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Selenium

Claims to prevent cancer, notably prostate cancer, and has other health benefits; acts as a potent antioxidant.

Evidence is accumulating that this mineral plays many important roles in the body. However, it's too early to recommend selenium supplements for everyone. Eat foods rich in selenium, and if you take a multivitamin, make sure it contains some selenium.

When first discovered in 1817, selenium was considered a poison. In the 1950s it was recognized as an essential trace mineral. And now it has become a superstar among nutrients. Headlines proclaim its ability to thwart cancer—particularly prostate cancer—and other conditions. But unlike many other health sensations, this one might have staying power, as evidence accumulates to back up at least some of the claims. While we don’t go so far as to recommend selenium supplements, we think it’s wise to make sure your diet contains adequate amounts.

Fortunately, that’s not hard to do in the U.S., since many foods here are good sources of selenium. And the recommended dietary allowance is small—only 55 micrograms a day (a microgram is one-millionth of a gram). We know that too little selenium impairs immunity and may affect thyroid function.

The amount of selenium in food depends on the amount in the soil, which varies widely from region to region. Early studies observed that there are fewer cancer deaths in areas where there’s more selenium in food, and that people with low blood selenium levels are at higher risk for several cancers.

In the first clinical trial, the Nutritional Prevention of Cancer Trial, selenium pills dramatically reduced the risk of prostate cancer, mostly in men with low selenium levels to start. And in the Physicians’ Health Study, men with the highest blood selenium levels were only half as likely to develop advanced prostate cancer as men with the lowest levels. The Selenium and Vitamin E Cancer Prevention Trial, from the National Cancer Institute, is currently testing the effects of these supplements against prostate cancer and should provide more insight.
Though the evidence is preliminary, some studies hint that selenium may provide other health benefits, including possible protection against asthma, heart attacks, rheumatoid arthritis, and male infertility. A pooling of data from three studies, recently reported in the Journal of the National Cancer Institute, linked higher blood selenium levels to a 34% reduced risk of the most common kind of colon polyp.

Researchers have proposed several ways that selenium may prevent cancer. As a component in antioxidant enzymes, it may help prevent DNA damage. It may also boost immune function. And selenium may even inhibit growth of blood vessels upon which tumors are dependent and induce cancer cells to self-destruct.

So is it time to supplement?

No. Study results have not been consistent, and there are still many unknowns. More research is needed.

• Some studies have found selenium protective against certain cancers, but not always against the same cancers. Other studies have found no benefits. And some even suggest harm, including a possible increased risk of some cancers.

• Selenium may benefit people who are deficient, but it’s still unclear if supplemental selenium will do any good if your diet supplies adequate amounts. Most Americans are already getting more than the RDA. It’s harder, though, to meet needs in some countries, such as Finland, China, and New Zealand, where soil concentrations of selenium are low.

• The optimal dose is unknown and may vary from person to person. Selenium may affect men and women differently. Moreover, it’s unclear which form of selenium is best, since studies have used different sources.

• Too much selenium can be harmful. The difference between an optimal and toxic dose is small. The safe upper limit for adults is 400 micrograms a day. Doses of 1,000 micrograms or more a day can cause nausea, vomiting, hair loss, and other problems.

Food, your best bet

Selenium levels in foods vary geographically, depending on the soil content. Because our food comes from all over the world, it’s hard then to know how much selenium is in particular foods and thus exactly how much we are getting. Nevertheless, the best sources are whole grains (eat several servings daily), nuts (particularly Brazil nuts), and seafood (eat two or three servings a week, also for its heart-health benefits). Poultry and red meats are other good sources. Eat foods rich in vitamin E, such as nuts, seeds, and fortified cereals, since this vitamin increases the effectiveness of selenium in the body. For a list of some of the best food sources, see below.

Selenium is promising, but it’s no miracle mineral. It won’t help against all cancers, let alone all the conditions it’s touted for, and it won’t help everyone. Much may depend on your initial selenium status. Look to foods first for selenium. Multivitamin/mineral pills typically provide 20 to 200 micrograms of selenium. If you do take a selenium supplement, don’t take more than 200 micrograms a day.

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