenue to
regulate net body water balance across a broad range of fluid
intake volumes and losses from other avenues. Whereas the
quantification of urine volume is impractical on a daily basis, the
quantitative (USG, UOsm) or qualitative (UCol) assessment of its
concentration is far simpler. As a screening tool to dichotomize
euhydration from dehydration, urine concentration (USG, UOsm,
UCol) is a reliable assessment technique9,17,182 with reasonably
definable thresholds.
In contrast, urine measures often correlate poorly with “gold
standards” like plasma osmolality and fail to reliably track documented
changes in body mass corresponding to acute dehydration
and rehydration.97,151 It appears that changes in plasma
osmolality that stimulate endocrine regulation of renal water and
electrolyte reabsorption are delayed at the kidney when there
occur acute fluxes in body water.151 It is also likely that drink
composition influences this response. Shirreffs and Maughan186
demonstrated that drinking large volumes of hypotonic fluids
results in copious urine production long before euhydration is
achieved. Urine concentration measurements can also be confounded
by diet, which may explain large cross-cultural differences
in urine osmolality.110 However, use of the first morning
void following an overnight fast minimizes confounding influences
and maximizes measurement reliability.9,182 Urinalysis of
specific gravity, osmolality, and color can therefore be used to
assess and distinguish euhydration from dehydration so long as
the first morning void is used.
Inexpensive and easy-to-use commercial instruments are available
for assessing USG and conductivity (osmolality equivalent)17,182;
a urine color chart is also available.9
The simplest of
FIGURE 70-1 W stands for “weight.” U stands for “urine.” T stands
for “thirst.” When two or more simple markers of dehydration are
present, it is likely that you are dehydrated. If all three markers are
present, dehydration is very likely.41
Likely
Very
likely
Likely Likely
W U
T
these, color, is included in the Venn diagram. Under ideal circumstances,
the urine (first morning) should be in a clean, clear
vial or cup and the color assessed against a white background.
Urine color can be compared against a urine color chart9
or
assessed relative to the degree of darkness. Paler color urine
(similar to lemonade) indicates adequate hydration and the darker
yellow/brown the urine color (similar to apple juice), the greater
degree of dehydration. Assessing urine that has been diluted in
toilet water or while in mid-flow may alter the urine color. When
in less than ideal conditions, urine in a urinal is less dilute and
in the field, snow can provide a suitable background. Example
photos of urine color with corresponding numerical color,9
USG,
and urine osmolality values are presented in Figure 70-2.
Body Mass. Body mass is an often used measurement for rapid
assessment of athlete hydration changes in both laboratory and
field environments. Changes in acute hydration are calculated as
the difference between pre- and postexercise body mass. The
level of dehydration is best expressed as a percentage of starting
body mass rather than as a percentage of TBW, since the latter
ranges widely.84 Use of this technique implies that 1 g of lost mass
is equivalent to 1 mL of lost water. So long as total body water
loss is of interest, failure to account for carbon exchange represents
the only small error (~10%) in this assumption.39 Indeed,
acute body mass changes (water) are frequently the standard
against which the resolution of other hydration assessment
parameters are compared in the laboratory. In fact, if proper controls
are made, body mass changes can provide a more sensitive
estimate of acute total body water changes than can repeat measurements
by dilution methods.78
There is also evidence that body mass is a sufficiently stable
physiological parameter for potential daily fluid balance monitoring,
even over longer periods (1-2 weeks) that include hard
exercise and acute fluid flux.35,102 Young, healthy men undergoing
daily exercise-heat stress maintain a stable first morning body
mass so long as they make a conscious effort to replace exercise
sweat losses.35 Similarly, ad libitum intakes of food and fluid will
balance sweat losses incurred with regular exercise, resulting in
a stable daily body mass.102 Over longer time frames, changes in
body composition (fat and lean mass) that occur with chronic
energy imbalance are also reflected grossly as changes in body
mass, thus limiting this technique. Clearly, if first morning body
mass stability is used to monitor changes in hydration, it should
be used in combination with another hydration assessment technique
(urine concentration) to dissociate gross tissue losses from
water losses if long-term hydration status is of interest.
Thirst. Although genuine thirst develops only after dehydration
is present and is alleviated before euhydration is achieved,75,84
thirst is one of the only reliable subjective feelings reported by
a person in response to fluid restriction.184 Plasma osmolality near
295 mmol/kg will produce an arginine vasopressin (AVP) level
of around 5 pg/mL, which results in a maximal urine concentrating
capacity.156 The average plasma osmolality at which thirst is
stimulated above baseline is also approximately 295 mmol/kg.156
If we assume that a normal resting plasma osmolality of 285
becomes concentrated to 295, the ratio 285/295 multiplied by a
normal 42 L total body water gives an estimated 40.5 L or 1.5 L
total body water deficit, which is 2.1% dehydration for a 70-kg
person. This is consistent with general observations of thirst
insensitivity below a “threshold” fluid deficit (notion that thirst
develops late). However, it is important to recognize that there
is substantial individual variability in the plasma osmolality “set
point” and the osmotic thresholds for AVP release and thirst
perception.157 For example, Robertson reports as much as a
10-fold difference between individuals in the slope of the line
relating AVP to plasma osmolality.157 Clearly, thirst is a qualitative
tool for hydration assessment, but positive thirst symptoms
coupled with at least one additional Venn Diagram marker suggests
an increasing likelihood of dehydration.
Although plasma osmolality and total body water measurements
are the hydration assessment measures for large-scale
fluid needs,84 there is presently no consensus for using any one
approach over another in a field or athletic setting. In most circumstances,
the use of first morning body mass combined with
some measure of first morning urine concentration (USG, urine
osmolality, and/or color) offers simple assessment and allows
ample sensitivity (low false negative) for detecting meaningful
deviations in fluid balance (>2% body mass). This approach is
represented using a Venn Diagram decision tool (Figure 70-2).41
It combines three of the simplest markers of hydration, including
weight, urine, and thirst (WUT). No marker by itself provides
enough evidence of dehydration, but the combination of
any two simple self-assessment markers means dehydration is
likely, and the presence of all three makes dehydration very
likely. In a field setting, where a scale may not be available for
body weight measures, the combination of first morning urine
color and thirst may provide a reasonable indication of the presence
of dehydration.
Sweat and Sweat Prediction
Muscular contractions involved with activity/exercise produce
metabolic heat that is transferred from the active muscles to blood
and then the body core. Subsequent body temperature elevations
elicit heat loss responses of increased skin blood flow and increased
sweat secretion so that heat can be dissipated to the environment.173,175
Heat exchange between the skin and environment is
governed by biophysical properties dictated by surrounding temperature,
humidity and air motion, sky and ground radiation, and
clothing.67 When ambient temperature is greater than or equal to
skin temperature, evaporative heat loss accounts for all body
cooling. Eccrine sweat glands secrete fluid onto the skin surface,
permitting evaporative cooling when liquid is converted to water
vapor. Sweat glands respond to thermal stress primarily through
sympathetic cholinergic stimulation, with catecholamines having
a smaller role in the sweat response.175 The rate of sweat evaporation
depends on air movement and the water vapor pressure
gradient between the skin and environment, so in still or moist
air, sweat does not evaporate readily and collects on the skin.
Sweat that drips from the body or clothing provides no cooling
benefit. If secreted sweat drips from the body and is not evaporated,
higher sweating will be needed to achieve the evaporative
cooling requirements.36,173 Conversely, increased air motion (wind,
movement velocity) will facilitate evaporation and minimize
wasted (dripping) sweat.36 Sweat losses can vary widely and
depend on the amount and intensity of physical activity and environmental
conditions.71,180 In addition, a number of factors can
alter sweat rates and ultimately fluid needs. Heat acclimatization
results in higher and more sustained sweating rates.173,175 Similarly,
aerobic exercise training has a modest effect on enhancing sweating
rate responses.173,175 Wearing heavy or impermeable clothing
or protective equipment can increase heat stress115 and sweat rate,
but can limit evaporation of sweat and ultimately heat loss. Likewise,
wearing heavy or impermeable clothing while exercising in
cold weather can elicit unexpectedly high sweat rates,63 which can
increase fluid needs. Conversely, factors such as wet skin (e.g.,
from high humidity) and dehydration can act to suppress the
sweating rate response.173
Sweat is hypotonic relative to plasma and is typically half of
plasma osmolality (~145 mmol/kg vs. ~290 mmol/kg, respectively).46
Losses of electrolytes in sweat depend on total sweat
losses (over a given period of time) and sweat electrolyte concentrations. Typical sweat sodium concentration averages
~35 mEq/L (range 10-70 mEq/L) and varies depending on genetic
predisposition, diet, sweating rate, and heat acclimatization state.*
Sweat concentration of potassium averages 5 mEq/L (range
3-15 mEq/L), calcium averages 1 mEq/L (range 0.3-2 mEq/L),
magnesium averages 0.8 mEq/L (range 0.2-1.5 mEq/L), and chloride
averages 30 mEq/L (range 5-60 mEq/L).22 Sex, maturation, or
aging do not appear to have discernible effects on sweat electrolyte
concentrations,120,130 although dehydration can increase the
sweat concentrations of sodium and chloride.128 Sweat glands
reabsorb sodium and chloride by active transport, but the ability
to reabsorb these electrolytes does not increase proportionally
with the sweating rate. As a result, the sodium and chloride concentrations
of sweat increase as a function of sweating rate.5,47
Heat acclimatization improves the ability to reabsorb sodium and
chloride; thus, heat-acclimatized individuals usually have lower
sweat sodium concentrations (e.g., >50% reduction) for any given
sweating rate.5
The ability to predict sweat losses is critical for calculating
water needs, particularly for individuals who are exposed to heat
stress. For large cohorts such as the U.S. military, fluid requirements
in the field are based on water tables generated from existing
sweat prediction equations. Sweat rates differ between various
work activities and between individuals.173 Specifics on determining
individual sweat rate and fluid requirements are covered
later in the chapter in the fluid replacement recommendations
section. Figure 70-384 depicts generalized modeling approximations
for daily sweating rates as a function of daily metabolic rate
(activity level) and air temperature. Metabolic rate and air temperature
both have marked effects on water needs. In addition to
air temperature, environmental factors such as relative humidity,
air motion, solar load, and protective clothing influence heat strain
and water needs.
Although Figure 70-3 shows approximations of water needs,
mathematical models exist that more specifically predict fluid
needs. The Shapiro equation180 has been used extensively to
estimate sweating rates and calculate daily water needs. This
model calculates sweat rate (Msw; expressed as g m−2 h−1
) and
thus fluid needs as the following:
M E sw = × req × E − 27 9 0 455 . ( max ) , .
where Ereq is the required evaporative cooling (in W/m2
) and Emax
is the maximal evaporative cooling capacity. Ereq is calculated
from metabolic heat production, clothing heat transfer characteristics,
and the environment. Emax is derived from the vapor transfer properties of the clothing worn and the environment. However,
this equation has been shown to have limitations,40 in that it often
overpredicts fluids needs when exercise is greater than 2 hours,
when improved uniforms and body armor are worn, and when
activity takes place in lower air temperatures and the activity is
of high intensity. In response to these limitations, Gonzalez et al.71
developed a correction for the Shapiro equation in addition to a
new prediction equation using independent data across a wide
range of environmental conditions, metabolic rates, and environmental
conditions. Figure 70-4, taken from Jay et al.,86 depicts the
adjustment to the sweat loss values yielded by the corrected
Shapiro equation derived by Gonzalez et al.71 Future work in this
area may be required to further increase the applicability of sweat
rate prediction equations, in particular under conditions with
variable solar loads, in lower air temperatures (~15° C [59° F]),
with clothing having low water vapor permeability or with specialized
equipment (e.g., American football), and with individuals
possessing greater body mass and surface areas.86 At present, use
of such equations has been predominantly limited to the military.
Future commercial applications of such equations to predict fluid
needs by the general population in hiking/trekking/wilderness
scenarios may be possible via commonly used devices, such as
personal digital assistants (PDAs).
Physiological Consequences
of Dehydration
By virtue of tonicity and volume changes, dehydration has negative
consequences on thermoregulation and performance. Dehydration
is brought about by voluntary fluid restriction, insufficient
rehydration following daily activity, or physical activity/exercise
in the form of thermoregulatory sweating. The most common
form of dehydration during exercise in the heat is that where
water deficit occurs without proportionate sodium chloride
loss.166 Individuals often start an exercise task with normal total
body water and dehydrate over an extended duration; however,
in some situations, an individual might initiate activity/exercise
with a body water deficit because the interval between exercise
sessions is inadequate or chronic fluid intake is insufficient to
replace losses. During multiple-day treks or expeditions where
individuals take part in prolonged daily sessions of activity/
exercise, possibly in hot conditions, a fluid deficit may be carried
from one activity/exercise session to the next or from one day
to the next.70 In addition, individuals medicated with diuretics
may be dehydrated prior to initiating exercise. Use of medications,
such as acetazolamide taken prophylactically or while at
altitude for acute mountain sickness, can have such an effect
(and may increase risk for hyponatremia). This drug causes the
kidneys to excrete bicarbonate, which acidifies the blood, increasing
ventilation and blood O2 content; however, it also increases
fluid and electrolyte losses. If large sodium chloride deficits occur
during exercise, then the extracellular fluid volume contracts and
causes “salt depletion dehydration.”
Dehydration increases physiological strain as measured by
core temperature, heart rate, and perceived exertion responses
during exercise heat stress.166 The greater the body water deficit,
the greater the increase in physiological strain for a given exercise
task.3,124,125,176 Dehydration can augment core temperature
elevations during exercise in temperate26,136,170 as well as in hot
environments.43,168,178 The typical reported core temperature augmentation
with dehydration is an increase of 0.1° to 0.2° C
(32.18° to 32.36° F) with each 1% of dehydration.167 The greater
heat storage associated with dehydration is associated with a
proportionate decrease in heat loss. Thus, decreased sweating
rate (evaporative heat loss) as well as decreased cutaneous
blood flow (dry heat loss) are responsible for greater heat storage
observed during exercise when hypohydrated.59,60,134 The degree
to which each of these mechanisms dissipates heat from the
body depends on environmental conditions. However, both
avenues of heat loss are unfavorably altered by dehydration.
When dehydrated, the sweating rate is lower for any given
core temperature, and heat loss via evaporation is reduced.176 In
addition, as dehydration increases, there is reduction in total
body sweating rate at a given core temperature during exercise
heat stress.176 During submaximal exercise with little or no
thermal strain, dehydration results in increased heart rate and
decreased stroke volume, and typically no change in cardiac
output relative to euhydration levels.163,188 The addition of heat
stress in combination with dehydration during exercise results in
decreased blood volume, reducing central venous pressure and
cardiac output,94,129 and creates competition between the central
and peripheral circulation for limited blood volume.133,161 As body
temperature increases during exercise, cutaneous vasodilation
occurs and superficial veins become more compliant, decreasing
venous resistance and pressure.161 A result of reduced blood
volume (due to dehydration) and increased blood displacement
to cutaneous vascular beds (due to heat stress) is decreased
central venous pressure, venous return and, ultimately, cardiac
output below euhydration values.135,169
Environmental Heat Stress,
Dehydration, and Performance
Physiological factors that contribute to dehydration-mediated
aerobic exercise performance decrements include increased body
core temperature, increased cardiovascular strain, increased glycogen
utilization, and perhaps altered central nervous system
function.141,166,175 Although each factor is unique, evidence suggests
that they interact to contribute in concert, rather than in
isolation, to degrade aerobic exercise performance.35,166,175 The
relative contribution of each factor may differ depending on the
specific activity, environmental conditions, heat acclimatization
status, and athlete prowess, but elevated hyperthermia probably
acts to accentuate the performance decrement.
In a field or wilderness setting, individuals may perform activities
that require anaerobic power or muscular strength. Dehydration
likely does not degrade muscular strength58,77 or anaerobic
performance.34,85 However a recent critical review of the related
literature89 suggests that hypohydration decreases (1%-3%)
strength, power, and high-intensity endurance activities. Dehydration
>2% of body mass has been shown to degrade aerobic exercise
performance in temperate, warm, and hot environments.31,37
As the level of dehydration increases, aerobic exercise performance
is degraded proportionally.84 The critical water deficit (>2%
body mass for most individuals) and magnitude of performance
decrement are likely related to environmental temperature, exercise
task, and the individuals’ unique biological characteristics
(e.g., tolerance to dehydration). Therefore, some individuals are
more or less tolerant to dehydration. Adolph3
was one of the first
to document that during long duration exercise in temperate or
slightly warm environments, thermoregulatory sweating would
lead to progressive dehydration and result in lower exercise
output (Figure 70-5). Adolph derived this figure from limited
exercise capability data and heart rate responses from a variety
of exercise, heat stress, and dehydration conditions.
Exercise tasks that primarily require aerobic metabolism and
that are prolonged will be more adversely influenced by dehydration
than are exercise tasks that require anaerobic metabolism
or muscular strength and power.171 As demonstrated by GonzalezAlonso
et al.,72 the greater the level of dehydration, the greater
the magnitude of cardiovascular and thermoregulatory strain. It
has previously been demonstrated that high levels of aerobic
fitness and acclimatization status provide some thermoregulatory
advantage. However, dehydration seems to cancel out this protective
effect during exercise heat stress.25,119,172 A comprehensive
review164 of a number of studies that investigated the impact of
dehydration on physical exercise capacity and maximal aerobic
power found that in the majority of studies, exercise capacity
decreased with levels of dehydration of as little as 1% to 2%
body mass, although maximal aerobic power was not altered.
In addition, the reduction in exercise capacity when dehydrated
was further accentuated by combination with heat stress.51,140,150
In temperate environments, body water deficits of <2 body="" div="">
mass did not have a significant impact on maximal aerobic power.
In a hot environment, however, a small to moderate water deficit
(≥2%body mass) resulted in a large decline in maximal aerobic
power.51,140 A review of studies37 that observed the effects of
progressive dehydration (>2% body mass) specifically on aerobic
exercise performance found that in environments of >30° C
(86° F), aerobic exercise performance was decreased by anywhere
from 7% to 60%. It also appears that the magnitude of the effect
increases as exercise extends beyond 90 minutes. Overall what
can be taken from this review is that the impact of dehydration
on prolonged work efforts is magnified by hot environments,
and probably worsens as the level of dehydration increases.
Few investigations have studied the impact of dehydration on
aerobic performance across a range of environmental temperatures.
Cheuvront et al.25 observed 8% reduction in total work
during a cycling time trial when dehydrated by ≥2% of body
mass in a 20° C (68° F) environment. However, in 2° C (35.6°
F), no effect of dehydration was observed. Kenefick et al.92
reported decrements in aerobic performance (15-minute cycling
time trial) of −3%, −5%, −12%, and −23% in 10° C (50° F), 20° C
(68° F), 30° C (86° F), and 40° C (104° F) respectively, when
volunteers were dehydrated by ≥2% of body mass. Figure 70-6
depicts the change in performance relative to euhydrated trials
and relative to the coefficient of variation (test variability; shaded
area) of the cycling test itself. Mean values that lie inside of the
shaded area are considered to be within the noise of the test and
those that lie outside are considered to be meaningful. Based on
the findings, it would appear that the temperature cusp where
dehydration of 4% of body mass altered aerobic exercise performance
occurred at 20° C (68° F). It is important to note that these
reported results are the minimal decrement in performance that
could be expected. Greater decrements could be expected during
more prolonged work or with greater levels of dehydration.
Table 70-4 depicts the decrement in aerobic time trial (<60 div="">
minutes) performance across a continuum of environmental temperatures
and at an altitude of ~3000 m with and without hypohydration
(>2% body mass loss). Without any degree of dehydration,
certain environments (warm/hot, high altitude) themselves have a
negative impact on aerobic exercise performance. It is important
to note that in combination with these environments, dehydration
further degrades aerobic exercise performance and that with
longer duration exercise (>60 minutes), greater degradations in
performance can be expected. However, by maintaining a well-hydrated state, the contribution of dehydration to the degradation
in exercise performance can be alleviated.
One explanation for the impact of dehydration on exercise performance
is that during exercise in the heat, sweat output can often
exceed water intake and lead to overall loss of body water and
reductions in plasma and blood volume. The amount of body fluid
lost through thermoregulatory sweating can vary widely, but commonly
is in the range of 0.5 to 1.5 L/h. The upper limits for fluid
replacement during exercise heat stress are set by the maximal
gastric emptying rates, which have been reported to be 1.0 to
1.5 L/hr for the average adult,121,132 but are reduced by exercise
heat stress and dehydration.33 Although gastric emptying may or
may not be sufficient to maintain hydration (depending on sweating
rate), people tend to drink only after thirst develops. As presented
earlier in the chapter, the sensation of thirst appears at
~295 mmol/kg156 or ~2% of body mass loss. Thus, a significant
amount of fluid loss occurs before the sensation of thirst drives
fluid intake. During activity, if fluid intake occurs after being signaled
by thirst sensation and is less than fluid loss through thermoregulatory
sweating, the outcome is progressive dehydration. As a
result of blood pooling in the skin and reduction in plasma volume
secondary to sweating, cardiac filling is reduced and larger fractional
utilization of oxygen is required at any given workload.11
Ultimately, these responses have a negative impact on exercise/
work performance, especially in warm/hot environments.
The negative impact of dehydration on work performance can
increase risk in a field or wilderness setting. Dehydration, in
combination with heat stress, reduces maximal oxygen uptake,
increases relative effort, and reduces work output. When dehydrated,
an individual will either not be able to trek as far or as
fast compared to when euhydrated. For example, when on a
hike, dehydration can increase the duration of time required to
complete the hike beyond what is to be expected for a given
distance and terrain, especially when in warm/hot environments.
In the scenario of a day hike, or a hike to a destination, this
increases the time to complete the hike and could result in a
hiker being caught unprepared. If the expected plan for the day’s
trek is to complete the hike during daylight hours, or to arrive at
a destination that has supplies, adequate food and water, proper
clothing, maps, then GPS, headlamps or a compass may not be
brought. Without the supplies and equipment mentioned, hikers
may run the risk of getting lost or injuring themselves in the dark
(trip or fall), becoming more dehydrated without sufficient food
and water, or becoming hypothermic as temperatures fall.
Dehydration and Work Productivity
As previously discussed, during physical work in the heat, sweat
output often exceeds water intake, which leads to body water
losses. Bishop et al.19 observed that in simulated industrial work
conditions, encapsulated protective clothing produced sweating
rates up to 2.25 L/hr. Likewise, wearing protective equipment
such as full- or half-face masks can make fluid consumption more
difficult and further contribute to dehydration in the workplace.
Firefighters wear heavy protective clothing and are exposed to
intense heat. Rossi160 reported that firefighters wearing protective
clothing and equipment while performing simulated work tasks
in the heat can have sweat rates up to 2.1 L/hr. It is also the
case that workers often not only become dehydrated on the job
but also may start the work day with a fluid deficit. Brake et al.21
observed fluid losses and hydration status of mine workers under
thermal stress working extended shifts (12 hours). By measuring
USG at the start of a work shift, they observed that 60% of the
miners reported to work dehydrated and that their hydration
status did not improve during the 10- to 12-hour shift.
While many studies have observed the effect of dehydration
on physical work capacity, few studies have observed dehydration’s
impact on manual labor productivity. Wasterlund and
Chaseling200 studied forest workers in a 15° C (59° F) environment
in two scenarios, one where subjects consumed fluid sufficient
to maintain a normal hydration state and a second where subjects
consumed limited fluid, which resulted in 0.7 kg body mass loss
(>1% dehydration). The measure of productivity was the amount
of time to stack and debark 2.4 cubic meters of pulpwood. When
subjects were dehydrated, productivity of stacking and debarking
pulpwood was reduced by 12%.
Dehydration and Cognitive Function
Cognitive/mental performance, which is important when concentration,
skilled tasks, and tactical issues are involved, is degraded
by dehydration and hyperthermia.80,158 The evidence is stronger for
a negative effect of hyperthermia than for mild dehydration on
degrading cognitive/mental performance,42 but the two are closely
linked when performing exercise in warm/hot weather. The relative
hyperthermia associated with dehydration could diminish
psychological drive24 or perhaps alter central nervous system function
independent of temperature. Adolph4
reported that dehydrated
subjects fainted more quickly when faced with a change in
body posture (orthostatic challenge test). Likewise, Carter et al.27
reported that subjects who were dehydrated by >2% of body mass
from heat exposure exhibited significant reduction in cerebral
blood flow velocity and possibly cerebral oxygen availability,
when going from a seated to a standing posture. Intracranial
volume is altered in response to dehydration,54 although the exact
functional consequence of this is unknown. Dehydration has been
shown to adversely influence decision making and cognitive performance,
which may contribute to decline in work capacity and
could possibly be associated with increased risk of accidents.
Dehydration has been reported to impair visual motor tracking,
short-term memory, attention, and arithmetic efficiency74 and to
bring about greater tiredness, reduced alertness, and higher levels
of perceived effort and concentration,192 with as little as 2% dehydration.
The negative impact of dehydration on short-term memory
and fatigue may persist for up to 2 hours following rehydration.42
It is possible that factors associated with dehydration such as
greater tiredness, reduced alertness, difficulty in concentrating/
decision making, or orthostatic intolerance could contribute to
accidents. Although there are no reported links, it is also possible
that dehydration-mediated reductions in cognitive function and
reaction time may be indirectly connected. In a classic study by
Vernon,198 accident rates were shown to be at their lowest at temperatures
of ~20° C (68° F) and increased by 30% in environments
of ~24° C (75° F). It is in warm/hot environments that fluid turnover
is highest and individuals most likely to become dehydrated.
Changes in reaction time have been reported to accompany
dehydration. Figure 70-7 depicts a 23% change in reaction time74
when subjects were 4% dehydrated, in comparison to a study131
that related the effects of blood alcohol content to driving skills.
A blood alcohol level of 0.08 (legal limit in all states) yielded
17% slowing of drivers’ response time.131 Although these two
studies74,131 are not equivalent, because different tests were used
to measure reaction time, a point of comparison can be made.
Blood alcohol content at or above the legal limit in all U.S. results
in significant impairment in the ability to operate a vehicle; it is
possible that the changes in reaction time reported with dehydration
may cause similar impairment and increase the potential risk
for accidents to occur.
Accidents that occur as a result of delayed reaction time or
those that are the result of a trip or fall can occur anywhere.
However, when these accidents occur in wilderness situations,
medical help may not be readily available and the consequences
may be dire. Accidents such as trips or falls (dehydration-related
orthostatic intolerance) can result in broken bones, lacerations,
or death (fall from a height). In addition, accidents occurring
during expeditions or when mountaineering can be traced to a
poorly made initial decision (dehydration-related mental fatigue,
reduced alertness and concentration) that led to subsequent bad
decisions, further compounding the severity of the situation.
Dehydration, Rehydration
Related Illness
Dehydration increases the risk for heat exhaustion3,118,177 and is a
risk factor for heat stroke.28,57,73,154 Other factors, such as lack of
heat acclimatization, certain medications, genetic predisposition,
and illness, often play a large role.28,55 Historically, unexpected
cases of heat-related illness were attributed solely to dehydration,
as dehydration has been shown to impair thermoregulation and
increase cardiovascular strain. However, it is now suspected that
previous sickness or injury might increase susceptibility to serious
heat illness.90 Dehydration was present in ~17% of all heat stroke
hospitalizations in the U.S. Army over a 22-year period.28 In a
series of 82 cases of heat stroke in Israeli soldiers, dehydration
was present in ~16%.57 Team physicians for American football
have observed during summer practice that dehydration, occasionally
brought on by emesis, contributes to heat stroke.55,155
Dehydration has been associated with reduced autonomic cardiac
stability,29 altered intracranial volume,54 and reduced cerebral
blood flow velocity responses to orthostatic challenge.27
Hyponatremia
Hyponatremia describes a state of lower than normal blood
sodium concentration; typically <135 also="" div="" is="" it="" meq="" to="" used="">
describe a clinical syndrome that can occur when there is rapid
lowering of blood sodium, usually to a level below 130 mEq/L
and accompanied by altered cognitive status. This is a serious
medical condition that can result in death. Exercise-associated
hyponatremia occurs as a consequence of prolonged work (typically
>5 hours), where sweating is the primary means of heat
dissipation. Because sweat contains not only water but small
quantities of electrolytes, there is a progressive loss of water,
sodium, chloride, and potassium. Hyponatremia most often
occurs when individuals consume low-sodium drinks or sodiumfree
water in excess of sweat losses (typified by body mass
gains), either during or shortly after completing exercise.
However, drinking sodium-free water at rates near to or slightly
less than the sweat rate can theoretically produce biochemical
hyponatremia when coupled with progressive loss of electrolytes.
Reductions in solute concentration of extracellular fluid promote
water movement from the extracellular space into cells. If this
fluid shift is of sufficient magnitude and occurs rapidly, it can
congest the lungs, result in brain swelling, and alter central
nervous system function. Signs and symptoms of hyponatremia
often mimic those of heat injury and include confusion, disorientation,
loss of faculties, headache, nausea, vomiting, aphasia, loss of coordination, and muscle weakness. In general, hyponatremia
can be distinguished from heat injury by the presence of
repeated vomiting and abdominal distention and production of
copious clear urine. Complications of severe and rapidly evolving
hyponatremia include seizures, coma, pulmonary edema, and
cardiorespiratory arrest.
Hyponatremia tends to be more common in long-duration
activities and is precipitated by consumption of hypotonic fluid
(water) alone. The interaction between drinking rate (water only)
and plasma sodium concentration is illustrated in Figure 70-8A
and B for a 70-kg individual, in 28° C (82° F) hiking at a moderate
pace (6 km/hr), drinking at three different rates (200, 400,
and 600 mL/hr). Figure 70-8A predicts the percentage change in
body mass over time for the three drinking rates, whereas Figure
70-8B predicts the expected plasma sodium concentration. The
slowest drinking rate (200 mL/hr) over the duration of the hike
(12 hours) predicts an elevated plasma sodium level well above
that of asymptomatic hyponatremia (135 mEq/L). However, this
drinking rate also results in a >4% level of dehydration, a level
of fluid loss that would substantially degrade performance (Figure
70-8A grey zone). Because the drinking rate is well in excess of
sweating rate, the fastest drinking rate (600 mL/hr) actually
results in a body mass gain and is predicted to result in asymptomatic
hyponatremia within 5 to 6 hours of activity and symptomatic
hyponatremia (sodium <130 div="" figure="" grey="" meq="" zone="">
70-8B) by 10 hours. It is important to note that predicted changes
in plasma sodium concentrations are different for individuals of
greater or lesser body mass, varying sweat rates and sweat
sodium concentrations. For example, in the case of individuals
who lose large amounts of sodium in their sweat (salty sweaters),
it is possible that matching fluid intake to sweat loss can still
result in hyponatremia because of the progressive sodium loss
over long-duration activity.123 Overdrinking hypotonic fluid is the
mechanism that leads to exercise-associated hyponatremia. In
general, consumption of water should never exceed 12 quarts
(~11 L). Consumption of an electrolyte-supplemented drink
should substantially delay or prevent this outcome. This may be
accomplished using prepared beverages or by adding electrolytes
(e.g., Elete) to water.
Exercise-associated hyponatremia has been observed during
marathon and ultramarathon competition,52,81,187 military training,68,145
and recreational activities.13 In athletic events, the condition
is more likely to occur in females and slower competitors,
both of whom gain weight (due to drinking) during the event.
The severity of the symptoms is related to the magnitude by
which serum sodium concentration falls, and the rapidity with
which it develops.95 If hyponatremia develops over many hours,
it might cause less brain swelling and less adverse symptoms.95
Unreplaced sodium losses contribute to the rate and magnitude
of sodium dilution and may in certain situations (e.g., salty sweaters)
be the primary reason for development of exercise-associated
hyponatremia.123,127 Nausea, which increases AVP (antidiuretic
hormone) secretion, and exercise heat stress, which reduces renal
blood flow and urine output, can negatively affect the ability of
kidneys to rapidly correct the fluid–electrolyte imbalance.205 The
syndrome can be prevented by not drinking in excess of sweat
rate, and by consuming salt-containing fluids or foods when
participating in exercise events that result in many hours of continuous
or near-continuous sweating.
Dehydration and Limits of Survival
Severe elevations in blood osmotic pressures are incompatible with
life. Just as hyponatremia (blood hypo-osmolality) can produce
fatal brain swelling, severe hypernatremia (hyper-osmolality) can
produce fatal brain shrinkage. The physical forces of each can
produce tearing of intracerebral veins and cerebral hemorrhage.8
Although other pathologic outcomes of severe dehydration may
also have fatal consequences, the effects of hyper-osmolality on
central nervous system function have long been suspected as
primary.20
Acute elevations in plasma osmolality (Posm) >350 mmol/kg
produce neurologic symptoms in animals, such as seizures and
coma; death in humans has been consistently observed in patients
with Posm >370 mmol/kg.8
Postmortem analysis of human vitreous
humor samples in cases of death from dehydration show marked
sodium elevations (>170 mmol/L).108 By using the formula 2.1 ×
Na+
to estimate osmolality,82 a value of 357 mmol/kg is obtained.
It therefore appears that a plasma osmolality value of 350 mmol/
kg can be considered as an approximate limit for human survival.
The level of lethal dehydration (Posm >350 mmol/kg) and the
time required to reach it can be estimated. If we assume that a
70-kg person possesses 42 L of body water and has a resting Posm
of 285 mmol/kg, then the degree of pure water loss required to
concentrate Posm to the lethal limit is (285/350) × 42 = 34.2 L, or
7.8 L water loss. However, since electrolytes are also lost in urine
and sweat, a reasonable correction can be applied (7.8/0.94),
which gives 8.3 L. This gives a level of dehydration of almost
12% body mass and 20% of total body water. Although higher
estimates have been made (~20% body mass), it is cautioned that
as much as half of fasting weight losses derive from nonwater
sources.23 Under fasting conditions, Brown et al.23 estimate that
urine losses will stabilize at 0.5 L/day after the first day. The
remaining losses from sweat depend on environmental temperature
and body heat production.
Under hospitable indoor conditions, obligatory urine23,84 and
insensible sweat losses84,98 add up to about 1.2 L/day, which
makes survival without water possible for almost 7 days. This is
longer than the 100-hour rule of thumb (about 4 days),149 but
highly dependent on environmental and behavioral factors. For
example, in a worst-case desert scenario where there is 10 hours of daytime temperature exposure (>40° C [104° F]) and 14 hours
of nighttime temperature exposure (<20 approximately="" c="" div="" f="">
3.0 L/day of sweat loss can be added to the 0.5 L/day
losses of urine when at rest.23 This would limit survival to about
2.5 days. If the lost desert sojourner was to travel by night (14
hours) on foot through sand69 at 4.8 km/hr (3 mph) and rest
unshaded during the day, 8.6 L/day fluid losses23,71 would limit
survival to less than 1 day (23 hours). If traveling by day and
night, sweat losses of approximately 0.60 L/hr (day) and 0.40 L/
hr (night)71 would limit survival to about 16 hours. In each case
when traveling, the distance covered would be roughly the same
(42-48 miles).
Dehydration and Susceptibility
to Cold Injury
A common response to cold exposure is cold-induced diuresis
(CID), an increase in urine production associated with shift in
fluid centrally induced by vasoconstriction.174 In addition, when
in a cold environment, attention to replacement of fluid losses is
often neglected. If skin temperatures fall significantly, thirst is less
noticeable in cold compared to hot weather.93 In addition, individuals
may voluntarily not drink fluid in an effort to decrease
the need to urinate brought on by CID. Given the fluid loss
brought on by CID, attenuation of thirst when exposed to cold,
and voluntarily not ingesting fluid, dehydration can result. Dehydration
in the cold may be more important during heavy exercise
in the cold when core temperature is elevated and blood flow to
skin increases to dissipate heat. If individuals in the cold are
heavily clothed and/or traversing in snow (resulting in high metabolic
rates),147 they may overheat more readily and increase fluid
losses due to thermoregulatory sweating. During cold-weather
outdoor activities, individuals can still become dehydrated by 3%
to 8% of their body mass 62. For these reasons, maintaining hydration
is important when performing work in cold environments.
It appears that the impact of dehydration while in a cold
environment does not have the same impact on exercise performance
as in temperature or warm/hot environments. Recent
data33 show that if the skin temperatures are low, 4% dehydration
has no effect on cycling performance in the cold. However, if
cold strain is minimized by clothing, thereby maintaining skin
and core temperatures near those observed in temperate or even
hot environments, dehydration will likely degrade performance.62
Dehydration does not alter heat conservation, heat production,
or CIVD responses143,144 and thus does not appear to increase the
likelihood of peripheral cold associated injuries. However, lack
of significant impact on exercise performance and injury does
not negate the importance of maintaining hydration while in a
cold environment. Little is known regarding the impact of long
term, chronic dehydration similar to that experienced on long
duration expeditions/missions in cold environments, where water
availability is limited and sense of thirst is diminished. Individuals
should drink adequately during endurance activity to replace
fluid losses and prevent dehydration, even when in a cold environment.
When returning to a warm environment, individuals
who have free access to food and fluid will rehydrate on their
own. When in the field, ice and snow can be melted. However,
the source of ice or snow should be known, because only clean
snow or ice should be melted for drinking water. If unsure, water
melted from snow or ice should either be filtered, boiled or
sanitized by addition of disinfection tablets.
Fluid Replacement (Before,
During, After)
The U.S. Army has developed fluid replacement and work pacing
guidelines that incorporate work intensity, environment, workto-rest
cycles, and fluid intake as shown in Table 70-5.126 These
guidelines use wet bulb globe temperature (WBGT) to mark
levels of environmental heat stress and emphasize both the need
for sufficient fluid replacement during heat stress and concern
for the dangers of overhydration. The WBGT takes environmental
variables such as solar radiation, humidity, and ambient temperature
into account in its calculation; automated systems for WBGT
measurement are commercially available. The fluid-replacement
guidelines in Table 70-5 were designed to be simple and practical
for use with large cohorts in situations where determining individual
sweat rates would be impractical. These recommendations
specify an upper limit for hourly and daily water intake, which
safeguards against overdrinking and water intoxication. However,
it is recommended that individuals performing endurance activities
validate their sweat rates, because the guidelines do not
account for individual variability.
Acsm Fluid Replacement
Recommendations
The most current knowledge regarding exercise with respect to
fluid replacement is presented in the 2007 American College of
Sports Medicine Position Statement on Exercise and Fluid
Replacement.165 The position statement summarizes current
knowledge regarding exercise with respect to fluid and electrolyte
needs and the impact of their imbalances on exercise performance
and health. The recent statement stresses the fact that
individuals have varying sweat rates and as such, fluid needs for
individuals performing similar tasks under identical conditions
can be very different. Specifically the ACSM Position Statement
provides recommendations in relation to hydration prior to,
during, and following exercise/activity.
Before Exercise. The objective is to begin the physical activity
euhydrated and with normal plasma electrolyte levels. If sufficient
beverages are consumed with meals and a protracted
recovery period (8-12 hours) has elapsed since the last exercise
session, then the person should already be close to being euhydrated.84
However, if the person has suffered substantial fluid
deficits and has not had adequate time or fluids/electrolytes in
quantities sufficient to reestablish euhydration, then an aggressive
pre-hydration program may be merited. When hydrating prior to
exercise the individual should slowly drink beverage (for
example, ~5-7 mL/kg body mass, 350-490 mL for a 70-kg individual)
at least 4 hours before the exercise task. If the individual
does not produce urine, or the urine is dark or highly concentrated,
the individual should slowly drink more beverage (e.g.,
another ~3-5 mL/kg body mass, 210-350 mL for a 70-kg individual)
about 2 hours before activity. By hydrating several hours
prior to exercise, there is sufficient time for urine output to return
toward normal before activity. Consuming beverages with sodium
(20-50 mEq/L) and/or small amounts of salted snacks or sodiumcontaining
foods at meals will help to stimulate thirst and retain
the consumed fluids.113,153,183
Hyper-hydration can be achieved either by overdrinking
or ingesting fluids (e.g., water) that expand the extra- and intracellular spaces. Simple overdrinking usually stimulates urine
production,84 and body water rapidly returns to euhydration
within several hours.61,142,183 This means of hyperhydrating greatly
increases the risk of having to void during activity/exercise61,142
and provides no clear physiological or performance advantage
over euhydration.91,99,100 In addition, hyperhydration can substantially
dilute and lower plasma sodium61,142 before starting exercise
and therefore increase the risk of dilutional hyponatremia if fluids
are aggressively replaced during exercise.123 Enhancing palatability
of ingested fluids is one way to help promote fluid consumption,
before, during, or after exercise. Fluid palatability is
influenced by several factors, including temperature (preferred
between 15° and 20° C [59° and 68° F]), sodium content, and
flavoring.
During Exercise. The objective is to drink enough fluid to
prevent excessive dehydration (>2% body mass loss from water
deficit) during exercise by replacing sweat losses to help sustain
performance. The amount and rate of fluid replacement depends
on the individual sweating rate, exercise duration, and opportunities
to drink. Individuals should periodically drink (as opportunities
allow) during activity; if it is expected, they will become
excessively dehydrated from not drinking. Care should be taken
in determining fluid replacement rates, particularly in prolonged
exercise lasting greater than 3 hours. The longer the exercise
duration, the greater the cumulative effects of slight mismatches
between fluid needs and replacement, which can exacerbate
dehydration or dilutional hyponatremia.123 It is recommended
that individuals should monitor body mass changes during
training/activity to estimate their sweat lost during a particular
exercise task with respect to the weather conditions. This allows
customized fluid replacement programs to be developed for each
person’s particular needs; however, this may not always be
practical.
The Institute of Medicine also provides general guidance for
composition of “sports beverages” for persons performing prolonged
physical activity in hot weather.83 They recommended that
fluid replacement beverages should contain ~20 to 30 mEq/L
sodium (chloride as the anion), ~2 to 5 mEq/L potassium and
~5% to 10% carbohydrate.83 The need for these different components
(carbohydrate and electrolytes) will depend on the specific
exercise task (e.g., intensity and duration) and weather conditions.
The sodium and potassium are to help replace sweat
electrolyte losses, while sodium also helps to stimulate thirst, and
carbohydrate provides energy. These components also can be
consumed using non-fluid sources such as gels, energy bars and
other foods.
Carbohydrate consumption can be beneficial to sustain exercise
intensity during high-intensity exercise events of ~1 hour or longer, as well as less intense exercise/activity sustained for
longer periods.18,49,50,88,202 Carbohydrate-based sports beverages
are sometimes used to meet carbohydrate needs, while attempting
to replace sweat water and electrolyte losses. Carbohydrate
consumption at a rate of 1 g/min has been demonstrated to
maintain blood glucose levels and exercise performance.49,50 Most
typical sport beverages contain carbohydrate sufficient to achieve
this goal if drinking a liter per hour or less. It should be noted
that this rate of carbohydrate consumption was observed in
highly fit, elite athletes. Most individuals would not work or
perform exercise at a high enough intensity or for long enough
duration to utilize 1g/min. The greatest rates of carbohydrate
delivery are achieved with a mixture of simple sugars (e.g.,
glucose, sucrose, fructose, maltodextrin). If fluid replacement and
carbohydrate delivery are going to be met with a single beverage,
the carbohydrate concentration should not exceed 8%, or even
be slightly less, as highly concentrated carbohydrate beverages
reduce gastric emptying.87,199 Finally, caffeine consumption might
help to sustain exercise performance48 and likely will not alter
hydration status during exercise.50,203
After Exercise. If recovery time and opportunities permit, consumption
of normal meals and snacks with a sufficient volume
of plain water will restore euhydration, provided the food contains
sufficient sodium to replace sweat losses.84 If dehydration
is substantial (>2% body mass) with a relatively short recovery
period (<12 aggressive="" an="" div="" hours="" may="" program="" rehydration="" then="">
be merited.112,113,183
Failure to sufficiently replace sodium losses prevents return
to a euhydrated state and stimulates excessive urine production.112,139,182
Consuming sodium helps retain ingested fluids and
stimulates thirst. Sodium losses are more difficult to assess than
are water losses and it is well known that individuals lose sweat
electrolytes at vastly different rates. Drinks containing sodium,
such as sports beverages, may be helpful, but many foods can
supply the needed electrolytes. A little extra salt may be added
to meals and recovery fluids when sweat sodium losses are high.
Table 70-6 presents the electrolyte content of common sport
drinks, tablets, and powdered additives.
Individuals looking to achieve rapid and complete recovery
from dehydration should drink ~1.5 L of fluid for each kilogram
of body mass lost.182 The additional volume is needed to compensate
for the increased urine production accompanying the
rapid consumption of large volumes of fluid.182 Therefore, to
maximize fluid retention, fluids should be consumed over time
(and with sufficient electrolytes) rather than being ingested in
large boluses.96,204 The use of intravenous fluid replacement after
exercise may be warranted in individuals with severe dehydration,
nausea, vomiting, or diarrhea, or who for some reason
cannot ingest oral fluids.
Education. Alleviating dehydration should involve a combination
of strategies that include assessment, education, and inclusion
of practices that encourage fluid intake. Education is a vital
TABLE 70-6 Electrolyte Content of Common Sport Drinks, Tablets, and Powdered Additives That Can Be Used to
Help Replace Electrolytes Lost During Activity/Exercise
Product Serving Size CHO Na+ K+ Ca2+ Mg2+
CeraSport fl. oz. 5 g 200 mg 100 mg 0 mg 0 mg
Ensure fl. oz. 42 g 200 mg 460 mg 375 mg 62.5 mg
Elete Electrolyte Add-in teaspoon 0 g 125 mg 130 mg 0 mg 45 mg
Elete Tablytes® 1 tablet 0 g 150 mg 95 mg 40 mg 30 mg
Gatorade (G2 Series) 8 fl. oz. 14 g 110 mg 30 mg 0 mg 0 mg
Gatorade (Pro Series) 8 fl. oz. 14 g 200 mg 90 mg 0 mg 0 mg
Lucozade Lite 8 fl. oz. 5 g 0 mg 0 mg 92.5 mg 0 mg
Nutrilite 8 fl. oz. 14 g 110 mg 3 0 mg 0 mg 0 mg
Pedialyte 8 fl. oz. 6 g 253 mg 192 mg 25 mg 2.5 mg
Powerade 8 fl. oz. 14 g 100 mg 25 mg 0 mg 0 mg
Powerade Zero 8 fl. oz. 0 g 55 mg 35 mg 0 mg 0 mg
Vitaminwater Essential 8 fl. oz. 13 g 0 mg 70 mg 50 mg 0 mg
Vitalyte 8 fl. oz. 10 g 68 mg 92 mg 2.1 mg 1.6 mg
1
2
component to help individuals maintain hydration before, during,
and after activity. Informing individuals, especially those who
perform work/activity in a hot environment, about hydration
assessment, signs and dangers of dehydration, and strategies in
maintaining hydration, can help to reduce incidences of dehydration.
Brake et al.21 reported that individuals working in a thermally
stressful environment were better able to maintain hydration when
they were educated about dehydration, assessed their hydration
state, and used a fluid replacement program while performing
working.
Modifying Factors
Diet. One important aspect of an education and hydration
program should stress the importance of consuming meals. Meal
consumption is critical to ensure full hydration on a day-to-day
basis.2,3,186 Eating food promotes fluid intake and retention84 and
sweat electrolyte (e.g., sodium and potassium) losses can be
replaced during meals with most individuals.106,139,183 De Castro53
observed food and fluid intake of 36 adults over 7 consecutive
days and concluded that the amount of fluid ingested was primarily
related to the amount of food ingested and that fluid intake
independent of eating was relatively rare. In addition, Maughan
et al.,114 among others, reported that meals play an important role
in helping to stimulate the thirst response, causing the intake of
additional fluids and restoration of fluid balance. Using established
meal breaks may help replenish fluids and can be important
in replacing sodium and other electrolytes.
Caffeine is contained in many beverages and foods. Recent
evidence suggests that caffeine consumed in relatively small doses
(<180 cause="" daily="" day="" div="" increase="" likely="" mg="" not="" or="" output="" urine="" will="">
dehydration.10 Maughan et al.111 performed a review of literature
concerning the effect of caffeine ingestion on fluid balance. They
concluded from their review of the literature that doses of caffeine
equivalent to the amount normally found in standard servings
of tea, coffee, and carbonated soft drinks appear to have
no diuretic action and that their consumption will not result in
fluid losses in excess of the volume ingested. Therefore, there
would appear to be no clear basis for refraining from caffeinecontaining
drinks in situations where fluid balance might be
compromised. However, because alcohol can act as a diuretic
(particularly at high doses) and increase urine output, it should
be consumed in moderation, particularly during the postexercise
period when rehydration is a goal.181
Clothing. Anecdotal statements and interviews have revealed
that individuals will purposefully not drink fluid (voluntary dehydration)
in certain situations, such as when bathroom facilities
are not available, or when in cold environments where exposure
to the environment may be an issue, or due to clothing systems
that are difficult to remove. While logistical factors and conditions
in the field may complicate access to facilities, there are a number
of alternatives (e.g., toilet tents) that can help to address this
issue and reduce the practice of voluntary dehydration.
Sex. Women typically have lower sweating rates and electrolyte
losses than men.12,172,179 Women appear to be at greater risk than
men to develop symptomatic hyponatremia when competing in
longer duration events such as marathon or ultra-marathon
races.6,81 This risk can be alleviated by not overdrinking fluid.
Age. Older (age >65 years) persons are generally adequately
hydrated.84 However, there is an age-related blunting of thirst
response to water deprivation,101,107,159 making older persons more
susceptible to becoming dehydrated.101 Older adults have agerelated
increase in resting plasma osmolality and are slower to
restore body fluid homeostasis in response to water deprivation148
and exercise107 than are younger adults. If given sufficient time
and access to water and sodium, older adults adequately restore
body fluids.105,107 Older persons are also slower to excrete water
following fluid loads.* This slower water and sodium excretion
increases sodium retention, which may lead to increased blood
pressure.105
While thirst sensitivity to a given extracellular fluid loss is
reduced in older adults, osmoreceptor signaling remains
intact.107,189,190 The osmotic and volume stimuli that result from
dehydrating impart important drives for thirst and drinking in
older adults just as they in younger people.15 Older adults should
be encouraged to rehydrate during or after exercise. Pre-pubescent
children have lower sweating rates than adults, with values rarely
exceeding 400 mL per hour.16,120 However, sweat electrolyte
content is similar (or slightly lower) in children compared to
adults.16 Lower sweating rates in children are probably the result
of smaller body mass and metabolic rate, depending on age and
the fact that thermoregulatory sweating is not fully developed
until adolescence. While older adults and young children represent
the extremes within the population, no matter the age, if
attention is paid to the hydration guidelines, overdrinking will not
occur and hydration can be maintained.
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