DEAR MAYO CLINIC:

I've heard that vitamin E can be harmful to diabetics with heart problems. Can you tell me more about this?

ANSWER:

The scientific evidence has been mixed regarding vitamin E's effect on a person's risk of heart disease. Early studies seemed to indicate that vitamin E could help prevent heart disease. Results of follow-up studies have been less promising, however. The vitamin E you consume as part of a healthy diet isn't a concern. But, based on the research information available now, I do not recommend that people with diabetes take vitamin E supplements.

Vitamin E is found in a variety of foods, including vegetable oils (corn, cottonseed, soybean, safflower), wheat germ, whole-grain cereals and green leafy vegetables. One of vitamin E's functions is to prevent a chemical reaction called oxidation, which can sometimes result in harmful effects in your body. Because of this, vitamin E is called an "antioxidant."

Substantial evidence from basic science and laboratory research has suggested that antioxidants such as vitamin E may reduce heart disease risk by several mechanisms. One is preventing low-density lipoprotein (LDL or "bad") cholesterol from producing plaques that can narrow the heart's arteries (atherosclerosis). Diabetes dramatically increases a person's risk of cardiovascular problems -- including coronary artery disease, heart attack, stroke, atherosclerosis and high blood pressure. Thus, it seemed appropriate, based on these initial studies, to recommend vitamin E supplements to people with diabetes.

Several subsequent studies have indicated otherwise. To follow up on initial observational studies that suggested possible benefits, researchers conducted a variety of controlled clinical trials that studied people who were randomly assigned to take vitamin E daily. These studies found that regular use of high-dose vitamin E supplements may increase the risk of death (from all causes) by a small amount, while other studies found no effect on mortality in people who took vitamin E daily.

Some of the clinical trials raised concerns about an increased risk of bleeding when people take high doses of vitamin E daily, particularly in patients who take blood-thinning medications, such as warfarin, heparin, or aspirin, and in patients with vitamin K deficiency.

As a result of these more recent findings, some researchers have argued that there may be subpopulations, including individuals with diabetes, who are affected differently by vitamin E, and some vitamin E preparations that have differing effects. These situations have yet to be studied.

The ongoing research regarding vitamin E illustrates several important points. First, recommendations based solely on laboratory studies do not always show the expected outcome when translated to real-life situations in people. Second, outcomes found in observational studies are not controlled in a way that allows researchers to determine the true impact of the use of a medication or supplement. For example, observational studies can suggest an association, but are not a true test, of possible benefits of vitamin E. Vitamin E may have been the basis for the findings, but other possible explanations exist.

Third, the highest level of evidence to support the use of a certain medication or supplement comes from clinical randomized controlled trials assessing not only intermediate outcomes -- such as antioxidant activity in this case -- but more significantly, patient-important outcomes, such as quality of life and morbidity.

Based on the information we have now, I would not recommend that a diabetic patient take additional vitamin E other than what would be included in a sensible heart-healthy diet. If you have questions about your diet or about ways to reduce your risk of heart disease, talk to your doctor.

-- Steven Smith, M.D., Endocrinology, Mayo Clinic, Rochester, Minn.

Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care.

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Copyright © 2010 Steven Smith, M.D., Endocrinology, Mayo Clinic