Effects of selenium and vitamin C were not significantly different from those of placebo, data show.

Antioxidant supplementation has no effect on the risk of prostate and certain other cancers, according to two large longitudinal trials, leading one of them to be cut short.

The findings are scheduled for publication this month in The Journal of the American Medical Association but were released online ahead of print “because of the public health implications.”

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JAMA describes the Selenium and Vitamin E Cancer Prevention Trial (SELECT) as “perhaps the largest cancer chemoprevention trial ever conducted.” It included 35,533 generally healthy men at 427 sites in the United States, Canada, and Puerto Rico. Black participants, who made up 13% of the sample, were at least 50 years old at baseline; other subjects were at least 55.

The men were randomly assigned to one of four groups: selenium (200 µg/day); vitamin E (400 IU/day); both selenium and vitamin E; or placebos.

Follow-up was supposed to be for 7-12 years. But after a median follow-up of 5.46 years, the researchers found no statistically significant differences in the absolute numbers or five-year incidence rates for prostate cancer among the four groups. The rates were 4.56% for selenium, 4.93% for vitamin E, 4.56% for the combined intervention, and 4.43% for placebo. Results for other cancers, including lung and colorectal, followed similar trends.

The study was suspended when an independent oversight committee decided that the lack of effect had been “convincingly demonstrated,” and there was no possibility of benefit if the study continued.

Meanwhile, The Physicians’ Health Study II looked at long-term supplementation with 400 IU of vitamin E every other day or 500 mg of vitamin C daily among 14,641 male physicians. Compared with placebo, neither vitamin had a significant effect on the incidence of prostate cancer and other malignancies.

In an accompany editorial, Peter H. Gann, MD, ScD, of the University of Illinois at Chicago, comments on the “disappointing news” from these trials, which were “conceived during a new wave of hope” in the wake of earlier studies.

“Regarding nutritional prevention of prostate cancer,” he wrote, “first-generation phase 3 trials were too reliant on biased interpretation of prior research, second-generation trials may have been too reliant on chance, yet there is every reason to believe that the next generation will have a firmer basis for causal hypotheses. Until then, physicians should not recommend selenium or vitamin E—or any other antioxidant supplements—to their patients for preventing prostate cancer.”