The RDA for selenium is set by assuming a coefficient of variation (CV) of 10 percent (see Chapter 1) because information is not available on the standard deviation of the requirement for selenium; the RDA is defined as equal to the EAR plus twice the CV to cover the needs of 97 to 98 percent of the individuals in the group (therefore, for selenium the RDA is 120 percent of the EAR). The calculated RDA is rounded to the nearest 5 µg.

The selenium content of food varies depending on the selenium content of the soil where the animal was raised or the plant was grown: organ meats and seafood, 0.4 to 1.5 µg/g; muscle meats, 0.1 to 0.4 µg/g; cereals and grains, less than 0.1 to greater than 0.8 µg/g; dairy products, less than 0.1 to 0.3 µg/g; and fruits and vegetables, less than 0.1 µg/g (WHO, 1987). Thus the same foodstuffs may have more than a ten-fold difference in selenium content. Plants do not appear to require selenium and most selenium metabolism by plants occurs through sulfur pathways in which selenium substitutes for sulfur. Thus, plant content of selenium depends on the availability of the element in the soil where the plant was grown. This means that wheat grown in a low-selenium soil will have a low selenium content, whereas the same wheat variety grown in a high-selenium soil will have a high selenium content. For this reason, food tables that reflect average selenium contents are unreliable. Much plant selenium is in the form of selenomethionine, selenocysteine, or selenocysteine metabolites. Other organic forms of the element are known to exist, including some that have not yet been identified.

Unlike plants, animals require selenium. Meat and seafood are therefore reliable dietary sources of selenium. Meat and seafood contain selenium in its functional form as selenoproteins. Virtually all animal proteins contain selenomethionine obtained when the animal consumes selenium from plants. This means that meat varies in its selenium content depending largely on the selenomethionine intake of the animal.