Selenium, Se

Selenium is an essential trace mineral the body needs in very small amounts to build a family of proteins that protect cells from oxidative damage and help regulate thyroid hormone. Needs are easily met through diet — a single Brazil nut can exceed a full day's requirement — with seafood, organ meats, and eggs as other concentrated sources.

What Is Selenium

Selenium works through selenoproteins, a group of roughly 25 human proteins built around the amino acid selenocysteine, according to the Linus Pauling Institute's Micronutrient Information Center. Key selenoproteins include glutathione peroxidases and thioredoxin reductases, which neutralize reactive oxygen species and support cellular antioxidant defense; iodothyronine deiodinases, which convert inactive thyroid hormone (T4) into its active form (T3); and selenoprotein P, the body's main selenium transport protein, particularly important for brain and testicular tissue.

Selenium content in plant foods varies with the selenium level of the soil they're grown in, so seafood and organ meats are more dependable sources. The foods measured in this database with the highest selenium content include Brazil nuts by a wide margin, followed by tuna, sardines, organ meats such as beef kidney, shiitake mushrooms, and enriched grain products like whole wheat bread.

Health Benefits of Selenium

Antioxidant defense. The Linus Pauling Institute describes selenium-dependent glutathione peroxidases and thioredoxin reductases as central to the body's system for reducing oxidative stress, working alongside vitamins C and E to regenerate each other's antioxidant activity.

Thyroid hormone regulation. According to Cleveland Clinic, the thyroid gland holds the body's highest concentration of selenium, which is converted into selenoproteins that help regulate thyroid hormone levels; the Linus Pauling Institute notes this includes the enzymes that convert T4 into active T3.

Immune function. The Linus Pauling Institute's research summary associates adequate selenium status with the normal migration, proliferation, differentiation, and activation of immune cells.

Cancer risk — evidence is mixed, not protective at supplemental doses. Early observational research suggested lower selenium status was associated with higher cancer risk, but the large SELECT trial (more than 35,000 men) found that 200 mcg/day of supplemental selenium did not reduce prostate, lung, or colorectal cancer risk in men who already had adequate selenium status, per the Linus Pauling Institute — and was associated with an increased risk of type 2 diabetes at that dose.

How Much Selenium You Need

An established RDA (or AI for infants) exists, per the NIH Office of Dietary Supplements' Selenium Health Professional Fact Sheet (figures corroborated via search; this ODS fact sheet blocks automated fetches, see Sourcing notes):

• Infants 0–6 months: 15 mcg/day (AI)
• Infants 7–12 months: 20 mcg/day (AI)
• Children 1–3 years: 20 mcg/day
• Children 4–8 years: 30 mcg/day
• Children 9–13 years: 40 mcg/day
• Teens 14–18 years: 55 mcg/day
• Adults 19+ years: 55 mcg/day
• Pregnancy: 60 mcg/day
• Lactation: 70 mcg/day

The Tolerable Upper Intake Level (UL) is 400 mcg/day for teens 14–18 and adults, including pregnancy and lactation. UL figures for children 1–3, 4–8, and 9–13 years were not independently confirmed this session (see Sourcing notes).

Signs of Selenium Deficiency

True deficiency is rare in the U.S. and mainly affects people on long-term intravenous feeding or with severe gastrointestinal disorders such as Crohn's disease that impair absorption, per MedlinePlus. In regions with selenium-poor soil, documented deficiency diseases include Keshan disease, a potentially fatal heart muscle condition historically affecting children and young women in parts of China; Kashin-Beck disease, a joint and bone disorder that has affected an estimated 2.5 million people across China, Tibet, North Korea, and Siberia per the Linus Pauling Institute; and myxedematous endemic cretinism. Cleveland Clinic also notes that selenium deficiency can worsen the effects of iodine deficiency on the thyroid.

Signs of Selenium Excess

Excess selenium causes selenosis, with signs including brittle or lost hair and nails, a metallic taste in the mouth, garlic-odor breath, gastrointestinal upset, fatigue, skin rashes, and mild nerve damage, per MedlinePlus and Cleveland Clinic. Severe toxicity, which is rare, can involve kidney failure and heart complications.

Warnings and Interactions

Thyroid and iodine. Because selenoproteins are required to activate thyroid hormone, selenium status interacts with iodine status; Cleveland Clinic notes selenium deficiency can compound the thyroid effects of low iodine intake.

Antioxidant nutrients. The Linus Pauling Institute notes selenium works in synergy with vitamin C, vitamin E, and coenzyme Q10, helping regenerate their active, reduced forms.

Populations who should consult a healthcare provider before supplementing, per Cleveland Clinic's patient guidance on selenium supplements: people who are pregnant or trying to conceive, people who are breastfeeding, and children — as well as anyone taking other medications or supplements, since selenium supplements may interact with them.

Above the UL. Supplementing beyond the RDA in people who are already selenium-replete has shown no added benefit and, in the SELECT trial, was linked to a higher risk of type 2 diabetes at 200 mcg/day — well under the 400 mcg/day UL, underscoring that "more is better" does not apply here. Sustained intake above the UL risks selenosis as described above, and severe overdose warrants contacting a poison control center or emergency care, per the same Cleveland Clinic guidance.

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