Depending upon whether men have a lot or little of the mineral selenium in their bodies, taking large doses of either selenium or vitamin E can almost double the risk of an aggressive form of prostate cancer, suggests a new study. Vitamin E supplements increase all prostate cancer risk among men with low levels; selenium supplements increase risk among those with high levels.
The study was published in the Journal of the National Cancer Institute. Its findings, somewhat unexpected, adds to a complex body of research on supplements and cancer risk.
AICR currently recommends not relying on supplements to prevent cancer; instead, taking in your nutrients, phytochemicals and other cancer-fighting compounds from food.
The study continues the findings from a large trial that was stopped back in 2008. Called the Selenium and Vitamin E Cancer Prevention Trial (SELECT), the study was investigating whether selenium and/or vitamin E could lower prostate cancer risk. Both are essential for good health and have antioxidant activity.
SELECT started in 2001 with approximately 35,000 healthy men who were randomly placed into supplement groups. But when the trial found that the supplements did not offer protection and vitamin E supplements may even increase risk, the trial was stopped.
The men were asked to stop taking the supplements but researchers continued to follow-up on their health. (In 2011, they found that healthy men who took vitamin E supplements were at increased risk of prostate cancer.)
This study used data from the 1,739 men who were diagnosed with prostate cancer. They compared these men to a random sample of approximately 3,000 men matched by age and other factors. The scientists looked at the men’s toenail selenium concentrations at the start of the study. Because selenium stays in our bodies for long periods of time, scientists use toenails as a long-term indicator of selenium status.
For men who began the study with a high amount of selenium, taking selenium supplements increased the risk of high-grade prostate cancer by 91 percent.
Among men with low selenium at the start, vitamin E supplementation increased their total risk of prostate cancer by 63 percent. It also increased risk of high-grade cancer by 111 percent.
There was no increased risk of prostate cancer among men who did not take supplements.
The dose of supplements the men took was relatively high. For selenium, it was about 200 mcg/day — about four times the recommended daily intake; vitamin E supplements were 400 IU/day.
Given the risks and lacking evidence of benefit for both prostate cancer and other diseases, men aged greater than 55 should avoid vitamin E or Selenium supplementation that go beyond the recommended dietary intake, the authors conclude.
For both men and women, there are plenty of health reasons you may need to take supplements. How you know is the top question here on the Office of Dietary Supplements.