Did You Know Just That Many Cancers Are Linked To A Vitamin Deficiency?
Vitamin E
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,
which is the measure of potency or functional use in the
body [1]. Alpha-tocopherol (a-tocopherol) is the name of
the most active form of vitamin E in humans. It is also
a powerful biological antioxidant [2-3]. Vitamin E in supplements
is usually sold as alpha-tocopheryl acetate, a form that
protects its ability to function as an antioxidant. The
synthetic form is labeled "D, L" while the natural
form is labeled "D". The synthetic form is only
half as active as the natural form [4].
Antioxidants
such as vitamin E act to protect your cells against the
effects of free radicals, which are potentially damaging
by-products of energy metabolism. Free radicals can damage
cells and may contribute to the development of cardiovascular
disease and cancer. Studies are underway to determine whether
vitamin E, through its ability to limit production of free
radicals, might help prevent or delay the development of
those chronic diseases. Vitamin E has also been shown to
play a role in immune function, in DNA repair, and other
metabolic processes [2-3].
What foods provide vitamin E?
Vegetable oils, nuts, green leafy vegetables, and fortified
cereals are common food sources of vitamin E in the United
States (U.S.). Table 1, Selected Food Sources of Vitamin
E, suggests many food sources of vitamin E [4]. Food values
are listed in alpha-tocopherol equivalents (ATE) to account
for the variation in biological activity of the different
forms of vitamin E.
DV
= Daily Value. DVs are reference numbers developed by the
Food and Drug Administration (FDA) to help consumers determine
if a food contains a lot or a little of a specific nutrient.
The DV for vitamin E is 30 International Units (or 20 mg
ATE). Most food labels do not list a food's vitamin E content.
The percent DV (%DV) listed on the table indicates the percentage
of the DV provided in one serving. A food providing 5% of
the DV or less is a low source while a food that provides
10-19% of the DV is a good source. A food that provides
20% or more of the DV is high in that nutrient. It is important
to remember that foods that provide lower percentages of
the DV also contribute to a healthful diet. For foods not
listed in this table, please refer to the U.S. Department
of Agriculture's Nutrient Database Web site: http://www.nal.usda.gov/fnic/cgi-bin/nut_search.pl.
What is the recommended intake for vitamin E?
Recommendations for vitamin E are provided in the Dietary
Reference Intakes developed by the Institute of Medicine
[5]. Dietary Reference Intakes (DRIs) is the general term
for a set of reference values used for planning and assessing
nutrient intake for healthy people. Three important types
of reference values included in the DRIs are Recommended
Dietary Allowances (RDA), Adequate Intakes (AI), and Tolerable
Upper Intake Levels (UL). The RDA recommends the average
daily dietary intake level that is sufficient to meet the
nutrient requirements of nearly all (97-98%) healthy individuals
in each age and gender group [5]. An AI is set when there
is insufficient scientific data available to establish a
RDA. AIs meet or exceed the amount needed to maintain a
nutritional state of adequacy in nearly all members of a
specific age and gender group. The UL, on the other hand,
is the maximum daily intake unlikely to result in adverse
health effects [5].
In
Table 2, RDAs for vitamin E are listed as Alpha-Tocopherol
Equivalents (ATE) to account for the different biological
activities of the various forms of vitamin E [5-6]. Table
2 also lists RDAs for vitamin E in International Units (IU)
because food and some supplement labels list vitamin E content
in International Units (1 mg ATE vitamin E = 1.5 IU).
There
is insufficient scientific data on vitamin E to establish
an RDA for infants. An Adequate Intake (AI) has been established
that is based on the amount of vitamin E consumed by healthy
infants who are fed breast milk. Table 2 lists the adequate
intakes for vitamin E for infants in mg ATE and IUs (1 mg
ATE vitamin E = 1.5 IU) [5].
Results
of two national surveys, the National Health and Nutrition
Examination Survey (NHANES III 1988-94) [7] and the Continuing
Survey of Food Intakes by Individuals (1994-96 CSFII) [8]
indicated that diets of most Americans do not provide the
recommended intake for vitamin E. However, an Institute
of Medicine (IOM) report on vitamin E published in 2000
states that intake estimates of vitamin E may be low because
energy and fat intake are often underreported in national
surveys and because the kind and amount of fat added during
cooking is often not known. The IOM states that most North
American adults get enough vitamin E from their normal diets
to meet current recommendations. However, they do caution
that low fat diets can result in a significant decrease
in vitamin E intake. "Low-fat diets can substantially
decrease vitamin E intakes if food choices are not carefully
made to enhance a-tocopherol intakes" [5].
Who is at risk for vitamin E deficiency?
Vitamin E deficiency is rare in humans. There are three
specific situations when a vitamin E deficiency is likely
to occur.
persons
who cannot absorb dietary fat due to an inability to secrete
bile or with rare disorders of fat metabolism are at risk
of vitamin E deficiency [9];
individuals with rare genetic abnormalities in the alpha-tocopherol
transfer protein are at risk of vitamin E deficiency [10];
and
premature, very low birth weight infants (birth weights
less than 1500 grams, or 3 pounds, 4 ounces) are at risk
of vitamin E deficiency [3,6].
Blood levels of vitamin E may also be decreased with zinc
deficiency [11]. Vitamin E deficiency is usually characterized
by neurological problems associated with nerve degeneration
in hands and feet [5]. These symptoms are also associated
with other medical conditions. A physician can determine
if they are the result of a vitamin E deficiency or are
from another cause.
Who may need extra vitamin E to prevent a deficiency?
Individuals who cannot absorb fat require a vitamin E supplement
because some dietary fat is needed for the absorption of
vitamin E from the gastrointestinal tract. Intestinal disorders
that often result in malabsorption of vitamin E and may
require vitamin E supplementation include [3]:
Crohn's
Disease is an inflammatory bowel disease that affects the
small intestines. People with Crohn's disease often experience
diarrhea and nutrient malabsorption.
Cystic Fibrosis is an inherited disease that effects the
lungs, gastrointestinal tract, pancreas, and liver. Cystic
fibrosis can interfere with normal digestion and absorption
of nutrients, especially of fat soluble vitamins including
vitamin E.
People who cannot absorb fat often pass greasy stools or
have chronic diarrhea. People with an inability to secrete
bile, a substance that helps fat digestion, may need a special
water-soluble form of vitamin E.
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. Individuals with abetalipoproteinemia may be
prescribed special vitamin E supplements by a physician
to treat this disorder [12].
Ataxia
and vitamin E deficiency (AVED) is also a rare inherited
disorder. It is caused by a genetic defect in a liver protein
that is responsible for maintaining normal alpha-tocopherol
concentrations in the blood. These individuals have such
severe vitamin E deficiency that without supplements they
are unable to walk (ataxia) [10].
Very
low birth weight infants may be deficient in vitamin E [3,6].
Necrotizing enterocolitits, a condition sometimes seen in
very low birth weight infants that is characterized by inflammation
of the lining of the intestines, may lead to a vitamin E
deficiency [4]. 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.
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
[13]. Researchers have reported that oxidative changes to
LDL-cholesterol (sometimes called "bad" cholesterol)
promote blockages (atherosclerosis) in coronary arteries
that may lead to heart attacks. Vitamin E may help prevent
or delay coronary heart disease by limiting the oxidation
of LDL-cholesterol [14]. 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.
Researchers found that the apparent benefit was mainly associated
with intake of vitamin E from dietary supplements. High
vitamin E intake from food was not associated with significant
cardiac risk reduction [15]. A 1994 review of 5,133 Finnish
men and women aged 30-69 years also suggested that increased
dietary intake of vitamin E was associated with decreased
mortality (death) from heart disease [16].
Even
though these observations are promising, randomized clinical
trials raise questions about the efficacy of vitamin E supplements
in the prevention of 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 [17]. 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 placebo (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, with the goal of determining
whether a longer duration of intervention with vitamin E
supplements will provide any protection against cardiovascular
disease.
In
a study sponsored by the National Heart, Lung, and Blood
Institute (NHLBI) of the National Institutes of Health,
postmenopausal women with heart disease who took supplements
providing 400 IU vitamin E and 500 mg vitamin C twice a
day, either alone or in combination with hormones, did not
have fewer heart attacks or deaths. There was also no change
in progression of their coronary disease. This study, The
Women's Angiographic Vitamin and Estrogen (WAVE) trial,
studied 423 postmenopausal women at seven clinical centers
in the U.S. and Canada. In postmenopausal women with coronary
disease enrolled in this trial, neither hormone replacement
therapy nor antioxidant vitamin supplements provided cardiovascular
benefit [18].
Results
of the Women's Health Study, the Women's Antioxidant and
Cardiovascular Study and the SuVIMAX study, all of which
are investigating the effects of vitamin supplements on
the progression of coronary heart disease, are due in 2005
and will provide additional information on the association
between vitamin E supplements and cardiovascular disease.
Vitamin
E and cancer
Antioxidants such as vitamin E are believed to help protect
cell membranes against the damaging effects of free radicals,
which may contribute to the development of chronic diseases
such as cancer [4]. 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
[19]. Unfortunately, human trials and surveys that have
tried to associate vitamin E intake 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 [20]. 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 [21].
A
study of women in Iowa provides evidence that an increased
dietary intake of vitamin E may decrease the risk of colon
cancer, especially in women under 65 years of age [22].
On the other hand, a study of 87,998 females from the Nurses'
Health Study and 47,344 males from the Health Professionals
Follow-up Study failed to support the theory that an increased
dietary intake of vitamin E may decrease the risk of colon
cancer [23].
The
American Cancer society recently released the results of
a long-term study that evaluated the effect of regular use
of vitamin C and vitamin E supplements on bladder cancer
mortality in almost 1,000,000 adults in the U.S. The study,
conducted between the years 1982 to 1998, found that subjects
who regularly consumed a vitamin E supplement for longer
than 10 years had a reduced risk of death from bladder cancer.
No benefit was seen from vitamin C supplements [24].
At
this time researchers cannot confidently recommend vitamin
E supplements for the prevention of cancer because the evidence
on this issue is inconsistent and limited.
Vitamin
E and cataracts
Cataracts are abnormal growths in the lens of the eye. These
growths cloud vision. They also 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 [25]. A study of middle-aged male
smokers, however, did not demonstrate any effect from vitamin
E supplements on the incidence of cataract formation [26].
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.
http://dietary-supplements.info.nih.gov/factsheets/vitamine.asp
