Too Much of a Good Thing?
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By Sarah Webb

Too Much of a Good Thing?

Excessive multivitamin use may be linked to fatal prostate cancer

By Sarah Webb


If one dose of a vitamin supplement might be healthy, more could be better, according to one train of logic. But sometimes, too much of a good thing might be, well, too much. According to a report in the May 16, 2007, Journal of the National Cancer Institute (JNCI), men who take multivitamins more than once a day may have almost twice the risk of fatal prostate cancer compared with men who don’t use multivitamins.

Michael Leitzmann, an internist and epidemiologist, and his colleagues at the National Cancer Institute followed almost 300,000 men who enrolled in the National Institutes of Health–AARP Diet and Health Study in 1995 and 1996. The men were surveyed about their use of vitamins and supplements, and then monitored. During the next five years, more than 10,000 men were diagnosed with prostate cancer. By the end of the sixth year, 179 men had died of the disease.

In addition to the doubled risk of fatal prostate cancer, taking a multivitamin more than once a day was linked to a 32 percent increased risk of non-localized, advanced cancers compared with the risk for men who never took multivitamins, according to Leitzmann. To be sure that men with undiagnosed cancer at the study’s onset weren’t skewing the analysis, Leitzmann and his colleagues performed new calculations that excluded men diagnosed with prostate cancer during their first two years of monitoring. The new results showed no significant increased risk of advanced cancer for excessive multivitamin users, but an even greater increased risk of fatal cancers.

The results suggest that excessive intake of supplements might speed growth of an existing tumor, Leitzmann says. But because the study was not a randomized trial, in which patients are randomly assigned into groups that take different amounts of multivitamins, the researchers can't provide conclusive evidence that excessive multivitamin use helps prostate cancer develop, Leitzmann adds.

Many of the vitamins and other compounds in multivitamins are collectively known as antioxidants. Under conventional wisdom, these antioxidants act like sponges, sopping up cancer-causing free radicals—reactive particles—in the body. But it's also possible that antioxidants could interfere with necessary processes that help the body eliminate unwanted cells, according to an editorial in JNCI. "In our own systematic review of all randomized trials, we have been unable to identify any beneficial effect [of antioxidant supplements] on mortality," says Christian Gluud, an internist at Copenhagen University Hospital in Denmark and a co-author of the editorial. In the absence of a physician's recommendation, he thinks the potential risks of taking supplements outweigh any possible benefits.

However, diet is one of the few facets of life with the disease that prostate cancer patients feel they can control, says Jim Kiefert, an 18-year prostate cancer survivor and the chairman of the board of directors of Us TOO International, a prostate cancer patient advocacy group based in Downers Grove, Ill. Men want "something that they can do to enhance their quality of life and extend their survival," he says.

Researchers need to examine more data to clearly determine a direct link between multivitamin use and prostate cancer before Kiefert will change his routine. “I'm still taking my multivitamin,” he says.

According to Leitzmann, if men follow a physician's or manufacturer's recommendations for vitamin intake, they shouldn't be concerned. "What we saw in our study was if men adhered to the normal recommended dose of up to one multivitamin per day, they were not at an increased risk or a decreased risk for prostate cancer," he says.