Selenium: needs, supplementation and risks

Selenium is a component of selenoproteins. They're involved notably in thyroid hormone metabolism, DNA synthesis, reproduction, and protecting cells against certain oxidative damage. These functions describe a nutritional need; they don't prove that a capsule improves a disease, prevents cancer, or boosts immunity.

The NIH sets 55 µg per day for adults, 60 µg during pregnancy, and 70 µg during breastfeeding. The European Food Safety Authority (EFSA) set in 2014 an adequate intake of 70 µg per day for adults and 85 µg during breastfeeding. The gap comes from the methods and criteria used; none of these figures is a universal tablet dose.

These values include all sources: food, fortified products, multivitamins, and supplements. In cases of digestive disease, dialysis, a very restricted diet, or suspected deficiency, the assessment should be made with a professional.

Fish and seafood, meat, poultry, eggs, dairy products, and grains provide selenium. For plants, the content depends heavily on the soil and the growing region.

Brazil nuts are exceptionally rich in selenium, but their content varies widely from batch to batch. They therefore don't provide a standardized dose: regularly eating large amounts, on top of a supplement, can push you past the safety limit.

Supplements can contain selenomethionine, selenium-enriched yeast, sodium selenite, or sodium selenate. The name of the form doesn't prove clinical superiority. To compare two labels, look at the amount of selenium per serving and the unit: 1 mg equals 1,000 µg, and a unit mistake can multiply the dose by a thousand.

A supplement is worth discussing when a professional identifies an insufficient intake or a deficiency to correct. It doesn't replace the diagnosis or treatment of a thyroid disorder.

The SELECT trial, one of the largest prevention trials, randomly assigned 34,887 healthy men to selenium (200 µg per day), vitamin E, the combination of both, or a placebo, with follow-up of 7 to 12 years. Selenium alone did not reduce the risk of prostate cancer: the observed increase was not significant (hazard ratio 1.09). Vitamin E alone increased that risk by 17%.

Prolonged excess causes selenosis. Its signs include garlic-smelling breath, a metallic taste, digestive disorders, a skin rash, fatigue, irritability, as well as hair loss and brittle nails. At very high exposure, neurological disorders and serious harm are possible.

Two upper limits coexist: 400 µg per day from the NIH and 255 µg per day from EFSA since 2023, for adults, including during pregnancy and breastfeeding. EFSA identified hair loss as the first observable adverse effect. These limits are ceilings not to be exceeded, not targets to reach.

1. 01Note your usual food sources and any supplements already in use.
2. 02Check the amount of selenium per serving and the unit on each label.
3. 03Add up the intake from all products.
4. 04Don't take selenium to treat a symptom or prevent cancer without a diagnosis.
5. 05Regularly reassess whether the product is still needed instead of automatically continuing it.

For another trace element often paired with selenium in "immunity" formulas, see the article on zinc.