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Vitamin supplements do not reduce incidence of cancer or heart disease

Deborah Josefson Nebraska

The evidence that vitamin supplements are useful in preventing cancer or heart disease is not conclusive, the US Preventive Services Task Force, an influential US government advisory panel, has said.

Moreover, b carotene supplementation may do more harm than good in patients with lung cancer, the panel found (Annals of Internal Medicine 2003;139:51-5).

A separate meta-analysis reported by researchers at Oregon Health and Science University who are affiliated with the Preventive Services Task Force also found no evidence that vitamins are useful in preventing cardiovascular disease (Annals of Internal Medicine 2003;139:56-70).

In reaching these findings the task force researchers searched medical reports from 1966 to September 2001, using the Cochrane controlled trials register and Medline. They looked at randomised controlled trials, cohort studies, and meta-analyses of trials that examined the role of vitamins A, C, or E, b carotene, multivitamins containing folic acid, or combinations of antioxidants, in the prevention of cancer and cardiovascular disease.

Only vitamin supplements were reviewed. The panel did not evaluate sources of vitamins in food and their role in the prevention of cancer and cardiovascular death and disease. Furthermore, they did not address the role of vitamin supplementation in people with potential nutritional deficiencies, including pregnant and lactating women, children, elderly people, and people with chronic illnesses.

Overall 932 citations were reviewed, of which 102 were reports from 36 randomised controlled trials. Ten of these trials were of sufficient quality for their findings to be included in the recommendations.

The effect of folate and multivitamins on the primary prevention of cardiovascular disease was not evaluated, because no studies that examined this had been completed at the time of this analysis. The researchers did evaluate the relation of vitamin supplementation to the development or prevention of tumours of the breast, prostate, lung, and colon and non-melanoma skin cancer.

The literature review failed to find any good quality studies that showed any beneficial effect of taking vitamins A or C on cardiovascular health or in reducing the risk of cancer. Some observational studies did show a reduction in the risk of breast and colon cancer in women taking vitamin A, but the researchers could not control for confounding factors.

Data on multivitamins and antioxidants were mixed. Folic acid supplementation did seem to reduce the risk of colon cancer in several retrospective studies, but the relation needed to be confirmed by prospective studies, the task force said.

Vitamin E was found to be useful in reducing cardiovascular disease mortality in the nurses health study and the physicians health study, two observational longitudinal surveys, but no such benefit was found in prospectively controlled trials that were designed to examine the vitamin’s effect in primary prevention.

The study found not only that b carotene failed to reduce the risk of lung, prostate, colon, and breast cancer or non-melanoma skin cancer in middle-aged and older adults, but also that it seemed to increase the risk of lung cancer and death in smokers who took b carotene supplements.

While the task force could not recommend for or against vitamins in most cases, they wrote: "With the exception of vitamins for which there is compelling evidence of net harm (for example, beta carotene supplementation in smokers), there is little reason to discourage people from taking vitamin supplements. Patients should be reminded that taking vitamins does not replace the need to eat a healthy diet."

The Preventive Services Task Force’s findings are accessible at www.ahcpr.gov/clinic/uspstfix.htm