Brazil nut lovers beware – there’s a dark side to this selenium-packed snack
Your body only needs a small amount of selenium. Even so, it’s absolutely essential for your thyroid, immune system and fertility. So which food products contain it? And why should you be careful with selenium-rich Brazil nuts?
Like iron, zinc, iodine and copper, selenium is a trace element. Basically, a microelement your body only needs in very small amounts (less than 100 milligrammes per day). Nevertheless, selenium’s crucial to vital metabolic processes.
Why does your body need selenium?
Selenium impacts countless enzymes and drives many metabolic processes. As a component of antioxidant enzymes, it protects against oxidative stress by rendering free radicals harmless, preventing cell damage as a result. The trace element’s also key to thyroid function. Enzymes containing selenium control the thyroid gland’s hormone balance, keeping the metabolism stable. Not only that, but selenium’s also important for male fertility. As an essential building block of sperm, it’s a significant factor in the ability to conceive children.
Which foods are rich in selenium?
It’s difficult to definitively say how much selenium’s contained in certain foods. Since the selenium content of soil varies enormously, the amount present in the plants that grow in it differs too. Generally speaking, cruciferous vegetables such as broccoli or white cabbage, onions, mushrooms, asparagus and legumes are rich in selenium. Seafood also contains a relatively high amount of the trace element.
The German Nutrition Society (DGE) (page in German) has also pointed out that selenium levels vary drastically. However, it’s set out some example figures (in German) as a rough guide. According to these figures, 100 g of rye bread contains around 3 μg of selenium; 100 g of rice or lentils equates to around 10 μg, an egg contains 6 μg, 500 g of mushrooms contains 35 μg, 150 g of oatmeal contains 14.6 μg, 150 g of white cabbage contains 3.6 μg and 100 g of peppers contains 4.3 μg.
Adding selenium to animal feed is permitted in both the EU and Switzerland. As a result, animal products such as meat and eggs provide relatively steady selenium quantities. So in Switzerland and Germany, animal-based foods, including fish, are considered more reliable sources of selenium than plant products, whose selenium levels vary depending on the selenium content of the surrounding soil.
Be careful with Brazil nuts
Packed with around 103 μg of selenium per 100 g, Brazil nuts are a great source of the trace element. Even so, you’d be ill-advised to eat them in large quantities. According to the Federal Office for Radiation Protection (BfS), Brazil nuts absorb and store more radioactive elements (especially radium) from their environment than other foods. The more Brazil nuts you eat, the higher the dose of radioactive radiation you ingest. Though insignificant in small quantities, infants, pregnant women, children and adolescents are advised to be particularly careful. In fact, the BfS recommends they should avoid Brazil nuts altogether.
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What if you have a selenium deficiency?
Your recommended selenium intake depends on your age and gender. It also increases as you get older: infants need 10 µg, while adolescents under the age of 15 need 60 µg. From age 15 onwards, the recommended daily amount differs by gender. Men require 70 µg, while women are supposed to consume 60 µg daily. Breastfeeding mothers need a little more (75 µg) to compensate for the quantity lost through breast milk.
Your body can’t produce selenium itself, so you have to get the substance from food. If you don’t get enough of it, your health will suffer. Selenium deficiency can weaken the immune system, impair muscle function and disrupt sperm formation. Deficiencies are more common in certain rural areas of Central Africa and Asia, where the soil doesn’t contain much selenium. In Europe, on the other hand, they’re rare. In this part of the world, deficiencies usually result from diseases that impair selenium utilisation or cause increased selenium loss.
With this in mind, it’s advised that premature babies or people with genetic defects, weakened immune systems or pre-existing conditions such as chronic inflammatory bowel disease, cystic fibrosis, renal insufficiency or rheumatoid arthritis pay particularly close attention to their selenium intake. Vegetarians and vegans should also keep an eye on their selenium levels.
Symptoms of a selenium deficiency include tiredness, impaired muscle function, weak immune defences and, in men, reduced fertility. Severe cases lead to skin and nail changes, anaemia and heart and joint problems. However, a balanced diet usually covers your selenium requirements. Taking supplements is only helpful in certain cases. Given the harmful nature of a selenium overdose, you should only take supplements after seeking advice from a doctor.
What if your selenium intake is too high?
As important as selenium is for your body, consuming too much of it is still harmful. If you take high doses of the element over a long period of time (most notably if you’re taking nutritional supplements), you’re at risk of selenosis or selenium poisoning. This starts out with neurological issues, exhaustion, joint pain, nausea and diarrhoea, and can later lead to hair loss, brittle nails and garlicky smelling breath.
Acute selenium poisoning, triggered by consuming several grammes of selenium, can actually be life-threatening. In the worst-case scenario, this can cause heart failure and ventricular fibrillation. However, your dietary intake isn’t expected to cause the condition. In fact, it’s even unlikely to happen as a result of taking dietary supplements.
The European Food Safety Authority (EFSA) recommends a maximum daily intake of 300 µg of selenium for adults. Since supplements usually don’t provide more than 200 µg of selenium per day, you’re still at a low risk of overdose if you’re in Europe and taking your supplements correctly. Nevertheless, you should still consult your doctor before taking selenium supplements.
Find out more about vitamins, minerals and fibre and why they’re essential for your health here:
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Science editor and biologist. I love animals and am fascinated by plants, their abilities and everything you can do with them. That's why my favourite place is always outside - somewhere in nature, preferably in my wild garden.
