There's more worrisome news about vitamins: Taking too many may increase men's risk of dying from prostate cancer.
The study, being published Wednesday, doesn't settle the issue. But it is the biggest yet to suggest high-dose multivitamins may harm the prostate, and the latest chapter in the confusing quest to tell whether taking various vitamins really helps a variety of conditions -- or is a waste of money, or worse.
Government scientists turned to a study tracking the diet and health of almost 300,000 men. About a third reported taking a daily multivitamin, and 5 percent were heavy users, swallowing the pills more than seven times a week.
Within five years of the study's start, 10,241 men had been diagnosed with prostate cancer. Some 1,476 had advanced cancer; 179 died.
Heavy multivitamin users were almost twice as likely to get fatal prostate cancer as men who never took the pills, concludes the study in Wednesday's Journal of the National Cancer Institute.
Here's the twist: Overall, the researchers found no link between multivitamin use and early-stage prostate cancer.
The researchers speculate that perhaps high-dose vitamins had little effect until a tumor appeared, and then could spur its growth.
While similar but smaller studies have suggested a link, too, more rigorous research is needed, caution the National Cancer Institute scientists. This newest study involves men who voluntarily took vitamins, and those most at risk -- perhaps because they had a family history of the disease -- may have been more likely to take the pills in hopes of avoiding their fate.
Still, "the findings lend further credence to the possibility of harm associated with increased use of supplements," Dr. Christian Gluud of Copenhagen University Hospital and Dr. Goran Bjelakovic of Serbia's University of Nis wrote in an accompanying editorial.