What Is Selenium and Why Do We Need It?

Selenium is a trace mineral — meaning your body only needs a tiny amount — but those tiny amounts are crucial. It helps regulate your thyroid, protect DNA from damage, support fertility, and strengthen your immune system (NIH, 2025).

The thyroid gland actually stores more selenium than any other part of the body, where it’s converted into special proteins that help regulate hormones. In men, selenium plays a key role in sperm health and fertility. It also helps protect cells from oxidative stress, acting like a shield against everyday wear and tear.

Where Does Selenium Come From?

Selenium is naturally found in soil, water, and air, which means it ends up in many foods. Rich sources include:

• Brazil nuts (the most concentrated source by far)
• Seafood (tuna, oysters, shrimp, sardines)
• Meat and poultry (beef, pork, turkey, chicken)
• Eggs and dairy products
• Grains, beans, and seeds (like sunflower seeds)

Because selenium levels in food depend on the soil where plants grow (or where animals are raised), the amount can vary widely. That’s why Brazil nuts from one region may have much more selenium than those from another.

How Much Selenium Do We Need?

• Adults: 55 mcg per day
• Pregnant: 60 mcg per day
• Breastfeeding: 70 mcg per day
• Safe upper limit: 400 mcg per day (U.S.) or 255 mcg per day (Europe) (NIH, 2025)

This narrow margin means it’s easy to get enough — and also easy to get too much.

Brazil Nuts: A Powerful But Risky Source

Brazil nuts are famously high in selenium. Just one nut can contain 68–91 mcg — already enough to meet (or exceed) your daily needs. A handful (6–8 nuts) may deliver up to 544 mcg, well above the safe upper limit (Thomson, Chisholm, McLachlan, & Campbell, 2008).

That’s why nutritionists often say: one nut a day is plenty.

Too Little Selenium: Rare But Serious

Most people in the U.S. and New Zealand get enough selenium through their diet. But deficiency can still happen, especially in areas with selenium-poor soil or in people with HIV or kidney dialysis.

Health problems linked to deficiency include:

• Keshan disease – a heart condition first seen in selenium-poor areas of China.
• Kashin-Beck disease – a type of arthritis causing joint pain and stiffness.
• Infertility in men – due to impaired sperm function.

Too Much Selenium: When “Healthy” Turns Harmful

Because Brazil nuts are so concentrated, overdoing them (or taking high-dose supplements) can lead to selenium toxicity or selenosis.

Early warning signs include:

• Garlic-like breath or metallic taste
• Hair loss or brittle nails
• Nausea, fatigue, or skin rashes

Severe, long-term overexposure can cause:

• Nerve damage (numbness, tingling)
• Breathing problems
• Tremors and mood changes
• Kidney or heart failure — and in extreme cases, death (Agency for Toxic Substances and Disease Registry, 2003).

The good news? Symptoms usually go away within a few weeks once the excess selenium source is removed.

Does Selenium Have Medical Uses?

Researchers have studied whether selenium supplements might help prevent cancer, support thyroid health, or slow cognitive decline. So far:

• Cancer prevention: Large studies show no clear benefit for healthy people.
• Hashimoto’s thyroiditis: Some studies suggest selenium may help reduce inflammation, but more research is needed.
• Proven use: Selenium sulfide is a common ingredient in anti-dandruff shampoos — it kills the fungus that causes flaky scalp.

What This Means for You

Think of selenium like salt: essential in the right amount, harmful in excess.

• A little: supports your thyroid, DNA, fertility, and immunity.
• Too much: can damage hair, nails, and even major organs.
• Brazil nuts: just one or two can cover your needs — more is not better.

For most people, a balanced diet with protein, grains, and seafood provides enough selenium without the need for supplements.

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FAQs About Selenium

Q: Can I get enough selenium from food without supplements?
Yes. Most people do, especially if they eat fish, eggs, or meat.

Q: How many Brazil nuts are safe to eat daily?
One or two nuts are usually enough. A handful may exceed the safe limit.

Q: What are the first signs of selenium overdose?
A garlic-like smell on the breath, brittle nails, and hair loss.

Q: Should I take selenium to prevent cancer?
There’s no strong evidence that supplements reduce cancer risk.

Q: Are Brazil nuts dangerous?
Not if eaten in moderation. But daily handfuls could put you at risk of toxicity.

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References

Agency for Toxic Substances and Disease Registry. (2003, September). Selenium ToxFAQs (CAS # 7782-49-2). U.S. Department of Health and Human Services.
National Institutes of Health, Office of Dietary Supplements. (2025, June 18). Selenium – Consumer.
Thomson, C. D., Chisholm, A., McLachlan, S. K., & Campbell, J. M. (2008). Brazil nuts: An effective way to improve selenium status. The American Journal of Clinical Nutrition, 87(2), 379–384. https://doi.org/10.1093/ajcn/87.2.379

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⚠️ Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider before making dietary or supplement changes.