Selenium

Selenium is an essential mineral, not a vitamin. The body builds it into 25 selenoproteins, says the NIH (the US National Institutes of Health). They include enzymes that protect cells from oxidative damage, enzymes that help make thyroid hormone, and selenoprotein P, which carries selenium in the blood.

Its chemistry is unusual: selenium enters a protein as selenocysteine. The code that normally means “stop” (UGA) is read as “insert selenocysteine” when a special RNA structure in the gene’s message tells the cell to.

Here is the part most people do not know: how much selenium a food holds depends on the soil it grew in. The NIH says plant foods vary widely by region. A study of Brazil nuts from the Brazilian Amazon found median selenium of 2.07 mg per kg in one state. In another state it was 68.15 mg per kg.

So a single nut could supply anything from 11% to 288% of a 70 µg (microgram) daily requirement. In the US, the issue is rarely too little. Average intake from food and drinks is 108 µg a day, nearly twice the 55 µg RDA (the recommended daily amount).

The gap between enough and too much is narrow. The US upper limit is 400 µg a day, and EFSA, the EU food safety agency, sets 255 µg. In 2008 a mislabeled liquid supplement holding 200 times the stated amount of selenium poisoned 201 people in 10 states.

Where soil selenium is very low, as in parts of China, deficiency is tied to Keshan disease. Where intake is already adequate, large trials found that extra selenium did not prevent cancer or heart disease. This page sets out what the studies show.

Health benefits of selenium

• ★★★No lower cancer risk from selenium supplementsIn 3 trials at low risk of bias (19,475 people), any cancer was 1.01 times as likely with selenium supplements, so no change. The largest trial found more hair loss and skin inflammation, and the trials showed slightly higher type 2 diabetes risk.
• ★★★No lower risk of heart disease or deathIn 12 trials of selenium-only supplements (19,715 people), death from any cause was 0.97 times as likely and heart events 1.03 times. Type 2 diabetes was slightly more common (1.06 times), which could be chance.
• ★★Protected against Keshan disease where it is commonIn 41 community studies in areas where Keshan disease is common, supplemented groups had 0.14 times the risk of controls. The authors conclude that selenium deficiency is a cause of the disease.
• ★★Lower thyroid antibodies in Hashimoto thyroiditisIn a pool of 35 randomized studies of selenium supplements, thyroid peroxidase antibodies fell, and TSH fell slightly in people not on thyroid hormone. These are blood markers, not symptoms.
• ★★Helped mild Graves’ eye diseaseIn 159 people with mild Graves’ orbitopathy, 6 months of selenium (100 µg twice daily) improved quality of life and eye involvement and slowed progression.

Foods high in selenium

Brazil nuts, seafood and meat lead. Brazil nuts top our USDA ranking by a wide margin. One ounce (28 g) holds 543 µg of selenium, 988% of the 55 µg Daily Value. A 100 g portion holds 1,917 µg.

Oysters follow at 65.5 µg per 3 ounces (85 g), then mackerel and sardines. Wild Atlantic salmon, tuna and herring give 31 µg per 3 ounces each. A chicken drumstick has 26.9 µg and a chicken breast 25.8 µg.

Per 100 kcal, Brazil nuts (291 µg) lead, followed by oysters (95.1 µg), white mushrooms (42.3 µg) and shrimp (41.6 µg). The ranking below has the full list.

Selenium is bound to protein in food, so protein-rich foods tend to be the best sources, the NIH says. Its table adds cooked yellowfin tuna at 92 µg per 3 ounces and shrimp at 42 µg. A broiled pork chop has 37 µg and a cup of cooked spaghetti 33 µg. A hard-boiled egg has 15 µg, a cup of cooked brown rice 12 µg and a slice of whole-wheat bread 8 µg.

Plant foods vary with the soil, and so do Brazil nuts. The NIH puts them at 68 to 91 µg per nut. The Amazon study found a nut could supply 11% to 288% of a 70 µg need, depending on where it grew. Animal foods are steadier, because animals keep predictable tissue levels.

Absorption is good: the NIH says up to about 90% of the selenium in supplement forms is absorbed. Fish selenium was highly bioavailable (well absorbed) in a study of 35 men. And 2 Brazil nuts raised selenium status as much as 100 µg of selenomethionine in a New Zealand trial.

How selenium works

Most of the biochemistry below is well established. But it comes from reviews, lab work and human physiology studies, not from trials of extra selenium. Four ideas cover most of it.

Selenium is built into 25 selenoproteins, and the thyroid depends on them. The liver steers selenium to the tissues that need it. And the body’s needs level off long before the upper limit. Then comes what has been seen in people.

It is built into 25 proteins as selenocysteine

The human genome codes for 25 selenoproteins, according to a 2003 survey of mammalian genomes. In the genetic code, UGA is normally a stop signal. But it can also stand for selenocysteine, and that is how selenium gets into the protein.

A 1986 study of the mouse glutathione peroxidase gene found that the selenocysteine in the enzyme’s active site is encoded by this “termination” codon. In 1973, researchers had shown that selenium is part of glutathione peroxidase, which protects red cells from oxidative damage.

The NIH lists three main families: glutathione peroxidases, thioredoxin reductases and selenoprotein P. These proteins have roles in thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection.

The thyroid depends on it

The thyroid holds more selenium than any other organ, says the NIH. Selenoproteins called deiodinases convert the prohormone T4 into active T3. Glutathione peroxidase and thioredoxin reductase protect the gland from the hydrogen peroxide made during hormone synthesis. Like iodine, selenium is part of thyroid hormone production.

The NIH adds that selenium deficiency could worsen iodine deficiency and raise the risk of congenital hypothyroidism (low thyroid from birth) in infants. A 2020 review notes that adverse effects have been seen at both extremes of intake, with a narrow optimum range.

The liver steers selenium to the tissues that need it

Once absorbed, selenomethionine and inorganic selenium are quickly turned into a common intermediate that makes selenocysteine, the NIH says. A 2015 review explains that the liver is the central organ for regulating selenium.

When selenium is short, the liver cuts excretion and sends selenoprotein P into the blood at the expense of its own selenoproteins. The tissues take it up in proportion to how many receptors they carry.

The NIH says the body keeps its level mainly through urinary excretion, and at higher intakes also through the lungs and in feces. About 28% to 46% of the body’s selenium sits in skeletal muscle.

Needs level off, and more is not better

The recommended amount is set where the markers of selenoproteins stop rising. In a trial in China, people ate only 14 µg a day. Selenoprotein P reached its top level with 49 µg a day in total, and glutathione peroxidase activity with 35 µg. In 119 UK adults who ate about 55 µg a day, 50 µg more was needed to optimize selenoprotein P.

EFSA used the levelling off of selenoprotein P to set 70 µg a day for adults. It says studies of health outcomes showed no added benefit beyond that.

At the other end, EFSA based its upper level of 255 µg on hair loss. A 2012 review in The Lancet describes a U-shaped link between selenium status and health. People with low status may gain from more, while people with adequate-to-high status might be harmed.

What has been seen in people

The clearest human finding is about deficiency. Where intakes were at most 10 µg a day, Keshan disease appeared, and supplements prevented it in community trials.

For people who already get enough, extra selenium has not prevented cancer or cardiovascular disease in large trials. Some analyses point to harm, so the answer depends on where you start. Supplements lower thyroid antibodies in Hashimoto thyroiditis but without a clear clinical gain, and they helped in one trial of mild Graves’ orbitopathy. All of these are among the study cards above.

How much selenium do you need?

Adults need 55 µg of selenium a day, according to the US recommended dietary allowance (RDA). EFSA’s adequate intake (an amount assumed to be enough) in the EU is 70 µg a day for adults. The two differ because they rest on different markers. The US value was set using glutathione peroxidase activity, and EFSA’s using the level at which plasma selenoprotein P stops rising.

Pregnancy raises the US allowance to 60 µg a day for ages 14 to 50, and breastfeeding raises it to 70 µg. EFSA says the body adapts its selenium metabolism in pregnancy, so the adult value of 70 µg applies. For breastfeeding, it adds 15 µg to cover the selenium secreted in milk, giving 85 µg.

For older adults, the US allowance stays at 55 µg from age 51. EFSA sets one adult value, with no separate figure for older people.

Most Americans already eat more than the RDA. The NIH reports an average of 108 µg a day from food and drinks. And 17% to 19% of adults also take a supplement that contains selenium.

Selenium deficiency: signs and who is at risk

Selenium deficiency on its own rarely causes illness, says the NIH. In some regions it is linked to two diseases.

Keshan disease is a heart muscle disease first identified in 1935 in parts of China where the soil is low in selenium. Adults there ate no more than 10 µg a day, and the NIH says at least 20 µg a day protects them. It mainly affects women of childbearing age and preschool children.

Kashin-Beck disease is a childhood form of osteoarthritis found in low-selenium and low-iodine areas of China, Tibet, Siberia and North Korea. Deficiency can also make iodine deficiency worse in infants.

Deficiency is very rare in the US and Canada. The NIH lists the groups most likely to be low. They are people living in selenium-poor regions, such as parts of China and some European countries, especially those on vegan diets. Others are people on long-term kidney dialysis, because dialysis removes selenium, and people with HIV.

Vegetarian and vegan diets and smoking are linked to lower intakes or status. Doctors usually test plasma or serum selenium, and 8 µg per dL or more is considered sufficient in healthy people. Whole blood, hair and nails reflect longer-term intake. The selenoproteins used as markers can be thrown off by inflammation, so a result needs a doctor’s reading.

Forms of selenium: absorption and effects

Selenium supplements come in four common chemical forms, says the NIH. They are selenomethionine, selenium-enriched yeast (grown in a high-selenium medium and mostly selenomethionine), sodium selenite and sodium selenate. Food holds mainly selenomethionine and selenocysteine.

The NIH says the body absorbs up to about 90% of the selenium from all four supplement forms. Selenium-only supplements typically hold 100 to 400 µg, it adds, while many multivitamins hold 55 µg.

Absorption is only part of the story. A 2010 review says most dietary selenium is absorbed efficiently. But the body retains the organic forms, such as selenomethionine, better than the inorganic forms, selenite and selenate.

The honest bottom line: the head-to-head studies are small, and they measure blood markers, not health. In three of them, selenomethionine was absorbed or kept better than selenite, and selenium yeast behaved mostly like selenomethionine. We found no trial that compared forms for cancer, thyroid disease or heart disease.

The two big cancer trials used different forms: selenium yeast in NPC and L-selenomethionine in SELECT. The Cochrane authors say possible differences between forms still need research.

The Keshan disease trials used sodium selenite tablets, and the Danish thyroid and diabetes trial used selenium yeast. The pregnancy thyroid trial used selenomethionine, and the Swedish heart trial used selenium yeast with coenzyme Q10.

Form	Share of the mineral by weight	What studies show	Source
Selenomethionine	Not stated	Absorbed better than selenite in three studies, with nearly twice the bioavailability in a trial in China. It raised plasma selenium in a dose-dependent way and was kept longer. It was the form in SELECT (200 µg a day) and in a pregnancy thyroid trial (200 µg a day).	1, 2, 3, 8, 9, 10, 12
Sodium selenite	Not stated	Absorbed less than selenomethionine in a small New Zealand study (0.46 against 0.75 of the intake). It did not raise plasma selenium in selenium-replete adults, and a trial in China needed 66 µg a day for full glutathione peroxidase activity against 37 µg as selenomethionine. It was the form in the Keshan disease prevention trials and in a small immune trial (50 or 100 µg a day).	1, 2, 3, 8, 13, 14
Sodium selenate	Not stated	The NIH lists it as a common supplement form. In a study of 35 men with labeled isotopes, absorption of selenate was similar to selenium from fish, but retention was lower. We found no trial comparing selenate with other forms for any health outcome.	1, 7
Selenium-enriched yeast	Not stated	Mostly selenomethionine (about 60% in one set of tablets). It raised plasma selenium in a dose-dependent way, but yeast selenium was less bioavailable than fish selenium in the isotope study. It was the form in the NPC trial (200 µg a day), the Danish trial and the Swedish KiSel-10 trial (200 µg a day, with coenzyme Q10). In one trial yeast, but not selenomethionine, lowered two oxidative stress markers, with yeast supplied by its maker.	1, 2, 4, 6, 7, 11, 15, 16, 17
Food selenium (Brazil nuts, fish)	Not applicable	Two Brazil nuts a day raised selenium status as much as 100 µg of selenomethionine, although the nuts held only about half the intended selenium. Fish selenium was highly bioavailable.	5, 7, 9

Here are the head-to-head and form-specific studies, strongest evidence first, 7 in all.

Is selenium safe? Supplements, dose and interactions

Selenium from ordinary food is not a safety concern: EFSA says selenium from the natural content of foods does not raise reasons for concern. The US tolerable upper intake level (UL, a likely-safe daily maximum) for adults is 400 µg a day from food and supplements together. It was set from the amounts linked to hair and nail brittleness and loss.

For infants it is 45 µg up to 6 months and 60 µg from 7 to 12 months. For children it is 90 µg at ages 1 to 3, 150 µg at 4 to 8 and 280 µg at 9 to 13. It is 400 µg from age 14, including pregnancy and breastfeeding. It does not apply to people taking selenium under a doctor’s care.

EFSA’s 2023 opinion is lower: 255 µg a day for all adults, including pregnant and lactating women. For children and teens it is 70 to 230 µg, depending on age.

It rests on hair loss, with a lowest-observed-adverse-effect level of 330 µg a day from SELECT and a safety factor of 1.3. That is the lowest dose seen to cause harm. EFSA adds that adults are unlikely to exceed it, except regular users of high-dose supplements or regular eaters of Brazil nuts.

• Signs of too much. Early signs are a garlic odor on the breath and a metallic taste. The most common sign of long-term excess, called selenosis, is hair loss and brittle or lost nails. Skin rash, nausea, diarrhea, fatigue, irritability and nerve problems can follow, the NIH says. In SELECT, hair loss and mild dermatitis were more common with 200 µg a day.
• A mislabeled supplement. In 2008 a liquid supplement held 200 times the labeled amount of selenium. Of 201 cases in 10 states, 1 person was hospitalized. The median estimated dose was 41,749 µg a day. The common symptoms were diarrhea (78%), fatigue (75%), hair loss (72%), joint pain (70%), nail discoloration or brittleness (61%) and nausea (58%). After 90 days, 52% still had nail discoloration or loss. The authors say pharmaceutical-grade standards might have prevented it.
• Brazil nuts. The NIH puts them at 68 to 91 µg a nut, and a trial estimated 288.75 µg for one nut. So regular eating can pass the upper levels. Yet a study found blood selenium above toxic benchmarks in 448 people in Amazon communities who eat Brazil nuts. They had no skin or breath signs of selenosis (selenium poisoning). The authors say form and route of exposure may matter.
• If you already get enough. Cochrane reviews found no benefit from supplements for cancer or heart disease and a small signal for type 2 diabetes. In SELECT, men with higher starting selenium had more high-grade prostate cancer on selenium. A 2012 review says people with adequate-to-high status might be harmed by extra selenium and should not take supplements.
• Thyroid, pregnancy and medicines. The American Thyroid Association made a weak recommendation in 2017 against selenium in pregnant women with thyroid antibodies, the NIH reports. Cisplatin, a chemotherapy drug, can lower selenium in hair and serum. Small studies suggest selenium reduces its toxicity, but a Cochrane review found the evidence insufficient. People on kidney dialysis often have low selenium, though it is unproven that supplements help. Ask a doctor before taking selenium with any of these.

How to get more selenium

• Seafood and meat. Oysters give 65.5 µg per 3 ounces, mackerel 49.4 µg per fillet and sardines 48.5 µg per can in our ranking. The NIH adds shrimp at 42 µg and a hard-boiled egg at 15 µg.
• Brazil nuts, in small amounts. A single nut can hold far more or far less than average. In the Amazon study, it supplied 11% to 288% of a 70 µg need, so a daily handful can pass the upper level. In a New Zealand trial, 2 nuts a day averaged 53 µg (range 20 to 84).
• Grains and plants depend on soil. A cup of cooked spaghetti has 33 µg and a cup of brown rice 12 µg, while lentils have 6 µg per cup. Vegan diets are linked to lower intakes in Europe, and New Zealand’s intake rose after it imported high-selenium wheat.

History of selenium

Selenium was discovered in 1817 by the Swedish chemists Berzelius and Gahn. Roasting 200 kg of sulfur from pyrite from the Falun mine, they obtained about 3 g of a precipitate. At first they took it for tellurium.

Berzelius doubted this, repeated the analysis months later and realized a new element was in his hands. He named it after Selene, the Greek word for the moon, because it resembled tellurium, named for the earth.

It took more than a century to show that people need it. Keshan disease was first identified in 1935 in parts of China with low-selenium soil. Large intervention trials from the 1970s to the 1990s gave sodium selenite tablets and cut the disease sharply. A 2012 review calls this critical evidence that selenium is an essential trace element for humans, and the basis for setting the RDA.

The biochemistry came in between. In 1973 researchers showed that selenium is a component of glutathione peroxidase. In 1986 a study of the mouse gene found the selenocysteine in the enzyme is coded by TGA, normally a “termination” codon. In 2003 a survey of genomes counted 25 selenoproteins in humans.

Frequently asked questions about selenium

How many Brazil nuts is safe to eat?

There is no single safe number, because the selenium in a nut varies so much. The NIH puts it at 68 to 91 µg per nut. A study of nuts from the Brazilian Amazon found one nut could supply 11% to 288% of a 70 µg daily need. The amount depended on the nut’s origin. In a New Zealand trial, 2 nuts a day averaged 53 µg (range 20 to 84). The US upper level is 400 µg a day and EFSA’s is 255 µg. EFSA names regular Brazil nut eaters as people who may exceed it, so a daily handful could pass the limit.

Is selenium a vitamin?

No. Selenium is an essential mineral, an element, while vitamins are organic compounds. It is often grouped with antioxidant nutrients because selenoproteins such as glutathione peroxidase protect cells from oxidative damage. Its US upper level is 400 µg a day.

Does selenium help the thyroid?

The thyroid holds more selenium than any other organ, and selenium-based enzymes switch on thyroid hormone. In Hashimoto thyroiditis, a 2024 meta-analysis found supplements lowered thyroid peroxidase antibodies and slightly lowered TSH. But those are blood markers, and the NIH notes other trials found no effect on quality of life. In mild Graves’ orbitopathy, the EUGOGO trial of 159 people found 6 months of selenium improved quality of life and slowed the disease. In people with enough selenium, a Danish trial saw only tiny changes in TSH. Ask an endocrinologist (a hormone specialist) before starting it.

Which foods are highest in selenium?

Brazil nuts lead our USDA ranking at 543 µg per ounce. Oysters (65.5 µg per 3 ounces), mackerel (49.4 µg per fillet) and sardines (48.5 µg per can) follow. Tuna, salmon and herring give about 31 µg per 3 ounces. Plant foods depend on the soil, so grain and nut values vary by region.

Does selenium prevent cancer?

In people who already get enough, no. A Cochrane review of 10 trials with 27,232 people found 1.01 times the risk of any cancer in the trials at low risk of bias. An early trial hinted at fewer cancers in subgroups, but the 35,533-man SELECT trial did not confirm it. In men who started with higher selenium, supplements raised high-grade prostate cancer risk by 91% in one analysis. Observational studies link higher status to less cancer, with very low certainty.

How much selenium is too much?

The US upper level is 400 µg a day for adults, and EFSA’s is 255 µg. Long-term excess causes hair loss and brittle nails. Selenium-only supplements typically hold 100 to 400 µg, so it is easy to overshoot with several products. In the 2008 outbreak, a supplement with 200 times its labeled selenium gave a median dose of 41,749 µg a day. Of the cases, 78% had diarrhea.

What are the symptoms of selenium deficiency?

Deficiency alone rarely causes illness. In very low-selenium areas of China, it is tied to Keshan disease, a heart muscle disease, and Kashin-Beck disease, a childhood joint disease. A plasma level of 8 µg per dL or more is considered sufficient. Most Americans get plenty (an average of 108 µg a day). The groups most at risk live in selenium-poor regions, receive kidney dialysis or have HIV.

Which form of selenium supplement is best?

No form has been shown best for health. In direct comparisons, selenomethionine was absorbed better than selenite. It was 0.75 against 0.46 of the intake in a small study, and nearly twice the bioavailability in a trial in China. Selenium yeast is mostly selenomethionine. No trial compared forms for cancer, thyroid or heart outcomes. Check the micrograms of selenium on the label, and your total intake against the upper levels.