Selenium is a mineral that is essential to good health but is required only in small amounts. Most people in the U.S. get plenty of selenium in their diets. But selenium intake is linked to the amount of selenium in the soil where plants grow, and some areas have very little selenium in their soil. In parts of China and Russia, for example, selenium deficiency is a common problem. In the U.S., deficiencies are generally seen only in people who have trouble absorbing selenium because of damage to their digestive system, such as with Crohn’s disease, or those who are being fed through a tube. Very low levels of selenium have been linked to heart disease and thyroid problems.

Selenium can be found primarily in plants but also in some meats and seafood. The body uses the mineral to make antioxidant enzymes that protect against cell damage. It may also play a role in maintaining a strong immune system.

After studies showed that people who had high levels of selenium intake had lower rates of cancer, selenium supplements have been tested to see if they could ward off all sorts of cancers. But results so far have been disappointing. In one study, those taking the supplements actually had a higher risk of prostate cancer. Studies of selenium supplements to prevent heart disease and arthritis have also shown no effect.

Nutritional guidelines have set the upper tolerable limit of selenium intake at 400 micrograms per day for adults, and much less than that for children. Experts recommend limiting intake of Brazil nuts, which can be very high in selenium.

— Markian Hawryluk, The Bulletin

Recommended daily allowance, in microgramsAdults (19+): 55 mcgPregnant women: 60 mcgBreast-feeding women: 70 mcgChildren (0-6 months): 15 mcg*Children (7-12 months): 20 mcg*Children (1-3 years): 20 mcgChildren (4-8 years): 30 mcgChildren (9-13 years): 40 mcgChildren (14-18): 55 mg*No RDA has been set for children younger than 12 months; estimated adequate intake provided instead

Good sourcesBrazil nuts (dried, 1 oz.): 544 mcgTuna (light, canned in oil, 3 oz.): 63 mcgBeef (cooked, 3 oz.): 35 mcgCod (cooked, 3 oz.): 32 mcgEgg (1 whole, medium): 14 mcgRice (white enriched, cooked): 12 mcg

Source: National Institutes of Health, Office of Dietary Supplements