What Selenium Is and Why You Need It (But Not Too Much of It)

The saying “a little bit goes a long way” can apply to all kinds of things: hand lotion, chili powder and hair gel, for instance.

It’s also an apt way to describe selenium, an essential mineral that supports whole-body health. You only need a small amount of it — but its health benefits are big by comparison.

Registered dietitian Kayla Kopp, RD, LD, explains what selenium is, why you need it and how to get enough of it in your diet.

What is selenium?

Selenium is an essential mineral, which means that your body needs it to function well but can’t produce it on its own. You have to get it through diet.

“Selenium plays a role in protecting our cells from damage, promoting thyroid function and reproductive function and playing a role in DNA synthesis,” Kopp says.

Let’s delve deeper into some of these key benefits.

Supports thyroid health

A small gland called the thyroid has a big role to play in keeping your body at peak function. It’s responsible for producing and releasing hormones, including the ones that control your metabolism (how fast your body transforms food into energy). And selenium helps it do its job.

“Your thyroid gland holds the highest concentration of selenium in your body,” Kopp explains. “Selenium is converted into selenoproteins, which help regulate your thyroid hormones.”

Protects reproductive health

When unstable molecules known as free radicals cause damage to your cells, the result is called oxidative stress. It’s thought to contribute to a wide variety of chronic diseases, including infertility. But selenium may be able to help.

“Selenium offers protection from oxidation damage and infection,” Kopp notes, “and some research has shown selenium supplementation to be beneficial in people who are trying to conceive.”

May help with asthma

Selenium could play a role in managing chronic asthma. But don’t go rushing out to buy supplements, as this possible benefit hasn’t yet been proven by research.

“Selenium plays a role in oxidative stress in tissues like the lungs,” Kopp says, “but research on this subject in humans is still inconclusive.”

May protect against cancer

Studies have shown an inverse relationship between selenium levels and the risk of certain types of cancer. That means that people with low levels of selenium had higher cancer risks, while people with higher levels of selenium had lower risks of:

• Breast cancer
• Lung cancer
• Esophageal cancer
• Gastric (stomach) cancer
• Prostate cancer

“Lower selenium intakes have been associated with increased cancer diagnoses,” Kopp says. “Selenium has been known to decrease cancer risk, as it helps to repair DNA damage that occurs in our bodies.”

How much selenium do you need?

For adults ages 19 and over, the recommended daily amount of selenium is 55 micrograms (mcg) per day. If you’re pregnant, you should be getting 60 mcg per day; if you’re lactating, that amount increases to 70 mcg per day.

Thinking about selenium supplements? Depending on where you live, you probably don’t need them.

“Even though selenium is essential, we only need small amounts of it,” Kopp states. “In North America, it’s very rare to need selenium supplementation because foods grown here typically provide plenty of it through the soil.”

Foods that are high in selenium

Get this: A single Brazil nut contains about 68–91 mcg. That means that just one or two of them will fulfill your daily selenium intake!

Organ meats and seafood (especially yellowfin tuna, halibut, sardines and shrimp) are also high in selenium. Other good sources include:

• Beans
• Beef
• Chicken
• Cottage cheese
• Eggs
• Fortified cereals
• Lentils
• Turkey
• Whole-wheat bread

It’s typically not harmful to get some extra selenium in your diet, as long as you’re not consuming more than 400 mcg a day (that’s the Tolerable Upper Intake Level, or UL, which we’ll get into momentarily).

Just keep in mind that high levels of selenium can be dangerous, and it can also interact with some medications. And you should always speak with a healthcare provider before starting a new supplement (so maybe just stick to a Brazil nut a day instead).

The risks and side effects of too much selenium

Let’s go back to old adages for a second: “A little bit goes a long way” is a good way to describe selenium, but so is “too much of a good thing.”

Because you only need such a small amount of selenium, going over the recommended daily amount usually isn’t a big deal. But you start to face some serious risks once you hit the Tolerable Upper Intake Level, or the maximum amount that you can consume without being likely to experience negative effects.

The UL of selenium is 400 mcg a day. Beyond that, possible side effects include:

• A metallic taste in your mouth
• Brittle hair and nails
• Diarrhea
• Discolored teeth
• Fatigue
• Garlicky breath
• Hair loss
• Muscle tenderness
• Nausea
• Rashes

Very high selenium consumption can lead to selenium toxicity, which is associated with severe issues, like:

• Breathing problems
• Tremors
• Kidney failure
• Heart attack
• Heart failure

Risks of selenium deficiency

Because our bodies require so little of this trace mineral, it’s typically not hard to consume enough of it in your diet if you live in a place with selenium-rich soil. But you may have a hard time getting enough if you:

• Have HIV
• Are going through kidney dialysis
• Mostly eat locally grown foods, which could come from soil that’s low in selenium

Selenium deficiency can cause:

• Infertility in men
• Kashin-Beck disease, a form of arthritis
• Keshan disease, a type of heart disease

If you have concerns about your selenium levels, speak with a healthcare provider to figure out safe next steps for you.

Are you getting enough selenium?

If you live in North America, you likely don’t have to think (or worry) too much about your selenium levels. Selenium deficiency is rare, and unless you’re eating organ meats for dinner every single day, you’re probably not getting too much of it, either.

So skip the supplements and munch a Brazil nut or two as you see fit. Your body will thank you, and your tastebuds will, too!