DEAR DOCTOR. K:

I try to eat a balanced diet but don't always succeed. Should I take a single vitamin supplement? How about a multivitamin?

DEAR READER:

This apparently simple question is tough to answer. Here's why. In the past 200 years, doctors discovered several diseases that were caused by severe deficiencies of particular vitamins. An example is scurvy, a disease that was caused by lack of vitamin C.

These days you don't need to worry about these vitamin-deficiency diseases, even if you're not always eating a perfectly balanced diet. Many of today's foods are fortified with vitamins. So you don't need to take vitamin pills to prevent these diseases.

In the past 40 years or so, doctors found some evidence that more subtle deficiencies of particular vitamins might increase the risk of various serious diseases. For example, B-vitamin or beta-carotene deficiency might increase the risk of heart disease. Vitamin E deficiency might increase the risk of Alzheimer's disease. Vitamin C deficiency might increase the risk of certain cancers. However, studies have shown no benefit from taking these vitamins as pills.

There are two vitamin pills, however, that you should consider taking. All women of childbearing age should get extra folic acid (400 micrograms daily) to protect against birth defects in their babies, should they get pregnant. Large scientific studies have proven the value of taking this vitamin.

The one other vitamin supplement that makes sense is vitamin D. The value of taking a vitamin D supplement has not been proven. Nevertheless, many people have low blood levels of vitamin D. Authorities recommend that most Americans should get between 800 international units (IU) and 1,000 IU of vitamin D daily.

Large doses of vitamins might even cause some real harm. For example, beta carotene, a form of vitamin A, can increase the risk of lung cancer in people at high risk, such as smokers.

What about taking a multivitamin pill each day? There is little evidence that it either helps you or hurts you. They're not very expensive, and they're an easy way to fill in any nutritional gaps.

We have more information on vitamins in our Special Health Report called "Healthy Eating: A Guide to the New Nutrition." You can find out more about it at my website.

So here's where I come down -- based on the scientific evidence available today. As more evidence comes in, I reserve the right to change my mind.

I'm still inclined to give daily multivitamins the benefit of the doubt. They're not likely to pose any risk. But I'm less optimistic today than I was 20 years ago that they offer a benefit. If you do decide to take a multivitamin, look for a brand that contains enough vitamin D.

Finally, if you're a woman of childbearing age, don't forget to take a folic acid supplement every day. That is the one case in a developed nation like the United States where the value of taking a daily vitamin supplement has been proven beyond doubt.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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