Selenium

What Is Selenium?

Selenium is an essential trace mineral that the body incorporates into proteins as selenocysteine, a unique amino acid sometimes called the 21st amino acid. The adult body contains approximately 15 mg of selenium, concentrated in the thyroid gland, liver, kidney, and testes. Selenium content in food varies widely depending on the selenium concentration of the soil in which crops were grown, making geographic location a key determinant of dietary selenium status.

Key Functions

• Antioxidant defense: Selenium is the catalytic component of glutathione peroxidase (GPx) enzymes, which neutralize hydrogen peroxide and lipid hydroperoxides, protecting cell membranes and DNA from oxidative damage.
• Thyroid hormone metabolism: Iodothyronine deiodinases — selenium-dependent enzymes — convert inactive thyroxine (T4) into the active thyroid hormone triiodothyronine (T3) in peripheral tissues.
• Immune function: Selenium supports the proliferation and activity of T cells and natural killer cells. Deficiency increases susceptibility to viral infections, and some viruses mutate more rapidly in selenium-deficient hosts.
• Reproduction: Selenoprotein P and phospholipid hydroperoxide GPx protect sperm from oxidative damage; selenium is important for male fertility.
• Cancer prevention (emerging evidence): Epidemiological data suggest inverse associations between selenium status and risks of colorectal, prostate, and lung cancers, though large clinical trials have produced mixed results.

Dietary Reference Values and Food Sources

The RDA for selenium is 55 mcg/day for adults. The UL is 400 mcg/day, as excess selenium (selenosis) is toxic. Brazil nuts are extraordinarily rich sources — a single nut can provide 68-91 mcg, so they should not be consumed in large quantities. Other sources:

• Brazil nuts (1 nut): ~68-91 mcg
• Yellowfin tuna (3 oz): ~92 mcg
• Halibut (3 oz): ~47 mcg
• Enriched pasta (1 cup): ~37 mcg
• Pork chop (3 oz): ~33 mcg
• Beef (3 oz): ~28 mcg
• Cottage cheese (1 cup): ~20 mcg

Deficiency and Toxicity

Selenium deficiency is most common in regions with low-selenium soil (parts of China, New Zealand, and Eastern Europe). Keshan disease — a potentially fatal cardiomyopathy — occurs in severely selenium-deficient populations, particularly children and pregnant women. Kashin-Beck disease is a selenium-associated osteoarthropathy. Selenosis (toxicity) causes brittle hair and nails, garlic-breath odor (from dimethyl selenide), skin rashes, fatigue, and neurological symptoms.