Assessing Hydration Status: The Elusive Gold Standard

Abstract

 

Acknowledging that total body water (TBW) turnover is complex, and that no measurement is valid for all situations, this review evaluates 13 hydration assessment techniques. Although validated laboratory methods exist for TBW and extracellular volume, no evidence incontrovertibly demonstrates that any concentration measurement, including plasma osmolality (P_osm), accurately represents TBW gain and loss during daily activities. Further, one blood or urine sample cannot validly represent fluctuating TBW and fluid compartments. Future research should (a) evaluate novel techniques that assess hydration in real time and are precise, accurate, reliable, non-invasive, portable, inexpensive, safe, and simple; and (b) clarify the relationship between P_osm and TBW oscillations in various scenarios.

 

Keywords:dehydration, osmolality, total body water, extracellular fluid, intracellular fluid

Side Effects and Symptoms of Dehydration

Dehydration ranges on  a scale from mild to severe, with symptoms and  consequences ranging in severity between those two extremes. In mild to moderate dehydration that can results from strenuous or prolonged activity, extended heat exposure, or illnesses and conditions that involve fluid loss ( high fever, vormiting, and/or diarrhoea), one of the first indications of the need for water is thirst.  Bear in mind and remember to drink your water before  you feel thirsty because if you wait for the first thirst signal to be triggered by your body, by then, you have already entered into 2 percent dehydration. as dehydration ensues, the following symptoms are commonly reported:
• Dry mouth
• Swollen tongue
• Fatigue
• Decrease in urination frequency
• Headache
• Dry skin
• Dizziness
• Hunger

  If the dehydration becomes severe, the signs and symptoms change to include indications that serious damage to the organs and systems is setting it. If you begin to experience these symptoms or are in the presence pf someone else exhibiting the symptoms, water should be consumed immediately, and you should seek emergency assistance. The following are symptoms of moderate to severe dehydration:
• Fever at or above 103℉
• Confusion
• Lethargy
• Seizures
• Difficulty breathing/ wheezing sound
• Chest pain or abdominal pain
• Fainting
• No urine for 12 hours or more.

The Importance of Hydration

When we're born, our bodies are composed of about 75 percent water. As we age, that percentage drops to about 65 percent, about two-thirds of our body's weight, and varies depending upon age, gender, and muscle and fat percentages. Containing essential nutrients that the human body requires in order to perform everyday functions, water is the clean hydration our bodieo need in order to survive and thrive, daily. Replenishing your body's cells, organs, and systems with adequate hydration is imperative, and while many people quench their thirst with sugary synthetic beverages such as sodas and sweetened fruit juices, these drinks offer little in terms of hydration and a great deal in terms of harm. When the essential nutrients provided by water are replaced with synthetic addictive, empty calories, and loads of sugar, the brain and body reap none of the benefits that plain water provide, and instead endure blood sugar spikes and crashes, over-whelming processing and storage of improper "nutrients" ( what some might refer to as "malnutrients"), and of course, inadequate hydration. All of these malnutrients wreak havoc on the human body's brain, cells, organs and systems, compromising the body's functions rather lah supporting them. If you don't think water is overly important for the human body's proper functioning, simply take a look at what water does for the body. 

Physical Processes That Depend upon Adequate Hydration
Adequate hydration is essential to the optimal performance of every process in the human body and the functioning of every cell, organ, and system. Clearly, a focus on consuming enough water (32.53 ml water for every 1 kilogram of body weight) daily is a step in the right direction toward achieving overall health. "To think, drink, and blink, you nee water" is a saying that sums up the human body's need for proper hydration in order to perform every function, whether simple or complex, voluntary or involuntary. Water supports the electrical impulses that allow the heart to beat regularly, the nerves and spinal cord to communicate, and the brain to process information to sustain good health. To move, jump, throw, and perform any physical activity, the body' muscles need water. The joints, eyes, nose, and mouth are lubricated by ( you guessed it right) water. in addition to all the benefits that result from hydrating the body's cells and systems, your brain and body experience far fewer difficulties thanks to the improved detoxification, proper digestion, and enhanced nutrient absorption that naturally occur when the body has adequate amounts of water. 

The Average Adult's Water Needs

The average adult doesn't consume enough water on a daily basis. Most people are simply unaware of how little water it takes to actually keep the body hydrated and operating at its best. In order to perform everyday functions such as breathing and muscle movement, and involuntary actions including digestion and heartbeats, the human body requires about 2 liters of water per day ( i.e. about 68 ounces or about 8½ cups, cup size is 8 ounces). Those 2 litres of water help the body perform everyday functions, and replenish the stores of water that are constantly escaping the human body through every exhaled breath, urination, and persperation. Factors such as gender, age, and body composition( the ratio of fat to muscle percentage) play into the amount of water needed by the body on a daily basis. but the fluctuation is  minimal, making the general rule of eight (8-ounce) cups  of water per day widely accepted for the general population over the age of eighteen(18). There are instances when the amount of water required by the human body increases, such as with an increase of physical activity, outdoor, extended exposure to hot and/or hudud climates, and illnesses that can lead to fluid loss, such as those that occur with vomiting and diarrhoea.

  Following Water Cure Protocol Formula, those 10 cups of 10 percent of water per dose, can be consumed during a twelve-hour period by simply drinking a single cup every hour and a half. Dividing the total daily amount into regular, 90 minutes, "doses" lets the human body achieve and maintain adequate hydration throughout the day. Energy levels, focus, digestion, and even mood can remain unaffected by the effects of dehydration. Making it even easier to fulfil those 1½ hourly water intake goals are the sweet treat that can be concocted with infused water recipes. You can choose 100 of easy, delicious recipes for detox, Weight Loss, healthy Skin, Better Immunity and More! all within this healthy wealth blog site. 

The Process and Effects of Human Body Dehydration

Most people have experienced the dry mouth or slight twinge of a headache that usually signals the need for fresh water, but few people are aware of the actual process that takes us to the point of dehydration. It all starts with the hypothalamus, the gland responsible for a number of functions, including:
~ Hormone production.
~ Body temperature regulation.
~ Constant evaluation and maintenance of the human body's fluid balance.


  All three of these functions play roles in the process of keeping the body hydrated. When the hypothalamus detects too little water in the blood, it signals to the pituitary gland to release an anti-diuretic hormone (ADH).





The ADH signals the kidneys to minimize their process of removing water from the blood while returning as much fluid as possible into the bloodstream. Once the kidneys pump the brakes on water extraction and instead focus on returning water content to the blood, the need to urine lessens and urine becomes more concentrated, thus the darker yellow hue that is a common indicator of dehydration. Once this process has reached a point where the blood volume is adversely affected, the brain signals thirst, and you feel the urge to drink water. Note that the first thirst signal was triggered when the body is already 2 percent dehydrated. If the process is prolonged, when you do not drink any fresh water right away, the percentage of water in the blood drops, leading to a reduced blood volume, this in turn leads to a drop in blood pressure and an increase in heart rate, both of which are detrimental to all of the cells, as well as the organs and systems throughout the human body.
As the process of becoming dehydrated continues to pile stress on the cardiovascular system, the supply and delivery to the brain is reduced, resulting in headaches, fatigue, inability to concentrate, sleepy, and fluctuations in mood. Once enough fresh water is consumed, though, the body is able to replenish the blood, cells, organs, and systems with the hydration they require, the human body can function as intended again.

Dehydration and Rehydration*

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 

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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