Livro DRI 2006 (Micronutrientes)

Livro DRI 2006 (Micronutrientes)


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the desaturation of
oleic acid is reduced, and synthesis of eicosatrienoic acid from oleic acid in-
creases. A lack of dietary n-6 polyunsaturated fatty acids is characterized by
rough scaly skin, dermatitis, and an elevated eicosatrienoic acid:arachidonic
acid (triene:tetraene) ratio.
n-3 Polyunsaturated Fatty Acids
A lack of a-linolenic acid in the diet can result in clinical symptoms of a defi-
ciency (e.g., scaly dermatitis). Unlike essential fatty acid deficiency (of both n-6
and n-3 fatty acids), plasma eicosatrienoic acid (20:3 n-9) remains within nor-
mal ranges, and skin atrophy and scaly dermatitis are absent when the diet is
only deficient in n-3 fatty acids.
Because of their function, growing evidence suggests that dietary n-3 poly-
unsaturated fatty acids (EPA and DHA) may reduce the risk of many chronic
diseases including CHD, stroke, and diabetes. For example, n-3 fatty acids may
reduce CHD risk through a variety of mechanisms, such as by preventing
arrhythmias, reducing atherosclerosis, decreasing platelet aggregation and plasma
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
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136 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
triacylglycerol concentration, slightly increasing HDL concentration, modulat-
ing endothelial function, and decreasing proinflammatory eicosanoids.
ADVERSE EFFECTS OF OVERCONSUMPTION
As mentioned earlier, there is no defined level of fat intake at which an adverse
effect, such as obesity, can occur. An AMDR for fat intake, however, has been
estimated based on potential adverse effects occurring from consuming low-fat
and high-fat diets (see Part II, \u201cMacronutrients, Healthful Diets, and Physical
Activity\u201d). High-fat diets in excess of energy needs can cause obesity. Several
studies have shown associations between high-fat intakes and an increased risk
of CHD, cancer, and insulin resistance. However, the type of fatty acid con-
sumed is very important in defining these associations. The potential adverse
effects of overconsuming fatty acids are summarized in Table 5.
Special Considerations
Individuals sensitive to n-3 polyunsaturated fatty acids: People who take hy-
poglycemic medications should consume n-3 fatty acids with caution. Because
n-3 fatty acids may excessively prolong bleeding time, DHA and EPA supple-
ments should be taken with caution by people who take anticoagulants, includ-
ing aspirin and warfarin.
Exercise: High-fat diets may result in a positive energy balance and therefore in
weight gain under sedentary conditions. Active people can probably consume
relatively high-fat diets while maintaining their body weight. Athletes may not
be able to train as effectively on short-term (fewer than 6 days) high-fat diets as
they could on high-carbohydrate diets. It is important to note that physical
activity may account for a greater percentage of the variance in weight gain than
does dietary fat.
Genetic factors: Some data indicate that genes may affect the relationship be-
tween diet and obesity. Some people with relatively high metabolic rates appear
to be able to eat high-fat diets (44 percent of energy from fat) without becoming
obese. Intervention studies have shown that people susceptible to weight gain
and obesity appear to have an impaired ability to oxidize more fat after eating
high-fat meals.
Alcohol: Significant alcohol intake (23 percent of energy) can depress fatty acid
oxidation. If the energy derived from alcohol is not used, the excess is stored as
fat.
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: DIETARY FAT 137
TABLE 5 Potential Adverse Effects of Fatty Acid
Overconsumption
Fatty Acid Potential Adverse Effects of Overconsumption
Saturated fatty acids In general, the higher the saturated fatty acid
intake, the higher the serum total and LDL
cholesterol concentrations. There is a positive
linear relationship between serum total and LDL
cholesterol concentrations and the risk of CHD or
mortality from CHD.
Cis monounsaturated fatty acids Overconsumption of energy related to a high-fat,
high-monounsaturated fatty acid diet is one risk
associated with excess monounsaturated fatty
acid intake. High intakes can also cause an
increased intake of saturated fatty acids, since
many animal fats that contain one have the other.
Cis polyunsaturated fatty acids:
n-6 polyunsaturated fatty acids An AMDR was estimated based on the adverse
effects from consuming a diet too high or low in
n-6 polyunsaturated fatty acids (see Part II,
\u201cMacronutrients, Healthful Diets, and Physical
Activity\u201d).
n-3 polyunsaturated fatty acids Data on the effects of EPA and DHA intakes on
bleeding times are mixed. Until more information
is available, supplemental forms of EPA and DHA
should be taken with caution. An AMDR was
estimated based on the adverse effects from
consuming a diet too high or low in n-3
polyunsaturated fatty acids (see Part II,
\u201cMacronutrients, Healthful Diets, and Physical
Activity\u201d).
Trans fatty acids There is a positive linear trend between trans fatty
acid intake and LDL concentration, and therefore
an increased risk of CHD. Recent data have
shown a dose-dependent relationship between
trans fatty acid intake and the LDL:HDL ratio. The
combined results of numerous studies have
indicated that the magnitude of this effect
is greater for trans fatty acids, compared with
saturated fatty acids.
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
138 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
Interaction of n-6 and n-3 fatty acid metabolism: Many studies, primarily in
animals, have suggested that the balance between linoleic and a-linolenic acids
is important in determining the amounts of arachidonic acid, eicosapentaenoic
acid (EPA), and docosahexaenoic acid (DHA) in tissue lipids. An inappropriate
ratio may involve too high an intake of either linoleic acid or a-linolenic acid,
too little of one fatty acid, or a combination leading to an imbalance between
the two. The ratio between the two is likely to be of most importance in diets
that are low in or devoid of arachidonic acid, EPA, and DHA. The importance of
this ratio is unknown in diets that are high in these three fatty acids.
n-6:n-3 polyunsaturated fatty acid ratio: The ratio of linoleic acid to a-
linolenic acid in the diet is important because the two fatty acids compete for
the same desaturase enzymes. Thus, a high ratio of linoleic acid to a-linolenic
acid can inhibit the conversion of a-linolenic acid to DHA, while a low ratio
will inhibit the desaturation of linoleic acid to arachidonic acid.
Although limited, the available data suggest that linoleic to a-linolenic acid
ratios below 5:1 may be associated with impaired growth in infants. Based on
limited studies, the linoleic to a-linolenic acid or total n-3 to n-6 fatty acid
ratios of 5:1\u201310:1, 5:1\u201315:1, and 6:1\u201316:1 have been recommended for infant
formulas. Based on limited studies, a reasonable linoleic to a-linolenic acid
ratio of 5:1\u201310:1 has been recommended for adults.
KEY POINTS FOR FAT AND FATTY ACIDS
3 A major source of energy for the body, fat aids in tissue
development and the absorption of the fat-soluble vitamins A,
D, E, K, and other food components, such as carotenoids.
3 Dietary fat contains fatty acids that fall into the following
categories: saturated fatty acids, cis monounsaturated fatty
acids, cis polyunsaturated fatty acids (n-6 fatty acids and n-3
fatty acids), trans fatty