A daily megadose of vitamin E -- once touted as an inexpensive way to prevent prostate cancer -- actually increases risk of developing the disease, researchers have found.
The study, appearing in Wednesday's Journal of the American Medical Association, is the second this week to raise alarming questions about dietary supplements.
In a massive clinical trial, more than 35,000 healthy, middle-aged men who took a daily 400-milligram capsule demonstrated a 17 percent increased risk of developing prostate tumors compared with those given placebos.
"The most important thing now is for men not to take vitamin E with the hope of preventing prostate cancer," said Dr. Iris Granek, who led an arm of the research at Stony Brook University Medical Center and chairs the center's department of preventive medicine.
The research was based on the premise that vitamin E might reduce the risk of prostate cancer by as much as 25 percent.
Federally funded at a cost of $12 million a year, the research began in fall 2000 and was designed to run 12 years, but was stopped in 2008 when no appreciable preventive benefit was seen among men taking the vitamin. It took until now to find the increased prostate cancer risk.
Dr. Eric Klein of the Cleveland Clinic and a team of researchers nationwide spent three years analyzing follow-up data.
Vitamin E is an antioxidant, which theories suggested could help prevent the kind of DNA damage associated with cancer formation.
But Dr. Deepak Kapoor, a Long Island urologist not connected with the research, noted the vitamin is also a fat-soluble nutrient. When taken in megadoses, it can accumulate in the body's tissues.
Still, even though a risk was found, Kapoor said, that does not yet prove vitamin E causes prostate cancer.
"Correlation is not causation," said Kapoor, who runs treatment centers throughout Long Island and New York City. "No one knows what causes prostate cancer."
Unrelated research earlier this week suggested older women who take multivitamins and other supplements are at increased risk of death.