Pulling together the research

The review, published in the American Journal of Clinical Nutrition, used data from 12 studies with a combined total of more than 13,000 participants. The studies looked at the relationships between prostate cancer risk and blood selenium levels, selenium content of toenail clippings, and selenium intake.

The reviewers combined the data and analyzed it to identify whether a relationship existed. They looked at:

• Blood levels: Men with higher levels of selenium in their blood had a lower risk of being diagnosed with prostate cancer and a lower risk of developing advanced prostate cancer.
• Toenail clipping content: The selenium content of toenail clippings had what is known as a U-shaped relationship (because it is represented by a graph that is U-shaped) to prostate cancer risk: more selenium appeared protect against risk up to a point, but after that point, the risk began to rise again as selenium increased.
• Dietary intake: In general, higher selenium intake through diet and supplements was linked to lower prostate cancer risk, but one study again found that at very high intakes, selenium was no longer protective. Differences in the ways the studies were done made it impossible for the researchers to recommend an ideal daily intake.

Selenium's complicated considerations

Although some of the studies did not show this tapering off of protection from selenium, the reviewers speculated that selenium levels and intakes in these studies may not have been high enough to show the possible detrimental effects of having too much.

The results led the reviewers to comment on the complex nature of the relationship between selenium and prostate cancer, saying, "We showed in our dose-response meta-analysis that a decreased risk of prostate cancer appears to be associated with a relatively narrow range of selenium status." They emphasized the importance of clearly identifying the ideal range in order to make safe recommendations about supplementing with selenium.

Get your selenium-but not too much

Selenium is a mineral micronutrient and a powerful antioxidant. Scientists believe it plays a role in cancer prevention and some studies have found that it protects against colon and lung cancers, as well as prostate cancer. It also keeps the immune system strong and appears to protect the heart and blood vessels.

Here are some ways to ensure that you get enough selenium every day:

• Have a Brazil nut. Brazil nuts are the most highly concentrated source of selenium. Just one Brazil nut per day can provide enough selenium to maintain a healthy level in your body.
• Add mushrooms. Crimini (button) and shitake mushrooms can be good sources of selenium if they come from selenium-rich soil.
• Include seafood. Cod, tuna, halibut, shrimp, scallops, sardines, and salmon are excellent sources of selenium.
• Consider the guidelines. The Recommended Dietary Allowance is 55 mcg per day for adults, and many healthcare providers recommend supplements with 100 to 200 mcg per day.

(Am J Clin Nutr 2012; doi: 10.3945/ajcn.111.033373)