Selenium is a trace element that is naturally present in many foods, added to others, and available as a dietary supplement. Selenium, which is nutritionally essential for humans, is a constituent of more than two dozen selenoproteins that play critical roles in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection [1].

Selenium exists in two forms: inorganic (selenate and selenite) and organic (selenomethionine and selenocysteine) [2]. Both forms can be good dietary sources of selenium [3]. Soils contain inorganic selenites and selenates that plants accumulate and convert to organic forms, mostly selenocysteine and selenomethionine and their methylated derivatives.

Most selenium is in the form of selenomethionine in animal and human tissues, where it can be incorporated nonspecifically with the amino acid methionine in body proteins. Skeletal muscle is the major site of selenium storage, accounting for approximately 28% to 46% of the total selenium pool [3]. Both selenocysteine and selenite are reduced to generate hydrogen selenide, which in turn is converted to selenophosphate for selenoprotein biosynthesis [4].

The most commonly used measures of selenium status are plasma and serum selenium concentrations [1]. Concentrations in blood and urine reflect recent selenium intake. Analyses of hair or nail selenium content can be used to monitor longer-term intakes over months or years. Quantification of one or more selenoproteins (such as glutathione peroxidase and selenoprotein P) is also used as a functional measure of selenium status [3]. Plasma or serum selenium concentrations of 8 micrograms (mcg)/dL or higher in healthy people typically meet needs for selenoprotein synthesis [5].

Sources of Selenium

Food

Seafoods and organ meats are the richest food sources of selenium [1]. Other sources include muscle meats, cereals and other grains, and dairy products. The amount of selenium in drinking water is not nutritionally significant in most geographic regions [2,6]. The major food sources of selenium in the American diet are breads, grains, meat, poultry, fish, and eggs [7].

The amount of selenium in a given type of plant-based food depends on the amount of selenium in the soil and several other factors, such as soil pH, amount of organic matter in the soil, and whether the selenium is in a form that is amenable to plant uptake [2,6,8,9]. As a result, selenium concentrations in plant-based foods vary widely by geographic location [1,2]. For example, according to the U.S. Department of Agriculture Food Composition Database, Brazil nuts have 544 mcg selenium/ounce, but values from other analyses vary widely [10-12].

The selenium content of soil affects the amounts of selenium in the plants that animals eat, so the quantities of selenium in animal products also vary [2,5]. However, selenium concentration in soil has a smaller effect on selenium levels in animal products than in plant-based foods because animals maintain predictable tissue concentrations of selenium through homeostatic mechanisms. Furthermore, formulated livestock feeds generally contain the same levels of selenium.

Several food sources of selenium are listed in Table 2.

*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet. The DV for selenium is 70 mcg for adults and children aged 4 and older. Foods providing 20% or more of the DV are considered to be high sources of a nutrient. The U.S. Department of Agriculture’s (USDA’s) Nutrient Database Web site [10] lists the nutrient content of many foods and provides a comprehensive list of foods containing selenium arranged by nutrient content and by food name.