Livro DRI 2006 (Micronutrientes)

Livro DRI 2006 (Micronutrientes)


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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:
\u2022 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\u221eF or 30\u221eC) and triple in very hot weather
(104\u221eF or 40\u221eC) to make up for water lost via sweating.
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
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PART II: WATER 161
\u2022 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.
\u2022 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\u2019s 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:
\u2022 People in very hot (e.g., desert) climates, who often have sweating rates
of 0.3\u20131.2 L/hour while performing occupational activities
\u2022 People wearing protective clothing, who often have sweating rates of
1\u20132 L/hour while performing light-intensity exercise in hot weather
\u2022 Male competitive runners, who can have sweating rates of 1 to > 2 L/hour
while training or racing in the heat
\u2022 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.
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
162 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
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\u2019 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
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
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\u2019 maximal excretion rate of approxi-
mately 0.7\u20131.0 L/hour.
According to NHANES III (1988\u20131994), 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:
\u2022 35\u201354 percent from drinking water
\u2022 49\u201363 percent from other beverages (with juice, carbonated drinks,
coffee, and milk being the major sources)
\u2022 19\u201325 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:
\u2022 Impaired mental function and motor control
\u2022 Diminished aerobic and endurance exercise performance
\u2022 Enhanced fever response (fever is a regulated rise in body temperature)
\u2022 Increased core temperature during exercise
\u2022 Reduced tolerance to the stress of exercise and heat
\u2022 Increased resting heart rate when standing or