Livro DRI 2006 (Micronutrientes)

Livro DRI 2006 (Micronutrientes)


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potassium, selenium, sodium chloride, sul-
fate, and zinc; there is also a chapter on other substances including arsenic,
boron, nickel, silicon, and vanadium.
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
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 III: VITAMINS AND MINERALS 169
DEFINITIONS USED IN TABLES IN PART III
EAR = Estimated Average Requirement. An EAR is the average daily nutrient
intake level estimated to meet the requirements of half of the healthy individu-
als in a group.
RDA = Recommended Dietary Allowance. An RDA is the average daily dietary
intake level sufficient to meet the nutrient requirements of nearly all (97\u201398
percent) healthy individuals in a group.
AI = Adequate Intake. If sufficient scientific evidence is not available to estab-
lish an EAR, and thus calculate an 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.
UL = Tolerable Upper Intake Level. The UL is the highest level of daily nutrient
intake that is likely to pose no risk of adverse health effects to almost all indi-
viduals in the general population. Unless otherwise specified, the UL repre-
sents total intake from food, water, and supplements. In the absence of a UL,
extra caution may be warranted in consuming levels above the recommended
intake. Members of the general population should be advised not to routinely
exceed the UL. The UL is not meant to apply to individuals who are treated
with the nutrient under medical supervision or to individuals with predispos-
ing conditions that modify their sensitivity to the nutrient.
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
TABLE 1 Dietary Reference Intakes for Vitamin A by
Life Stage Group
DRI values (mg RAEa/day)
EARb RDAc AId ULe,f
males females males females
Life stage group
0 through 6 mo 400 600
7 through 12 mo 500 600
1 through 3 y 210 210 300 300 600
4 through 8 y 275 275 400 400 900
9 through 13 y 445 420 600 600 1,700
14 through 18 y 630 485 900 700 2,800
19 through 30 y 625 500 900 700 3,000
31 through 50 y 625 500 900 700 3,000
51 through 70 y 625 500 900 700 3,000
> 70 y 625 500 900 700 3,000
Pregnancy
£ 18 y 530 750 2,800
19 through 50 y 550 770 3,000
Lactation
£ 18 y 885 1,200 2,800
19 through 50 y 900 1,300 3,000
a RAE = Retinol activity equivalent. 1 mg RAE = 1 mg retinol, 12 mg b-carotene, and
24 mg a-carotene or b-cryptoxanthin. The RAE for dietary provitamin A carotenoids in
foods is twofold greater than retinol equivalents (RE), whereas the RAE for preformed
vitamin A in foods is the same as RE.
b EAR = Estimated Average Requirement.
c RDA = Recommended Dietary Allowance.
d AI = Adequate Intake.
e UL = Tolerable Upper Intake Level.
f The UL for vitamin A applies only to preformed vitamin A (e.g., retinol, the form of
vitamin A found in animal foods, most fortified foods, and supplements). It does not
apply to vitamin A derived from carotenoids.
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 III: VITAMIN A 171
VITAMIN A
Vitamin A is a fat-soluble nutrient that is important for vision, gene ex-pression, reproduction, embryonic development, growth, and immunefunction. Forms of vitamin A include retinol (preformed vitamin A),
retinal, retinoic acid, and retinyl esters. The term vitamin A also includes pro-
vitamin A carotenoids that are dietary precursors of retinol. The term retinoids
refers to retinol and its metabolites, and any synthetic analogues that have a
similar structure.
The requirements for vitamin A are now denoted in retinol activity equiva-
lents (RAEs), such that 1 mg RAE = 1mg all-trans-retinol, 12 mg b-carotene, and
24 mg a-carotene or b-cryptoxanthin. This recognizes that 50 percent less bio-
conversion of carotenoids to vitamin A occurs than was previously thought
when vitamin A was expressed in retinol equivalents (REs). The change means
that twice the amount of provitamin A\u2013rich carotenoids contained in leafy green
vegetables and certain fruits is required to provide a given amount of vitamin A
activity.
The requirements for vitamin A are based on the assurance of adequate
liver stores of vitamin A. The Tolerable Upper Intake Level (UL) is based on
liver abnormalities as the critical endpoint. For women of childbearing age, the
UL is based on teratogenicity as the critical adverse effect. DRI values are listed
by life stage group in Table 1.
Preformed vitamin A (retinol) is naturally found in animal-based foods,
whereas dietary carotenoids (provitamin A carotenoids), which are converted
to vitamin A in the body, are present in oils, fruits, and vegetables. Common
dietary sources of preformed vitamin A in the United States and Canada
include liver, dairy products, and fish. Foods fortified with vitamin A are
margarine and low-fat and nonfat (skim and partly skimmed) milk. Provita-
min A carotenoids are found in carrots, broccoli, squash, peas, spinach, and
cantaloupe.
The most specific clinical effect of vitamin A deficiency is xerophthalmia
and its various stages, including night blindness, conjunctival xerosis, Bitot\u2019s
spots, corneal xerosis, corneal ulceration, and scarring. Preformed vitamin A
toxicity (hypervitaminosis A) due to high vitamin A intakes may be acute or
chronic.
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
172 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
VITAMIN A AND THE BODY
Function
Vitamin A is a fat-soluble vitamin that is important for normal vision, gene
expression, reproduction, embryonic development, growth, and immune func-
tion. Forms of vitamin A include retinol (preformed vitamin A), retinal, retinoic
acid, and retinyl esters. Some examples of vitamin A functions include retinal,
which is required by the eye to transduce light into the neural signals necessary
for vision; retinoic acid, which is required to maintain normal differentiation of
the cornea and conjunctival membranes, thus preventing xerophthalmia; and
retinioic acid, which is required to regulate the expression of various genes that
encode for structural proteins (e.g., skin keratins), enzymes (e.g., alcohol dehy-
drogenase), extracellular matrix proteins (e.g., laminin), and retinol binding
proteins and receptors.
The term vitamin A also includes provitamin A carotenoids that are the
dietary precursors of retinol. The term retinoids refers to retinol and its me-
tabolites, and any synthetic analogues that have a similar structure to retinol.
Of the more than 600 forms of carotenoids found in nature, several have provi-
tamin A nutritional activity, but food composition data are available for only
three (a-carotene, b-carotene, and b-cryptoxanthin). The proposed functions of
provitamin A carotenoids are described in Part III, \u201cCarotenoids.\u201d
Absorption, Metabolism, Storage, and Excretion
Preformed vitamin A (retinol) is absorbed in the small intestine. The efficiency
of absorption of preformed vitamin A is generally high, ranging from 70 to