selenium

Selenium is an essential trace mineral the body needs in small amounts to stay healthy. According to the NIH Office of Dietary Supplements, it is a component of selenoproteins that support thyroid hormone production, reproduction, DNA synthesis, and protection of cells from oxidative damage and infection. It occurs naturally in protein-rich foods such as seafood, meat, poultry, eggs, dairy, and grains, and is also sold as a stand-alone supplement and in multivitamin/mineral products in forms including selenomethionine, selenium-enriched yeast, sodium selenite, and sodium selenate.

Most people in the US already get enough selenium from food, and for several conditions people commonly take it for, the NIH says supplements have not shown a clear benefit. According to the NIH ODS, studies have found that selenium supplements "do not lower the risk of prostate cancer," and other research suggests they "does not reduce the risk of heart disease," especially in people who already get enough from food. Most studies have also not found that selenium supplements reduce the risk of cognitive decline or Alzheimer's disease. For thyroid disease and male fertility the NIH notes the data are limited and "more research is needed," and supplements have shown "little if any benefit" in people with HIV. While getting enough selenium from the diet "may lower" the risk of some cancers, the NIH cautions this does not mean supplements provide the same effect; it also notes more research is needed to understand selenium's effects on cancer risk.

The Recommended Dietary Allowance for adults is 55 mcg per day (rising to 60 mcg in pregnancy and 70 mcg while breastfeeding), and the Daily Value used on supplement labels is 55 mcg. Multivitamin/mineral products often contain around 55 mcg, but selenium-only supplements typically contain 100-400 mcg, which can be far more than most people need. Because most Americans already meet their needs through food and excess selenium can be harmful, talk to a doctor, pharmacist, or registered dietitian before taking a selenium supplement, especially at higher doses.

Selenium can be harmful when too much is taken over time. The NIH sets a Tolerable Upper Intake Level of 400 mcg per day for adults from all sources combined (food, beverages, and supplements). Brazil nuts are very high in selenium (about 68-91 mcg per nut) and can push intake over the upper limit if you eat too many. Very high intakes can cause serious effects including difficulty breathing, tremors, kidney or heart failure, heart attacks, and, in rare cases, death. The NIH notes these upper limits do not apply to people taking selenium under the care of a doctor.

Getting too much selenium over time (sometimes called selenosis) can cause garlic breath, a metallic taste in the mouth, hair loss, brittle nails or nail loss, skin rash, nausea, diarrhea, extreme tiredness, irritability, and nervous system problems. Consuming very high amounts can lead to more severe effects such as trouble breathing, tremors, lightheadedness, flushing, muscle tenderness, and kidney or heart failure.

Selenium may interact with certain medications. The NIH notes that cisplatin, a chemotherapy drug, can lower selenium levels, although the effect this has on the body is not clear. Tell your doctor, pharmacist, and other providers about all supplements and prescription or over-the-counter medicines you take so they can check for possible interactions.