Selenium: Benefits, Dosage, Food Sources and Upper Limit

Incorporated into selenoproteins including glutathione peroxidases and thioredoxin reductases, which handle antioxidant defence, and the deiodinases that convert thyroxine to the active triiodothyronine. That thyroid role is why it appears in thyroid formulas.

• Correcting deficiency: unambiguous where intake is genuinely low, which depends heavily on regional soil selenium.
• Thyroid function: required for the deiodinases; supplementation reduces thyroid antibody titres in autoimmune thyroiditis in some trials.
• Antioxidant defence: essential to glutathione peroxidase activity.
• Cancer prevention: not supported. SELECT found no reduction in prostate cancer and reported an increased risk of high-grade disease and of type 2 diabetes.

The SELECT trial randomised 35,533 men to 200 mcg/day selenomethionine with or without vitamin E and was stopped after 5.5 years; selenium did not lower prostate cancer risk and was associated with increased high-grade disease and type 2 diabetes. The NIH Office of Dietary Supplements fact sheet documents the 400 mcg upper limit and the selenosis threshold behind it.

The RDA is 55 mcg/day for adults. Supplements typically supply 100-200 mcg. The tolerable upper intake level is 400 mcg/day, derived by halving the 800 mcg/day intake at which selenosis appeared. Two Brazil nuts can supply more than the RDA, so food sources count towards the total.

Selenosis at high chronic intakes: brittle hair and nails or hair loss, a garlic odour on the breath, GI upset, skin rash, fatigue and irritability. These appeared around 800 mcg/day. SELECT also reported a non-significant increase in type 2 diabetes risk in the selenium arm.

May reduce the efficacy of some chemotherapy and radiotherapy through antioxidant activity; oncology patients should ask first. Additive with other selenium sources including multivitamins and Brazil nuts, which is the usual route to accidental excess. Statin and niacin combinations have shown blunted HDL responses in some trials.