Published guidelines by the U.S. Preventive Services Task Force find that vitamin supplements do not prevent cardiovascular disease or cancer and should not be taken with the assumption that the supplements will lower the risk of either of these diseases. In an analysis of clinical trials that investigated the effects of vitamin A, vitamin B, vitamin E, beta-carotene, folic acid, antioxidant combinations, and multivitamins, the task force said that there is no solid evidence to recommend using these supplements to prevent heart disease or cancer.

Studies have found that low dietary intake and blood levels of certain antioxidant vitamins have been associated with a higher number of cases of certain cancers as well as a higher risk of deaths from these cancers, according to the task force. Furthermore, shortages of antioxidant vitamins (vitamins A, C, and E; beta-carotene; and folic acid) can cause blood vessel changes associated with cardiovascular disease. Therefore, the U.S. Preventive Services Task Force conducted a systematic review of the use of vitamin supplements to determine whether they decrease the risk of cancer and cardiovascular disease.

The review on cardiovascular disease found that "highest-quality studies did not show that vitamins consistently or meaningfully decreased cardiovascular disease," according to the task force. Furthermore, the task force found no evidence that vitamin supplements prevent cancer. In fact, two high-quality studies found that people who smoked and took beta-carotene developed cancer more often than people who did not take beta-carotene.

Based on the review, the U.S. Preventive Services Task Force recommends the following:

• Do not take beta-carotene supplements to lower the risk of developing cardiovascular or cancer.
• The task force recommends neither for nor against taking vitamins A, C, or E; multivitamins with folic acid; or combinations of these vitamins for the primary purpose of preventing cardiovascular disease or cancer.

The analysis of studies used to develop these recommendations was based on research examining the use of vitamin supplements. According to the task force, there is stronger evidence to support the benefit of vitamins obtained from foods against disease.

The task force also identified a number of areas where further research is required. The most promising finding involved people who smoked and took vitamin E supplements. There was some evidence to suggest that vitamin E may lower the risk of developing prostate cancer and lower the number of deaths from the disease. A large U.S. trial is currently underway to investigate the possible benefits of vitamin E against prostate cancer. Other vitamins that require further research in large clinical trials include vitamin A and vitamin B for breast cancer and colon cancer.

Because large amounts of vitamin supplements can cause health problems, and the use of supplements can interfere with certain disease treatments, people should talk to their physicians about the continued use of vitamin supplements. Most experts recommend a diet high in vegetables and fruits; both are good sources of vitamins.

• The U.S. Preventive Services Task Force report on "Vitamin Supplementation to Prevent Cancer and Cardiovascular Disease," is available at http://www.ahcpr.gov/clinic/uspstfix.htm