Selenium is an essential trace mineral that is important for many bodily processes, including cognitive function, a healthy immune system, and fertility in both men and women.

It contributes to thyroid hormone metabolism and DNA synthesis, and it helps protect against oxidative damage and infection, according to the United States Office of Dietary Supplements.

It is present in human tissue, mostly in skeletal muscle.

Dietary sources are varied. They include Brazil nuts, seafood, and meats.

The amount of selenium in food often depends on the selenium concentration of the soil and water where farmers grew or raised the food.

Fast facts on selenium

Here are some key points about selenium. More detail is in the main article.

  • Selenium is a mineral that plays a role in many bodily functions.
  • It may protect against cancer, thyroid problems, cognitive decline, and asthma, but more research is needed.
  • Brazil nuts, some fish, brown rice, and eggs are good sources.
  • The best source of nutrients is food. Any supplement use should first be discussed with a doctor.

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Selenium is an essential trace mineral that assists with cognitive function and fertility.

Selenium may help prevent cardiovascular disease, thyroid problems, cognitive decline, which means disorders related to thinking, cancer, and others.

Cardiovascular disease: According to the Office for Dietary Supplements, selenoproteins can protect against cardiovascular disease, because they prevent the oxidative modification of lipids, or fats, in the body.

This reduces inflammation and prevents the buildup of platelets.

However, clinical evidence does not support the use of selenium supplements for this purpose.

Cognitive decline: Selenium's antioxidant activity may help reduce the risk of cognitive, or mental, decline, as people get older.

Evidence from studies is mixed, however, and selenium supplements are not yet prescribed for people at risk of diseases such as Alzheimer's, although it may have a role in prevention that is still under investigation.

Thyroid disorders: Selenium has an important role in producing and metabolizing thyroid hormone.

There is some evidence that women with higher selenium levels have fewer thyroid problems, but this has not been proven for men, and other studies have produced mixed results.

More studies are under way to decide whether selenium supplements might support thyroid health.

Cancer: The role played by selenium in DNA repair and other functions may mean that is can help prevent cancer. However, studies have produced mixed results.

The U.S. Food and Drug Administration (FDA) concluded in 2003:

"Some scientific evidence suggests that consumption of selenium may reduce the risk of certain forms of cancer."

Studies have suggested that selenium may also help to:

  • prevent HIV from progressing to AIDS
  • reduce the risk of miscarriage
  • protect against asthma

There has been some investigation into whether a woman's selenium levels during pregnancy might predict her child's risk of asthma.

While selenium is clearly element for many aspects of human health, there is too little evidence to indicate that supplements could be of use in preventing these conditions.

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An adequate selenium intake during pregnancy may reducing the risk of childhood asthma.

The recommended Daily Value (DV), or daily allowance, for selenium is 55 micrograms (mcg) per day for adults.

During pregnancy, a woman should consume 60 mcg, and lactating women should consume 70 mcg a day.

Selenium deficiency is rare worldwide. It often takes years to develop, and it usually only occurs in regions with severely low selenium content in the soil.

Several regions in China have low soil selenium content, but deficiencies in the population have been eradicated through supplementation programs.

Selenium supplements are available, but it is best to obtain any vitamin or mineral through food.

It is not the individual vitamin or mineral alone that make certain foods an important part of our diet, but the way the nutrients work together.

Isolating specific nutrients in supplement form does not necessarily provide the same health benefits as consuming the nutrient from a whole food.

The daily requirements of any nutrient should first come from food.

Selenium is most likely to be found in whole grains and animal produce, rather than fresh fruit and vegetables.

The following foods are a good source:

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Brazil nuts are rich in selenium.
  • Brazil nuts: 1 ounce provides 544 micrograms (mcg), or 777 percent of the daily recommended value (DV)
  • Tuna: 3 ounces of yellowfin tuna, cooked dry, contains 92 mcg, or 131 percent of DV
  • Halibut, baked: 3 ounces, cooked dry, contains 47 mcg, or 68 percent of DV
  • Brown rice, cooked: 1 cup contains 19 mcg
  • Egg: One large egg contains 15 mcg
  • Bread, white: 1 slice provides 10 mcg

The amount of selenium in grains and grain-based foods depends on the soil content where the grains grew.

The upper limit per day for selenium is 400 mcg for adults.

Selenium toxicity due to overdose is rare, especially from dietary sources, but an overdose of highly concentrated supplements could have negative effects.

These may include:

  • a garlic-like smell on the breath and a metallic taste in the mouth
  • brittle nails
  • mottled or decaying teeth
  • gastrointestinal problems such as nausea
  • neurological abnormalities
  • fatigue and irritability
  • skins lesions and rashes
  • hair loss
  • In extreme cases, it could lead to kidney failure, heart failure, and death.

    Selenium supplements can also interact with some medications, including cisplatin, a chemotherapy drug. The use of this drug can reduce selenium levels in the body.

    Use of selenium supplements

    The 2015-2020 Dietary Guidelines for Americans encourage people to get their nutrients from food sources rather than supplements.

    Eating a varied and healthful diet is more important than concentrating on individual nutrients as the key to good health.

    When taking any supplement, it is important to purchase from a reputable source. In the U.S., the Food and Drug Administration (FDA) does not monitor supplements for quality, purity, packaging, or strength.