Vitamin E: What is it?
Vitamin E is a fat-soluble vitamin that
exists in eight different forms. Each form has its own biological
activity, the measure of potency or functional use in the body (1). Alpha-tocopherol
is the most active form of vitamin E in humans, and is a powerful
biological antioxidant (2,3). Antioxidants such as vitamin E act to
protect your cells against the effects of free radicals, which are
potentially damaging by-products of the body’s metabolism. Free radicals
can cause cell damage that may contribute to the development of
cardiovascular disease and cancer. Studies are underway to determine
whether vitamin E might help prevent or delay the development of those
chronic diseases (2,3).
What foods provide vitamin E?
Vegetable oils, nuts, and green leafy vegetables are the main dietary
sources of vitamin E. Fortified cereals are also an
important source of vitamin E in the United States. The table of selected
food sources of vitamin E suggests foods that contain vitamin E (4).
When can vitamin E deficiency occur?
Vitamin E deficiency is rare in humans. There are three specific
situations when a vitamin E deficiency is likely to occur. It is seen in
persons who cannot absorb dietary fat, has been found in premature, very
low birth weight infants (birth weights less than 1500 grams, or
3 1/2 pounds) (3,6), and is seen in individuals with rare disorders
of fat metabolism (9). A vitamin E deficiency is usually characterized by
neurological problems due to poor nerve
conduction.
Who may need extra vitamin E to
prevent a deficiency?
Individuals who cannot absorb fat may require a vitamin E
supplement because some dietary fat is needed for the absorption of
vitamin E from the gastrointestinal tract. Anyone diagnosed with cystic
fibrosis, individuals who have had part or all of their stomach removed,
and individuals with malabsorptive problems such as Crohn’s disease may
not absorb fat and should discuss the need for supplemental vitamin E with
their physician (3). People who cannot absorb fat often pass greasy stools
or have chronic diarrhea.
Very low birth weight infants may be
deficient in vitamin E (3, 6). These infants are usually under the care of
a neonatologist, a pediatrician specializing in the care of newborns, who
evaluates and treats the exact nutritional needs of premature infants.
Abetalipoproteinemia is a rare
inherited disorder of fat metabolism that results in poor absorption of
dietary fat and vitamin E (9). The vitamin E deficiency associated with
this disease causes problems such as poor transmission of nerve impulses,
muscle weakness, and degeneration of the retina that can cause blindness
(10). Individuals with abetalipoproteinemia may be prescribed special
vitamin E supplements by a physician to treat this disorder.
What are some current issues and
controversies about vitamin E?
Vitamin E and heart disease
Preliminary research has led to a widely held belief that vitamin
E may help prevent or delay coronary heart disease (11). Researchers are
fairly certain that oxidative modification of LDL-cholesterol (sometimes
called "bad" cholesterol) promotes blockages in coronary
arteries that may lead to atherosclerosis and heart attacks. Vitamin E may
help prevent or delay coronary heart disease by limiting the oxidation of
LDL-cholesterol (12). Vitamin E also may help prevent the formation of
blood clots, which could lead to a heart attack. Observational studies
have associated lower rates of heart disease with higher vitamin E intake.
A study of approximately 90,000 nurses suggested that the incidence of
heart disease was 30% to 40% lower among nurses with the highest intake of
vitamin E from diet and supplements. The range of intakes from both diet
and supplements in this group was 21.6 to 1,000 IU (32 to 1,500 mg), with
the median intake being 208 IU (139 mg) (13). A 1994 review of 5,133
Finnish men and women aged 30 - 69 years suggested that increased dietary
intake of vitamin E was associated with decreased mortality (death) from
heart disease (14). But even though these observations are promising,
randomized clinical trials raise questions about the role of vitamin E
supplements in heart disease. The Heart Outcomes Prevention Evaluation
(HOPE) Study followed almost 10,000 patients for 4.5 years who were at
high risk for heart attack or stroke (15). In this intervention study the
subjects who received 265 mg (400) IU of vitamin E daily did not
experience significantly fewer cardiovascular events or hospitalizations
for heart failure or chest pain when compared to those who received a
sugar pill. The researchers suggested that it is unlikely that the vitamin
E supplement provided any protection against cardiovascular disease in the
HOPE study. This study is continuing, to determine whether a longer
duration of intervention with vitamin E supplements will provide any
protection against cardiovascular disease (5).
Vitamin E and cancer
Antioxidants such as vitamin E help protect against the damaging
effects of free radicals, which may contribute to the development of
chronic diseases such as cancer (5). Vitamin E also may block the
formation of nitrosamines, which are carcinogens formed
in the stomach from nitrites consumed in the diet. It also may protect
against the development of cancers by enhancing immune function (16).
Unfortunately, human trials and surveys that tried to associate vitamin E
with incidence of cancer have been generally inconclusive.
Some evidence associates higher intake of
vitamin E with a decreased incidence of prostate cancer and breast cancer
(17). However, an examination of the effect of dietary factors, including
vitamin E, on incidence of postmenopausal breast cancer in over 18,000
women from New York State did not associate a greater vitamin E intake
with a reduced risk of developing breast cancer (18).
A study of women in Iowa provided
evidence that an increased dietary intake of vitamin E may decrease the
risk of colon cancer, especially in women under 65 years of age (19). On
the other hand, vitamin E intake was not statistically associated with
risk of colon cancer in almost 2,000 adults with cancer who were compared
to controls without cancer (20). At this time there is limited evidence to
recommend vitamin E supplements for the prevention of cancer.
Vitamin E and cataracts
Cataracts are growths on the lens of the eye that cloud vision.
They increase the risk of disability and blindness in aging adults.
Antioxidants are being studied to determine whether they can help prevent
or delay cataract growth. Observational studies have found that lens
clarity, which is used to diagnose cataracts, was better in regular users
of vitamin E supplements and in persons with higher blood levels of
vitamin E (21). A study of middle aged male smokers, however, did not
demonstrate any effect from vitamin E supplements on the incidence of
cataract formation (22). The effects of smoking, a major risk factor for
developing cataracts, may have overridden any potential benefit from the
vitamin E, but the conflicting results also indicate a need for further
studies before researchers can confidently recommend extra vitamin E for
the prevention of cataracts.
What is the health risk of too much
vitamin E?
The health risk of too much vitamin E is low (23). A recent review of
the safety of vitamin E in the elderly indicated that taking vitamin E
supplements for up to four months at doses of 530 mg or 800 IU (35 times
the current RDA) had no significant effect on general health, body weight,
levels of body proteins, lipid levels, liver or kidney function, thyroid
hormones, amount or kinds of blood cells, and bleeding time (24). Even
though this study provides evidence that taking a vitamin E supplement
containing 530 mg or 800 IU for four months is safe, the long term safety
of vitamin E supplementation has not been tested.
The Institute of Medicine has set an upper tolerable intake level for
vitamin E at 1,000 mg (1,500 IU) for any form of supplementary alpha-tocopherol
per day because the nutrient can act as an anticoagulant and increase the
risk of bleeding problems. Upper tolerable intake levels "represent
the maximum intake of a nutrient that is likely to pose no risk of adverse
health effects in almost all individuals in the general population"
(5).
Table of Selected Food Sources of
vitamin E (4)
As the 2000 Dietary Guidelines for Americans state, "Different
foods contain different nutrients. No single food can supply all the
nutrients in the amounts you need" (25). The following table lists
selected sources of vitamin E. As the tables indicate, vegetables oils,
nuts, and green leafy vegetables are good dietary sources of vitamin E.
Including these foods in your diet will help you meet your daily need for
vitamin E, but it is still important to moderate total fat intake as
recommended by the Dietary Guidelines for Americans.
Food manufacturers fortify many foods
with vitamins and minerals. It is important to read the nutrition facts
panel of the food label to determine whether a food provides vitamin E. If
you want more information about building a healthful diet, refer to the
Dietary
Guidelines for Americans and
the Food
Guide Pyramid.
Table of Food Sources
Therapeutic
Dosage
