Antioxidants fail to prevent prostate cancer

Selenium, vitamins C, E don’t lower incidence of prostate cancer in two large trials

Despite much hope generated by earlier studies, vitamins C and E and the element selenium have failed to reduce the incidence of prostate cancer. The disappointing news from two huge trials is reported online December 9 in the Journal of the American Medical Association.

In one trial, more than 14,000 male physicians age 50 or older were randomly assigned to take vitamin E, vitamin C, both or placebos starting in the 1990s. Researchers monitored the men’s health over eight years on average.

Nearly 2,000 cases of cancer arose in the men during the trial, roughly half of which were prostate cancer. But taking a vitamin didn’t affect a man’s likelihood of developing prostate cancer or any other malignancy, the data show.

In the other trial, started in 2001, more than 35,000 men also age 50 or older were randomly assigned to get vitamin E, selenium or a placebo. After more than five years of follow-up per participant, on average, more than 1,700 cases of prostate cancer occurred. A panel monitoring the ongoing results stopped the trial after determining that the supplements had no effect on prostate cancer risk.

“This does put us back to the drawing board in terms of looking for an agent that might prevent prostate cancer,” says Scott Lippman, a physician and cancer researcher at the University of Texas M.D. Anderson Cancer Center in Houston who coauthored the selenium/vitamin E study. “This is really fairly definitive.”

Hopes had been high for all three supplements, which have antioxidant properties and therefore were thought to curb cell damage that can lead to cancer. Earlier studies hinted at a lower prostate cancer risk in people with high selenium levels in their bodies. And a previous randomized trial showed less prostate cancer among men taking selenium than among men not taking it.

Vitamins C and E had their own adherents, who noted that inside the body, antioxidants perform the essential duty of sopping up free radicals that cause the oxidative damage to cells. Adding more antioxidants — such as vitamins C and E — seemed an obvious strategy.  

But based on the new findings, “there’s really no scientific basis for recommending antioxidant supplementation for prostate cancer prevention,” says physician and cancer epidemiologist Peter Gann of the University of Illinois at Chicago. He notes that the selenium/vitamin E trial is the largest cancer prevention trial ever conducted.

Sometimes research doesn’t follow its natural order. “This is an example of massive clinical trials that are going to require people to go back to the lab,” Gann says.

Lippman agrees. “One has to rethink the whole antioxidant hypothesis,” he says. Part of that re-evaluation will be a deeper analysis of antioxidants themselves.

For example, research has shown that the body on its own makes some antioxidants, supplies of which can rise or fall with stress, exercise and other factors. “Our bodies respond very nicely to exercise with these natural antioxidants,” says Michael Gaziano, a cardiologist at Brigham and Women’s Hospital and the Veterans Affairs Boston Healthcare System who was a coauthor on both reports. “I don’t think micronutrient [supplements] harm these natural defenses or diminish them. It just doesn’t seem like a single micronutrient materially augments our antioxidant defense in a substantial way.”

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