SAN FRANCISCO – Healthy middle-aged and older men who take vitamin E supplements have an increased risk of prostate cancer, although the risk takes some time to emerge, suggests an update of the randomized SELECT prevention trial.
At a median follow-up of 7 years – or 1.5 years after the trial had been closed early for futility and men had been told to stop taking supplements – those who had taken vitamin E had a significant 17% greater risk of prostate cancer than those who had taken a placebo. The risk had been increased, although not significantly so, at the time of trial closure.
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More than half of U.S. men over age 60 take vitamin E, with about one-quarter taking at least 400 IU daily.
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In absolute terms, the elevated risk from taking vitamin E translated to 11 more cancers for every 1,000 men over a 7-year period, Dr. Eric A. Klein said at the Genitourinary Cancers Symposium.
The findings serve as a cautionary tale, he said. "Nutritional supplements are biologically active and may in fact be harmful, and importantly, the effect may continue after the intervention stops," Dr. Klein commented.
More than half of U.S. men over age 60 take vitamin E, with about one-quarter taking at least 400 IU daily (Ann. Intern. Med. 2005;143:116-20), the dose used in the trial. "Consumers should be skeptical about health claims for unregulated over-the-counter products in the absence of strong evidence of benefit from clinical trials," said Dr. Klein at the meeting, which was sponsored by the American Society for Clinical Oncology, the American Society for Radiation Oncology, and the Society of Urologic Oncology.
In other updated results, men who took selenium with vitamin E and men who took selenium alone did not have any significant increase in the risk of prostate cancer. And baseline plasma levels of various tocopherols (forms of vitamin E) modified prostate cancer risk: Within the vitamin E group, for example, higher levels of alpha-tocopherol were protective, whereas higher levels of gamma-tocopherol were deleterious.
The obvious question now is, how does vitamin E increase prostate cancer risk? said Dr. Klein, who is chairman of the Glickman Urological and Kidney Institute at the Cleveland Clinic. Theories that have been advanced include the possibility that antioxidants become carcinogenic pro-oxidants at high doses; there may be interplay between the different types of tocopherols; high doses of vitamin E may inhibit absorption of other, protective fat-soluble vitamins; and genetic susceptibility may affect the actions of antioxidants. A related, as yet unanswered question is why adding selenium to vitamin E abolishes the excess risk.
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Dr. Eric A. Klein
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"We need to sort all of this out," Dr. Klein said, pointing to the SELECT (Selenium and Vitamin E Cancer Prevention Trial) Biorepository, which contains a wealth of information about the men studied and is being offered as a resource. "I invite anybody in the scientific community to float a hypothesis. If it passes scientific muster, we will give you access to the Biorepository to help us answer [these questions]." More than a half dozen studies have already been approved, he said.
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