Selenium: Can Eating Too Many Brazil Nuts Be Bad For You?

What is selenium?

Selenium is a trace element that was discovered in 1817, and named after Selene, the Greek goddess of the Moon. It occurs naturally in the Earth’s crust, but it doesn’t stay there – selenium can be found in air, water, and soil.

As a result, selenium is also found in a whole host of food sources (and dietary supplements). It occurs at particularly high levels in Brazil nuts, which, according to some sources, contain a whopping 544 micrograms of selenium per 28-gram (1-ounce) portion – that’s 989 percent of the recommended daily intake for most people over the age of 14 (more on that later).

Beyond that, selenium tends to be at its highest in animal products that are high in protein, such as seafood – yellowfin tuna and oysters are rich sources – meats like pork and beef, and poultry, including turkey and eggs.

What role does selenium have in the body?

Though we only need a small amount of it, selenium is an essential mineral for humans. That’s because it’s a critical component of selenoproteins, a group of proteins that play a number of important roles in the body, including: the production of DNA; protecting DNA from damage; regulating thyroid function; and reproduction.

Selenium’s importance in the body also means that not getting enough of it in your diet can lead to health problems, though this is rare in countries like the US. However, for people who live in regions where there’s not much selenium in the soil, people with HIV, or people undergoing long-term dialysis, there’s an increased risk of severe selenium deficiency and two particular conditions: Keshan disease and Kashin-Beck disease.

Keshan disease is a type of heart condition known as cardiomyopathy, and involves the death and scarring of the heart muscle, affecting the heart’s ability to effectively pump blood. Kashin-Beck disease, on the other hand, is a type of arthritis, with the degeneration of cartilage causing pain, swelling, and loss of motion in the joints.

Does selenium have any medical uses?

Given the critical roles it plays in the body, it’s a pretty logical step to then wonder if selenium could be applied in medicine.

Cancer

One course of investigation involves selenium’s part in protecting DNA from damage. When damaged DNA is left unrepaired, or is repaired incorrectly, it can lead to cancer; and so, researchers have questioned if selenium treatments could be used to prevent cancer by exploiting its protective role.

However, research carried out so far has been unable to support that possibility. The most recent version of a Cochrane review on the matter concluded that “there is no evidence to suggest that increasing selenium intake through diet or supplementation prevents cancer in humans,” although it did also note that further research needs to be done to determine if selenium can affect risk in people with “a specific genetic background or nutritional status”.

Hypothyroidism

Selenium supplementation is also being investigated as a way of preventing the progression of Hashimoto’s disease; this is a condition that sees the immune system attack the thyroid gland, eventually leading to an underactive thyroid, known as hypothyroidism.

The jury’s still out on whether or not selenium could be useful here. While a recent systematic review and meta-analysis of 35 studies found that selenium supplementation in patients with Hashimoto’s disease showed promise – it appeared safe, and slightly reduced the levels of a hormone that’s normally elevated in this condition – it still concluded that the data was limited, and more research was needed to confirm its efficacy.

Dandruff

One area where selenium has a proven track record, though, is in treating dandruff. Selenium sulfide is a common ingredient in anti-dandruff shampoo, as it can kill off the fungus that causes the scalp to become itchy and flaky.

Can you have too much selenium?

While having too little selenium can be a problem given its roles in the body, is there such a thing as having too much selenium as well? The answer is yes – and it can be just as serious as not having enough.

Taking in too much selenium can be surprisingly easy to do, depending on where you get it from. One of the easiest ways to do so is by regularly eating Brazil nuts, which often contain anywhere between 68 to 91 micrograms of selenium in just one nut – that’s between 123 to 165 percent of the recommended daily intake for selenium for most people over the age of 14.

The National Institutes of Health advises the upper limit for daily intake of selenium – from all sources – is 400 micrograms for adults, so eating anything more than a few Brazil nuts in a day can easily send you beyond both the recommended amount and upper limit.

Doing so on a regular basis can lead to what’s known as selenium toxicity or selenosis, the early symptoms of which are often a metallic taste in the mouth and breath that smells like garlic, followed by changes to the hair and nails, like brittleness or loss. Chronic intake of too much selenium can also lead to nausea and diarrhea, mood changes, and skin rashes, and in severe cases, difficulty breathing, tremors, and even kidney failure, heart failure, and death.

The bottom line? A little bit of selenium = good. A daily dinner of Brazil nuts, oysters, and tuna = definitely not good (for more reasons than excess selenium).

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.

The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.