Selenium

No. The 2013 SACN review of selenium and health considered the evidence on UK intakes and did not find that the population needed routine supplementation. Average adult intake has declined since the 1970s and many adults sit below the RNI in dietary surveys, but most are not deficient. NHS guidance is that adults eating meat, fish, eggs, or nuts will generally obtain enough selenium from a varied diet. People most likely to benefit are those with consistently low intake or specific clinical reasons to consider it — discuss with your GP or pharmacist. ^[3,1]

On average yes, but they are not a reliable way to hit a specific daily amount. Selenium content in a single Brazil nut ranges roughly 70-90 µg in nuts grown in selenium-rich Brazilian soil — but the variation between batches and origins is around tenfold. Eating a couple every day is a reasonable dietary approach if you tolerate them; eating a large handful daily for years risks selenosis. If you need a defined dose for a clinical reason, a labelled supplement is the more controllable route. ^[5]

This is a discussion to have with your GP or endocrinologist. Trials (Gärtner 2002) and the Toulis 2010 meta-analysis show that selenium 200 µg/day reduces TPO antibody titres in autoimmune thyroiditis over 3 months — but whether this slows the underlying disease, reduces the need for levothyroxine, or prevents progression is not established. UK NHS, the British Thyroid Association, and NICE do not currently recommend routine selenium for Hashimoto''s. If you decide to try it, your clinician can help avoid stacking with other selenium sources and watch for the U-shaped diabetes-risk signal at long-term high intake. ^[7,8]

At the doses used in supplements, the practical clinical differences between these forms are modest. Selenomethionine is well absorbed and accumulates in tissues over weeks because it goes into the body''s general protein pool; sodium selenite and selenate are inorganic, less retained, but functionally adequate. Selenium-enriched yeast is mostly selenomethionine with smaller fractions of selenocysteine and methylselenocysteine, but the proportions vary by manufacturer and are rarely declared on the label. The labelled µg of elemental selenium is what matters most; chase that number and the named form, not branded positioning.

EFSA''s 2023 review set the tolerable upper intake level for adults at 255 µg/day (alopecia is the early adverse effect). WHO and historical UK guidance reference 400 µg/day as the upper safe level. Sustained intakes above ~800 µg/day risk selenosis: the classic features are garlic-smelling breath (the body exhales dimethylselenide), hair loss, brittle white-flecked nails, GI symptoms, and peripheral neuropathy. NHS guidance is that supplementation up to 350 µg/day is unlikely to cause harm — but this is an upper safety bound, not a target. The right target is the RNI, not the ceiling. ^[1]

The GB-authorised health claim is that selenium "contributes to the normal function of the immune system". That is a claim about adequacy supporting normal function — i.e. selenium is required for normal immune function — not about supplementation producing an above-normal effect in adults who are already replete. People who are deficient gain by correction; above adequacy, immune-function gains have not been demonstrated. ^[2]

The original signal came from the 1996 Nutritional Prevention of Cancer trial in adults with low baseline selenium. The much larger SELECT trial (Lippman 2009 JAMA, 35,533 men) did not replicate any anti-cancer effect of selenium in adults with adequate baseline status. Long-term high-dose selenium in already-replete adults has also been linked to a higher rate of new-onset type 2 diabetes (Stranges 2007). The fair summary is that selenium does not reduce cancer risk in the general adult population. ^[9,11]