Selenium is a trace element that is essential for good human health. Essential means that selenium is a micronutrient that the body must have but cannot synthesize for itself. Its supply must come from food and, if necessary, from supplements. What the body does synthesize when it has an adequate supply of the element selenium is first the amino acid selenocysteine and then selenoproteins into which the selenocysteine molecules are incorporated [Bai 2024].
Accordingly, selenium is an indispensable component of selenoprotein P and numerous other selenoproteins. Among these selenoproteins are several vital selenoenzymes, e.g., the glutathione peroxidases (GPxs), the thioredoxin reductases (TrxRs), and the iodothyronine deiodinases (DIOs). Through the activity of the selenoproteins, selenium has many biological functions, i.e., antioxidant activity, anti-inflammatory effects, enhanced immune system function, enhanced reproductive capacity, protection of the cardiovascular system, and regulation of thyroid function. In figure 6 of their paper, Bai et al list the best documented selenoproteins and their known functions [Bai 2024].
Narrow Safe Intake Range for Selenium
Adequate supply of selenium is critical to human health. Even so, it is important to remember that more is not always better. The distribution of selenium in the soil and in crops and fruits is extremely uneven around the world. There are significant differences in soil and food selenium content between and within different regions of the world. That means that the need for selenium supplementation will differ considerably from region to region [Bai 2024].
In a 2024 review, Bai et al report that more than 40 countries and regions of the world, with 500 million to 1 billion residents, are selenium-deficient to one degree or another. Many countries in Europe and the Middle East are among these selenium-poor countries [Stoffaneller & Morse 2015].
Compared to other substances used in nutritional substances, the range between nutritional doses and toxic doses of selenium is quite narrow. There seems to be a U-shaped relationship between selenium intake/status and human health. Some adverse health effects such as increased mortality risk are associated with both low and high selenium status. Selenium intakes to maintain a serum selenium concentration of approximately 125 mcg/L seem to be optimal [Rayman 2020, Fig. 2].
Different Forms and Species of Selenium
Selenium in food and in supplements consists of both inorganic and organic selenium. Generally, the body absorbs organic selenium more efficiently than it does inorganic selenium [Bai 2024].
In simplest terms, the most commonly encountered forms of selenium in supplements are the inorganic sodium selenite supplements, the organic 100% selenomethionine supplements, and the organic selenium-enriched yeast supplements. Selenium-enriched yeast supplements have two distinct advantages:
- They supply the body with more than 20 different selenium species in addition to the selenomethionine that they deliver. The various other selenium species in the selenium-enriched yeast preparations seem to have biological effects beyond the biological effects of selenomethionine [Larsen 2004].
- Selenium-enriched yeast supplements confer antioxidant effects that pure selenomethionine supplements do not [Richie 2014].
Biological Functions of Selenium
Antioxidant Activity
Bai et al report that nearly half of the known selenoproteins have antioxidant functions. These selenoproteins include the GPxs, the TrxRs, the DIOs, Selenoprotein P, Selenoprotein H, Selenoprotein M, Selenoprotein O, and Selenoprotein V. In particular, the GPxs are notable for their antioxidant activity. In the paper by Bai et al, there is an extensive review of the antioxidant properties of various selenoproteins [Bai 2024].
Immune System Activity
Bai et al report that selenium is widely present in all immune cells. Selenium has a role in protecting the thymus, maintaining lymphocyte activity, and promoting antibody formation. Supplementing with selenium where necessary can enhance the body’s immune system, both the innate immune system and the adaptive immune system [Bai 2024].
Thyroid Hormone Synthesis
The three deiodinase selenoenzymes have different roles in the interconversion of the thyroid hormones T3 and T4. Thus, selenium is essential for thyroid functionality and normal body functioning. In periods of low selenium intake, the thyroid gland has high priority in the hierarchy of selenium distribution within the body [Schomburg 2020].
Other Biological Functions of Selenium
In addition to the above mentioned biological functions, Bai et al list the following selenium-related activities:
- Enhancement of male fertility
- Neutralization of toxic heavy metals
- Reduction of cardiovascular disease mortality
- Regulation of inflammatory responses
Conclusion: Selenium and Human Health
Selenium has several critical biological roles in the body.
Maintaining an appropriate level of selenium intake is very important.
If serum selenium status is below 90-100 mcg/L, then supplementation may be advisable [Al-Mubarak 2021].
Sources
Al-Mubarak AA et al. Selenium, selenoproteins, and heart failure: current knowledge and future perspective. Curr Heart Fail Rep. 2021 Jun;18(3):122-131.
Bai S et al. Effects and impact of selenium on human health, a review. Molecules. 2024 Dec 26;30(1):50.
Larsen EH et al. Speciation and bioavailability of selenium in yeast-based intervention agents used in cancer chemoprevention studies. J AOAC Int. 2004 Jan-Feb;87(1):225-32.
Rayman MP. Selenium intake, status, and health: a complex relationship. Hormones. 2020;19:9–14.
Richie JP Jr et al. Comparative effects of two different forms of selenium on oxidative stress biomarkers in healthy men: a randomized clinical trial. Cancer Prev Res (Phila). 2014 Aug;7(8):796-804.
Schomburg L. The other view: the trace element selenium as a micronutrient in thyroid disease, diabetes, and beyond. Hormones (Athens). 2020 Mar;19(1):15-24.
Stoffaneller R, Morse NL. A review of dietary selenium intake and selenium status in Europe and the Middle East. Nutrients. 2015 Feb 27;7(3):1494-537.
The information presented in this review article is not intended as medical advice. It should not be used as such.