Selenium

Selenium is a trace mineral built into around 25 selenoproteins in the human body, including the glutathione peroxidases that quench peroxides and the iodothyronine deiodinases that activate thyroid hormone. Selenium is the most-studied nutrient interaction with thyroid function after iodine.

UK soils are relatively selenium-poor, and the selenium content of UK-grown wheat and other plant foods reflects that. The dietary supply has historically been topped up by imports from selenium-rich-soil regions (notably North American wheat and feed grains) and by animal foods. Population-level UK selenium status sits around adequate but not generous, and brazil nuts have become a popular concentrated source.

Values per 100g from USDA SR Legacy and McCance and Widdowson 7th edition; the site's food entries currently store macros only.

Brazil nut practical guidance. A single brazil nut typically carries around 95 micrograms of selenium, which is more than a day's intake. NHS standing advice: "Don't have more than a few brazil nuts a day for a long period of time." A couple of brazils a day covers the requirement comfortably; six or eight a day, every day for months, is enough to push intake into the selenosis range.

UK Reference Nutrient Intake (RNI) for selenium is set by SACN (1991).

Selenium deficiency from UK diets is uncommon in adults, but population status sits closer to the lower edge than for most nutrients. NDNS data has shown UK selenium intakes below the RNI in some groups in recent surveys.

Signs of clinical deficiency include reduced immune function, increased susceptibility to infections, and (in severe deficiency, historically in selenium-deficient regions of China) Keshan disease, a form of cardiomyopathy.

At higher risk in the UK

• People who eat very little fish, meat, or eggs and rely only on UK-grown plant foods.
• People with severe malabsorption.
• People on long-term parenteral nutrition without added selenium.

Selenium has a narrow safety window between adequate and too much. NHS supplement guidance: taking 350 micrograms or less a day of selenium from supplements is unlikely to cause any harm.

NHS standing advice on brazil nuts, restated: do not have more than a few brazil nuts a day for a long period of time, because they contain very high levels of selenium.

Excessive selenium intake (selenosis) causes:

• Hair loss and brittle, peeling nails
• A distinctive garlic-like breath odour
• Fatigue and irritability
• Nausea and digestive symptoms
• Skin rashes
• In severe cases: peripheral neuropathy and other nerve symptoms

Reported UK cases come from over-eating brazil nuts, and from concentrated supplements (sometimes labelled with milligram, not microgram, doses, the difference being a factor of 1,000).

Routine selenium supplementation is not needed for healthy UK adults eating a varied diet that includes fish, meat, eggs, or a couple of brazil nuts.

If supplementing, a typical reasonable dose is 50 to 100 micrograms a day; stay at or below 350 micrograms a day. Look at the label twice: micrograms (mcg or ug) is the right unit; milligrams (mg) would be 1,000 times too much.

People with thyroid conditions, particularly autoimmune thyroid disease, should follow personalised advice rather than supplementing selenium speculatively.

• The thyroid pair: Iodine is the other nutrient that affects thyroid hormone directly.
• Other trace minerals: Zinc, Copper, Manganese.
• Best food sources: Tuna, Salmon, Eggs, Beef.

• NHS. Vitamins and minerals: Others. nhs.uk/conditions/vitamins-and-minerals/others. Selenium and the brazil-nut guidance.
• SACN. Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. Department of Health Report 41 (1991).
• National Diet and Nutrition Survey (NDNS) rolling programme. Public Health England / OHID.
• Public Health England. McCance and Widdowson's The Composition of Foods, 7th summary edition (2015).

This page is reference information for UK shoppers. It is not medical advice. People with thyroid conditions should follow personalised advice from their GP or endocrinology team.