Selenium

Until 1979, it was not known that selenium was an essential mineral. The first RDA for selenium was announced in 1989. Selenium is an important part of antioxidant enzymes that protect cells against the effects of free radicals that are produced during normal oxygen metabolism. Antioxidants help control levels of free radicals, which can damage cells and contribute to the development of some chronic diseases. Selenium is also essential for normal functioning of the immune system and thyroid gland.

Plant foods are the major dietary sources of selenium. The amount of selenium in soil, which varies by region, determines the amount of selenium in the plant foods that are grown in that soil. Researchers know that soils in the high plains of northern Nebraska and the Dakotas have very high levels of selenium. People living in those regions generally have the highest selenium intakes in the United States.

Selenium can also be found in some meats and seafood (see Table 7-8). Animals that eat grains or plants that were grown in selenium-rich soil have higher levels of selenium in their muscle. In the United States, meat and bread are common sources of dietary selenium. Some nuts, in particular Brazil nuts and walnuts, are also very good sources of selenium.

Selenium deficiency is most commonly seen in parts of China where the selenium content in the soil, and therefore selenium intake, is very low. Selenium deficiency is linked to Keshan disease (it is named after the province in China where it was studied), in which the heart becomes enlarged and does not function properly. Selenium deficiency also may affect thyroid function because selenium is essential for the synthesis of active thyroid hormone. Researchers also believe selenium deficiency may worsen the effects of iodine deficiency on thyroid function, and that adequate selenium nutritional status may help protect against some of the effects of iodine deficiency. Selenium deficiency is rare in the United States.

There is a moderate to high health risk associated with too much selenium. High blood levels of selenium can result in a condition called selenosis. Symptoms include gastrointestinal upset, hair loss, white blotchy nails, and mild nerve damage. Selenium toxicity is rare in the United States.

Selenium Content of Foods 

Food	Micrograms	% Daily Value
Brazil nuts, unblanched, 1 oz.	840	1200
Tuna, canned in oil, drained, 3-1/2 oz.	78	111
Beef/calf liver, 3 oz.	48	69
Cod, cooked with dry heat, 3 oz.	40	57
Noodles, enriched, boiled, 1 cup	35	50
Macaroni and cheese (box mix), 1 cup	32	46
Turkey breast, oven roasted, 3-1/2 oz.	31	44
Macaroni, elbow, enriched, boiled, 1 cup	30	43
Spaghetti with meat sauce, 1 cup	25	36
Chicken, meat only, 1/2 breast	24	34
Beef chuck roast, lean only, oven-roasted, 3 oz.	23	33
Bread, enriched, whole-wheat, 2 slices	14	20
Rice, enriched, long-grain, cooked, 1 cup	14	20
Cottage cheese, lowfat (2%), 1/2 cup	11	16
Walnuts, black, 1 oz.	5	7
Cheddar cheese, 1 oz.	4	6
Source: Facts About Dietary Supplements. Office of Dietary Supplements, National Institutes of Health, 2001.