Selenium is an essential mineral that facilitates normal cellular function, helps make our DNA, and protects us against infections, cell damage, and other health problems. 

It’s also vital for thyroid health. Getting healthy amounts of the mineral has been associated with a reduced risk of Hashimoto’s disease, Graves’ disease, and thyroid cancer.

Clearly, selenium is essential. But that doesn’t mean you need a ton of it. In fact, our bodies only require a tiny amount of selenium—you can get plenty by eating a single Brazil nut per day.

On the flip side, eating too much selenium-rich foods or taking too much via supplements can cause some pretty serious health issues.

Selenium deficiency is extremely rare in the United States because our soil is rich in the mineral. Most Americans get plenty of it in our food, and a supplement could easily put you well over the daily limit.

Nevertheless, according to the NIH Office of Dietary Supplements, nearly one-fifth of the US population takes a daily supplement that contains selenium. The recommended dietary allowance for adults over the age of 19 is 55 micrograms every day. The only exception is women who are pregnant and lactating—they require approximately 60 and 70 micrograms daily, respectively.

That’s a pretty easy range to hit with most regular diets—and it means if you take a supplement that contains selenium, you could get too much without realizing it.

In one study that investigated an outbreak of selenium toxicity, researchers found that the source was a liquid dietary supplement that contained 200 times the labeled concentration of selenium. 

Partly because the supplement industry isn’t regulated like the pharmaceutical industry is, this mislabeled product made it onto store shelves.

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Any amount of more than 400 micrograms daily for adults (including pregnant and lactating women) can cause health problems including muscle tremors, upset stomach, lightheadedness, hair loss, and respiratory distress. In severe cases, chronically high intakes of selenium can cause a heart attack or kidney failure.

If you’re a fan of Brazil nuts, be especially careful that your supplements don’t contain extra selenium. Brazil nuts contain the highest amount of selenium of any food, and eating just one (a single nut has 96 micrograms) will put you over the recommended daily amount.

That can be a good thing if you’re concerned about thyroid health, but eating too many Brazil nuts on a regular basis can cause your body to reach toxic levels. 

Other foods high in selenium include seafood and organ meats. Protein foods from animals are also good sources of selenium, as are breads, cereals, poultry, red meat, and eggs.

Bottom line: it’s always a good idea to check with your doctor before taking any supplement, and to read the ingredients list carefully.

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