Selenium

What it is

Selenium is a trace mineral found in Brazil nuts, fish, eggs, meat and some wholegrains. How much you get from food depends on selenium levels in the soil where crops grew, and UK soil is relatively low in it.

It is used by the body in selenoproteins, which help protect cells from oxidative damage and help the thyroid produce hormones. People take supplements for thyroid support, immunity, fertility, hair and general antioxidant protection.

Brazil nuts are such a rich source that one or two a day can meet your needs, but their selenium content varies a lot.

How it works

Selenium is built into enzymes such as glutathione peroxidases, which neutralise damaging oxidants, and iodothyronine deiodinases, which convert thyroid hormone to its active form. Intake matters most if you start low; above adequate levels the extra does little.

What the evidence says

• Strong Correcting low selenium
  Deficiency is uncommon but treatable; supplements restore selenium enzymes.
• Limited Autoimmune thyroid disease (Hashimoto's)
  Some trials show lower antibody levels, but clinical benefit is unclear and research is mixed.
• Limited Fertility and sperm quality
  Small studies suggest possible benefits in men with low levels; results are inconsistent.
• Weak Cancer prevention
  Large trials have not shown prevention benefits, and some showed possible harm at higher doses.
• Limited Immune function
  Matters when levels are low; no clear benefit when adequate.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Typical supplements provide 50-200 mcg. Many people need only a small top-up.
• UK guidance suggests that supplemental intakes up to around 350 mcg a day are unlikely to cause harm; check current guidance, and remember food counts too.
• Brazil nuts: one or two a day is generally enough; eating a handful daily can overshoot.
• Do not combine multiple selenium-containing products without adding up.

Forms and what to look for

• Selenomethionine: well absorbed and retained by the body.
• Selenium yeast: contains mixtures; quality varies.
• Sodium selenite: cheaper, less well retained, more irritating at high doses.
• Brazil nuts: a food option with variable content.

What to expect, and when

• You will not feel selenium working.
• Blood levels shift within weeks if you were low.
• Thyroid or fertility effects, if any, take months.

Side effects

• Garlic breath or metallic taste
• Nausea, diarrhoea
• Brittle nails and hair loss with excess
• Skin rashes, fatigue and nerve symptoms with chronic high intake (selenosis)

Interactions

• Can add to effects of blood thinners
• May interact with some chemotherapy drugs
• Large doses combined with other antioxidants may blunt some treatments; ask your care team
• High intakes may alter thyroid medication needs

Who should avoid it

• Anyone eating Brazil nuts in large amounts daily
• People taking high-dose supplements for long periods
• Anyone undergoing cancer treatment without oncology advice
• People with thyroid conditions unless advised

Risks and safety

• Toxicity (selenosis) is possible with doses much above guidance, and mislabelled products have caused serious poisoning.
• Not banned in sport.
• Brazil nuts can vary widely in selenium and may contain traces of radium.
• Possible links between high intake and type 2 diabetes risk are debated.

Buying well: quality and testing

• Choose products with a clear mcg per serving.
• Third-party testing helps because overdose cases have come from mislabelled products.
• Avoid mega-dose products.
• Check multivitamins for existing selenium.

Bottom line

Selenium is a trace mineral where moderate intake is good and excess is harmful. A couple of Brazil nuts or a low-dose supplement is plenty for most people; megadoses do not offer extra benefits.