Selenium still helps preventing cancer

January 26, 2009

A huge U.S. study showed that supplementation of selenium do not prevent cancer of the prostate. But this result is only valid if you get plenty of selenium in advance.

12 years ago it aroused hope and optimism when American Larry Clark could tell that the mineral selenium prevents cancer, particularly prostate cancer, the second most common cause of death from cancer in men. He had to stop his trial before expiry when he learned that far fewer selenium-treated than placebo-treated patients (placebo: Inert tablets) got cancer.

Now a second, much larger, selenium trial has been stopped prematurely. Also in this case the focus of interest was the effect against prostate cancer. This was also an American study. But SELECT, as the trial was named, unfortunately showed that selenium had no effect. One could even not exclude an, admittedly very little, harmful effect. So, it was stopped.

In the meantime, Clark’s trial has been studied more closely. Was it really as convincing as was first believed? With 1.312 participants it was not nearly as large as SELECT where 35.000 attended. Very important was that the final report which came in 2003 showed that the benefit was smaller than first believed. Some cases of prostate cancer among selenium treated had for various reasons been overlooked.

What was left was a statistically significant benefit among those who at the beginning of the experiment had the least selenium in the blood and with most certainty did not have incipient cancer of the prostate. The latter could be concluded from the very low values of PSA (Prostate-Specific Antigen) in the blood of these people. In this group, while the trial was in progress, the incidence of cancer of the prostate was three times less than in the placebo group.

More selenium in the U.S.
Now the question is whether the much larger SELECT trial cancels Clark’s trial. It seems to be the general opinion as for example reflected in the leading article of the same issue of the American medical journal, JAMA, where SELECT was published. So far physicians should not recommend selenium as a prevention against prostate cancer, it says.

And yet one can rightly come to the diametrically opposite conclusion: There is every reason to believe that selenium prevents cancer of the prostate, and presumably also other kinds of cancer.

The fact is that Americans, but not all Americans, get far more selenium in their diet than we Scandinavians. In Clark’s study, participants were selected on the basis of consistently having relatively little selenium in their diet for U.S. standards. Two-thirds had less than 122 micrograms of selenium per. liter of serum. In the SELECT study only one in five had that low values. In other words, it is conceivable that most of the SELECT participants already got plenty of selenium so that additional supplementation would not benefit them. In Denmark almost everybody get less selenium than the participants of both the first and the second study mentioned. Our values are typically 80 micrograms per liter.

This is in excellent compliance with the fact that incredible few participants died from prostate cancer during the SELECT study. Statistically one would have expected 75 to 100 deaths for this reason, during the 5.6 years duration of the study. But only one died (!).

A contributory cause may have been that the vast majority of participants in the SELECT study on their own underwent PSA measurement annually. Possible prostate cancer was therefore detected and treated early. On the other hand, other studies have shown that annual PSA measurement does not reduce mortality. Therefore it is not recommended in Denmark.

Despite the termination of the SELECT study, as a Dane you should still remember that the research that involves us – as opposed to Americans we get very little selenium in our diet – suggests that supplementation with selenium in the order of 1-2 tablets (100-200 micrograms) per day seems to reduce the risk of prostate cancer to a third.

By: Niels Hertz, M.D.

1. Lippman SM et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers. JAMA online December 9, 2008: E1-E13
2. Gann PH. Randomized trials of antioxidant supplementation for cancer prevention. JAMA online December 9, 2008: E1-E2.
3. Selenium supplementation, baseline plasma selenium status and incidence of prostate cancer: An analysis of the complete treatment of the Nutritional Prevention of Cancer Trial. BJU Int. 2003;91:608-12.