Continuing Education
Hydration and Health Promotion
Kathryn M. Kolasa, PhD, RD, LDN
Carolyn J. Lackey, PhD
Ann C. Grandjean, EdD, FACSM
While the relation between hydration status and physical
activity (military operations, sports performance) has been an
area of extensive research, more recently, researchers have
begun examining the relation between hydration status and
health, acute and chronic diseases, and cognitive
performance. On November 29 and 30, 2006, the International
Life Sciences Institute North America Technical Committee
on Hydration organized a conference on hydration and health
promotion. This article is an overview of that conference.
Nutr Today. 2009;44(5):190–201
Background
For years, it was assumed that an individual’s response
to thirst would ensure sufficient water intake to maintain
euhydration. In fact, for most healthy individuals, thirst
does ensure adequate hydration. In 2004, the Institute of
Medicine (IOM) established age and sex recommendations
for total water intake. Definitions of terms used herein
are defined elsewhere1 and in the published workshop
proceedings.2Y15
On November 29 and 30, 2006, the North American
Branch of the International Life Sciences Institute
sponsored a workshop to review available science on
hydration and related health and performance issues. About
50 participants from academia, government, industry, and
scientific associations participated. Fourteen papers were
presented on current knowledge related to the role of water
in disease, methods of determining hydration status,
sources of water in the diet, hydration and cognition, and
specific considerations for infants, children, and the
physically active. The proceedings are published
elsewhere.2Y15 With the goal of advancing the science
regarding hydration, health, and performance, discussion
by all participants identified future research and the need
to draw on that research to develop evidence-based advice
190
for consumers. This article serves as a summary of the
presentations and the participants’ discussions.
Current Recommendations for
Water Intake
Adequate Intake
For the first time, the 2004 IOM recommendations for
daily water intake were quantified and were sex- and
age-specific. Although the IOM acknowledged that a low
total water intake has been associated with some chronic
diseases, the weight of the evidence did not support
establishing water intake recommendations as a means
of reducing the risk of chronic diseases. In addition,
given the extreme variability in water requirements
(resulting from differences in metabolism, environmental
conditions, and activity), there is not a single level of
water intake that would ensure adequate hydration and
optimal health for half of all apparently healthy persons
in all environmental conditions. Hence, an estimated
average requirement could not be established. Because
an estimated average requirement is necessary to
determine a recommended dietary allowance, an
adequate intake (AI) was established instead. The AI was
based on median intakes of total water reported in the
Third National Health and Nutrition Survey (Table 1).
Healthy individuals have considerable ability to excrete
excess water and thereby maintain water balance;
therefore, an upper tolerable intake level was not set.
For the first time, the 2004 dietary
recommended intakes for water were
expressed as measurable amounts
(eg, liters and cups) and are sex- and
age-specific.
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Table 1. Total Water Adequate Intake (AI)
Life Stage Group Total Daily AIa
Infants
0Y6 mo
7Y12 mo
Children
1Y3 y
4Y8 y
Males
9Y13 y
14Y18 y
19 to 9 70 y
Females
9Y13 y
14Y18 y
19 to 9 70 y
Pregnancy
14Y50 y
Lactation
14Y50 y
Assumptions
0.7
0.8
Assumed to be from human milk
Assumed to be from human milk and complementary foods and beverages; this includes
approximately 0.6 L (~3 cups) as total fluid, including formula, juices, and drinking water
1.3
1.4
Approximately 0.9 L (~4 cups) as total beverages, including drinking water
Approximately 1.2 L (~5 cups) as total beverages, including drinking water
2.4
3.3
3.7
Approximately 1.8 L (~8 cups) as total beverages, including drinking water
Approximately 2.6 L (~11 cups) as total beverages, including drinking water
Approximately 3.0 L (~13 cups) as total beverages, including drinking water
2.1
2.3
2.7
Approximately 1.6 L (~7 cups) as total beverages, including drinking water
Approximately 1.8 L (~ 8 cups) as total beverages, including drinking water
Approximately 2.2 L (~9 cups) as total beverages, including drinking water
3.0
Approximately 2.3 L (~10 cups) as total beverages, including drinking water
3.8
Approximately 3.1 L (~13 cups) as total beverages, including drinking water
a
All values are in liters per day.
Source: Grandjean and Campbell.23
Numerous factors affect the types and amount of fluid
consumed, such as availability, ambient temperature,
beverage temperature, and flavor, to name a few. Cultural
variations have been reported as to types of beverages
consumed; beer, for example, is a popular beverage in many
countries (Table 2). Usual daily intake of total water
appears to be less for Canadians than for Americans
(Table 3). It is not clear if the differences in amounts are
due to climate, body size, culture, or other factors.
Additional research is needed to further elucidate water
requirements of different subgroups of the population
(ie, very active groups such as the military, laborers, and
athletes; people working under extreme environmental
conditions; children, elderly, and those with chronic
Table 2. Market Shares (in liters per capita) of Beverages in Various Countriesa,b
Beverage
Soft drinks
Milk
Beer
Mineral water
Fruit juice
Winec
Liquorsd
Coffeed
Tead
United States
Great Britain
Federal Republic of Germany
Italy
Finland
173
102
89
?
28
16
V
98
25
91
126
108
2
16
11
5
13
40
78
55
147
62
27
23
19
96
5
47
68
2
50
4
80
11
57
2
31
104
63
8
29
5
35
156
3
a,b
Adapted from Tuorila, H. Individual and cultural factors in the consumption of beverages. In: Ramsay, Booth, eds. Thirst: Physiological and psychological
aspects. London: Springer-Verlag; 1991.
The USA consumption from 1985,24,25 European figures from 1986.26
c
In the US data includes liquors.
d
Volumes estimated from dry substance with dilution ratios given.
b
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Table 3. Mean and Selected Percentiles for Usual Daily Intake of Total Water (in milliliters): Canada and the
United States
Canadian
Sex and Age Category
n
M, 19Y30 y
M, 31Y50 y
M, 51Y70 y
M, Q71 y
F, 19Y30 y
F, 31Y50 y
F, 51Y70 y
F, Q71 y
All individuals (including pregnant and/or lactating)
United States
Mean (SD)
316
566
712
424
371
857
842
401
4,489
3,039
2,961
2,706
2,394
2,455
2,553
2,406
2,139
2,667
n
(124)
(79)
(87)
(68)
(130)
(61)
(76)
(83)
(37)
1,872
2,495
1,872
1,186
1,885
2,906
2,002
1,317
28,178
Mean (SD)
3,908
3,848
3,551
2,994
2,838
3,101
3,024
2,617
3,011
(65)
(57)
(64)
(45)
(41)
(43)
(49)
(35)
(24)
Abbreviations: F, female; M, male.
Source: Institute of Medicine and Food and Nutrition Board.1
illnesses). In addition, the methods for determining the
AI for water may benefit from refinement and validation.
The 1998 National Health and Nutrition Examination
Survey data were derived from individuals’ responses to
the question: ‘‘How much plain drinking water do you
usually drink in a 24-hour period? Include only plain, tap,
or spring water.’’ Although the IOM collapsed the adult AI
into one number, there was wide variation from 2.3 L to
up to 6.2 L. The AI describes only the probability that an
individual would have an AI. Because there appears to
be wide variations in daily intake, the use of a 2-day usual
intake to estimate a person’s water intake (using the SIDE,
Software for Distribution Estimation) may not be
sufficient. Even with considerable variation in total water
intake, serum osmolality varies little across deciles of
total water intake, indicating that variation in water intake
in a broad range is not physiologically important.1
It should be recognized that there are limitations and
potential sources of error in using total water intake
estimates. Even so, these data are the best available.
Table 4. Water intake (in liters per day) From Foods and Beverages, NHANES 1999Y2002a
Mean ± SE
Children, 4Y18 y
All
Boys
Girls
Non-Hispanic whites
Non-Hispanic blacks
Hispanics
Other
Adults, Q19 y
All
Men
Women
Non-Hispanic whites
Non-Hispanic blacks
Hispanics
Other
1.40
1.55a
1.24b
1.42a
1.25b
1.41a
1.38ab
2.01
2.40a
1.76b
2.20a
1.68b
1.82c
1.86c
5th Percentile
50th Percentile
95th Percentile
T 0.02
T 0.03
T 0.02
T 0.03
T 0.02
T 0.03
T 0.07
0.51
0.58
0.45
0.51
0.44
0.58
0.52
T
T
T
T
T
T
T
0.03
NE
NE
0.04
0.02
0.03
0.07
1.24
1.37
1.14
1.25
1.13
1.25
1.19
T
T
T
T
T
T
T
0.02
0.03
0.02
0.03
0.02
0.04
0.06
2.79
3.13
2.34
2.91
2.39
2.60
2.76
T
T
T
T
T
T
T
0.18
0.23
0.13
0.15
0.16
0.19
0.24
T
T
T
T
T
T
T
0.72
0.86
0.64
0.79
0.51
0.64
0.67
T
T
T
T
T
T
T
0.04
0.05
0.04
0.05
0.03
0.05
0.12
1.85
2.15
1.59
1.97
1.49
1.64
1.75
T
T
T
T
T
T
T
0.02
0.02
0.03
0.03
0.02
0.04
0.05
4.16
4.75
3.30
4.29
3.50
3.85
3.48
T
T
T
T
T
T
T
0.25
0.31
0.15
0.28
0.33
0.28
0.39
0.02
0.03
0.02
0.03
0.03
0.03
0.03
Abbreviations: NE, nonestimatable; NHANES, National Health and Nutrition Examination Survey.
a
Data are from single 24-hour recall: 6,705 children: 3,359 boys and 3,347 girls; and 8,836 adults: 4,477 men and 4,359 women.
a,b,c
Means with different superscript letters are different, P G 0.05 from regression model including sex, age, race/ethnicity, and body mass index status.
Source: Fulgoni VL III.11
192
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Hydration and Health Promotion
Table 4 shows mean water intake from food and
beverages for specific groups. The mean (SD) for all
adults older than 19 years was 2.01 (0.02) L/d. Males’
average consumption was significantly higher than that
of females at 2.40 (0.03) L/d versus 1.76 (0.02) L/d.
Children’s intake varied by sex and ethnicity, ranging
from 1.25 (0.02) L/d for non-Hispanic blacks to
1.42 (0.03) L/day for non-Hispanic whites (Table 4).
There was no effect of body mass index on food and
beverage water intake. It is estimated that an additional
1.28 L of water is consumed per day as drinking water
in adults (Table 5).11
It appears that, for most adults and children, thirst
and consumption of beverages at meals are adequate to
maintain hydration for healthy people who are not
under environmental or physical stress and that most
individuals are in a euhydrated state because of normal
food and fluid consumption. However, the volume
needed to avert some acute and chronic diseases and to
maximize mental and physical performance may be
higher. In addition, the amounts may be different for
individuals performing at different environmental
conditions. For example, a sedentary person in a
climate-controlled environment will have lower water
turnover compared with a worker doing strenuous
physical work in a hot atmosphere.
While most individuals pay attention to their
hydration state, there are exceptions where workers
Continuing Education
become dehydrated but do not rehydrate before the next
work shift. They may require large amounts of fluid to
stay euhydrated from day to day. In response to these
observations, some work-site recommendations have
been made. In North America, occupational health and
safety associations use guidelines established by the
American Conference of Governmental Industrial
Hygienists, Occupational Safety & Health Administration
(OSHA), and the National Institute of Occupational Safety
& Health (NIOSH). The American Conference of
Governmental Industrial Hygienists and OSHA both
recommend that cool water or any cool liquid (except
alcoholic beverages) be provided to workers and that they
be encouraged to drink small amounts frequently, such as
1 cup (250 mL) every 20 minutes.13 The NIOSH
recommends that drinking water be readily available to
workers exposed to a hot work environment. It is
important that these organizations have made
recommendations regarding fluid intake and hydration.
However, because these recommendations specify exact
amounts and frequency of consumption, strictly adhering
to the guidelines may provide too much or too little fluid,
depending on the environment, the individual, and the
work intensity. The US Army has replacement guidelines
for warm weather training that take into account
environment, work intensity, and amount of fluid
needed and indicate that fluid intake should not exceed
1.5 qt (1.4 L) per hour or 12 qt (11.3 L) per day.13
Table 5. Water Intake (in liters per day) From Plain Water, NHANES 1999Y2002a
Mean ± SE
Children, 4Y18 y
All
Boys
Girls
Non-Hispanic whites
Non-Hispanic blacks
Hispanics
Other
Adults, Q19 y
All
Men
Women
Non-Hispanic whites
Non-Hispanic blacks
Hispanics
Other
50th Percentile
95th Percentile
0.78 T 0.04
0.82a T 0.03
0.73b T 0.03
0.79a T 0.04
0.65b T 0.02
0.80a T 0.06
0.87a T 0.08
0.47
0.47
0.45
0.47
0.43
0.44
0.52
T
T
T
T
T
T
T
0.01
0.03
0.01
0.02
0.01
0.04
NE
2.47
2.81
2.21
2.47
2.06
2.57
2.83
T
T
T
T
T
T
T
0.20
0.27
0.22
0.23
0.20
0.33
0.63
1.28 T 0.03
1.34a T 0.03
1.22b T 0.03
1.30a T 0.03
1.13b T 0.06
1.24ab T 0.07
1.43a T 0.11
0.94
0.96
0.94
0.95
0.86
0.94
0.96
T
T
T
T
T
T
T
0.01
0.02
0.01
0.03
0.03
0.04
0.04
3.76
3.77
3.50
3.73
3.56
3.77
4.23
T
T
T
T
T
T
T
0.02
0.13
0.24
0.03
0.14
0.21
0.73
Abbreviation: NE, nonestimatable; NHANES, National Health and Nutrition Examination Survey.
a
In response to NHANES food frequency question: ‘‘Total plain water drank yesterday, including plain tap water, water from a drinking fountain, water from a
water cooler, bottled water, and spring water.’’ Data include 6,647 children: 3,329 boys and 3,318 girls; 8,798 adults: 4,459 men and 4,339 women.
a,b
Means with different superscript letters are different, P G 0.05 from regression model including sex, age, race/ethnicity, and body mass index status.
Source: Fulgoni VL III.11
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The OSHA and NIOSH both
recommend that cool water be
provided to workers and that they be
encouraged to drink small amounts
frequently.
Special Considerations for Infants and Children
Total water intake, in relation to body weight, is 4 times
greater in infants than in adults, while the osmolar load
related to energy intake is 4 times lower. Currently, it is not
possible to define optimal water intake for infants and
children. Growth patterns differ for exclusively breast-fed
and formula-fed infants. It is unknown if hydration status
contributes to some of the differences in growth patterns.
A remaining challenge is to find the appropriate fluid
management strategy to support a preterm infant.
Hydration may be optimal only in breast-fed infants. An
individual’s spontaneous drinking behavior appears to be
set as a toddler and may have a cultural basis because it
differs by country.3,7 Urine volume and osmolality in
validated 24-hour urine samples of healthy children could
provide insight into the definition of optimal intakes to
prevent certain diseases.16,17
Measuring Hydration Status
All hydration assessment methods (Table 6) attempt to
describe the human body’s complex network of
interconnected fluid compartments (eg, intracellular,
extracellular, circulating blood). Single measurements
are inadequate because fluid gain and loss are dynamic
and alter total body water continuously throughout life.
Table 6. Eleven Hydration Assessment Techniques
Isotope dilution
Neutron activation analysis
Bioelectrical impedance spectroscopy
Plasma osmolality
Hematocrit plus hemoglobin
Urine osmolality
Urine conductivity
Urine specific gravitya
Urine colora
Body weight changea
Rating of thirsta
a
The average person can effectively use these techniques during daily activities.
194
The ever-fluctuating nature of total body water explains
why no single technique validly represents hydration
status in all situations.
In the laboratory, where environmental conditions and
human activities may be controlled, certain hydration
assessment techniques are effective. For example, when
posture, exercise, diet, fluid intake, and air temperature
are constant, total body water and plasma osmolality
(Posm) measurements are objective distinction indicators
of volume and concentration at a single point in time.
During daily activities or exercise, when fluid
compartments are constantly fluctuating, an evaluation
of a single body fluid is insufficient to provide valid
information about either total body water or the
concentration of body fluids. Individual fluid
measurements are simply ‘‘snapshots’’ of a complex,
dynamic fluid matrix. When an estimate of hydration
status is required, the best approach involves
comparing information from 2 or more hydration
indices. The following sections describe 5 techniques
that require minimal technical proficiency and can be
used by most people to evaluate hydration status
during daily activities. All of these techniques are safe,
portable, and inexpensive and require just a few
minutes to perform.
Body Weight Difference
The change of body weight represents a straightforward,
effective assessment of hydration status and is especially
appropriate for measuring dehydration that occurs over a
period of 1 to 4 hours. Very simply, body weight loss
equals sweat loss (corrected for the weight of fluid and
food consumed and urine and fecal losses).
When using body weight to represent water loss, one
should consider 2 factors. First, in both clinical and
athletic settings, an accurate baseline body weight is
required. Surprisingly, few individuals know this basic
value. A simple way to determine baseline body weight is
to weigh on 5 or 6 consecutive mornings, after
eliminating but before eating. When 3 measurements are
recorded that lie within 0.5 lb of each other, the average
value is a valid representation of body weight. Unless
sweating causes loss of water that exceeds 3% of body
weight, the regulation of body weight is consistent from
day to day, varying only 0.5%.9 Because of potential
confounders, nonsequential body weight measurements
cannot be used to determine a baseline weight.
If kidney function is normal, urine is concentrated
and output is low when the body is dehydrated and
conserving water. When a temporary excess of body
water exists, urine is dilute and plentiful. This
physiological function provides 3 options to evaluate
human hydration status using urine.
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24-Hour Urine Volume
Urine volume can be used as an indicator of hydration
status. Urine output varies inversely with body hydration
status, with urine output generally averaging 1 to 2 L/d,
but can reach 20 L/d in those consuming large quantities
of fluid. The minimum urine output is approximately
500 mL/d, although for dehydrated subjects living in hot
weather, minimum daily urine outputs can be less.
Physical activity and climate affect urine output. Exercise
and heat strain will reduce urine output by 20% to 60%,
while cold and hypoxia will increase urine output.
Healthy older individuals cannot concentrate urine as
well as young individuals do and thus, assuming
euhydration, have a higher minimum urine output.1
Urine Specific Gravity
The density (mass per volume) of a urine sample relative
to water can be measured using a handheld
refractometer. Any fluid that is denser than water has a
specific gravity greater than 1.000. Normal urine
specimens usually range from 1.013 to 1.029 in healthy
adults. When serious dehydration or hypohydration
exists, urine specific gravity exceeds 1.030.9 Conversely,
when one consumes excess water, the values range from
1.001 to 1.012. To measure urine specific gravity, place a
few drops of a urine specimen on the glass slide of the
refractometer and point it toward a light source. While
dipsticks and other techniques are sometimes used to
measure specific gravity, their validity and reliability
require future research.
Urine Color
A numbered scale has been developed that includes
colors ranging from very pale yellow (number 1) to
brownish green (number 8). A ‘‘pale yellow’’ or
‘‘straw-colored’’ visual reading indicates that the
individual is within 1% of his/her baseline body weight
(see above). Urine color does not offer the same precision
and accuracy as urine specific gravity but provides a
useful estimate of hydration state during everyday
activities.1,9,13
Thirst
As a physiological response to dehydration, thirst is a
reliable indicator of 1% to 2% dehydration. When
instrumentation or technical expertise is unavailable, it is
meaningful to recognize that one has attained this level of
dehydration. Recent publications have reported the
negative effects of mild dehydration on health and human
performance, when just 1% or 2% of body weight is lost.1
Exercise performance capacity (see below), cognitive
function, and alertness decline, while physiological strain
(ie, heart rate, tissue heat storage) increases.
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When one is ‘‘a little thirsty’’ or ‘‘moderately thirsty,’’
an individual can assume that he/she is mildly
dehydrated. However, it is important to realize that many
other factors may influence thirst. These include fluid
palatability, time allowed for fluid consumption, gastric
distention, age, and sex. Although thirst offers an
estimate of mild dehydration, it better serves to remind
individuals to drink adequate fluids.
Combining Techniques
During daily activities, when fluid compartments are
constantly fluctuating, an evaluation of a single body
fluid will not provide valid information about either total
body water or the concentration of body fluids (see
above). Thus, the best approach involves comparing
information from 2 or more of the above hydration
indices. When the data agree, one can be confident of the
information at that time. If these techniques do not
agree and assuming that no water overload exists, it is wise
to continue to drink fluids and repeat the measurements
within a few hours. To further ensure accuracy, multiple
observations should be made throughout the day.
Future Research
Future research should focus on novel hydration
assessment techniques that (a) measure fluid volume and
concentration in real time; (b) have excellent precision,
accuracy, and reliability; (c) are noninvasive; (d) are
interpreted in concert with other hydration indices;
(e) are portable, inexpensive, safe, and simple to use;
(f) evaluate the validity of the relation between Posm
and body water gain/loss in a variety of settings; and
(g) compare the ability of Posm (and other hematologic
indices) to track body water change versus other
hydration assessment techniques.
Hydration and Physical Performance
Hydration status has a major impact on physical
performance. This section reviews the current consensus
on influence of environmental temperature, impact of
relatively low levels of dehydration, hyperhydration, and
directions for future research.
Hypohydration reduces the performance of
high-intensity endurance activities more than it impacts
strength and power exercises. Hydration influences are
not likely to be noticed by a recreational athlete or fitness
enthusiast who exercises to maintain health or improve
strength. However, even small reductions in exercise
performance significantly alter the outcome of athletic
events, especially in elite competitions where small
differences separate winning from losing. Reductions of
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peak strength, power, and endurance also may affect
military, police, and fire personnel, when they attempt to
maximize their performance for public or personal safety.
Specifically, a chronic 3% to 4% loss of body weight
reduces muscular strength by approximately 2% and
reduces muscular power by approximately 3%.18 The
same 3% to 4% level of hypohydration reduces
high-intensity muscular endurance by approximately
10%.18 There is a direct relationship between the
magnitude of hypohydration-induced performance decline
and exercise duration: the longer the event, the greater the
impact. Even when conditions of environment, exercise
intensity, fitness level, and the level of heat acclimation are
the same, there is substantial variation in the sweating
rates of individuals.4 In addition, the rate of sweating and
therefore the total amount of sweat lost differ markedly
from day to day in the same individual. Because sweat loss
and fluid intake during physical activity vary significantly,
some individuals may encounter significant dehydration,
while others have minimal dehydration when performing
the same task.
Although it may seem reasonable to prepare for
dehydration by overdrinking, there are 2 significant
reasons why this practice is not a good idea. First, the
human brain guards the body from overhydration. When
an individual drinks too much fluid, the central nervous
system and fluid-balance hormones cause urine volume
to increase rapidly. Because of the body’s built-in
mechanisms, it is virtually impossible to store excess
water in the body. Within a few minutes or hours, total
body water returns to normal. Second, some individuals
disregard advice to avoid overdrinking or believe that
overhydration protects them from heat illness or helps
them maintain performance. Sadly, exactly the opposite
is true. If a person of average build drinks 2 or more
quarts (1.9 L) of water or dilute fluid within a few hours,
it is possible to develop a serious medical illness known
as water intoxication, also known as hyponatremia or
low blood-sodium level. Water intoxication results from
extreme dilution of body fluids.
The human brain guards the body
from overhydration.
Glycerol (glycerine) provides an effective means of
overhydrating. Glycerol is a sugar-alcohol compound
that is used commercially to sweeten or moisten foods. In
preparation for competitions in hot environments,
athletes consume glycerol syrup and water to encourage
water retention. For years, physiologists have debated
196
whether glycerol improves performance in the heat. The
scientific literature does not provide a definitive answer.
However, because of the cardiovascular influence of an
expanded blood volume, it is clear that endurance
athletes are more likely to benefit than are competitors in
strength or power events. The timing of glycerol intake is
important because the body retains the excess water for
only 3 to 4 hours after consumption.9,19
Current Consensus
Professional recommendations from a variety of
professional organizations are consistent. That is,
dehydration of more than 2% has a negative impact on
physiological function and performance in a wide variety
of exercise tasks over a wide variety of exercise durations
and intensities. Therefore, it is recommended that fluid
replacement occur during exercise to maintain hydration
at less than 2% of body weight reduction.
Existing professional recommendations regarding
hydration arise from an abundant body of research
conducted over 7 decades. Although few, there have been
studies showing no effect of fluid replacement on
exercise performance. Furthermore, it is likely that this
effect (or lack thereof) is influenced by exercise duration
and level of dehydration. Future research is needed to
delineate the complex relation of hydration status and
the multiple factors that must be considered when
evaluating physical performance.
There may not be universal agreement, but there is a
preponderance of evidence to support staying well
hydrated during physical activity as it affects performance
capacity. It does not matter whether the cause of
dehydration is exercise, heat exposure, diuretics, or fluid
restriction; dehydration’s negative impacts decrease
muscular endurance and maximum work capacity.
Dehydration does not appear to have a significant effect
on anaerobic high-power, short-duration exercise and
strength exercise. Dehydration does have a significant
impact on cardiovascular function by lowering cardiac
output and thus maximal aerobic capacity and work
capacity decline. In addition, the interaction has indirect
effects on performance through alterations in muscle
metabolism, central nervous system function, and central
drive, motivation, and perception of effort that lead to
impaired performance.
Not only is dehydration associated with prolonged
athletic events under severe circumstances, but it also
occurs under less severe circumstances if the athlete does
not replenish fluid losses. Dehydration’s impact on
cardiovascular and thermoregulatory functions can
occur early in exercise (eg, within 30 minutes) at a body
water loss equal to 1% of body mass. As the level of
dehydration increases, so does deterioration in
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physiological functions. During 2 hours of cycling in the
heat, heart rate, core temperature, and perceived exertion
ratings increased over time with progressive dehydration
to j4.9% of body mass, while blood volume, stroke
volume, cardiac output, and skin blood flow all
decreased.4
Dehydration’s impact on a variety of cardiovascular
and thermoregulatory functions can be measured early in
exercise and at a relatively low level of dehydration. As
the level of dehydration increases, so does deterioration
in physiological functions. During 2 hours of cycling in
the heat, heart rate, core temperature, and perceived
exertion ratings increased over time with progressive
dehydration to j4.9% of body mass, while blood
volume, stroke volume, cardiac output, and skin blood
flow all decreased.4
The literature is replete with studies demonstrating
the ill effects of dehydration on physical performance
and with studies showing that consuming fluids in
adequate amounts can effectively maintain euhydration
and positively affect performance, be it occupational,
military, or sport. A number of position stands and
consensus papers provide reviews of the scientific
literature and guidance on prevention and
intervention.18,20Y22
Effect of Ambient Temperature
The impact of dehydration on performance is less under
cooler environmental conditions than under hot
conditions. Exercise in heat itself, even with no
dehydration, impairs performance.14,19 When dehydration
increases beyond a relatively low level, the impact on
performance and physiological function gets progressively
worse as dehydration increases. In cold circumstances, a
measurable impact may not be discernable at low levels
of dehydration, but impact is seen as dehydration
progresses. Impacts of dehydration on performance are
more pronounced in temperate than in cold environments.
Degree of Dehydration Needed to
Impact Performance
The arbitrarily defined cutoff point, based on the
preponderance of literature, is that dehydration should
be kept at less than 2% of body weight. Several studies
were cited that provide increasing evidence that even
very small levels of dehydration can negatively affect
exercise performance.4
Future Research
Future research areas include the effect of performance in
the following situations: dehydration of less than j2%
body weight, dehydration at less than 20-C, graded
dehydration and age, and dehydration on muscle strength.
The Contribution of Food to Water
Intake and Hydration
Data from the Continuing Survey of Food Intakes by
Individuals reveal that about 20% to 25% of total fluid
intake comes from food, whereas the National Health and
Nutrition Examination Survey III data show that
approximately 19% of total fluid intake is from foods.
As exemplified in Table 7, the specific foods consumed
determine the water contribution from food. Moreover,
interactions with food components may affect fluid
consumption.12
Researchers have evaluated the effect of the type of
replacement fluids on volume consumed and hydration
status, specifically evaluating the affect of carbohydrate,
flavoring, and sodium chloride. Studies that have allowed
exercising subjects free access to either water, flavored
water, or a flavored beverage containing carbohydrate and
sodium chloride have shown that, by the end of exercise,
the beverage consumed in the largest volume was the
carbohydrate-electrolyte (CE) beverage. This was followed
by flavored water, with the least consumed beverage being
Table 7. Water Content (Percentage of Total Weight) of Commonly Consumed Foods
High Water Content
Food
Low Water Content
Water Content, %
Food
96
94
92
90
87
86
84
Steak, cooked
Cheese, cheddar
Bread, white
Cookies
Walnuts
Corn flakes
Peanuts, roasted
Lettuce, iceberg
Squash, cooked
Pickle
Cantaloupe
Orange
Apple
Pear
Water Content, %
50
37
36
4
4
3
2%
Source: Sharp.12
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unflavored water. The impact on hydration status reflects
volume consumed, with a slight increase with the CE
beverage and a slightly negative balance in the unflavored
water trial.12
Although studies exploring the effect of adding food to
the rehydration protocol are limited, those conducted have
reported similar results.12 Rehydration trials of these
studies have evaluated water, CE beverages, and a meal in
various combinations. Collectively, results have shown
that consuming food during rehydration can reduce urine
production and accelerate the recovery of body water
stores. In one study, the meal provided consisted of rice,
beans, and beef. In another, the investigators compared
combinations of plain tap water, CE drink, chicken broth,
and chicken noodle soup. The effect was likely due to the
higher total sodium content of the meals. Sodium is the
major extracellular ion and has been shown to accelerate
recovery of plasma volume after exercise in the heat.12
Hydration and Mental Performance
The relation between hydration and mental performance
has not been extensively examined. The available literature
supports the conclusion that dehydration will degrade
mental performance in areas such as psychomotor, motor
control, dexterity, and cognitive functioning. However,
numerous methods of assessing mental performance exist,
and many confounding factors affect performance, making
it difficult to reach definitive conclusions. Table 8 lists
cognitive tests that have been used to measure mental
performance in dehydrated individuals.
Measurement of mental performance is complicated by
a variety of factors: motivation, thermal stress, physical
stress, IQ, sex, diet, disease states, drugs, use of caffeine,
beverage consumption during testing, time of day, degree
of dehydration, practice effect, and other physical and
mental conditions affect the outcome of cognitive
function assessments. Additional confounders should be
considered. For example did the subject(s) begin the
study in a state of euhydration? Adverse effects of
dehydration increase as the water deficit increases and
can become cumulative over time, and that at extreme
levels of dehydration, a state of delirium or coma is
reached and cognitive function cannot be assessed.
Dehydration induced by acute heat/exercise exposure
appears to impair cognitive performance at 1% body
weight loss or less based on consistent dose-response
functions across tests and studies.5,6 The effects of
dehydration induced by water deprivation need further
study and a larger data pool to draw any conclusions.
Studies of hydration and mental performance have
demonstrated that the techniques chosen to measure
hydration level produce different results,1,5,6 therefore
affecting ability to compare data from different studies.1
198
Table 8. Examples of Measures to Assess Mental
Performance5,6
Processing speed and accuracy
Discrimination
Tracking
Short-term memory
Working memory
Recall
Sustained and selective attention
Mathematic calculations
Motor function
Perceived effort and concentration
Motor control and dexterity
Speed to exhaustion
Confusion and disorientation
In addition, the presence of multiple confounders makes
it difficult to conduct tightly controlled studies. Most
studies have critical limitations such as substantial
individual and group variability in level of hydration
induced and use of behavioral tests that are neither
state-of-the-art or have limited sensitivity. Most had
small sample sizes and thus low statistical power, and
many did not have a placebo control.
Future Research
Research recommendations included conducting studies
to determine at what level of dehydration cognitive
decrements first appear, determine if a person’s state of
hydration impacts the diagnoses of dementia, evaluate
practice effect as a confounder, determine the portions of
the brain affected by dehydration, determine what aspects
of cognitive performance and mood states are altered
by dehydration, and examine the involvement of
cardiovascular, thermoregulatory, and the central nervous
system’s responses to dehydration and diminished
cognitive performance. Study design recommendations
included conducting adequately powered dose-response
studies using state-of-the-art, sensitive cognitive
performance and mood tests; conducting studies using
heat/exerciseY and fluid deprivationYinduced dehydration;
considering using a positive control where another
treatment in addition to dehydration is used to set up an
internal matrix; and considering novel techniques for
placebo control and blinding.
Hydration and Disease
The extremes of water metabolism, severe dehydration
and water intoxication, are known causes of morbidity
and mortality in children and adults.3 There is increasing
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Hydration and Health Promotion
evidence that even mild dehydration may lead to an
increased risk of negative health outcomes. Most of the
evidence published comes from descriptive studies such
as comparative, correlation, and case-control studies
(Table 9). Although not conclusive, there is emerging
evidence that small sex differences do exist. Conditions
associated with acute systemic mild dehydration include
an increased risk of morbidity and mortality in
pregnancy and in infants with acute gastroenteritis and
heat injury in individuals with cystic fibrosis. There is
strong evidence for an association between chronic
systematic mild dehydration and urolithiasis, urinary
tract infection, venous thromboembolism, hyperglycemia
in diabetic ketoacidosis, and mitral valve prolapse.
There is evidence, but less strong, for an association
between chronic mild dehydration and constipation,
hypertension, coronary heart disease, stroke, dental
disease, gallstones, and glaucoma. The evidence for a link
between local mild dehydration and bronchopulmonary
Continuing Education
disorders, such as exercise asthma and cystic fibrosis,
is moderately strong. The evidence of the role of
dehydration in the development of bladder and colon
cancers is inconsistent. Interestingly, the relation
between fluid intake and bladder cancer may be due
more to the amount of fluid than to dehydration.3
The extremes of hydration, severe
dehydration and water intoxication,
are known causes of morbidity and
mortality in children and adults.
Although the evidence clearly points to a link between
hydration status and chronic disease prevention,
Table 9. Relationships of Acute, Chronic, and Local Mild Dehydration on Various Disease or Body States and
Categorya of Evidence Classification
Category of Evidence
Ia
Acute systemic mild dehydration
Oligohydramnios
Prolonged labor
Hypertonic dehydration in infants
Cystic fibrosis
Renal toxicity of xenobiotics
Chronic systematic mild dehydration
Urolithiasis
Urinary tract infections
Constipation
Hypertension
Venous thromboembolism
Coronary heart disease
Stroke
Dental disease
Hyperosmolar hyperglycemic diabetic ketoacidosis
Gallstones
Mitral valve prolapse
Glaucoma
Local mild dehydration
Bronchopulmonary disorders
Exercise asthma
Cystic fibrosis
Ib
IIa
IIb
III
IV
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
a
Category IVevidence from a randomized controlled trial (Ia, meta-analysis; Ib, at least 1 trial); category IIVevidence from controlled studies (IIa, no
randomization; IIb, other type of quasi-experimental study); category IIIVevidence from descriptive studies such as comparative, correlation, and case-control
studies; category IVVevidence from expert committee reports, opinions or clinical experience of respected authorities, or both.
Source: Manz.3
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199
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additional work to answer the research questions posed by
the IOM on the effects of hydration status and fluid intake
on chronic disease is warranted. For example, in the case of
urolithiasis, the risk for recurrence may be related to urine
output, but urine output may not be the indicator of risk for
other conditions. Clinicians need electronic access to data
from human studies that help them answer patients’
questions about the amount and kinds of fluids needed to
prevent conditions such as stone recurrence, urinary tract
infections, and constipation.
The role of hydration in pregnancy and early life is
being elucidated. In animals, maternal dehydration
increased risk for intrauterine growth retardation.
There has been significant media attention given to water
intoxication and mortality, but there is good evidence that
the short half-life of water excretion and the high renal
dilution capacity are effective mechanisms to protect
healthy people from negative consequences of high fluid
intakes. When it occurs, it is usually due to a mixture of high
water intake, low nutritional osmolar load, an increased
minimum urine osmolality, and/or iatrogenic errors. People
in situations that can lead to water intoxication include
those working in high-heat situations with large sodium
losses, marathon runners taking anti-inflammatory
medications, and individuals consuming large volumes of
fluid within a period of 3 to 8 hours. There is evidence that
water should be limited for individuals with selected
chronic conditions such as chronic renal failure.
Water intoxication in infants and children is very rare
but may be experienced with cerebral hemorrhage,
persistent ductus arteriosus, dehydration, and necrotizing
interpolates or secondary surfactant deficiency.
There are concerns regarding the types of fluids that
children and adolescents consume. Most recommendations
classify alcohol as unsuitable because of its toxicity and
because it is illegal for those below the official drinking
age. Some recommendations include the avoidance of
caffeinated beverages because of the disruption of sleep.
The consumption of sugar-sweetened beverages by children
has been associated with obesity, dental caries, lower bone
density, urolithiasis, and a severe form of ketoacidosis in
diabetes mellitus.7 More than 80 years ago, it was
demonstrated that postabsorption water status may be
different after the consumption of the same amount
of different beverages. An understanding of how
postabsorption status affects beverage intake is needed to
develop recommendations that pediatricians can make to
parents. Office-based health advice about fluid intake
needs to be evidence based.
Advice for Consumers
There was general agreement by conference attendees
that myths about water requirements abound. While the
200
evidence is insufficient to provide an recommended
dietary allowance for total water intake, there are
scientifically based messages to inform consumers about
fluid intake and hydration. They include the following:
1. Water is essential for life.
2. Relying on the perception of thirst does not always
guarantee appropriate total water intake.
3. Foods and beverages contribute varying amounts of
water in the diet.
4. Consuming a variety of noncaffeinated and caffeinated
beverages including water, milk, tea, coffee, juice, soft
drinks, and sport drinks can contribute to meeting the
body’s water requirement.
5. Foods including certain fruits, vegetables, soups, and
dairy products can also contribute to meet the body’s
water requirement.
6. Appropriate beverages and food choices for an
individual may vary based on energy, nutrient, and
water needs, as well as consumer preference.
Kathryn M. Kolasa, PhD, RD, LDN, is a professor at the Departments of
Family Medicine and of Pediatrics, Brody School of Medicine at East
Carolina University, Greenville, North Carolina, and teaches evidence-based
nutrition to medical students, residents, practicing physicians, and allied
health professionals. She has studied the messages health professionals
provide to their clients about water and hydration. She has been active in
developing policies for ensuring that schoolchildren and workers have access
to healthy food and beverage options.
Carolyn J. Lackey, PhD, is a professor and a food and nutrition
specialist, Emerti at North Carolina State University, Raleigh, North Carolina.
Her career has focused on translating nutrition and food science into
appropriate consumer messages.
Ann C. Grandjean, EdD, FACSM, is the executive director of the Center
for Human Nutrition, a nonprofit organization committed to the enhancement of
human health, performance, and quality of life through better nutrition. As
executive director of Center for Human Nutrition, she is responsible for the
center’s research and community services divisions. Dr Grandjean is also a
clinical assistant professor in internal medicine at the University of Nebraska
Medical Center, Omaha.
The conference in November 2006 and this summary article were organized by
the North American branch of the International Life Sciences Institute North
America Technical Committee on Hydration and were supported in part by
educational grants from Campbell Soup Company, the Coca-Cola Company,
Danone Group, Dr Pepper/Snapple Group, the Gatorade Company, Kraft Foods,
and Nestle USA. At the conference, nonindustry speakers were reimbursed for
their travel expenses and offered a small honorarium for their participation and
preparation of a manuscript. In addition, the authors of this summary article
were paid for some of their time in preparing and editing the manuscript.
Conference Sponsor. The North American branch of the International Life
Sciences Institute (ILSI) is a public, nonprofit scientific foundation. Its Hydration
Committee was established in 2001 to advance the understanding and
application of scientific issues related to hydration. The committee supported
the workshop and the development of this article. The ILSI North America, a
public, nonprofit scientific foundation, advances the understanding and
application of scientific issues related to the nutritional quality and safety of
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Hydration and Health Promotion
the food supply. The organization carries out its mission by sponsoring relevant
research programs; professional education programs; and workshops,
seminars, and publications, as well as providing a neutral forum for
government, academic, and industry scientists to discuss and resolve scientific
issues of common concern for the well-being of the general public. The ILSI
North America’s programs are supported primarily by its industry membership.
Workshop speakers and panelists included Lawrence Armstrong, Maxime
Buyckx, Sheila Campbell, Victor Fulgoni, Ann Grandjean, Kathryn Kolasa,
Robert Kenefick, Florian Lang, Harris Lieberman, Friedrich Manz, Ron Maughan,
Bob Murray, Irv Rosenberg, and Rick Sharp.
Corresponding author: Kathryn M. Kolasa, PhD, RD, LDN, Department of Family
Medicine and of Pediatrics, Brody School of Medicine at East Carolina
University, 600 Moye Blvd, Suite 4N-70, Greenville, NC 27834
([email protected]).
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For more than 22 additional continuing education articles related to Nutrition topics, go to http://www.nursingcenter.com/CE.
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