Selenium

Selenium is a chemical element and a micronutrient that the body needs in trace quantities ^[1]. Rather than acting freely, it exerts its biological effects after being incorporated as the amino acid selenocysteine into selenoproteins, and the human genome encodes around twenty-five of them ^[1][2]. Key examples include the glutathione peroxidases, which neutralize hydrogen peroxide and other reactive oxygen species, thioredoxin reductases, which support antioxidant defense and DNA synthesis, and the iodothyronine deiodinases, which activate and inactivate thyroid hormones ^[2].

Through these proteins, selenium contributes to antioxidant protection, healthy immune and inflammatory responses, thyroid hormone metabolism, and reproduction in both sexes ^[1][2]. Low selenium status has been associated with increased mortality, impaired immunity, cognitive decline, and a higher risk of autoimmune thyroid disease, while adequate status supports these systems ^[1].

The selenium content of food depends heavily on the selenium in the soil where crops are grown or animals are raised. Rich dietary sources include Brazil nuts, seafood such as tuna and shellfish, organ meats, eggs, and cereal grains ^[1]. In regions with selenium-poor soils, deficiency has historically contributed to conditions such as Keshan disease, a heart muscle disorder, and Kashin-Beck disease, a bone and joint disorder, though pure selenium deficiency is uncommon and often interacts with other factors such as iodine status ^[1].

Selenium supplements have been widely studied, with mixed results. Observational studies have linked higher selenium status to lower risk of some cancers, but a Cochrane review of randomized trials found no convincing evidence that supplementation prevents cancer, and some trials reported increased risks of high-grade prostate cancer and type 2 diabetes in already-replete individuals ^[3]. This reinforces the central lesson that supplementation is likely to help only those whose intake is genuinely low ^[1][3].

Selenium is available in foods and in supplements as inorganic forms such as sodium selenite and selenate and organic forms such as selenomethionine and selenium-enriched yeast ^[1]. It is regulated as a nutrient, with recommended intakes and a defined safe upper limit ^[1]. Excess intake causes selenosis, whose signs include a garlic-like odor on the breath, gastrointestinal upset, hair and nail loss, fatigue, irritability, and, in severe cases, neurological damage, underscoring the narrow window between beneficial and harmful intake ^[1].

At normal intakes selenium is safe, but chronic high intake causes selenosis: hair and nail brittleness or loss, garlic breath, GI upset, and neurological issues. The tolerable upper limit for adults is around 400 mcg per day from all sources. Brazil nuts vary widely in content, so eating many daily can push intake high.