Selenium

Selenium was discovered in 1817 by Swedish chemist Jöns Jacob Berzelius, who named it after the moon goddess Selene because of its association with tellurium (named for Earth) [1]. Its essentiality to mammals wasn't established until 1957, when Klaus Schwarz showed it prevented liver necrosis in vitamin E-deficient rats. The dramatic demonstration of selenium's geographic variation came from contrasting Chinese provinces: Keshan disease (a fatal cardiomyopathy affecting children) was endemic in low-selenium soil regions, while Kashin-Beck disease (a disabling joint disease) was linked to selenium deficiency in separate regions — and industrial selenosis with hair loss and nerve damage occurred in selenium-rich areas of China [2]. The United States has similar geographic variation: Great Plains soils are selenium-rich (supporting cattle health but capable of causing selenosis in livestock), while Pacific Northwest soils are relatively selenium-poor [1]. Industrial selenium sources include copper and lead smelting (selenium is a byproduct), electronics manufacturing (selenium rectifiers, photoelectric cells), and glass decolorization [2].

Dietary intake from food grown in selenium-sufficient soils is the primary healthy exposure route — the U.S. RDA is 55 µg/day and the tolerable upper limit is 400 µg/day [1]. Brazil nuts from the Amazon basin are extraordinarily selenium-rich, with a single nut often containing 50-90 µg — multiple daily Brazil nuts can approach or exceed the upper limit [2]. Selenium supplements, widely marketed for cancer prevention (following the Nutritional Prevention of Cancer trial in the 1990s), have been a source of inadvertent excess — the SELECT trial showed high-dose selenium supplements failed to prevent prostate cancer and potentially increased risk in some groups [1]. Occupational inhalation occurs in copper smelting, electronics manufacturing, and glass production. Contaminated groundwater from mining or industrial areas is an environmental exposure pathway [2].

Selenium functions primarily through selenoproteins — proteins containing the unusual amino acid selenocysteine encoded by UGA codons [1]. Glutathione peroxidase (GPx) uses selenium to reduce peroxides and oxidized lipids, playing a central role in antioxidant defense. Thioredoxin reductase uses selenium in the reduction of thioredoxin, critical for ribonucleotide reductase activity in DNA synthesis [2]. At deficient levels: impaired thyroid hormone activation (selenoenzyme iodothyronine deiodinase), impaired antioxidant defense, and at severe deficiency, Keshan disease cardiomyopathy. At toxic levels (selenosis): hair and nail loss, peripheral neuropathy, garlic-smelling breath (dimethylselenide), and at extreme doses, acute respiratory distress and liver failure [1]. The therapeutic window is narrow: the difference between the recommended dietary intake (55 µg/day) and the tolerable upper intake level (400 µg/day) is roughly 7-fold [2].

Populations in low-selenium soil regions (parts of China, New Zealand, Finland before food fortification, some Pacific Northwest areas) are at risk of deficiency [1]. People taking high-dose selenium supplements (>200 µg/day) risk selenosis over time [2]. Residents in selenium-hyperaccumulator plant regions or near selenium-rich agricultural drainage (e.g., Kesterson Reservoir contamination in California) face elevated environmental exposure [1]. Workers in copper smelting and electronics manufacturing face occupational exposure via inhalation [2]. Patients with chronic kidney disease on dialysis often become selenium-deficient [1].

1. Do not exceed selenium supplement doses of 200 µg/day — the tolerable upper limit is 400 µg/day and chronic toxicity (hair loss, neuropathy) occurs with regular excess [1]. 2. Limit Brazil nut intake to 1-3 per day maximum given their highly variable and potentially very high selenium content [2]. 3. Test your groundwater for selenium if you live near mining operations, fly ash ponds, or selenium-rich agricultural drainage areas [1]. 4. For selenium deficiency: increase intake of seafood, Brazil nuts (1/day), meat, whole grains, and dairy — selenium content depends heavily on soil selenium where food was grown [2].