Selenium is a nutritionally essential mineral for humans, that plays critical roles in thyroid hormone metabolism, reproduction, DNA synthesis and protection from oxidative damage and infection. Selenium may assist to regulate the immune system as an immunomodulator and is also a co-factor for glutathione peroxidase, an important antioxidant enzyme within the immune system. Both animal studies and human trials have demonstrated its ability to improve activation and proliferation of white blood cells such as B and T lymphocytes and Natural Killer cells.
Besides Brazil nuts which are relatively rich in selenium, the major sources of dietary selenium include seafood and meat, especially organ meats. In general, plant-based foods tend to have lower levels of selenium compared to animal products, and the amount of selenium contained in plant sources tends to vary widely depending on the selenium content of the soil the plants are grown in. Several other factors affect the selenium concentrations in plant-based foods, including soil pH, whether the form of selenium in the soil is amenable to plant uptake, and the amount of organic matter contained in the soil.
Research suggests that people at highest risk of deficiency are vegetarians who consume diets in regions with selenium deficient soil. For instance, the lowest selenium intakes in the world exist in certain regions of China, where large proportions of the population consume primarily vegetarian diets and selenium content of soil is very low. In addition, average selenium intakes are low in some European countries, especially among those consuming vegan diets. Historically, New Zealand had relatively low intakes of selenium, until the country increased importation of foreign wheat with higher levels of selenium.
In terms of bioavailability, studies have demonstrated that selenomethionine tends to have the best absorption characteristics with more than 90% selenomethionine absorbed by the human body, compared to only 50% selenium absorbed from selenite.
Selenium. National Institutes of Health. Office of dietary supplements. [cited 23 Nov 2021). Available from: https://www.nrv.gov.au/nutrients/selenium