Megadoses of Vitamin E may increase your risk of prostate cancer.
Credit: Joseph Devenney/Getty

Megadoses of Vitamin E may increase your risk of prostate cancer.

If you've been taking vitamin E because you heard it helps prevent prostate cancer or heart disease, a recent study suggests you should stop immediately. The long-term study by the National Cancer Institute found that men who took a vitamin E supplement – which is already taken in some form by almost 40 percent of all American men – had a 17 percent higher rate of prostate cancer than men who took only a placebo.

A fat-soluble nutrient, vitamin E is essential for immune function and also plays a part in dilating blood vessels, and so may help prevent the type of blood clots that lead to heart attacks. It's also a powerful antioxidant, meaning in theory it ought to help mop up cancer-causing free radicals. As such, many experts have until now routinely advised older men to take a daily mega-dose of vitamin E to ward off prostate cancer.

In the NCI study, participants took 400 international units (UI) per day, or some 20 times than the FDA's daily recommendation for the nutrient. Trial co-author Dr. Eric Klein, a urologist at the Cleveland Clinic, notes however such a dose isn't uncommon. "Lots of men in this over-50 age group use these supplements at the dose levels [that we did in the study]," he says.

So how could it be that vitamin E increases cancer risk? One possibility, says Klein, is that when people take too much vitamin E, their bodies aren't able to absorb other cancer-fighting vitamins. Still another theory is that vitamin E somehow spurs budding cancer cells to grow, literally "feeding the tumor," says Dr. Lisa Ganjhu, a gastroenterologist and internist at St. Luke's–Roosevelt Hospital in New York City.

It's not just vitamin E that is sending up alarms, though. Recent studies have called into question the safety of several commonly consumed supplements. A 2007 analysis of 47 clinical trials published in the Journal of the American Medical Association reported that the rate of dying was 5 percent higher among those participants who used vitamin E, beta-carotene, and vitamin A supplements. A separate study in 2009 of nearly 7,000 men found that taking folate and vitamin B12 pills were associated with an increased cancer risk in people with heart disease. And a large trial funded by the National Cancer Institute discovered that beta-carotene and vitamin A supplements increase the risk of fatal lung cancer in smokers.

Still, researchers caution that these studies shouldn't cause you to swear off vitamin supplements altogether. "For the average healthy guy with a reasonable diet, I recommend about 1,000 IU of vitamin D whenever daily sun exposure is not happening, as well as one gram of an omega-3 oil supplement from algae or krill," says Dr. David Katz, founder of Yale University's Prevention Research Center. But, he cautions, "It's important to remember: Supplement, not substitute. There is nothing in a pill that can replace eating well."