The heavily promoted cancer-preventive effect of selenium, the essential trace mineral sold separately or within multivitamin products, hasn’t held up to scientific scrutiny. A new review of all selenium studies found no protective effect and some hints that taking selenium supplements over a long period could have toxic effects. If this sounds vaguely familiar, recall a similar scientific judgment leveled at the rest of the major antioxidants (vitamins A, C, E, and beta carotene) several years ago. Until now, selenium was the lone remaining antioxidant with promise. Click here

The marketing of selenium supplements for cancer prevention stems largely from the many observational or population studies, indicating that people with diets high in selenium have lower rates of cancer. Promoters of selenium products have successfully turned this around to: increasing selenium intake will decrease the risk of cancer. Hopes were particularly high for the prevention of prostate cancer.

The new systematic review of all selenium studies comes from the Cochrane Collaboration, which is an independent, international organization that evaluates existing research. After all studies on a given topic are found, the reviewers’ task is to separate the wheat from the chaff. They are expected to come to a conclusion after analyzing the quality of the studies and the accuracy of the results.

Here is what they found: There are 55 observational studies with a combined total of more than one million participants. Forty-nine of them went so far as to collect blood and toenail samples from the healthy participants to determine levels of selenium intake and then follow them to see who developed cancer. These studies did find a lower incidence of cancers of the bladder and prostate in people with higher selenium levels. But observational studies are universally considered by researchers to be unreliable because they do not separate the factor under review—in this case, selenium—from many other factors, such as a high standard of living or an exceptionally nutritious diet, which could account for the health benefit—in this case, lower cancer incidence. Here is how the Cochrane reviewers put it: “No reliable conclusions can be drawn regarding a causal relationship between low selenium exposure and an increased risk of cancer.”

There were only six versions of the more convincing, “gold standard” of medical research: the randomized controlled trial (RCT). These placebo vs. selenium supplements RCTs had a combined total of 43,408 participants (94% men). The two RCTs considered to be the most reliable (relatively free of bias) concluded that organic selenium supplements did not prevent prostate cancer and they increased the risk of skin cancers (other than melanoma) in women and men.

The Cochrane reviewers concluded: “Currently, regular intake of selenium supplements for cancer prevention cannot be recommended to either the selenium-replete or deficient populations.”

Selenium’s cancer prevention effects may not have held up, but the supplement showed a benefit to people with a mild version of a thyroid-associated eye disorder, known as Graves’ orbitopathy. The New England Journal of Medicine just published a six-month Italian study showing that people with this autoimmune inflammatory disorder who were randomly assigned to take selenium supplements did much better than those randomly assigned to take either a drug (pentoxifylline) or a placebo. Improvements included a better quality of life and a slowed progression of this condition characterized by bulging eyes. These improvements were confirmed six months after the study participants stopped taking the supplements. They did not experience any adverse effects.

Maryann Napoli, Center for Medical Consumers ©