Selenium is an essential mineral substance necessary for the body to function. It plays a key role in the functioning of thyroid glands, immunity, DNA synthesis and the antioxidant system. (1) It can be found in various foods, but the selenium content of food is greatly influenced by the selenium content of the soil. Some areas are extremely deficient in selenium, which can be detected in the selenium content of the food originating from there. (1) Where food has a low selenium concentration, people often suffer from selenium deficiency, while in other areas people usually obtain  a sufficient quantity. (1) High dose selenium intake can significantly lower the risk of several chronic illnesses, but one must be careful, as it can be quite easily overdosed. (2,3,4) 

How much do we need? 

The necessary daily intake of selenium is prescribed between 55-70 mcg, but many people consume more. (5,6) It’s not an issue, as higher doses of selenium are good for health up to a point, and consequently a daily intake of ~100-200 mcg can even be beneficial. However, bear in mind that the maximum amount of recommended daily intake is 400 mcg.  

How to provide sufficient selenium intake?  

Food of animal origin is the best source of selenium. Seafood is highly recommended, as not only does it have a consistently high selenium concentration, but it can be absorbed very well. (1,5) The selenium content of other  types of meat, eggs and dairy depend on the animals’ access to selenium from feed. The selenium content of plant products depends  on the soil. Brazil nut is an outstanding example, as a single nut can provide the recommended daily intake of selenium, provided it’s from a good agricultural land. (5) 

Selenium deficiency is the most common among people who live in a selenium deficient area and have a plant based diet. It’s often the case in China, for example, where selenium deficiency used to affect a high portion of the population and resulted in heart disease, infections and impotence. (7) After selenium supplementation has been introduced, these conditions were successfully suppressed.  

Selenium supplements come in many forms: the most popular are organic selenomethionine and yeast enriched with selenium, as well as various inorganic selenium salts, such as sodium selenite. Organic forms are recommended, as their absorption is more effective and their consumption is more realistic/life-like. (8) 

The health effects of selenium 

Many meta-analyses, which summarize multiple studies, came to the conclusion that higher selenium intake can have a protective effect against certain cardiovascular- or tumorous diseases. (2, 3, 4) 

A meta-analysis summarizing 49 studies pointed out that the risk of cancer was 31% lower and mortality as a result of cancer was 45% less likely for people with higher selenium levels. (3) However, selenium supplementation had mixed results in certain cancer prevention studies, probably because the tests were conducted on American citizens whose selenium levels were optimal to begin with. (9) 

Higher levels of selenium intake decreases the risk of cardiovascular diseases. A meta-analysis found that optimal selenium intake decreased the risk of cardiovascular diseases by 15% while selenium supplementation decreased it by 11%. (2) A more recent meta-analysis observed similar results: higher levels of selenium lead to a 13% decrease in the risk of cardiovascular diseases. (4) 

A recent study, where 929 participants were monitored , helps us deduce the optimal range of selenium intake. Researchers observed a U-shape graph between the participants’ selenium levels and mortality, meaning that too high and too low amounts of selenium both have a negative impact — which explains why selenium supplementation has not been effective for everybody. (10) 

Selenium is essential for proper thyroid  function. In a study, daily supplementation of 200 mcg selenium significantly decreased thyroiditis and hypothyroidism in women with autoimmune thyroid disease . (11) A meta-analysis made an additional discovery: selenium supplementation decreases the number of antibodies created against the thyroid glands in people with autoimmune thyroid disease . (12) 

The dangers of too much selenium supplementation  

The optimal range of selenium is relatively limited, so both deficiency and over-supplementation can be dangerous. Too much selenium can lead to poisoning, symptoms of which include garlicky breath, fragile nails, split ends of hair and various digestive issues. (13) Due to a manufacturing error, 201 people experienced  selenium poisoning after taking a dietary supplement which contained 200 times  the amount on the label. Hair loss and gastrointestinal symptoms were common, but only one person was hospitalized. (14) 

Overdosing selenium through diet is unlikely, but there are certain foods, such as the above-mentioned Brazil nut, which can easily lead to overconsumption. It’s not currently known how much selenium is too much selenium exactly. In an interesting study, Amazonian tribes with extremely high selenium levels were examined, but none of them had any symptoms of selenium poisoning. (15) 

It must be noted that even 200-300 mcg of daily selenium supplementation can cause issues, for example, it can have a negative influence on blood sugar values , which can in turn increase the risk of diabetes. (16, 17) An observational study found that 200-300 mcg doses of selenium can decrease the activity of certain immune cells. (13)

[The cover photo of the note shows the mineral form of selenium.]