A single Brazil nut can contain up to 400 µg of selenium, more than the daily safety limit set by EFSA. That is selenium's paradox: this trace element is essential, but the margin between the requirement of 70 µg per day for adults and the limit of 255 µg is narrow. Roles, requirements, foods, deficiency, excess and major trials: here are the essentials, with the figures.
Key takeaways
- Adults need around 70 µg of selenium per day; the safety limit set by EFSA in 2023 is 255 µg.
- Fish, seafood, offal and eggs provide it; five or six Brazil nuts supply 100 to 2,000 µg.
- Large trials did not reduce cancer risk and found a slightly increased risk of type 2 diabetes with supplementation.
- The European Union permits six claims, including “Selenium contributes to normal thyroid function”; none concerns a disease.
What is selenium and what does it do?
Selenium is a trace element: the body needs only tiny amounts. ANSES sets 70 µg per day for an adult, compared with 300 to 380mg of magnesium and 3,500mg of potassium.
It acts within selenoproteins, which incorporate it as selenocysteine. According to EFSA, the main ones are:
- glutathione peroxidases, which remove hydrogen peroxide and contribute to defences against oxidative stress;
- thioredoxin reductases, another pillar of this antioxidant network;
- deiodinases, which activate or deactivate thyroid hormones;
- selenoprotein P, which transports selenium from the liver to the organs.
Selenium, iodine and glutathione: teamwork
In the thyroid, iodine supplies the raw material for hormones. Selenium supports the deiodinases that activate them and the glutathione peroxidases that help thyroid cells manage oxidative stress. Neither replaces the other. These peroxidases work with glutathione, an antioxidant made by our cells, whose precursor is N-acetylcysteine (NAC).
Which selenium benefits are recognised in Europe?
European Regulation 432/2012 permits six health claims for selenium:
- “Selenium contributes to normal spermatogenesis”;
- “Selenium contributes to the maintenance of normal hair”;
- “Selenium contributes to the maintenance of normal nails”;
- “Selenium contributes to the normal function of the immune system”;
- “Selenium contributes to normal thyroid function”;
- “Selenium contributes to the protection of cells from oxidative stress”.
They apply only to a product that provides at least a “source” of selenium. They describe a physiological role: EFSA found no evidence of additional benefit beyond the intake that saturates selenoprotein P.
How much selenium is needed each day?
EFSA (2014) and ANSES (2025) set an adequate intake of 70 µg per day for adults. At this intake, selenoprotein P stops increasing in the blood, a sign that tissues are adequately supplied. In 2023, EFSA lowered the upper safety limit, set at 300 µg in 2000, to 255 µg per day for adults, including during pregnancy and breastfeeding.
| Population | Adequate intake (µg/day) | Safety limit (µg/day) |
|---|---|---|
| Children aged 1 to 3 years | 15 | 70 |
| Children aged 4 to 6 | 20 | 95 |
| Children aged 7 to 10 | 35 | 130 |
| Adolescents aged 11 to 14 | 55 | 180 |
| Adolescents aged 15 to 17 years | 70 | 230 |
| Adults and pregnant women | 70 | 255 |
| Breastfeeding women | 85 | 255 |
ANSES reference values; safety limits from EFSA's 2023 opinion.
Which foods are rich in selenium?
According to ANSES, the main sources are certain fish and seafood, meat, eggs, and nuts and seeds, particularly Brazil nuts. Offal such as kidney is among the richest sources.
| Food | Per 100 g (µg) | Serving | Per portion (µg) |
|---|---|---|---|
| Brazil nuts | highly variable | 5 to 6 nuts | 100 to 2,000 |
| Raw beef kidney | 118 | 100 g | ≈ 118 |
| Tuna in oil, drained | 100 | 80 g | ≈ 80 |
| Cooked mussels | 56 | 100 g of flesh | ≈ 56 |
| Sardines in olive oil, drained | 42 | 80 g | ≈ 34 |
| Cooked pollock | 22 | 120 g | ≈ 27 |
| Fried eggs | 26 | 2 eggs (100 g) | ≈ 26 |
| Cooked pasta | 4 | 200 g | ≈ 8 |
Average contents from the Ciqual database (ANSES); illustrative portions. Brazil nuts: EFSA estimate (2023). These averages conceal substantial variation: for canned tuna in water, Ciqual reports 28 to 660 µg per 100 g depending on the samples.
According to EFSA, food content depends largely on where crops and fodder grow: soil is a major determinant of selenium levels in plants. Soils are poorer than average in New Zealand, Nordic countries and parts of Eastern Europe. In 1984, Finland made it mandatory to add sodium selenate to fertilisers. Average intake there rose from 38 µg per day to 80 µg in 2001.
Brazil nuts: unpredictable content
The richest food source is also the most variable. According to EFSA, published averages range from 5 to 72 µg per gram of dry weight depending on the production region, with large differences between nuts in the same batch. In a New Zealand trial with 59 adults, two nuts per day for 12 weeks provided an average of 53 µg, ranging from 20 to 84 µg depending on the nuts, and raised blood selenium as much as 100 µg of selenomethionine.
The practical consequence: it is impossible to know what a particular nut supplies. Eating a few occasionally is better than several every day, especially if you already take a supplement.
Selenium deficiency: who is affected?
In France, confirmed deficiency is rare. In the SU.VI.MAX cohort of 13,017 adults recruited in the 1990s, fewer than 2% had blood selenium below the “sub-deficiency” threshold of 0.75 µmol/L. However, nearly 80% remained below the level then considered optimal for glutathione peroxidase. ANSES cites the example of people receiving intravenous nutrition without selenium who developed muscle weakness, among other symptoms.
The historical reference is Keshan disease, an often fatal heart disease observed in Chinese regions where cereals contain very little selenium. Large-scale trials with sodium selenite tablets substantially reduced the number of cases, although other factors are not ruled out. Unexplained muscle weakness warrants medical advice.
Selenium excess: what warning signs matter?
Like vitamin B6, selenium becomes toxic at high doses. The most frequent signs of chronic poisoning, or selenosis, are brittle, thickened nails with white spots or ridges, and brittle hair that falls out. EFSA also lists garlic-smelling breath, skin lesions, discoloured teeth, fatigue, tingling and an unsteady gait.
The 255 µg limit comes from the SELECT trial: hair loss was more frequent in men receiving 200 µg per day in addition to their diet, around 330 µg in total, with a relative risk of 1.28. EFSA considers exceeding the limit unlikely in adults, except with regular consumption of Brazil nuts or high-dose supplements. If these signs occur, stop the supplement and seek medical advice.
What do the large selenium trials show?
No cancer benefit, an unfavourable diabetes signal and lower thyroid antibody levels of uncertain clinical relevance.
SELECT: no effect on prostate cancer
SELECT assigned 35,533 men aged 50 or over to four groups: selenium (200 µg of L-selenomethionine per day), vitamin E, both, or placebo. After a median follow-up of 5.5 years, selenium did not change prostate cancer risk (hazard ratio 1.04; 99% confidence interval 0.87 to 1.24) or the risk of other cancers. A later analysis found more high-grade prostate cancers with selenium among men whose initial selenium status was high. The 2018 Cochrane review of 10 trials with 27,232 participants concluded, with high certainty, that selenium does not reduce cancer risk.
Type 2 diabetes: the U-shaped curve
The NPC trial tested 200 µg of selenium per day in dermatology patients in the eastern United States, where intake is low. A secondary analysis of 1,202 participants without diabetes recorded, over an average of 7.7 years, 58 cases of type 2 diabetes with selenium versus 39 with placebo (hazard ratio 1.55). This was especially evident in the third of participants with the highest initial blood selenium (hazard ratio 2.70). In SELECT, the trend went in the same direction but was not significant. In 2023, EFSA pooled 5 trials with 22,227 participants: relative risk 1.11, with moderate certainty for a cause-and-effect relationship.
Hence the hypothesis of a U-shaped relationship proposed by researcher Margaret Rayman: additional intake might help when status is low, but harm when it is already sufficient.
Autoimmune thyroiditis: lower antibodies, then what?
In Hashimoto thyroiditis, the immune system produces antibodies against the thyroid, including anti-TPO. A 2016 meta-analysis of 16 trials observed a decrease in these antibodies with selenium at 3, 6 and 12 months in patients treated with levothyroxine, but only at 3 months in others; evidence quality was considered low. The 2024 analysis of 35 studies confirmed the reduction in anti-TPO, with considerable variation between studies, and noted a slight TSH decrease in patients without hormone treatment, with no effect on thyroid hormones.
These are biological markers: a clinical benefit remains to be demonstrated, stress the 2016 authors. In thyroid disease, the decision belongs to the doctor.
Frequently asked questions
Should you take a selenium supplement?
Not routinely. A supplement should be discussed with a health professional in cases of a very restricted diet or documented deficiency. According to EFSA, supplements surveyed in Europe provide an average of 49 µg per day and up to 220 µg. Add up all your sources to remain below 255 µg.
Organic or inorganic selenium: what is the difference?
Selenomethionine, an organic form, predominates in food and selenium-enriched yeast; sodium selenite and selenate are inorganic forms. According to EFSA, all these forms are well absorbed (50 to 90%). Selenomethionine can also replace methionine in any protein, without performing a selenoprotein's role there.
Does selenium make hair grow?
There is no evidence of this. The authorised claim refers to the “maintenance of normal hair”, not growth. Paradoxically, hair loss is the sign of excess EFSA used to set the safety limit. Unusual hair loss warrants medical advice.
Can you take selenium during pregnancy?
Requirements remain 70 µg per day during pregnancy because the body adapts, then rise to 85 µg during breastfeeding to compensate for what passes into milk. The 255 µg limit also applies. Consult your doctor or midwife before any supplement.
When should you take selenium: morning or evening?
Neither ANSES nor EFSA gives timing instructions. What matters is the total daily dose, including food. Follow the instructions and choose a regular time, such as a meal.
Why is selenium often combined with zinc?
The two trace elements share several authorised claims: immune function, hair, nails and protection of cells against oxidative stress. This explains their frequent presence in the same products. Check each dose and add it to your other sources.
Conclusion
Selenium supports selenoproteins essential for antioxidant defences and the thyroid. Adult requirements, 70 µg per day, are modest and the margin is narrow: the safety limit is 255 µg. Fish, seafood, offal and eggs are good sources; Brazil nuts, with unpredictable contents, should be consumed in moderation. Large trials have shown no benefit from adding selenium when status is sufficient.
Sources
- ANSES (2025). Nutritional reference values for vitamins and minerals. anses.fr
- ANSES (2025). Ciqual food composition database (accessed 14 October 2026). ciqual.anses.fr
- EFSA Panel on Dietetic Products, Nutrition and Allergies (2014). Scientific Opinion on Dietary Reference Values for selenium. EFSA Journal, 12(10), 3846. doi:10.2903/j.efsa.2014.3846
- EFSA Panel on Nutrition, Novel Foods and Food Allergens (2023). Scientific opinion on the tolerable upper intake level for selenium. EFSA Journal, 21(1), e07704. doi:10.2903/j.efsa.2023.7704
- Commission Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children's development and health. Official Journal of the European Union, L 136, 1–40 (consolidated version of 19 August 2024). eur-lex.europa.eu
- Arnaud J et al. (2006). Serum selenium determinants in French adults: the SU.VI.M.AX study. British Journal of Nutrition, 95(2), 313-320. doi:10.1079/bjn20051528
- Chen J (2012). An original discovery: selenium deficiency and Keshan disease (an endemic heart disease). Asia Pacific Journal of Clinical Nutrition, 21(3), 320-326. PubMed 22705420
- Thomson CD et al. (2008). Brazil nuts: an effective way to improve selenium status. American Journal of Clinical Nutrition, 87(2), 379-384. doi:10.1093/ajcn/87.2.379
- Lippman SM et al. (2009). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA, 301(1), 39-51. doi:10.1001/jama.2008.864
- Vinceti M et al. (2018). Selenium for preventing cancer. Cochrane Database of Systematic Reviews, 1, CD005195. doi:10.1002/14651858.CD005195.pub4
- Stranges S et al. (2007). Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Annals of Internal Medicine, 147(4), 217-223. doi:10.7326/0003-4819-147-4-200708210-00175
- Rayman MP (2012). Selenium and human health. The Lancet, 379(9822), 1256-1268. doi:10.1016/S0140-6736(11)61452-9
- 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(12), 1681-1692. doi:10.1089/thy.2016.0256
- Huwiler VV et al. (2024). Selenium supplementation in patients with Hashimoto thyroiditis: a systematic review and meta-analysis of randomized clinical trials. Thyroid, 34(3), 295-313. doi:10.1089/thy.2023.0556
Food supplements do not replace a varied, balanced diet or a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, pregnant or breastfeeding.
