Healthnotes Newswire (August 2, 2007)—Research published over the past few decades suggests that taking selenium supplements can help prevent heart disease and certain types of cancer. And while selenium is known to be toxic when taken in large amounts, the general consensus has been that moderate doses of this mineral, such as 200 mcg per day, do not cause adverse effects.
A recent study, published in the Annals of Internal Medicine, has called the safety of taking selenium into question. This new study was a retrospective analysis of a double-blind trial published in 1996, in which people who received 200 mcg of selenium per day had a 50% higher risk of developing diabetes, compared with people who received a placebo. However, the study had serious weaknesses, and it is impossible to draw any reliable conclusions from it.
In the original 1996 study, elderly patients with a history of skin cancer were randomly assigned to receive 200 mcg per day of selenium from high-selenium yeast or a placebo for an average of 4.5 years. Selenium treatment did not affect the recurrence rate of skin cancer. However, compared with placebo, selenium treatment reduced total cancer mortality by 50%, and the incidence of lung, colorectal, and prostate cancers by 46, 58, and 63%, respectively.
In the new analysis, people from the original study who did not have diabetes were followed for an average of 7.7 years. During that time, type 2 diabetes developed in 9.7% of people receiving selenium and in 6.5% of those receiving the placebo, leading authors to conclude that the selenium was at fault.
The first important weakness of this study is that it was a retrospective analysis of research that was not originally designed to examine the incidence of diabetes. Statisticians will tell you that if one combs through data after the fact, looking for associations that had not been thought of originally, some seemingly important associations will appear simply by chance. Therefore, the higher incidence of diabetes among people who took selenium may have been just a random occurrence, rather than any indication that selenium is harmful.
The second weakness is that the placebo was baker’s yeast (Saccharomyces cerevisiae), a substance similar, if not identical, to brewer’s yeast (also Saccharomyces cerevisiae), which contains two different compounds that improve the body’s ability to regulate blood sugar. If the placebo used in the study had a beneficial effect on blood sugar control, then high-selenium yeast might have appeared to increase the risk of diabetes only because the placebo decreased the risk.
In addition to these problems, there are other factors that call the study’s findings into question: For example, in animal studies, selenium supplementation has been found to improve glucose metabolism, not to promote diabetes. Moreover, diabetes is not one of the reported manifestations of selenium toxicity in humans. Those facts cast further doubt on the idea that taking moderate doses of selenium causes diabetes.
One cannot say for certain that supplementing with selenium does not increase diabetes risk in elderly patients who have a history of skin cancer. However, the bulk of the evidence seems to indicate that the benefits of taking moderate amounts of selenium outweigh the risks.
(Ann Intern Med 2007;147:217–23)
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