Selenium is an essential mineral which is found in changing concentrations in the soil. It's found in foods such as Brazil nuts, yellowfin tuna, sardines, sunflower seeds and garlic, but the quantity in any food is determined by the selenium content of the soil in which it was grown.
Selenium is also found in water and therefore is also prevalent in different quantities in seafood. However, regardless of its relatively common presence in foods, many people are deficient of selenium. One of the main reasons for selenium deficiency is a high processed-food diet because selenium is destroyed during refining and processing.
Despite the fact that our body has a very small requirement for selenium, it's estimated that almost 1 billion people globally suffer from lack of selenium whilst a greater amount consume less selenium than is necessary to provide adequate protection against cancer and severe infectious diseases.
While small quantities of selenium provides significant benefits to our body, taking too much (for instance, 450 mcg daily) of it can potentially increase the risk of diabetes. However, it’s virtually impossible to overdose on selenium merely through nutrition.
Selenium acts as a powerful antioxidant (with increased effects when combined with vitamin E), and as such plays an important role in preventing chronic diseases as well as being important for thyroid and immune system function. According to one research:
"Selenium [Se] is critical to the health of living organisms. It has been postulated that the vast majority of the world's population has suboptimal Se intakes, and hence is at increased risk of several diseases such as cancer, heart disease, viral diseases and other conditions that involve increased levels of oxidative stress.
There are several disease conditions (e.g. diabetes, several infectious diseases and possibly asthma) where … good Se status in combination with an adequate intake of other antioxidative nutrients may help cells and tissues better to cope with harmful oxidative stress caused.
For instance, by some toxic heavy metal or other environmental pollutants, by hyperglycaemia, or by the immune system's reaction to infection. Efforts to increase Se concentration in the diet are urgent for both current and future generations."
In a recent research it was published that higher selenium levels are linked to a lower risk of colorectal cancer, particularly in women. The same study also found that selenium status is suboptimal in many Europeans.
A meta-analysis of 69 studies that was published this year found that high selenium exposure from food (not from supplements), had a protective effect on cancer risk and reduced the risk of breast, lung, oesophageal, gastric and prostate cancers.
Some of the scientific explanations for selenium's anti-cancer effects include:
Increased antioxidant protection and immune system support
Triggering DNA repair in damaged cells
Suppression of growth of blood vessels supplying the cancer with nourishment
Inhibition of tumour cell invasion
Selenium’s anti-cancer effects derive from its antioxidant properties as well as its ability to boost immune system function. For example, it may stimulate the immune system so it's able to eliminate early cancers. Aside from cancer, this immune stimulation may be advantageous for inhibition of infectious diseases. Selenium is often mentioned in tandem with HIV, since HIV-infected people frequently have low selenium levels.
Some studies have also found a relationship between selenium deficiency and evolution of HIV to AIDS, while others have found selenium supplementation may reduce hospitalisations and improve white blood cell counts among this population. It may also be useful for other viral infections, including influenza, as well as potentially bacterial infections.
Although there is a lot more which is yet to be discovered about selenium, it's known that it plays a role in thyroid function. The thyroid gland contains more selenium (per gram of tissue) than any other organ and expresses specific selenoproteins.
There is current research conducted into selenium's role in Hashimoto's disease and Graves' disease as well as its use in pregnant women with anti-TPO (thyroid peroxidase) antibodies. There is also some suggestion that selenium may be useful for people with chronic asthma.
Selenium is a nutrient which is best acquired through balanced nutrition. However, there is some inconsistent and confusing research surrounding selenium and its role in human health, and much of this stems from the consumption of selenium supplements. Until more is understood about selenium's role in the body, it makes sense to focus on eating a variety of whole, unprocessed foods, which will naturally optimises selenium levels (along with other important nutrients). Good sources of selenium include:
Brazil nuts – 30 grams(6-8 nuts): 544 mcg (over 100% DV)
Yellowfin tuna – 90 grams: 92 mcg (over 100% DV)
Halibut, cooked - 90 grams: 47mcg (67% DV)
Sardines, canned - 90 grams: 45mcg (64% DV)
Grass-fed beef - 90 grams: 33 mcg (47% DV)
Turkey, boneless - 90 grams: 31 mcg (44% DV)
Beef liver - 90 grams: 28 mcg (40% DV)
Chicken - 90 grams: 22 mcg (31% DV)
Egg - 1 large, 15 mcg (21% DV)
Spinach - 1 cup: 11 mcg (16% DV)
Selenium levels in the soils in areas such as China, Russia, Australia, New Zealand and Europe tend to be much lower than in the US, and if you eat food primarily grown in these areas, a high-quality selenium supplement may be beneficial. Even parts of the U.S. have been identified as selenium-deficient regions, including The Pacific Northwest, parts of the Great Lakes region and east of it toward New England and parts of the Atlantic Coast.
If you live in one of these areas and focus your diet on locally grown foods, you might want to test that you’re not low on selenium. Low levels of selenium are also associated with cigarette smoking, alcohol drinking and in people who have undergone weight loss surgery, or have Crohn's disease or ulcerative colitis.
It is generally recommended that when selenium is taken as a supplement for health maintenance, a high-quality bioavailable form in a low dose (such as 200 mcg) is used. However, as always…. It’s best to consult your healthcare professional regarding what is most appropriate for your own diet.
 The minimum adult daily recommended dietary allowance (RDA) for selenium is 55 micrograms
 Int J Cancer. 2015 Mar 1;136(5):1149-61
 Sci Rep. 2016 Jan 20;6:19213.
 Nutr Rev. 2010 Nov;68(11):671-81.