Dietary supplements have little to no effect in preventing cancer and may increase cancer risk, according to a review published in the Journal of the National Cancer Institute (2012;104:732-739, PMID: 22534785).

Restricting their review to supplements that have been researched in sufficiently powered clinical trials or large observational studies, the authors focused on antioxidants, folate and folic acid, vitamin D and calcium.

Despite early evidence suggesting an anticancer benefit from antioxidants, clinical studies have not borne out that promise, the authors argue. For example, b-carotene does not prevent recurrence of non-melanoma skin cancer (N Engl J Med 1990;323:789-795, PMID: 2202901); b-carotene and vitamin A do not protect against lung cancer (N Engl J Med 1996;334:1150-1155, PMID: 8602180); vitamins C and E do not protect against total cancer incidence (JAMA 2009;301:52-62, PMID: 19066368); and a-tocopherol, vitamin C, and b-carotene do not protect against total cancer or cancer mortality (J Natl Cancer Inst 2009;101:14-23, PMID: 19116389).

However, several trials have shown evidence of an increased cancer risk from antioxidants, the review authors, led by María Elena Martínez, PhD, at the University of California-San Diego, reported. One such study, conducted in a population at high risk for lung cancer, found a 39% increase in lung cancer incidence in the b-carotene arm compared with the placebo arm (N Engl J Med 1996;334:1150-1155, PMID: 8602180).

Similarly, the authors did not find evidence that folic acid and folate protect against cancer, whereas they did note evidence of increased risk for cancer from long-term folic acid supplementation. They found insufficient evidence to draw conclusions about vitamin D, and “diverse results” regarding calcium.

Many expert groups have reached a “general consensus” that “nutritional supplements have little to no benefit in preventing cancer,” the authors wrote. Even so, much of the public continues to use dietary supplements, a fact that the authors attribute in large part to the marketing influence of supplement manufacturers. The authors call for “efforts by scientists and government officials to encourage the public to make prudent decisions based on sound evidence with respect to the use of dietary supplements for cancer prevention.”

—George Ochoa