Selenium

Selenium is an essential mineral that occurs naturally in many foods. It is also available as a dietary supplement. Selenium is a component of 25 selenoproteins, including thioredoxin reductase, glutathione peroxidase, and selenoprotein P. Selenoproteins play a key role in thyroid hormone metabolism, DNA synthesis, reproduction, and protection against oxidative damage and infection.

Soil and groundwater contain inorganic forms of selenium (e.g., selenites, selenates) that plants accumulate and convert to organic forms, mainly selenomethionine and selenocysteine ​​and their methylated derivatives. In food, selenium is present primarily as selenomethionine along with selenocysteine. Dietary selenium is readily absorbed by the body. Selenomethionine is the organic form of selenium with high bioavailability and excellent absorption.

Plasma and serum selenium concentrations are commonly used to assess selenium status, and concentrations of 8 mcg/dL or higher are usually considered sufficient in healthy individuals. The selenium content of hair and nails can be used to estimate long-term selenium intake over months or years. Two plasma selenoproteins, glutathione peroxidase 3 and selenoprotein P, can be used as functional biomarkers of selenium status, but they can be influenced by factors such as inflammation, making them potentially unreliable.

Recommended intakes: The Food and Nutrition Board of the National Academies of Sciences, Engineering, and Medicine has established recommended daily allowances and adequate intakes for selenium. These values ​​range from 55 to 70 mcg for adults and from 15 to 70 mcg for infants, children, and adolescents, depending on age and life stage. Protein-rich foods, such as seafood, meat, and poultry, are rich sources of selenium. Brazil nuts are particularly high in selenium. Selenium concentrations in plant foods vary by geographic location because the amount and form of selenium in the soil and other factors can affect the selenium content of these foods. Forms of selenium commonly found in dietary supplements include selenomethionine, selenium-enriched yeast, sodium selenite, and sodium selenate. The body is able to absorb up to about 90% of the selenium from these forms.

Selenium intake and status: Although selenium intake and serum concentrations may vary by region, most people consume adequate amounts of selenium. However, people who follow a vegetarian or vegan diet may have lower selenium intake, and people who smoke cigarettes tend to have lower selenium status than non-smokers, possibly because cigarette smoking increases oxidative stress.

Selenium deficiency: Selenium deficiency alone rarely causes disease. However, in certain populations, selenium deficiency can lead to the development of conditions such as Keshan disease (a heart disease caused by chronic selenium deficiency) and Kashin-Beck disease (a chronic osteoarthropathy that leads to dwarfism and severe arthritis). In addition, selenium deficiency can increase the risk of congenital hypothyroidism in infants by exacerbating iodine deficiency. Although selenium deficiency is rare, some groups of people are at higher risk of inadequate intake. These groups include people living in selenium-deficient regions, those undergoing kidney dialysis, and those living with HIV.

Selenium and health: Selenium supplementation focuses on six diseases and conditions in which selenium may play an important role: cancer, cardiovascular disease (CVD), cognitive decline and Alzheimer’s disease (AD), HIV infection, male fertility, and thyroid disease.

Cancer: Some epidemiological studies suggest that people with higher selenium status have a lower risk of several types of cancer, and one randomized controlled trial found that men who took selenium had a lower risk of prostate cancer over a multi-year follow-up period. Observational studies examining the association between low selenium status and cardiovascular disease risk have yielded conflicting results. Available evidence from clinical trials does not support the use of selenium supplementation to reduce the risk of cardiovascular disease, especially in people who get enough dietary selenium. However, taking selenium as part of an antioxidant formula may reduce the risk of cardiovascular mortality.

Cognitive decline and Alzheimer’s disease: Researchers are investigating the role of selenium in maintaining cognitive function in older adults, as chronic selenium deficiency has been linked to cognitive decline. Results from observational studies have been mixed, with some reporting an association between lower plasma selenium concentrations and neurological impairment or a higher risk of Alzheimer’s disease, while others have found no such association.

HIV infection: Although selenium deficiency may increase the risk of morbidity and mortality among people with HIV, selenium supplementation appears to provide limited benefit in this population. Some evidence suggests that selenium supplementation may reduce the risk of preterm birth in pregnant women with HIV.

Male fertility: Some clinical trials have reported improvements in certain aspects of sperm quality in men who received selenium supplements.

Thyroid disease: Selenium plays an important role in the synthesis and metabolism of thyroid hormones. In women, low selenium status appears to be associated with an increased risk of thyroid disease; this association has not been found in men. Clinical trials have found that selenium supplementation may reduce levels of certain thyroid antibodies in people with chronic autoimmune thyroiditis. Results from clinical trials examining the effects of selenium supplementation on thyroid antibodies in pregnant women have been mixed, and the American Thyroid Association recommends that selenium supplementation not be taken by pregnant women with thyroid autoimmunity.

Health risks of excessive selenium intake: Selenosis results from chronically high selenium intake. It is most commonly characterized by hair loss and brittle or falling out nails, but other signs and symptoms may include a garlicky breath odor, a metallic taste in the mouth, skin rashes, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities. The upper permissible amount of selenium for adults is 400 mcg, and for infants, children and adolescents it ranges from 45 mcg to 400 mcg, depending on age.

Drug Interactions: Selenium supplements can interact with drugs. In addition, certain drugs, such as cisplatin, can affect the level of selenium in the body.