Livro DRI 2006 (Micronutrientes)

Livro DRI 2006 (Micronutrientes)


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trans fatty acid intake and LDL
cholesterol concentration and therefore an increased risk of coronary heart dis-
eases. It is recommended that individuals maintain their trans fatty acid con-
sumption as low as possible without compromising the nutritional adequacy of
their diet. Foods that contain trans fatty acids include traditional stick marga-
rine and vegetable shortenings subjected to partial hydrogenation and various
bakery products and fried foods prepared using partially hydrogenated oils.
Milk, butter, and meats also contain trans fatty acids but at lower levels.
A lack of either of the two essential fatty acids (EFAs), linoleic or
a-linolenic acid, will result in symptoms of deficiency that include scaly skin,
dermatitis, and reduced growth. Such deficiency is very rare in healthy popula-
tions in the United States and Canada. Certain types of fatty acids, such as trans
and saturated, have been shown to heighten the risk of heart disease in some
people by boosting the level of LDL cholesterol in the bloodstream. DRI values
are listed by life stage group in Table 1.
FAT, FATTY ACIDS, AND THE BODY
Background Information
Dietary fat consists mainly of triacylglycerol (98 percent) and small amounts of
phospholipids and sterols. Triacylglycerols are made up of one glycerol mol-
ecule esterified with three fatty acid molecules. In this publication, total fat
refers all to forms of triacylglycerol, regardless of fatty acid composition.
Fatty acids are hydrocarbon chains that contain a methyl (CH
3
\u2014) and a
carboxyl (\u2014COOH) end. Table 2 shows the major fatty acids found in the diet.
Fatty acids vary in their carbon chain length and degree of unsaturation (the
number of double bonds in the carbon chain) and can be classified as follows:
\u2022 saturated fatty acids
\u2022 cis monounsaturated fatty acids
\u2022 cis polyunsaturated fatty acids
\u2014 n-6 fatty acids
\u2014 n-3 fatty acids
\u2022 trans fatty acids
A very small amount of dietary fat occurs as phospholipids, a form of fat that
contains one glycerol molecule that is esterified with two fatty acids and either
inositol, choline, serine, or ethanolamine. In the body, phospholipids are mainly
located in the cell membranes and the globule membranes of milk.
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126 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
TABLE 2 Major Dietary Fatty Acids
Category of Fatty Acid Specific Fatty Acids Found in the Diet
Saturated fatty acids \u2022 caprylic acid, 8:0a
\u2022 caproic acid, 10:0
\u2022 lauric acid, 12:0
\u2022 myristic acid, 14:0
\u2022 palmitic acid, 16:0
\u2022 stearic acid, 18:0
Cis monounsaturated fatty acids \u2022 myristoleic acid, 14:1 n-7
\u2022 palmitoleic acid, 16:1 n-7
\u2022 oleic acid, 18:1 n-9 (account for 92% of
monounsaturated dietary fatty acids)
\u2022 cis-vaccenic acid, 18:1 n-7
\u2022 eicosenoic acid, 20:1 n-9
\u2022 erucic acid, 22:1 n-9
Cis polyunsaturated fatty acids
n-6 polyunsaturated fatty acid \u2022 linoleic acid,b 18:2
\u2022 g-linoleic acid, 18:3
\u2022 dihomo-g-linolenic acid, 20:3
\u2022 arachidonic acid, 20:4
\u2022 adrenic acid, 22:4
\u2022 docosapentaenoic acid, 22:5
n-3 polyunsaturated fatty acid \u2022 a-linolenic acid,b 18:3
\u2022 eicosapentaenoic acid, 20:5
\u2022 docosapentaenoic acid, 22:5
\u2022 docosahexaenoic acid, 22:6
Trans fatty acid \u2022 9-trans, 18:1; 9-trans, 16:1; 9-cis,11-trans,
18:2; 9-trans,12-cis, 18:2; 9-cis,12-trans, 18:2
a The first value refers to chain length or number of carbon atoms and the second value
refers to the number of double bonds.
b Linoleic acid and a-linolenic acid cannot be synthesized in the body and are therefore
essential in the diet.
Function
A major source of energy for the body, fat aids in the absorption of fat-soluble
vitamins A, D, E, K, and other food components, such as carotenoids. Fatty
acids function in cell signaling and alter the expression of specific genes in-
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
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PART II: DIETARY FAT 127
volved in lipid and carbohydrate metabolism. Fatty acids, the major constitu-
ents of triglycerides, may also serve as precursors or ligands for receptors that
are important regulators of adipogenesis, inflammation, insulin action, and neu-
rological function. Table 3 summarizes the functions of fat and fatty acids.
Absorption, Metabolism, Storage, and Excretion
TOTAL FAT
In the intestine, dietary fat is emulsified with bile salts and phospholipids (se-
creted into the intestine by the liver), hydrolyzed by pancreatic enzymes, and
TABLE 3 The Functions of Fat and Fatty Acids
Fat and Fatty Acids Function
Total fata \u2022 Major source of energy
\u2022 Aids in absorption of the fat-soluble vitamins
and carotenoids
Saturated fatty acids \u2022 Sources of energy
\u2022 Structural components of cell membranes
\u2022 Enable normal function of proteins
Cis monounsaturated fatty acids \u2022 Key components of membrane structural lipids,
particularly nervous tissue myelin
Cis polyunsaturated fatty acids
n-6 polyunsaturated fatty acids \u2022 Substrates for eicosanoid production, including
prostaglandins
\u2022 Precursors of arachidonic acid
\u2022 Components of membrane structural lipids
\u2022 Important in cell signaling pathways
\u2022 Vital for normal epithelial cell function
\u2022 Involved in the regulation of genes for proteins
that regulate fatty acid synthesis
n-3 polyunsaturated fatty acids \u2022 Precursors for synthesis of eicosapentaenoic
acid (EPA) and docosahexaenoic acid (DHA).
EPA is the precursor for n-3 eicosanoids
Phospholipids \u2022 Major constituents of cell membranes
a Total fat refers to all forms of triacylglycerol, regardless of fatty acid composition.
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Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
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128 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
almost completely absorbed. Following absorption, the fats are reassembled
together with cholesterol, phospholipids, and apoproteins into chylomicrons,
which enter the circulation through the thoracic duct. Chylomicrons come into
contact with the enzyme lipoprotein lipase (LPL) (located on the surface of
capillaries of muscle and adipose tissue) and LPL hydrolyzes the chylomicron
triacylglycerol fatty acids. Most of the fatty acids released in this process are
taken up by adipose tissue and re-esterfied into triacylglycerol for storage.
When fat is needed for fuel, free fatty acids from the liver and muscle are
released into the circulation to be taken up by various tissues, where they are
oxidized to provide energy. Muscle, which is the main site of fatty acid oxida-
tion, uses both fatty acids and glucose for energy. Fatty acids released from fat
tissue can also be oxidized by the liver.
As fatty acids are broken down through oxidation, carbon dioxide and
water are released. Small amounts of ketone bodies are also produced and ex-
creted in the urine. The cells of the skin and intestine also contain fatty acids.
Thus, small quantities are lost when these cells are sloughed.
SATURATED FATTY ACIDS
When absorbed along with fats containing appreciable amounts of unsaturated
fatty acids, saturated fatty acids are absorbed almost completely by the small
intestine. In general, the longer the chain length of the fatty acid, the lower the
efficiency of absorption. Following absorption, long-chain saturated fatty acids
are re-esterified along with other fatty acids into triacylglycerols and released in
chylomicrons. Medium-chain saturated fatty acids are absorbed, bound to al-
bumin, transported as free fatty acids in the portal circulation, and