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Jan. 24, 2008 -- With one possible exception, many antioxidants in pill form do not appear to protect against cancer, according to pooled data from some of the most rigorous studies ever to examine the issue.
There was some evidence linking the mineral selenium to a reduced risk of cancer in men, but not in women. But the findings are not yet conclusive enough to recommend that men take selenium supplements, says Mayo Clinic physician Aditya Bardia, MD, MPH.
Bardia and colleagues combined 12 clinical trials including more than 100,000 participants for their systematic analysis, which appears in the January issue of the journal Mayo Clinic Proceedings.
Among their other major conclusions:
"Because we were able to pool the results from several different studies, we were able to show associations that individual studies might not show," Bardia tells WebMD.
The analysis included two widely reported clinical trials published in the mid-1990s designed to determine if beta-carotene lowered lung cancer risk in smokers when combined with vitamins A or E.
Both studies showed an increased risk for lung cancer in smokers who took beta-carotene supplements.
Bardia says the evidence overall is now strong enough to recommend that smokers avoid beta-carotene supplements.
Based on this evidence, the health advocacy group Center for Science in the Public Interest (CSPI) last month petitioned the Food and Drug Administration to require labels on beta-carotene supplements warning smokers about the risk.
"Smokers are already at a high risk for lung cancer, and it appears that they are at even greater risk if they take beta-carotene supplements," CSPI senior nutritionist David Schardt tells WebMD. "Smokers need to know this, and it should be on the labels of these products."
The analysis did suggest a lower cancer risk in men who took selenium supplements, but the finding was based on the results of just three trials, and only one was of high quality, Bardia says.
Findings from two large, ongoing randomized trials assessing selenium supplementation for the prevention of prostate cancer should help clarify the issue, he adds.
Bardia also points out that the studies included in the analysis did not examine whether eating antioxidant-rich foods such as fruits, vegetables, and whole grains can lower cancer risk.
Many health groups, including the American Cancer Society (ACS), recommend consuming antioxidants through foods instead of supplements.
"It is an old-fashioned message but it is important to eat a variety of foods, including plenty of fruits and vegetables," ACS nutritional epidemiologist Marjorie McCullough, ScD, tells WebMD. "If people really would do this it would go a long way to ensuring that they get the nutrients and phytochemicals that may be important for reducing their cancer risk."