Vitamin E, Selenium Supplements Tied to Cancer Risk
Risk was highest when men had high or low levels of selenium already in their body
Researchers in this new study used toenail samples taken from SELECT participants to determine the level of selenium already in their bodies before taking the supplements.
They then re-analyzed the data to see if the amount of selenium already present in the body made any difference when it came to prostate cancer risk. They focused in on 1,739 men from the SELECT trial who were diagnosed with prostate cancer and, for comparison purposes, a random sample of 3,117 men without prostate cancer who were matched to the others by race and age.
It turns out that existing selenium levels did make a difference.
Taking selenium supplements increased the risk of high-grade cancer by 91 percent among men with already high selenium levels.
Among men with low selenium levels, taking vitamin E increased their total risk of prostate cancer by 63 percent and their risk of aggressive cancer by 111 percent.
The researchers aren't sure why high doses of these supplements increase prostate cancer risk. "I don't think there's any real understanding why that occurs," Brooks said.
But these findings jibe with other studies that have linked taking large amounts of supplements with increased cancer risk, Kristal said. Such studies have tied large amounts of omega-3 fatty acids to endometrial cancer, excess folate to colon cancer and beta carotene supplementation to lung cancer, he said. High doses of selenium have also been linked to skin cancer, he added.
"As humans, we evolved with the levels of micronutrients you'd normally get with food," Kristal said. "There's no benefit to taking high doses of these micronutrients. There's only risk."
Prostate cancer, the most common cancer among U.S. men, strikes about 200,000 Americans each year.