Selenium and Vitamin E Supplements May Increase Risk of Prostate Cancer in Some Men
A multicenter study led by Fred Hutchinson Cancer Research Center has found that high-dose supplementation with both the trace element selenium and vitamin E may increase the risk of high-grade prostate cancer. This risk is dependent upon a man’s selenium status prior to taking the supplements. The findings by Kristal et al, published in the Journal of the National Cancer Institute, are based on data from the randomized, placebo-controlled SELECT (Selenium and Vitamin E Cancer Prevention) trial, which sought to determine whether taking high-dose vitamin E and/or selenium supplements could protect men against prostate cancer.
The SELECT trial, which began in 2001 and was designed to last 12 years, stopped early, in 2008, because it found no protective effect from selenium and there was a suggestion that vitamin E increased risk. Although use of the study supplements stopped, men were still followed, and after an additional 2 years the men who took vitamin E had a statistically significant 17% increased risk of prostate cancer.
The data for the current analysis compared the effect of selenium and vitamin E, taken either alone or combined, on prostate cancer risk among 1,739 men 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 cases by race and age.
Selenium Increases Risk in Men With High Selenium Status
When the study started, there was some evidence that selenium supplementation would not benefit men who already had an adequate intake of the nutrient. For that reason, researchers measured the concentration of selenium in participants’ toenails and planned to test whether selenium supplementation would benefit only the subset of men with low selenium status at baseline.
Instead, they found that taking selenium supplements had no effect among men with low selenium status at baseline and increased the risk of high-grade cancer by 91% among men with high selenium status at baseline. When selenium supplements were taken by men who had high selenium status to begin with, the levels of selenium became toxic.
Vitamin E Increases Risk in Men With Low Selenium Status
The study also found that only a subgroup of men was at increased risk of prostate cancer from taking vitamin E. Among men with low selenium status at baseline, vitamin E supplementation increased their total risk of prostate cancer by 63% and increased the risk of high-grade cancer by 111%.
This explained one of the original SELECT findings, which was that only men who received vitamin E plus a placebo pill, and not those who received both vitamin E and selenium, had an increased prostate cancer risk. Selenium, whether from dietary sources or supplements, protected men from the harmful effects of vitamin E.
No Known Benefits—Only Risks
“Many people think that dietary supplements are helpful or at the least innocuous. This is not true,” said corresponding and first author Alan Kristal, DrPH, of the Public Health Sciences Division of Fred Hutchinson. “We know from several other studies that some high-dose dietary supplements—that is, supplements that provide far more than the daily recommended intakes of micronutrients—increase cancer risk. We knew this based on randomized, controlled, double-blinded studies for folate and beta carotene, and now we know it for vitamin E and selenium.”
“These supplements are popular—especially vitamin E—although so far no large, well-designed and well-conducted study has shown any benefits for preventing major chronic disease,” Dr. Kristal said.
“Men using these supplements should stop, period. Neither selenium nor vitamin E supplementation confers any known benefits—only risks,” he continued. “While there appear to be no risks from taking a standard multivitamin, the effects of high-dose single supplements are unpredictable, complex and often harmful. Taking a broad view of the recent scientific studies there is an emerging consistency about how we think about optimal intake of micronutrients. There are optimal levels, and these are often the levels obtained from a healthful diet, but either below or above the levels, there are risks.”
Dr. Kristal is the corresponding author of the Journal of the National Cancer Institute article.
The study was funded by the National Cancer Institute. The study authors reported no potential conflicts of interest.
The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.
