According to the results of most large clinical studies, there is little evidence that any type of vitamin supplement reduces the risk of developing cancer, and some may actually increase the risk. This was the conclusion of the United States Preventive Services Task Force (USPSTF), an independent panel of experts in primary care and prevention.

Reviewing studies of antioxidants such as vitamins A, C, and E, as well as multivitamins, the task force concluded that there wasn’t enough evidence to recommend for or against supplement use specifically to prevent cancer. The panel did recommend against the use of beta carotene supplements, based on studies showing it may actually increase the risk of lung cancer among heavy smokers.

The Task Force’s recommendations were published in the Annals of Internal Medicine (Vol. 139, No. 1: 51-55).

The notion that vitamins may have an effect on cancer risk is based mainly on studies that have found cancer rates are lower in those who consume diets rich in naturally occurring vitamins (such as those including plenty of vegetables and fruits). Some laboratory studies have supported this link, showing that certain vitamins function as antioxidants, reducing the damage cells normally undergo over time, and possibly protecting against cancer.

But healthy foods such as vegetables are made up of a variety of chemicals. It’s still not clear whether the benefits might be due solely to the vitamins themselves, to other important nutrients, or to combinations of these. For this reason, the American Cancer Society currently recommends that, whenever possible, important nutrients should be obtained from a well-balanced diet based mostly on plant sources.

The types of studies needed to determine if specific vitamins may be helpful are expensive, difficult to perform, and often take many years to complete. Still, several large studies have now been completed, and many more are ongoing.

The Task Force researchers looked at the studies completed thus far on several of the most commonly-used vitamin supplements, and noted the following:

The Task Force members stressed that they examined only vitamin supplements, not the levels of vitamins people consumed in foods. To date, there is no solid evidence that any vitamin or other supplement can reduce a person’s risk of developing cancer.

However, the researchers were quick to point out that they were looking at vitamin supplement use only for specific purposes, and that they may be appropriate for other reasons in certain people with unmet nutritional needs. What’s more, with the exception of beta carotene, there appears to be little potential harm from taking vitamin supplements in doses generally considered to be safe.

Certain vitamins (such as A and D) may be harmful at high doses. People considering taking high doses need to weigh the possible benefits and risks accordingly, say the experts, and may want to speak with their health care providers before taking them.

People "should be reminded that taking vitamins does not replace the need to eat a healthy diet," the Task Force members concluded. "Although the health benefits of vitamin supplementation remain uncertain, there is more consistent evidence that a diet high in fruits, vegetables, and legumes has important benefits; other constituents besides vitamins may account for the benefits of such diets."