Wednesday, December 13, 2006

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.

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. 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. 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.

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. 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.

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. 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. 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. 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.

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. 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.

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.

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. A study of middle-aged male smokers, however, did not demonstrate any effect from vitamin E supplements on the incidence of cataract formation. 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.

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