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OCR for page 156
TABLE 1 Dietary Reference Intakes for Water by
Life Stage Group
DRI values (L/day)a
AIb
Life stage groupc
0 through 6 mo 0.7, assumed to be from human milk
0.8 of totald water, assumed to be from human milk,
7 through 12 mo
complementary foods and beverages. This
includes approximately 0.6 L (about 3 cups) as
total fluid, including formula or human milk, juices,
and drinking water.
1 through 3 y 1.3 of total water. This includes approximately 0.9 L
(about 4 cups) as total beverages, including
drinking water.
4 through 8 y 1.7 of total water. This includes approximately 1.2 L
(about 5 cups) as total beverages, including
drinking water.
9 through 13 y
males 2.4 of total water. This includes approximately 1.8 L
(about 8 cups) as total beverages, including
drinking water.
females 2.1 of total water. This includes approximately 1.6 L
(about 7 cups) as total beverages, including
drinking water.
14 through 18 y
males 3.3 of total water. This includes approximately 2.6 L
(about 11 cups) as total beverages, including
drinking water.
females 2.3 of total water. This includes approximately 1.8 L
(about 8 cups) as total beverages, including
drinking water.
19 through > 70y
males 3.7 of total water. This includes approximately
3.0 L (about 13 cups) as total beverages, including
drinking water.
females 2.7 of total water. This includes approximately 2.2 L
(about 9 cups) as total beverages, including
drinking water.
OCR for page 157
TABLE 1 Continued
DRI values (L/day)a
AIb
Pregnancy
14 through 50 y 3.0 of total water. This includes approximately 2.3 L
(about 10 cups) as total beverages, including
drinking water.
Lactation
14 through 50 y 3.8 of total water. This includes approximately 3.1 L
(about 13 cups) as total beverages, including
drinking water.
a Conversion factors: 1 L = 33.8 fluid oz; 1 L = 1.06 qt; 1 cup = 8 fluid oz.
b AI = Adequate Intake. If sufficient scientific evidence is not available to establish
an Estimated Average Requirement (EAR), and thus calculate a Recommended Dietary
Allowance (RDA), an AI is usually developed. For healthy breast-fed infants, the AI is
the mean intake. The AI for other life stage and gender groups is believed to cover the
needs of all healthy individuals in the group, but a lack of data or uncertainty in the
data prevents being able to specify with confidence the percentage of individuals
covered by this intake.
c Life stage groups through 8 years of age represent males and females.
d Total water (as italicized) includes all water contained in food, beverages, and
drinking water. For infants, 7 through 12 months, total water assumed to be from
human milk, complementary foods and beverages.
OCR for page 158
158
WATER
W
ater, vital for life, is the largest single constituent of the human body,
averaging approximately 60 percent of body weight. It is essential
for cellular homeostasis and for maintaining vascular volume. It
also serves as the medium for transport within the body by supplying nutrients
and removing waste.
Since data were insufficient to establish an Estimated Average Requirement
(EAR) and thus calculate a Recommended Dietary Allowance (RDA) for water,
an Average Intake (AI) was instead developed. The AIs for water are based on
the median total water intake from U.S. survey data. (Total water intake in-
cludes drinking water, water in beverages and formula, and water that is con-
tained in food.) These reference values represent the total water intake that is
considered likely to prevent deleterious, primarily acute, effects of dehydra-
tion, including metabolic and functional abnormalities. Although a low intake
of total water has been associated with some chronic diseases, this evidence is
insufficient to establish water intake recommendations as a means to reduce the
risk of chronic diseases.
Higher intakes of total water will be required for those who are physically
active or exposed to hot environments. Because healthy individuals have a con-
siderable ability to excrete excess water and thereby maintain water balance, a
Tolerable Upper Intake Level (UL) was not set for water. DRI values for water
are listed by life stage group in Table 1.
Over the course of a few hours, body water deficits can occur due to re-
duced intake or increased water loss from physical activity and environmental
(heat) exposure. However, on a day-to-day basis, fluid intake, usually driven by
the combination of thirst and mealtime beverage consumption, helps maintain
hydration status and total body water (TBW) at normal levels.
Sources of water include beverages, food, and drinking water. Inadequate
water intake leads to dehydration. Excessive water intake can lead to hyponatre-
mia, an extremely rare condition marked by a low concentration of sodium in
the blood.
OCR for page 159
PART II: WATER 159
WATER AND THE BODY
Function
Water is the solvent for biochemical reactions and represents the largest single
constituent of the human body, averaging approximately 60 percent of body
weight. Water absorbs the body heat from metabolic processes, maintains vas-
cular volume, and serves as the medium for transport within the body by sup-
plying nutrients and removing waste. It is also essential for cellular homeosta-
sis. Cell hydration has been suggested to be an important signal in the regulation
of cell metabolism and gene expression.
Daily water intake must be balanced with water loss in order to maintain
total body water (TBW). TBW is comprised of both the intracellular (ICF) and
the extracellular (ECF) fluids and varies by individual due to differences in
body composition.
Absorption, Metabolism, Storage, and Excretion
Water that is consumed via liquid and food is digested and absorbed within
the gastrointestinal tract. Body water is distributed between the ICF and the
ECF which contain 65 and 35 percent of TBW, respectively. Body water bal-
,
ance depends on the net difference between water gain and water loss. Pertur-
bations such as exercise, heat exposure, fever, diarrhea, trauma, and burns will
greatly affect the net volumes and water turnover rates between these fluid
compartments.
TBW gain occurs from consumption and as a by-product of the
metabolization of energy-yielding nutrients from foods. Production of meta-
bolic water is proportional to daily energy expenditure for people eating a mixed
diet. TBW loss results from respiratory, skin, renal, and gastrointestinal tract
water losses, which are described as follows:
Respiratory: Physical activity generally has a greater effect on water loss through
evaporation within the lungs than do environmental factors, such as ambient
air temperature and humidity. Daily loss averages about 200–350 mL/day for
sedentary people and can increase to 500–600 mL/day for active people who
live in temperate climates at sea level.
Urinary and gastrointestinal: Renal output can vary depending on specific
macronutrient, salt, and water loads. Urine output inversely varies with body
hydration status (usually averaging 1–2 L/day) and also generally increases in
healthy older individuals because they are unable to concentrate urine as well
as younger individuals. Exercise and heat reduce urine output, while cold and
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
160
hypoxia increase output. Fecal water loss in healthy adults is approximately
100–200 mL/day.
Skin: Water loss through skin occurs by insensible diffusion and secreted sweat.
For the average adult, loss of water by insensible diffusion is approximately 450
mL/day. In hot weather, sweat evaporation is the primary avenue of heat loss to
defend the body’s core temperature. Daily sweat loss considerably varies due to
differences in metabolic rate and environment (e.g., clothing worn, ambient
temperatures, air motion, and solar load).
DETERMINING DRIS
Determining Requirements
Since data were insufficient to establish an EAR and thus calculate an RDA for
water, an AI was instead developed. The AIs for water are based on median total
water intakes using survey data from the Third National Health and Nutrition
Examination Survey (NHANES III, 1988–1994). These reference values repre-
sent total water intakes that are considered likely to prevent deleterious, prima-
rily acute, effects of dehydration, including metabolic and functional abnor-
malities. Although a low intake of total water has been associated with some
chronic diseases, the evidence is insufficient to establish water intake recom-
mendations as a means to reduce the risk of chronic diseases
As with AIs for other nutrients, for a healthy person, daily consumption
below the AI may not confer additional risk because a wide range of intakes is
compatible with normal hydration. In this setting, the AI should not be inter-
preted as a specific requirement. Higher intakes of total water will be required
for those who are physically active or exposed to hot environments.
Over the course of a few hours, body water deficits can occur due to re-
duced intake or increased water loss from physical activity and environmental
(heat) exposure. However, on a day-to-day basis, fluid intake, usually driven by
the combination of thirst and mealtime beverage consumption, helps maintain
hydration status and TBW at normal levels.
Special Considerations
Generally, groups that are more active will have a greater total water intake:
• Active adults: Physical activity, particularly when performed in hot
weather, increases daily fluid needs. Daily water requirements for adults
can double in hot weather (86∞F or 30∞C) and triple in very hot weather
(104∞F or 40∞C) to make up for water lost via sweating.
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PART II: WATER 161
• Active children: Children who are active produce considerably less sweat
than active adults, even when exercising in hot environments. This dif-
ference in sweat production prevails until midpuberty and should be
considered when determining the water requirements of active children
and adolescents.
• Elderly: Hydration status continues to be normal in elderly individuals
over a wide range of intakes. However, a deficit in thirst and fluid intake
regulation, age-related impairments in renal-concentrating and sodium-
conserving ability, prior history of stroke, or evidence of hypothalamic
or pituitary dysfunction may contribute to increased incidence of dehy-
dration and hypernatremia.
Factors Affecting Water Requirements
Physical activity and heat strain: Physical activity and heat strain can substan-
tially increase water loss through sweating. The daily water requirement in-
creases that arise from activity and ambient temperature are the result of
increased sweating to meet evaporative cooling requirements. A person’s sweat-
ing rate depends on climatic conditions, the clothing worn, and exercise inten-
sity and duration. Physical fitness level has a modest effect on sweat loss, unless
accompanied by heat acclimation. Studies have shown broad ranges in fluid
requirements based on these influences. Examples include:
• People in very hot (e.g., desert) climates, who often have sweating rates
of 0.3–1.2 L/hour while performing occupational activities
• People wearing protective clothing, who often have sweating rates of
1–2 L/hour while performing light-intensity exercise in hot weather
• Male competitive runners, who can have sweating rates of 1 to > 2 L/hour
while training or racing in the heat
• Female competitive runners may increase their sweat losses from approxi-
mately 0.7 L/hour in temperate weather to approximately 1.1 L/hour in
warm weather when performing the same event
Altitude and cold temperature: Altitude exposure increases respiratory water
loss and hypoxia-induced diuresis. There may also be reduced fluid consump-
tion and, for persons traversing rugged mountain terrains, elevated sweating
due to high metabolic rates. The net effect can lead to dehydration. Body fluid
loss in cold climates can be as high as loss in hot climates due to high rates of
energy expenditure and the use of highly insulated heavy clothing. Fluid loss
during cold exposure is thought to result from cold-induced diuresis and in-
creased respiratory loss.
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
162
Diabetes mellitus: Dehydration is clearly associated with the worsening control
of diabetes. In addition, uncontrolled diabetes dramatically contributes to de-
velopment of severe dehydration and volume depletion due to osmotic diure-
sis. In people with poorly controlled diabetes, reduced water intake can also
lead to dehydration owing to infection or hypotension, which can lead to de-
lirium and an impaired ability to seek water.
Cystic fibrosis: People with cystic fibrosis have high concentrations of sodium
chloride in their sweat. They may lose excessive amounts of sodium and chlo-
ride when their sweating rates are high and, unlike healthy people, their body
fluid osmolality does not increase due to the high concentrations of sodium
chloride in their sweat. Without elevated serum osmolality, a major trigger for
thirst, cystic fibrosis patients can quickly become dehydrated during physical
activity, particularly in the heat.
Diuretics and other medications: There are no medications that directly stimu-
late water intake. When decreased fluid intake has occurred due to illness,
medications that improve metabolic and cognitive function should indirectly
help people increase their fluid intake. Examples include antibiotics for infec-
tions, insulin for unstable diabetics, and analgesics for delirium-inducing pain.
However, some drugs, such as diuretics, cause excess water loss. Diuretics are
commonly used medications that are prescribed for the treatment of condi-
tions such as hypertension, heart failure, and chronic kidney disease. Dehy-
dration may occur in people who do not modify their use of diuretics in hot
weather or in other situations where excess water loss occurs. Other medica-
tions, such as lithium, may interfere with the kidneys’ regulatory systems, lead-
ing to excessive water loss.
Criteria for Determining Total Water Requirements,
by Life Stage Group
Life stage group Criterion
0 through 6 months Average consumption from human milk content
7 through 12 months Average consumption from human milk + complementary foods
and other beverages
1 through > 70 y Median total water intake using data from NHANES III
Pregnancy Same as age-specific values for nonpregnant women
Lactation Same as age-specific values for nonpregnant women
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PART II: WATER 163
The UL
The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient
intake that is likely to pose no risk of adverse effects for almost all people.
Because healthy individuals have considerable ability to excrete excess water
and thereby maintain water balance, a UL was not set for water. However, acute
water toxicity has been reported from the rapid consumption of large quantities
of fluids that greatly exceeded the kidneys’ maximal excretion rate of approxi-
mately 0.7–1.0 L/hour.
According to NHANES III (1988–1994), the highest total water intake (99th
percentile) reported was 8.1 L/day. No adverse intakes have been reported with
chronic high intakes of water in health people consuming a normal diet, as long
as fluid intake is approximately proportional to losses.
DIETARY SOURCES
Sources of water include beverages, food, and drinking water. Fruits and veg-
etables contain a high percentage of water. According to data from NHANES
III, adults in the United States obtained total water from the following sources:
• 35–54 percent from drinking water
• 49–63 percent from other beverages (with juice, carbonated drinks,
coffee, and milk being the major sources)
• 19–25 percent from foods (such as fruits, vegetables, soups, ice cream,
and meats)
Dietary Interactions
There is evidence that water may interact with certain nutrients and dietary
substances (see Table 2).
INADEQUATE INTAKE
Inadequate water intake leads to dehydration, the effects of which include the
following:
• Impaired mental function and motor control
• Diminished aerobic and endurance exercise performance
• Enhanced fever response (fever is a regulated rise in body temperature)
• Increased core temperature during exercise
• Reduced tolerance to the stress of exercise and heat
• Increased resting heart rate when standing or lying down
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
164
TABLE 2 Potential Substances That Affect Water Requirements
Substance Potential Interaction Notes
SUBSTANCES THAT AFFECT WATER REQUIREMENTS
Caffeine Due to its diuretic effect, Available data were inconsistent. Unless future
caffeine in high amounts research proves otherwise, caffeinated beverages
may lead to a total body appear to contribute to total water intake to the
water (TBW) deficit. same degree as noncaffeinated fluids do.
Alcohol Alcohol intake appears to Based on limited data, ethanol ingestion did not
increase water excretion. appear to result in appreciable fluid loss over
a 24-hour period. An increased excretion of
water due to ethanol ingestion was transient.
Sodium Increased sodium intake Based on limited data, it was not possible to
may increase urine volume. determine the extent to which sodium intake
influences water intake.
Protein Increased protein Studies showed that increased protein intake did
consumption may increase not affect water intake or urine volume in
water needs. Urea, a major the setting of ad libitum water consumption.
end product of the
metabolism of dietary
proteins and amino acids,
requires water for excretion
by the kidneys.
Fiber Fecal water loss is increased Limited studies showed significant increases in
with increased dietary fiber. fecal water loss with high-fiber diets.
Carbohydrate The presence of dietary On average, 100 g/day of carbohydrates (the amount
carbohydrates may affect needed to prevent ketosis) has been shown to
decrease body water deficit by decreasing the
quantity of body solutes (ketone bodies) that need to
be excreted. This response is similar when ketosis
occurs with the consumption of very low
carbohydrate diets.
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PART II: WATER 165
• Impaired ability to maintain blood pressure when presented with vas-
cular challenges
• Fainting (in susceptible people)
• Reduced cardiac output during exercise and heat stress
• Apparent increased risk of life-threatening heat stroke
EXCESS INTAKE
No adverse effects have been reported with chronic high intakes of water by
healthy people who consume a normal diet, as long as fluid intake is approxi-
mately proportional to fluid loss. Excessive water intake can lead to hyponatre-
mia, which is a low concentration of sodium in the blood (defined as serum
sodium concentration of less than 135 mmol/L). The lowering of the extracel-
lular fluid sodium concentration causes fluid to move into the intracellular fluid
space, resulting in central nervous system edema, lung congestion, and muscle
weakness. Hyponatremia can also occur from excessive fluid intake, the under-
replacement of sodium, or both, during or after prolonged endurance athletic
events. In severe cases, hyponatremia can be life-threatening.
Hyponatremia is rare in healthy persons who consume an average North
American diet. The condition is most often seen in infants, psychiatric patients
with psychogenic polydipsia (chronic excessive thirst and fluid intake), pa-
tients on psychotropic drugs, women who have undergone surgery using a uter-
ine distension medium, and participants in prolonged endurance events, such
as military recruits.
A series of case studies has suggested that gross overconsumption of fluids
(for example, more than 20 L/day) is associated with irreversible bladder le-
sions and possibly thinner bladder muscles, delayed bladder sensation, and
flow rate impairment.
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
166
KEY POINTS FOR WATER
Water, vital for life, is essential for cellular homeostasis and for
3
maintaining vascular volume. It also serves as the medium for
transport within the body by supplying nutrients and removing
waste.
Since data were insufficient to establish an EAR and thus
3
calculate an RDA for water, an AI was instead developed.
The AIs for water are based on the median total water intake
3
from U.S. survey data. These reference values represent total
water intakes that are considered likely to prevent deleterious,
primarily acute, effects of dehydration, including metabolic and
functional abnormalities.
Although a low intake of total water has been associated with
3
some chronic diseases, this evidence is insufficient to establish
water intake recommendations as a means to reduce the risk
of chronic diseases.
Over the course of a few hours, body water deficits can occur
3
due to reduced intake or increased water loss from physical
activity and environmental (heat) exposure. However, on a day-
to-day basis, fluid intake, driven by the combination of thirst
and mealtime beverage consumption, helps maintain hydration
status and total body water at normal levels.
Because healthy individuals have a considerable ability to
3
excrete excess water and thereby maintain water balance, a UL
was not set for water.
Acute water toxicity has been reported from the rapid
3
consumption of large quantities of fluids that greatly exceeded
the kidneys’ maximal excretion rate of approximately 0.7–1.0 L/
hour.
Sources of water include drinking water, beverages, and food.
3
Inadequate water intake leads to dehydration, which can impair
3
mental function, exercise performance, exercise and heat
stress tolerance, and blood pressure regulation.
Excessive water intake can lead to hyponatremia, which is a
3
low concentration of sodium in the blood. This condition leads
to central nervous system edema, lung congestion, and muscle
weakness.
