Selenium: Benefits, Dosage, and Safety

What selenium is

Selenium is an essential trace mineral naturally present in many foods, added to some, and available as a dietary supplement. In the body it is incorporated into selenocysteine, the form found in 25 human selenoproteins, including the glutathione peroxidases, thioredoxin reductases, and selenoprotein P. In foods it occurs primarily as selenomethionine along with selenocysteine, and dietary selenium is readily absorbed regardless of selenium status. The selenium content of plant foods varies widely by geographic location because it depends on the amount and form of selenium in the soil.

What it does

• Serves as a component of 25 selenoproteins, including glutathione peroxidases, thioredoxin reductases, and selenoprotein P
• Provides antioxidant protection, helping protect cells from oxidative damage
• Plays a critical role in thyroid hormone metabolism, including the conversion of the prohormone T4 to the active T3
• Supports DNA synthesis
• Is needed for normal reproduction
• Contributes to protection from infection and normal immune function

How much selenium per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Selenium deficiency on its own rarely causes overt illness but produces biochemical changes that may predispose people under additional stress to certain conditions. In selenium-poor regions it is associated with Keshan disease, an endemic cardiomyopathy first identified in low-selenium areas of China (adults there had average intakes of no more than 10 mcg per day; at least 20 mcg per day is needed to protect against it), and with Kashin-Beck disease, a type of osteoarthritis presenting in childhood and puberty in certain low-selenium, low-iodine areas. Selenium deficiency can also exacerbate iodine deficiency, potentially raising the risk of congenital hypothyroidism in infants.

Who may need more

• People living in selenium-deficient geographic regions
• People undergoing long-term hemodialysis
• People living with HIV
• People following vegetarian or vegan dietary patterns, who may have lower selenium intakes

Food sources

Brazil nuts are exceptionally high (about 544 mcg per ounce, roughly 6 to 8 nuts). Other rich sources are protein-rich foods: seafood such as yellowfin tuna (92 mcg per 3 ounces), sardines, and shrimp; meat and poultry such as pork, beef, turkey, and chicken; organ meats such as beef liver; and eggs (15 mcg per large egg). Grains and dairy also contribute. In US diets the major sources are breads, grains, meat, poultry, seafood, and eggs. Plant-food content varies widely by soil selenium.

Forms and absorption

Common supplement forms are selenomethionine, selenium-enriched yeast (grown in a high-selenium medium, predominantly as selenomethionine), sodium selenite, and sodium selenate. The body absorbs up to about 90% of selenium from all of these forms. Many multivitamin/mineral products contain 55 mcg; selenium combined with vitamin E or other ingredients generally provides 50 to 200 mcg, and selenium-only supplements typically contain 100 to 400 mcg. Organic forms (selenomethionine, selenium-enriched yeast) appear to be somewhat better absorbed than inorganic sodium selenite in some comparisons.

Safety, excess and interactions

Too much

Chronically high intakes cause selenosis. The most common clinical signs are hair loss and nail brittleness or loss. Early indicators of excess intake are a garlic odor in the breath and a metallic taste in the mouth. Other signs and symptoms include skin rash, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities. Acute selenium toxicity, seen with misformulated over-the-counter products, can cause severe gastrointestinal and neurological symptoms, acute respiratory distress syndrome, myocardial infarction, hair loss, muscle tenderness, tremors, lightheadedness, facial flushing, kidney failure, cardiac failure, and, in rare cases, death. Brazil nuts contain very high amounts (68 to 91 mcg per nut) and could cause toxicity if consumed regularly.

Interactions to know

• Cisplatin: this platinum chemotherapy agent can reduce selenium levels in hair and serum; whether these reductions are clinically significant is not known, and a Cochrane Review found insufficient evidence that selenium supplementation alleviates chemotherapy side effects.
• Selenium supplements may interact with medications, and certain medications can affect body selenium levels; people taking medications regularly should discuss their selenium status with a health care provider.

How status is measured

Selenium status is most commonly measured by plasma or serum selenium concentration; a level of 8 mcg/dL or higher is considered sufficient for selenoprotein synthesis in healthy people. Plasma, serum, and urine reflect recent intake, while whole blood reflects long-term status, and hair and nail content indicate intake over months to years. The plasma selenoproteins glutathione peroxidase 3 and selenoprotein P can serve as functional biomarkers but may be unreliable because inflammation and oxidative stress affect them.

Regulated as a dietary supplement ingredient under DSHEA, not as a drug; supplements are not FDA-approved to diagnose, treat, cure, or prevent disease. The FDA Daily Value for selenium is 55 mcg for adults and children age 4 and older. Since 2003 the FDA has permitted limited qualified health claims that selenium "may reduce the risk of certain forms of cancer" and "may produce anticarcinogenic effects," while stating the evidence is limited and not conclusive; in 2009 it denied claims for many cancers and characterized remaining claims as highly uncertain or highly unlikely.

What the evidence supports

We grade the strength of evidence behind each common reason people take selenium, mirroring how the NIH describes it.