Also known as: Selenomethionine, Sodium selenite, Selenium yeast, Brazil nut selenium, Se
MineralsStrong evidence
A trace mineral central to the body's own antioxidant enzymes and to thyroid hormone handling. Has the narrowest gap between requirement and upper limit of any mineral here, which makes casual high dosing genuinely unwise.
Typical dose
55.0–200.0 mcg per day
With food; any time of day
What it is
Selenium is a trace mineral, needed in microgram rather than milligram quantities. Content in food depends almost entirely on the selenium content of the soil where it was grown, which varies enormously by region.
UK soils are relatively low in selenium, and intake here has fallen over recent decades as wheat sourcing shifted away from high-selenium North American grain. That makes it one of the more plausible shortfalls in a British diet.
Brazil nuts are the standout source, and a striking one: a couple of nuts can supply a full day's requirement. That concentration cuts both ways, since content varies hugely between nuts and eating a handful daily can push intake well past sensible levels. Selenomethionine is the better-absorbed supplemental form.
How it works
Selenium is incorporated into around 25 selenoproteins, most notably the glutathione peroxidases, which use glutathione to neutralise peroxides. This is why selenium status affects antioxidant capacity directly rather than by acting as an antioxidant itself.
It is also required by the enzymes that convert thyroid hormone into its active form. This is a genuine and specific role, and it is why selenium and iodine are usually discussed together.
Selenoproteins are also involved in immune function and in reproductive tissue, where selenium concentrations are notably high.
What the evidence shows
Selenium's essentiality and its role in antioxidant enzymes and thyroid hormone conversion are firmly established.
Supplementation in people with genuinely low status improves the activity of selenium-dependent enzymes, which is a clear and measurable effect.
Large trials of selenium supplementation in populations that already had adequate status have generally been disappointing, and in some cases raised concerns rather than showing benefit. The pattern is now familiar across this index: correcting a shortfall helps, exceeding sufficiency does not, and with selenium the margin for exceeding it is unusually narrow.
Dosing
Reference intake is around 60-75 micrograms daily for adults. Supplements commonly supply 100-200 micrograms.
The tolerable upper intake is 300-400 micrograms daily depending on which assessment you follow. That is a much narrower gap between requirement and ceiling than for most minerals, and it is the reason to be deliberate rather than casual with selenium.
Brazil nuts are an excellent source but a genuinely unpredictable one, since content varies several-fold between nuts depending on where they grew. Two or three a day is sensible; a handful daily is not. Selenomethionine absorbs better than selenite. Take with food.
At a glance
Common forms
Selenomethionine, Selenium-enriched yeast, Sodium selenite, Sodium selenate, Brazil nuts
Half-life / duration
Status adjusts over weeks to months
Timing
With food; any time of day
Competition status
Permitted — not on the WADA Prohibited List
Category
Minerals
Safety and side effects
At intakes near requirement, selenium is safe. The concern is how close the upper limit sits to ordinary supplemental doses, which is unusual among minerals.
Sustained excess produces a recognisable pattern: a garlic-like odour on the breath, a metallic taste, brittle or shedding hair and nails, and digestive upset. These resolve on stopping, but they are a clear signal to do so.
Check your total intake across multivitamins and combination products, and factor in Brazil nuts if you eat them regularly. Speak to your doctor before use if you take regular medication or if your thyroid function is being monitored.
Who should avoid it
Do not exceed 300-400 micrograms daily from all sources combined — the gap between requirement and upper limit is narrower than for most minerals. Stop and seek advice if you notice a garlic-like breath odour, metallic taste or brittle hair and nails. Count Brazil nuts and multivitamins toward your total. Speak to your doctor before use if your thyroid function is being monitored or you take regular medication.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
