Selenium

Selenium is incorporated into 25 known selenoproteins as the amino acid selenocysteine, earning it the designation of the 21st amino acid. The most important selenoproteins include glutathione peroxidases (GPx), which protect against oxidative damage, and iodothyronine deiodinases, which convert thyroid hormone T4 into the active T3 form. Selenium also supports selenoprotein P, the primary selenium transport protein in plasma. Soil selenium content varies dramatically by region, creating geographic disparities in dietary intake. Brazil nuts are the richest known food source, with a single nut providing 68–91 µg. The tolerable upper intake level for adults is 400 µg/day, above which selenosis—characterized by garlic breath, hair loss, and nail brittleness—can occur.