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Vitamins
and
Minerals Home
Vitamins,
Minerals
and the RDA
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Minerals
and the RDA
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Minerals
are inorganic substances derived from soil and water and incorporated
into our diet. Those that are required in large amounts in the diet are
called macro-minerals and include calcium, magnesium, and phosphorus.
The Recommended Dietary Allowances for calcium and phosphorus in adults
are between 800 and 1200 mg per day. Magnesium requirements range between
270 and 400 mg per day in adults. Magnesium and phosphorus are found in
all plant and animal foods since they are both major intracellular ions.
Deficiency is rare except in malnourished alcoholics. Calcium supplementation
is required in many women who fail to ingest adequate calcium in their
diet.
There
are also a number of trace minerals which serve critical roles in body
metabolism but are required in much smaller amounts. These trace minerals
and their roles include:
Iron
Iron is needed for red cell formation and is also a pro-oxidant used
in catalase and other peroxidase-type enzymes. The use of oral iron cannot
lead to overload but parenteral iron can be toxic. Iron is commonly deficient
in menstruating females, and should be replaced when anemia is identified,
or as a preventive in individuals with low range hematocrits. The Recommended
Dietary Allowance for iron is between 10 and 15 mg per day.
Selenium
This is a structural component of enzymes including glutathione peroxidase
and type I iodothyronine monodeiodinase. The Recommended Dietary Allowance
is between 40 and 70 micrograms per day in adults.
Zinc
Zinc is an essential component of many different enzymes and is necessary
for a variety of metabolic processes. Zinc deficiency was first observed
in adolescent boys in Egypt who were eating unleavened whole grain bread
containing phytate which bound to zinc in the intestinal tract preventing
its absorption. The boys demonstrated growth retardation and delayed puberty.
Zinc supplements have been marketed as a cure for impotence without any
substantiation of that claim. The daily requirements are comparable to
those for iron and range between 12 and 15 mg per day in adults. The major
sites of potential losses are in semen and from the gastrointestinal tract
with severe diarrhea or fistula. Zinc should not be administered in the
absence of any copper and should not be consumed as a separate supplement
except in individuals with a demonstrated zinc deficiency.
Copper
The elevation of serum copper seen in Wilson's Disease is due to
a deficiency of ceruloplasmin, the copper-binding protein. This leads
to a syndrome of hepatolenticular degeneration. As mentioned above a copper
deficiency can be induced with megadoses of zinc. There is no Recommended
Dietary Allowance for copper but an Estimated Safe and Adequate Dietary
Daily Intake of 3 mg per day in adults was issued by the National Research
Council of the National Academy of Sciences in 1980.
Iodine
The only major role of iodine is to serve as an essential part of
the basic structure of thyroid hormones. Iodine deficiency in areas away
from sea water (e.g. highlands around Mexico City, inland areas of Southeast
Asia) can lead to goiter formation. Since the late 1950's iodine has been
used as a bleaching agent for white flour and iodine deficiency in this
country is rare. It is not unusual for individuals immigrating to the
U.S. to develop thyroid disease due to the increase in dietary iodine
compared to their country of origin. Iodine supplementation in areas that
are normally iodine deficient has led to an increase in various types
of thyroid diseases including Grave's Disease, Hashimoto's Thyroiditis,
and thyroid nodules. It is possible for susceptible individuals to develop
thyroid nodules after taking kelp tablets which are rich in iodine.
Chromium
Chromium is also called glucose tolerance factor, but its effectiveness
in enhancing glucose tolerance is unproven. A recent study demonstrated
enhanced lifespan in small number of rats given chromium picolinate. The
increased interest in this trace mineral based on this small study is
probably not justified. A natural source of chromium is brewer's yeast.
There is no evidence that it enhances muscle building or fat loss in humans.
There is no Recommended Dietary Allowance for humans.
Many
other trace minerals including arsenic, cobalt, manganese, molybdenum,
nickel, silicon, tin, and vanadium have a variety of metabolic functions.
Some of these are not yet well understood. Fortunately, most foods in
a varied diet will provide adequate amounts of these trace minerals without
the necessity for supplementation. In fact, supplementation in the absence
of additional knowledge may be dangerous. For example, in trace amounts
arsenic is a required trace mineral, but it is a well-known poison at
higher doses.
Nutritional
Adequacy
Despite
the epidemic of overnutrition in the United States today, there are still
some groups at risk for nutritional deficiencies. These groups include:
1) pregnant women 2) the elderly, 3) individuals who smoke, drink excess
amounts of alcohol, or abuse drugs. The laboratory assessment of possible
deficiencies in these groups is summarized in Table 2.
Nutritional
Optimization - Future Directions
While classical
vitamin deficiency diseases are rare today in the U.S. except in certain
high risk groups (e.g. alcoholics, pregnant teenagers, institutionalized
elderly), there are a variety of individuals whose dietary intake is inadequate
to maintain optimal health. For instance, it is recommended that Americans
eat 25 grams of fiber per day, but the average intake is only about 10
grams. In California, only about one in five people consumes five servings
a day of fruits and vegetables as recommended by the US Department of
Agriculture. As a result there are a number of micronutrient vitamins
and minerals which are deficient but not at levels that would cause disease.
Examples include: carotenoids, vitamin E, vitamin C, folate, and selenium.
While it is established what nutrient levels constitute deficiency, there
is little information on what is suboptimal or what types of responses
can be expected following nutritional intervention. It is also unclear
why there are individual variations in the absorption of a beta-carotene
oral load, the effects of dietary fiber eaten at the same time, or the
effects of various fats in the diet on absorption.
Many
of the micronutrients contribute to antioxidant effects. For the measurement
of the host response to oxidant stress, there are a number of markers
including: the measurement of lipid oxidation products such as malondialdehyde
or thiobarbituric acid reactive substances (TBARS) in blood or urine;
modified DNA bases and/or DNA adducts in peripheral blood cells or urine;
vitamin E or vitamin C levels in blood fractions; catalase or superoxide
dismutase levels in blood fractions; lipid peroxides in blood; volatile
gases such as ethane and pentane in expired breath; total peroxyl radical
trapping antioxidant power of serum (TRAP assay); autoxidative (non-cyclooxygenase-derived)
eicosanoids in plasma; and the in vitro oxidation of blood fractions such
as LDL. The usefulness of such measurements in assessing and managing
micronutrient deficiencies is not yet clear. Elevated oxidative stress
is found in a number of disease states. For example, hospitalized patients
with myocardial infarction have higher oxidative stress than hospitalized
controls. Hydrogen peroxide is increased in expired breath from patients
with adult respiratory distress syndrome. Micronutrient nutrition also
has implications for the prevention and treatment of cardiovascular diseases
where oxidized forms of LDL are implicated in the cellular basis for atherosclerosis.
It is likely that the amounts of antioxidants that can be derived from
the diet and/or supplements will be potent in reducing the formation of
oxidized species of LDL. However, it remains unclear what the ultimate
impact of increased micronutrient intake will be on the overall health
and longevity of the U.S. population.
REFERENCES
1. Moore, T. Vitamin
A. Elsevier, Amsterdam, 1957.
2. American Institute of Nutrition. Nomenclature policy: generic descriptors
and trivial names for vitamins and related compounds. J. Nutr. 117:7-14,
1987.
3. Petkovich, M., Brand, N.J., Krust A., Chambon P. A human retinoic acid
receptor which belongs to the family of nuclear receptors. Nature 330:
444-450, 1987.
4. Frolik C.A. Metabolism of retinoids: In The Retinoids (M.B.
Sporn, A.B. Roberts,and D.S. Goodman eds.) pp 177-208, Academic Press,
Orlando Fla., 1984.
5. Fong S-L, Liou GI, Landers RA, et.al. The characterization, localization,
and biosynthesis of an interstitial retinol-binding protein in the human
eye. J. Neurochem. 42:1667-1676, 1984.
6. Olson JA Recommended dietary intakes of vitamin A in humans.Am J Clin
Nutr. 45: 704-716, 1987.
7. World Health Organization. Control of Vitamin A deficiency and Xerophthalmia.
Technical Report Series 672. WHO, Geneva, 1982.
8. Olson JA Vitamin A: In Present Knowledge in Nutrition 6th Edition (1990)
Internat. Life Sciences Institute Nutrition Foundation, Washington,D.C.
9. Wuest, HM The history of thiamine. Ann NY Acad Sci 378:576-601, 1982.
10. Haas, RH Thiamin and the brain. Ann. Rev. Nutr. 8:483-515, 1988.
11. Merrill, A.H., Lambeth J.D., Edmonson, D.E.,and McCormick D.B..Formation
and mode of action of flavoproteins. Ann. Rev. Nutr.1:281-317, 1981.
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