Selenium is a trace mineral, needed only in small amounts in the body. It is needed on a daily basis and plays an important role in human health, as it is required for the function of many enzymes.
Selenium plays an important role as an antioxidant. It protects the body against damage by free radicals, and works in association with other antioxidants such as vitamin C and vitamin E. Thus, it may have a protective role against cancer, heart disease and joint inflammation due to its antioxidant function.
Other functions of selenium include support and proper functioning of thyroid gland as it is essential for the production of thyroid hormone. Selenium also helps to maintain and promote the immune system.
 Food Sources
The selenium content of foodstuffs varies widely. Plants do not appear to require selenium and hence, are not a very good dietary source of this important mineral. However, the selenium content in plants is dependent on the soil in which they grow. Generally, plants grown in a selenium rich soil will have high concentration of selenium.
Animals feeding on selenium rich plants will also have high selenium and contribute to the dietary sources. Some of the good sources of this nutrient are organ meats, muscle meats, fish such as salmon, shrimp and tuna, plant foods such as grains, sunflower seeds, mushrooms and oats. Some nuts have considerable amount of selenium, the richest being Brazil nuts.
Loss of selenium on cooking and processing varies with the procedure and the kind of food. Selenium from plant foods is more susceptible to loss than that from animal foods on cooking.
 Recommended Dosage
The Recommended Dietary Allowances (RDA) for selenium on a daily basis, set in 2000 by the Institute of Medicine at the National Academy of Sciences, is as follows:
- Males and females, 1-3 years: 20 micrograms
- Males and females, 4-8 years: 30 micrograms
- Males and females, 9-13 years: 40 micrograms
- Males and females, 14 years and older: 55 micrograms
- Pregnant females: 60 micrograms
- Lactating females: 70 micrograms
For infants, an Adequate Intake (AI) has been established which is 15 micrograms for the first six months of life, and 20 micrograms for the next six months. Adequate Intake is given when there is not sufficient data to establish the RDA.
Selenium deficiency has been observed in areas where the soil is poor in selenium content. People, especially children, in large areas of China suffer from selenium deficiency, as the soil concentration of selenium is very low. The conditions associated with selenium deficiency are Keshan Disease, which involves the heart and leads to enlargement and arrhythmias of the heart.
Another specific disease due to selenium deficiency is Kashin-Beck Disease, which involves deterioration of the joint tissues. Apart from dietary deficiency, other causes can also lead to selenium deficiency. These include severe gastrointestinal disorders when absorption of selenium is affected and severe overall malnutrition. Symptoms associated with this type of selenium deficiency include muscle pain, white nails, weakness and hair discoloration.
Selenium toxicity from dietary sources is quite rare. It has been associated more with supplements and leads to a condition called selenosis. Symptoms observed include skin inflammation, hair loss, nail brittleness, fatigue and irritability. The upper intake level for selenium to prevent toxicity has been established by the Food and Nutrition Board of the Institute of Medicine as 400 micrograms daily for adults.
- What can high-selenium foods do for you?
- Micronutrient Information Center
- Nutrition Fact Sheet: Selenium