Livro DRI 2006 (Micronutrientes)

Livro DRI 2006 (Micronutrientes)


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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.
TABLE 1 Continued
DRI values (L/day)a
AIb
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
158
WATER
Water, vital for life, is the largest single constituent of the human body,averaging approximately 60 percent of body weight. It is essentialfor 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.
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 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\u2013350 mL/day for
sedentary people and can increase to 500\u2013600 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\u20132 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
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
160 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
hypoxia increase output. Fecal water loss in healthy adults is approximately
100\u2013200 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\u2019s 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\u20131994). 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