According to a report by two Harvard Medical School experts, 30% of Americans take vitamin supplements.

Walter Willett, MD, DrPH, and Meir Stampfer, MD, DrPH, addressed the question of what vitamins Americans should consider taking in a recent issue of the New England Journal of Medicine (Vol. 345, No. 25: 1819-1824). They point out there is little good information on the value of vitamin supplements.

But that doesn't mean they aren't valuable. It just means the studies are hard to do. Willett and Stampfer believe that some vitamins can be very helpful to our health. In their article, they list which vitamins they think are useful.

First on their list is folic acid. It is useful for pregnant women because it prevents a type of spinal cord defect in babies. It also may reduce the risk of leukemia in children.

For some adults, folic acid may lower the risk of colon cancer and breast cancer. It may be particularly useful in women who drink. Alcohol increases breast cancer risk and folic acid may reverse this effect.

The authors said B vitamins, particularly B12, are valuable. Vitamin B12 may lower cancer risk. It also can prevent the development of a type of anemia in people who avoid animal sources in their diet.

Vitamin D probably has no cancer-preventing effect. But it does keep bones strong, particularly for people who live in places where the sun seldom shines.

Vitamin A has been touted as a cancer-stopper. Many studies have tried to show that this vitamin could prevent cancer. None have been successful.

Recently Willett and colleagues from Harvard showed that elderly women taking high doses of vitamin A were more prone to hip fractures. There is also some concern about birth defects if this vitamin is taken in high doses during pregnancy.

Vitamin E has also carried great hope of preventing cancer, but there is only some weak evidence that it may help men avoid prostate cancer. This has led to the large National Cancer Institute-sponsored SELECT trial. It will test whether vitamin E reduces prostate cancer risk. In the trial, the mineral selenium is also being studied for any prostate cancer-preventing action.

Taking vitamin E has also been regarded as a possible way to reduce coronary heart disease. But a recent study showed that in large doses, it blocked the benefits of cholesterol-lowering drugs in opening diseased arteries. It hasn't been shown to have value in preventing cancer.

At one time, vitamin C was heralded as a treatment for cancer, but many studies found it had no value. In fact, there have been no studies proving vitamin C has any value in preventing or treating cancer, no matter what the dose. In high doses, it also may block the beneficial effects of cholesterol-lowering drugs.

The bottom line, according to Willett and Stampfer, is that vitamins are low cost when taken as a multivitamin supplement (about $20 to $40 per year), are safe in low doses, and may be helpful in certain instances.

But, they cautioned, "A vitamin pill is no substitute for a healthful lifestyle or diet, because foods contain additional components, such as fiber and essential fatty acids. In particular, a vitamin supplement cannot begin to compensate for the massive risks associated with smoking, obesity, or inactivity."