Men who take vitamin E supplements have a slight but statistically significant increased risk of developing prostate cancer, according to a study published today in the Journal of the American Medical Association (JAMA).
These findings add to the growing evidence that when used by people who do not have specific nutrient deficiencies, dietary supplements may do more harm than good.
They also point out how health recommendations can change — often doing an about-face — as the result of more vigorous research.
After all, early epidemiological and other studies had suggested that taking vitamin E would reduce the risk of prostate cancer.
Triggered by a troubling finding
The JAMA findings are based on 10-year follow-up data from a large randomized clinical trial known as SELECT (Selenium and Vitamin E Cancer Prevention Trial). It involved some 35,000 men aged 50 and older, and was originally designed to determine whether vitamin E (400 IU daily) or selenium (200 micrograms daily) — or a combination of the two — could prevent prostate cancer.
The SELECT trial was discontinued in 2008, seven years after it began, when it became apparent — quite unexpectedly — that the vitamins offered no cancer-prevention benefit. But the trial also turned up another bit of unexpected data: a small increase in the risk of prostate cancer among the men who were taking vitamin E.
At that time the increase was considered statistically insignificant, but researchers decided to continue to follow the trial’s participants. The current JAMA study reports on that follow-up.
It found that the SELECT participants who were assigned to the vitamin E supplements were 17 percent more likely to have been diagnosed with prostate cancer 10 years after the study began than the almost equal number of men in the study who were given a placebo pill.
Here are the absolute numbers: 620 men assigned to the vitamin E group developed prostate cancer compared with 529 men in the placebo group.
As explained on SELECT’s website, another way of looking at these findings is this: “[F]or every 1000 participants who were given study supplements for five years and then followed for an additional two years, we would observe 65 prostate cancers in the placebo group, and 76 in the vitamin E alone group.”
That makes the finding statistically significant and not the result of chance alone.
No increased risk was seen in the group of men who took both vitamin E and selenium. This may mean that the selenium dampened the effect of the vitamin E on the risk of prostate cancer, write the authors of the JAMA study.
It doesn’t mean, however, that selenium protects against prostate cancer.
Be skeptical of supplement claims
Around half of American adults aged 60 and older take supplements containing vitamin E, and almost a quarter of them are taking at least the same amount of the vitamin that was given to the men in the SELECT study.
Most do so not because of a specific nutrient deficiency, but as a kind of health “insurance” — a way, they believe, to protect themselves from chronic disease.
But, as the authors of the JAMA study note, consumers need to be “skeptical of health claims for unregulated over-the-counter products in the absence of strong evidence of benefit demonstrated in clinical trials.”
“The observed 17 percent increase in prostate cancer incidence demonstrates the potential for seemingly innocuous yet biologically active substances such as vitamins to cause harm,” they add.