Taking high doses of selenium or vitamin E can increase the risk of prostate cancer, a new study found. Vitamin E supplements can raise the risk of prostate cancer by as much as 63 percent in certain men, while selenium can actually double the risk of a high grade cancer in others, according to a report published Friday in the Journal of the National Cancer Institute.
It’s the latest in a growing body of research that shows taking high doses of supplements can cause harm. In the new study, the researchers initially assumed that supplementation with selenium might lower prostate cancer risk in men with low levels of the trace micronutrient, said Dr. Alan Kristal, the lead author of the study and a faculty member in the Public Health Sciences Division at Fred Hutchinson Cancer Research Center.
“But we found there’s no benefit for anyone,” Kristal said. “All we did find was a heightened risk. I’m now willing to go on the record and say that there is no evidence that high doses of supplements of anything are good for you.”
The data in the new report came from a larger study that included more than 35,000 men: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). That trial, conducted by the SWOG cancer research cooperative group, began in 2001 and was originally designed to last 12 years, but it was halted early when it looked like vitamin E might be increasing instead of decreasing the risk of cancer. The study volunteers were randomly assigned to one of four groups: selenium and vitamin E, selenium alone, vitamin E alone or placebo.
In the new analysis of the SELECT data researchers examined the impact of vitamin E and selenium in a subset of the original group. In this more focused look Kristal and his colleagues found that supplementation with vitamin E alone raised the risk of prostate cancer by 63 percent in men who had low levels of selenium at the beginning of the study. Further, selenium supplementation raised the risk of a high grade cancer by 91 percent in men who had adequate levels of the nutrient at the outset.
Other studies have also found that vitamins originally thought to prevent cancer might actually cause it.
In 1996 researchers were surprised to discover that beta carotene and vitamin A supplementation not only didn’t prevent lung cancer, but also increased the risk of its development.
“At that time there was a lot of interest in using micronutrients and vitamins to help prevent cancer,” said Dr. Gary Goodman, a lead researcher on the beta carotene study who is a member of the Public Health Sciences Division at Fred Hutch and an oncologist at the Swedish Cancer Institute. “There was some suggestion from epidemiological studies that people with low levels of micronutrients in their blood were at a higher risk for cancers.”
'We thought vitamins were good for you'
Goodman’s study found an increased risk of lung cancer in smokers who had been randomized to take beta carotene and vitamin A. “That was kind of a shock to everyone. We thought vitamins were good for you, and they certainly didn’t cause harm.”
Just a year ago, Kristal took a closer look at the impact of the highly touted omega-3 fatty acids and found that they, too, increased the risk of aggressive prostate cancer.
The take-home message, Kristal said, is that supplementation isn’t a good idea. The best way to stay healthy is to eat healthy.
“The micronutrients in food are at a level that you need,” he said.
What all these studies are telling us is that we need to look at supplements as bioactive compounds, Goodman said.
“What I learned is that you need to regard these as you would any other drug we study,” he added.
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