Selenium Explained Simply

1. What Is Selenium?

Selenium is a trace element. This means: the body needs it in very small amounts but cannot produce it itself. It takes it in through food. Selenium is found in soils and in plants. Animal foods also contain it. In the body, selenium is a building block of its own proteins, the enzymes. In studies, selenium is tested as a dietary supplement. Mostly it concerns people who take in little selenium, or people with certain diseases.

2. Where Is It Found, and What Is It Made From?

Selenium occurs in soils and in plants. How much selenium is in the soil varies greatly from region to region. Through plant and animal foods it enters the body. The amounts differ depending on the food and its origin. For supplements, selenium is mainly obtained as a byproduct of metal processing. This produces salts such as sodium selenite and sodium selenate or organic compounds such as selenomethionine. In studies, mainly these forms as well as selenium-enriched yeast are tested.

3. What Does Selenium Do? What Research Shows

Heart and Circulation

There are many studies on the heart and circulation. Some found more favorable blood lipid levels, for example in metabolic disorders 1721 and for HDL cholesterol 19. Other studies found no difference in mortality, neither in people with heart disease 3 nor in prevention 5 nor after heart surgery 74251. Also for cardiovascular events, no difference was shown several times 1538. A review even found a slight increase in the upper blood pressure value 28.

Thyroid and Hashimoto's Thyroiditis

In Hashimoto's thyroiditis, an autoimmune disease of the thyroid, TPO antibodies fell in several review articles 81113162027. Other studies found no difference here 526071757778. Tg antibodies fell in some studies 1316617682, but not in two reviews 811. For the thyroid hormone TSH, there was sometimes a slight decrease 81382, sometimes no difference 1135657778. In a large study, quality of life did not improve more with selenium than with a sham treatment (placebo) 49.

Immune Function

There are many studies on immune function, and the results are mostly favorable. In controlled studies (with a comparison group), inflammation markers such as C-reactive protein fell 63646780. In HIV, selenium slowed the decline of certain immune cells 50. In very low birth weight preterm infants, late-onset bloodstream infection occurred less frequently 32. Two reviews, by contrast, found no difference in infection risk in critically ill people 2537. The results are inconsistent depending on form, amount, and population.

Cancer Prevention and Cancer

In cancer prevention and cancer diseases, selenium mostly showed no benefit. In a large study with selenomethionine, prostate cancer risk did not fall 41. A large review found no difference in cancer incidence and cancer deaths 4. Side effects of treatment occurred less frequently in two studies 5859. An older review found lower cancer incidence at low baseline levels 2, but the large review did not confirm this 4.

Pregnancy

In pregnant women with gestational diabetes, blood glucose and insulin values improved in several studies 6381. One study found no difference here 68. In a study with selenium-enriched yeast, neonatal jaundice and hospital admissions of newborns occurred less frequently 79. In HIV-positive pregnant women, the risk of preterm birth fell 54. A review found preeclampsia less frequently 36. In two studies, blood flow in the uterine artery improved 6669.

Male Fertility

Regarding male fertility, the data situation is unclear. A review found better semen values under selenium 29. Another review compared eight antioxidants and found confirmed advantages only for coenzyme Q10 and carnitine 12. A study with selenium, vitamin E and folic acid in men with few motile sperm found no difference from a sham treatment 62. The chemical form of the selenium is not named in these studies. Some of the preparations contained additional substances. The strength of evidence is unclear because there is only one small study with a comparison group.

Other Investigations

In this field, very many studies on various questions were examined. Several works found a better selenium status 104345465556. The activity of the enzyme glutathione peroxidase also rose several times 33140577073. For mortality of critically ill people, two reviews found an advantage 625, other reviews no difference 18233037. In isolated cases there were adverse findings: higher mortality in older people under the highest amount 44, worse blood sugar control in type 2 diabetes 72 and worse insulin action in PCOS 74.

4. How Was Selenium Taken in the Studies?

In the studies, selenium was predominantly swallowed, that is, as a tablet or capsule. Mainly selenomethionine, selenium-enriched yeast, sodium selenite, and sodium selenate were used. Individual works gave selenium directly into the vein, for example in heart surgery 42 or in critically ill people 6141539. Some of the studies tested selenium only together with other substances, for example with coenzyme Q10 4748. From such mixed preparations, no separate effect of selenium can be derived.

5. Is Selenium Safe? Side Effects in the Studies

The results depend strongly on which chemical form of selenium was used and how much of it. The baseline supply of the participants also plays a role: In regions with selenium-poor soils, different results are to be expected than in well-supplied areas. Some of the studies examined selenium in mixed preparations with several substances. From this, no statement about selenium alone can be derived. Blood values such as the selenium concentration in the blood are only measured quantities (measured value). They are not to be equated with a clinical endpoint (target value of the study), that is, not with a noticeable health outcome. In several reviews, selenium was well tolerated 618. In the largest prevention study, however, hair loss and skin inflammation occurred more frequently under selenium 4. The risk for type 2 diabetes also rose slightly in studies 434. A study with older people in Denmark found higher mortality under the highest amount 44. The large review also reports an indication of a higher risk for black skin cancer (melanoma) 4.

All details with tables and sources: full expert page on Selenium