Selenium

A trace mineral built into antioxidant enzymes and into the conversion of thyroid hormone. The gap between too little and too much is narrower than for almost any other nutrient.

Forms the selenoproteins, including glutathione peroxidase — the body’s main antioxidant defence — and is required to activate thyroid hormone.

• Brazil nuts — one or two supply a full day
• Tuna and sardines
• Eggs
• Whole grains
• Sunflower seeds

55 µg for most adults. The upper limit, past which harm becomes likely, is 400 µg.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

A sustained shortage has a name here: Keshan disease. That is worth pausing on — nutrients whose deficiency earned a name are the ones where the consequences were severe enough, historically, to be described as a disease.

Most at risk: People in low-selenium regions, kidney dialysis, and severe gut disease.

Low soil selenium in the region food is grown, malabsorption, and long-term parenteral nutrition without it. Genuine deficiency is uncommon in North America.

A form of heart muscle disease seen where soils are selenium-poor, notably parts of China. Also causes muscle weakness and impaired immunity.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

Plasma or serum selenium can be measured but is seldom needed outside specific clinical situations, and falls during inflammation like several other trace elements.

Brazil nuts are exceptionally rich — one or two a day is generally plenty and a handful can exceed the upper limit. Seafood, meat, eggs and wholegrains contribute steadily.

This is the nutrient where casual supplementation is least advisable. Chronic excess causes hair loss, brittle nails, a garlic odour on the breath and nerve damage. Combination supplements sometimes stack more than intended.

The margin between enough and too much is narrow. Excess causes garlic breath, hair and nail loss, and nerve damage. Eating a large handful of Brazil nuts daily can genuinely reach that.