Minerals · 4 min read ·

Selenium: two Brazil nuts instead of a tablet

The thyroid and the body’s cell defences need selenium. The gap between enough and too much is small — and supplements did not help against cancer.

78.971 Se 34

In a nutshell

1What it does and how much you need

Selenium is part of 25 proteins, the so-called selenoproteins. They protect cells from oxidation and take part in immunity, the conversion of thyroid hormones and the work of skeletal muscle and the heart.1

EFSA recommendation 70 µg EFSA set the adequate intake at 70 µg a day for adults and pregnant women and 85 µg when breastfeeding. At this intake the blood level of selenoprotein P levels off, which shows that tissues have enough selenium.1

2Where to find it

Data 1,917 µg Brazil nuts contain a huge amount of selenium: 1,917 µg per 100 g. Wheat germ (79 µg), tuna (71–76 µg), wholemeal flour (62 µg) and egg yolk (56 µg) follow. But the selenium content of foods varies a lot and tables often do not give it accurately.1,3

3Forms and absorption

EFSA recommendation well Selenium is well absorbed from the diet in all its forms. Selenomethionine, however, is built into proteins in place of the amino acid methionine, so it raises blood selenium more than its effect corresponds to.1,4
Weak evidence — a small study 53 µg In a study of 59 people, two Brazil nuts a day raised plasma selenium as much as 100 µg of selenomethionine, and the activity of the enzyme glutathione peroxidase even more. Yet the nuts provided only 53 µg on average, ranging from 20 to 84 µg.5

In the EU, supplements may contain selenium as L-selenomethionine, selenium-enriched yeast, selenious acid, sodium selenate, sodium hydrogen selenite or sodium selenite.6

4When it harms

EFSA recommendation 255 µg The upper level is 255 µg of selenium a day for adults including pregnant and breastfeeding women. EFSA derived it from the large SELECT trial, in which harmful effects appeared at 330 µg a day — according to a Cochrane review, mainly hair loss and skin inflammation.2,7
Strong evidence — Cochrane 10 studies A Cochrane review (10 randomised trials with 27,232 people): selenium supplements did not reduce cancer or cancer deaths. Instead, type 2 diabetes increased slightly in the trials.7
Moderate evidence — a randomised trial +55% In a trial of 1,202 people taking 200 µg of selenium a day, 55% more people developed type 2 diabetes than with placebo — most of all those who already had high blood selenium at the start.8
Weak evidence — a poisoning report 201 people In the US in 2008, a liquid supplement with 200 times more selenium than the label stated poisoned 201 people. They had diarrhoea and fatigue, lost hair and had brittle nails; in some, the problems lasted beyond 90 days.9
Moderate evidence 16 studies A summary of 16 studies in people with autoimmune thyroiditis (Hashimoto’s): selenium lowered the level of antibodies against the thyroid. But people taking it reported side effects more often.10

What to take from this

Sources

Written on , based on studies available at that date. Newer research may change the conclusions.

  1. EFSA NDA Panel (2014). Scientific Opinion on Dietary Reference Values for selenium. EFSA Journal 12:3846. doi.org/10.2903/j.efsa.2014.3846
  2. EFSA NDA Panel (2023). Scientific opinion on the tolerable upper intake level for selenium. EFSA Journal 21:e07704. doi.org/10.2903/j.efsa.2023.7704
  3. USDA ARS (2018). FoodData Central — SR Legacy, selenium per 100 g: brazil nuts 1,917 µg (FDC 170569), wheat germ 79.2 µg (FDC 168892), tuna in oil, drained 76 µg (FDC 173708), tuna in water, drained 70.6 µg (FDC 173709), whole wheat flour 61.8 µg (FDC 168893), egg yolk 56 µg (FDC 172184). fdc.nal.usda.gov
  4. Burk R. F. et al. (2006). Effects of chemical form of selenium on plasma biomarkers in a high-dose human supplementation trial. Cancer Epidemiol Biomarkers Prev 15:804–810. doi.org/10.1158/1055-9965.EPI-05-0950
  5. Thomson C. D. et al. (2008). Brazil nuts: an effective way to improve selenium status. Am J Clin Nutr 87:379–384. doi.org/10.1093/ajcn/87.2.379
  6. Directive 2002/46/EC on food supplements, Annex II — forms of vitamins and minerals permitted in supplements (consolidated version 26 Nov 2025). eur-lex.europa.eu
  7. Vinceti M. et al. (2018). Selenium for preventing cancer. Cochrane Database Syst Rev CD005195. doi.org/10.1002/14651858.CD005195.pub4
  8. Stranges S. et al. (2007). Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med 147:217–223. doi.org/10.7326/0003-4819-147-4-200708210-00175
  9. MacFarquhar J. K. et al. (2010). Acute selenium toxicity associated with a dietary supplement. Arch Intern Med 170:256–261. doi.org/10.1001/archinternmed.2009.495
  10. Wichman J. et al. (2016). Selenium supplementation significantly reduces thyroid autoantibody levels in patients with chronic autoimmune thyroiditis: a systematic review and meta-analysis. Thyroid 26:1681–1692. doi.org/10.1089/thy.2016.0256

This is general information and does not replace a doctor. With thyroid disease, talk to a doctor about selenium.