Friday, 6 May 2011

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 (α-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 of alpha-tocopherol 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 the alpha-tocopherol form of vitamin E.

Table 1: Selected Food Sources of Vitamin E [4]
FOOD Milligrams (mg)
Alpha-tocopherol
per serving Percent
DV*
Wheat germ oil, 1 tablespoon 20.3 100
Almonds, dry roasted, 1 ounce 7.4 40
Sunflower seed kernels, dry roasted, 1 ounce 6.0 30
Sunflower oil, over 60% linoleic, 1 tablespoon 5.6 30
Safflower oil, over 70% oleic, 1 tablespoon 4.6 25
Hazelnuts, dry roasted, 1 ounce 4.3 20
Peanut butter, smooth style, vitamin and mineral fortified, 2 Tablespoons 4.2 20
Peanuts, dry roasted, 1 oz 2.2 10
Corn oil (salad or vegetable oil), 1 tablespoon 1.9 10
Spinach, frozen, chopped, boiled, ½ cup 1.6 6
Broccoli, frozen, chopped, boiled, ½ cup 1.2 6
Soybean oil, 1 tablespoon 1.3 6
Kiwi, 1 medium fruit without skin 1.1 6
Mango, raw, without refuse, ½ cup sliced 0.9 6
Spinach, raw, 1 cup 0.6 4
*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 about 20 mg alpha-tocopherol). 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/foodcomp/search/.
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 based only on the alpha-tocopherol form of vitamin E [5]. Table 2 also lists RDAs for vitamin E in International Units (IU) because food and most supplement labels list vitamin E content in International Units (1 mg alpha-tocopherol vitamin E = 1.49 IU).

Table 2: Recommended Dietary Allowances for Vitamin E for Children and Adults [5]
Age
(years) Children
(mg/day) Men
(mg/day) Women
(mg/day) Pregnancy
(mg/day) Lactation
(mg/day)
1-3 6 mg
(=9 IU)
4-8 7 mg
(=10.5 IU)
9-13 11 mg
(=16.5 IU) 11 mg
(=16.5 IU) 15 mg
(=22.5 IU) 19 mg
(=28.5 IU)
14 + 15 mg
(=22.5 IU) 15 mg
(=22.5 IU) 15 mg
(=22.5 IU) 19 mg
(=28.5 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 alpha-tocopherol and IUs (1 mg alpha-tocopherol = 1.49 IU) [5].

Table 3: Adequate Intake for Vitamin E for Infants [5]
Age
(months) Males and Females
(mg/day)
0 to 6 4 mg
(=6 IU)
7 to 12 5 mg
(=7.5 IU)

Results of two national surveys, the National Health and Nutrition Examination Results of two national surveys, the National Health and Nutrition Examination Survey (NHANES III 1988-94) [6] and the Continuing Survey of Food Intakes by Individuals (1994-96 CSFII) [7] 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 α-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.
1. 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 [8];
2. individuals with rare genetic abnormalities in the alpha-tocopherol transfer protein are at risk of vitamin E deficiency [9]; and
3. premature, very low birth weight infants (birth weights less than 1500 grams, or 3 pounds, 4 ounces) are at risk of vitamin E deficiency [3,10].
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 affects 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 [8]. 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) [9].

Very low birth weight infants may be deficient in vitamin E [3,10]. 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].

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.
What is the health risk of too much vitamin E?
Most studies of the safety of vitamin E supplementation have lasted for several months or less, so there is little evidence for the long-term safety of vitamin E supplementation.

The Food and Nutrition Board of the Institute of Medicine has set an upper tolerable intake level (UL) for vitamin E at 1,000 mg (1,500 IU) for any form of supplementary alpha-tocopherol per day. Based for the most part on the result of animal studies, the Board decided that because vitamin E can act as an anticoagulant and may increase the risk of bleeding problems this UL is the highest dose unlikely to result in bleeding problems.

Table 4 lists the Tolerable Upper Intake Levels (UL) of vitamin E in mg alpha-tocopherol and IUs for children and adults (1 mg alpha-tocopherol vitamin E = 1.49 IU). A UL for vitamin E for infants up to 12 months of age has not been established.

Table 4: Tolerable Upper Intake Levels (UL) of vitamin E for Children and Adults [5]
Age (years) Males
(mg/day) Females
(mg/day) Pregnancy
(mg/day) Lactation
(mg/day)
1-3 200
(=300 IU) 200
(=300 IU) N/A N/A
4-8 300
(=450 IU) 300
(=450 IU) N/A N/A
9-13 600
(=900 IU) 600
(=900 IU) N/A N/A
14-18 800
(=1,200 IU) 800
(=1,200 IU) 800
(=1,200 IU) 800
(=1,200 IU)
19-70 1,000
(=1,500 IU) 1,000
(=1,500 IU) 1,000
(=1,500 IU) 1,000
(=1,500 IU)
> 70 1,000
(=1,500 IU) 1,000
(=1,500 IU) N/A
(=1,500 IU) N/A
(=1,500 IU)

Vitamin E intakes and healthful diets
Many people are concerned about their fat intake today. Your overall diet should be moderate in fat, but it is important to include some healthful sources of fat, including those oils and nuts that provide vitamin E. Including these foods in your diet will help you meet your daily need for vitamin E. Meats, grain products, dairy products, and most fruits and vegetables are generally not good sources of vitamin E. According to the 2005 Dietary Guidelines for Americans, "Nutrient needs should be met primarily through consuming foods. Foods provide an array of nutrients and other compounds that may have beneficial effects on health. In certain cases, fortified foods and dietary supplements may be useful sources of one or more nutrients that otherwise might be consumed in less than recommended amounts. However, dietary supplements, while recommended in some cases, cannot replace a healthful diet."

The Dietary Guidelines for Americans describes a healthy diet as one that:
• emphasizes a variety of fruits, vegetables, whole grains, and fat-free or low-fat milk and milk products;
• includes lean meats, poultry, fish, beans, eggs, and nuts;
• is low in saturated fats, trans fats, cholesterol, salt (sodium), and added sugars; and
• stays within your daily calorie needs.

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