Selenium (Elemental Se)

Direct Answer

Selenium is an essential trace element incorporated as Selenocysteine (the 21st amino acid) into 25 human selenoproteins including Glutathione Peroxidases (GPx) and Iodothyronine Deiodinases (T4->T3 conversion). Adult RDA is 55 µg, with a Tolerable Upper Limit of 400 µg/day.

Source: NIH ODS / EFSA · Last reviewed Aug 28, 2026

Specs

Nutrient Type	Essential Trace Element
Adult RDA / RDI	55 µg / day (60 µg pregnancy, 70 µg lactation)
Tolerable Upper Limit (UL)	400 µg / day (Avoid selenosis / garlic breath, hair/nail brittleness)
Chemical Forms	Selenomethionine (Plant organic), Selenocysteine (Animal selenoprotein), Sodium Selenite / Selenate (Inorganic)
Primary Biological Function	Glutathione Peroxidase (GPx1-4) lipid hydroperoxide neutralization, Iodothyronine Deiodinases (Converts thyroid T4 into active T3), Thioredoxin Reductase (TXNRD)
Deficiency Condition	Keshan Disease (Endemic juvenile cardiomyopathy), Kashin-Beck Disease (Deforming osteoarthropathy), Impaired thyroid metabolism
Top Whole Food Sources	Brazil Nuts (1 single nut provides ~70–90µg / 130–160% RDA), Eastern Oysters, Yellowfin Tuna, Halibut, Sardines, Beef Liver, Eggs
Soil Selenium Geographical Variability	Plant selenium reflects regional soil content; volcanic soils (South America, Great Plains) are rich, while parts of China, Scandinavia, and New Zealand are depleted

The Brazil Nut Powerhouse Warning

A single Brazil nut (Bertholletia excelsa) can contain up to 90 µg of selenium. Eating more than 3–4 Brazil nuts daily chronically exceeds the 400 µg Tolerable Upper Limit and can cause selenosis.