Selenium

How much you need

The official Recommended Dietary Allowance for adults, per day:

People aged 18 fall in the official 14–18 group, which has the same values — 55 mcg a day, 60 mcg in pregnancy and 70 mcg when breastfeeding — and the same upper limit, 400 mcg. Pregnancy and breastfeeding change what you need — confirm yours with your midwife, obstetrician or doctor.

Upper limit

The upper limit covers selenium from food and supplements combined. It is the most that is unlikely to cause harm, not a target, and was set from the amounts linked to brittle and lost hair and nails. It does not apply to people taking selenium under a doctor's care. One ounce of Brazil nuts, the first food listed below, provides 544 mcg — more than the limit — and the fact sheet warns they could cause toxicity if eaten regularly. The European Food Safety Authority set a lower limit in 2023; see Where official guidance disagrees, below.

What it does

Selenium is part of 25 selenoproteins, which play critical roles in thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection ^[1]. The thyroid holds more selenium than any other organ, and selenoproteins convert the thyroid hormone T4 into its active form, T3 ^[1].

Forms

In food, selenium is mainly selenomethionine, with some selenocysteine ^[1]. Supplements contain selenomethionine, selenium-enriched yeast, sodium selenite or sodium selenate, all up to about 90% absorbed ^[1]. Many multivitamins contain 55 mcg; selenium-only supplements typically contain 100 to 400 mcg ^[1].

Where it comes from

1. Brazil nuts (6–8 nuts)1 ounce544 mcg
2. Tuna, yellowfin, cooked3 ounces92 mcg
3. Sardines, canned in oil, drained, with bone3 ounces45 mcg
4. Shrimp, cooked3 ounces42 mcg
5. Pork chop, bone-in, broiled3 ounces37 mcg

How these five were chosen: The five foods with the most selenium per serving in the NIH fact sheet's food table. Fortified foods are marked.

Getting the most from it

Selenium in food is readily absorbed, largely regardless of selenium status ^[1]. The amount in plant foods depends on the soil they grow in and varies widely by region, but animal foods vary much less, because animals regulate their tissue levels ^[1]. Breads, grains, meat, poultry, seafood and eggs are the main sources in the U.S. diet ^[1].

In the United States

Deficiency is very rare in the United States and Canada, because food travels widely ^[1]. In 2017–March 2020 data, average intake from food and drink was 132 mcg a day for men and 94 mcg for women, and 17% to 19% of adults take a supplement containing selenium ^[1]. People who follow vegetarian or vegan diets may get less, and people who smoke tend to have lower selenium status ^[1].

Who is most likely to run low

• People in regions with low-selenium soils whose diets come mainly from local vegetables, such as parts of China ^[1]
• People on long-term kidney dialysis, which removes some selenium from the blood ^[1]
• People living with HIV, who often have low selenium ^[1]

What deficiency looks like

Selenium deficiency alone rarely causes overt illness, but it may make people under other stresses more likely to develop certain diseases ^[1]. It is linked to Keshan disease, a heart muscle disease in low-selenium parts of China, and to Kashin-Beck disease, a form of osteoarthritis, and it could worsen iodine deficiency ^[1]. Plasma or serum selenium of 8 mcg/dL or more is considered sufficient ^[1].

What the evidence supports, and what it doesn't

Established. Selenium is essential for selenoproteins and thyroid hormone metabolism ^[1]. Selenium supplements greatly reduced Keshan disease where deficiency is endemic — by 86% in a 2018 meta-analysis ^[1].

Not supported by trials. Cancer prevention: in SELECT, 35,533 men aged 50 and older who took 200 mcg a day did not have less prostate cancer, and a 2018 Cochrane review of 10 trials concluded that 200 mcg a day does not reduce the overall risk of cancer or of any individual type ^[1]. Heart disease: a Cochrane review of 12 trials in 19,715 mostly male adults found that supplements did not reduce cardiovascular events ^[1]. Cognitive decline: in 7,540 healthy men aged 60 and older, 200 mcg a day for 5.4 years did not reduce dementia ^[1].

Mixed. Thyroid: in chronic autoimmune thyroiditis, supplements lowered thyroid antibodies in some trials, but a systematic review found no effect on thyroid function or quality of life, and the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid antibodies ^[1]. In SELECT, men with higher selenium status at the start had more high-grade prostate cancer when supplemented, and the researchers recommended that men over 55 not take more than the recommended amount from supplements ^[1]. Male fertility: trials conflict ^[1].

Where official guidance disagrees

In 2023, the European Food Safety Authority set an upper limit for selenium of 255 mcg a day for all adults, including women who are pregnant or breastfeeding, based on systematic reviews of hair loss ^[1]. The U.S. limit is 400 mcg ^[1].

Too much

Chronically high intakes cause selenosis: early signs are a garlic smell on the breath and a metallic taste, and the most common are hair loss and brittle or lost nails, with skin rash, nausea, diarrhea, fatigue, irritability and nervous system problems ^[1]. Brazil nuts contain 68 to 91 mcg per nut ^[1]. Acute toxicity has come from wrongly formulated products — in 2008, 201 people had severe reactions to a liquid supplement with 200 times the labelled amount — and can cause respiratory distress, heart attack, kidney and heart failure and, rarely, death ^[1].

Medicines that interact

Cisplatin, a chemotherapy drug, can lower selenium levels, although whether that matters clinically is not known, and a Cochrane review found too little evidence that selenium eases chemotherapy side effects ^[1]. If you take cisplatin or other medicines regularly, talk to your doctor or pharmacist before taking a selenium supplement.