Health Benefits of Selenium

Selenium (Se) is an essential trace element (for example, you only need to eat 1-2 Brazil Nuts per day; see Table 2) of high importance for human health . Studies have already identified associations between selenium deficiency and increased morbidity and mortality from viral infections, cardiovascular, and thyroid diseases, as well as prostate, gastrointestinal, and breast cancers. But, you do need to avoid overdosing on Se as warned by CDC:^[17]

Brazil nuts contain very high amounts of selenium (68–91 mcg per nut) and could cause selenium toxicity if consumed regularly. Acute selenium toxicity has resulted from the ingestion of misformulated over-the-counter products containing very large amounts of selenium. In 2008, for example, 201 people experienced severe adverse reactions from taking a liquid dietary supplement containing 200 times the labeled amount. Acute selenium toxicity can cause severe gastrointestinal and neurological symptoms, acute respiratory distress syndrome, myocardial infarction, hair loss, muscle tenderness, tremors, lightheadedness, facial flushing, kidney failure, cardiac failure, and, in rare cases, death.

Figure 1.  10 foods high in Selenium (Source: dietingwell.com)

Figure 2.  Selenium absorption, metabolism, and distribution (Source: [12])

Absorption/ Metabolism / Distribution

Selenium is found in foods and nutritional supplements in 2 forms

• Organic form
• selenomethionine, selenocysteine, seleniummethylselenocysteine
• Inorganic form
• selenate and selenium

After intestinal absorption, selenium forms are converted into hydrogen selenide (H2Se), a metabolic intermediate incorporated into selenoproteins, in the form of selenocysteine. 

Figure 3.  Selenium supplementation boosts T_FH cells in mice and humans (Source: [7])

Important Roles of Selenoproteins

Se performs its main functions in the form of selenoproteins.  A wide range of these selenoproteins are linked to redox signaling, oxidative burst, calcium flux, and the subsequent effector functions of immune cells being grouped into families such as:

• Glutathione peroxidases (GPXs)
• Iodothyronine deiodinases (DIOs) 
• Thioredoxin reductases (TrxRs)
• Methionine sulfoxide reductase B1 (MSRB1)
• Selenophosphate synthetase 2 (SEPHS2)

In addition, the Selenoprotein P (or SELENOP) acts as the main selenium transporter for peripheral tissues also performing extracellular antioxidant function (see Figure 2). 

Thus, selenium plays a role in antioxidant, anticarcinogenic, anti-inflammatory, redox and immune-cell function as well as in the regulation of thyroid hormone metabolism:

• Anti-inflammatory
• Findings suggest that UV damage to the epidermis affects deeper layers of the skin and even blood and other tissues
• Diets enriched with antioxidant nutrients, including selenium, beta-carotene, vitamin E, and vitamin C, inhibit the formation of UV-induced tumors.^[4]
• A major role of the exogenous antioxidants, vitamins C and E (a-tocopherol), b-carotene, and selenium, is to act as efficient scavengers of reactive oxygen radicals, thereby protecting against oxidative damage.^[14]
• Antiviral
• Se/selenoproteins are relevant in the viral pathogenicity, notably reducing proliferation of T cells, lymphocyte-mediated toxicity and NK cell activity, all of which are crucial for antiviral immunity. 
• Selenoproteins partly reduce oxidative stress generated by viral pathogens.
• The available studies support the belief that selenium may be of relevance in the infection with SARS-CoV-2 and disease course of COVID-19.
• There is a mechanism proposed by Zhang et al.^[10] by which selenium might suppress the life cycle and mutation to virulence of SARS-COV-2 while attenuating viral-induced oxidative stress, organ damage and the cytokine storm.
• Redox
• Selenoproteins also regulate or are regulated by cellular redox tone, which is a crucial modulator of immune cell signaling. 
• The cellular redox environment is a balance between the production of reactive oxygen species (ROS), reactive nitrogen species (RNS), and their removal by antioxidant enzymes and small-molecular-weight antioxidants.
• Redox-active selenium metabolites are involved in the anti-viral action of selenium in mice and humans.^[15]
• Immune-cell activity
• Se is useful for the competency of the cellular component of both innate and adaptive immunity.
• Inhibition of ferroptosis (i.e., a form of regulated cell death) via selenium supplementation promotes the survival of follicular helper T cells (T_FH), boosting the germinal center and antibody response following vaccination in mice and people (see Figure 3).^[7]
• On a cellular level, Se status may influence various leukocytic functions including adherence, migration, phagocytosis, and cytokine secretion. 
• Regulation of thyroid hormone metabolism
• Selenium in iodothyronine deiodinase, as selenocysteine, plays a crucial role in determining the free circulating levels of T3. Selenium deficiency can have implications in fall of T3 levels.
• In target tissues, T4, the most abundant circulating thyroid hormone, can be converted to T3 by selenium-containing enzymes known as deiodinases.^[11]

Table 1: Selenium Content of Selected Foods [16]

Food	Micrograms (mcg) per serving	Percent DV*
Brazil nuts, 1 ounce (6–8 nuts)	544	989
Tuna, yellowfin, cooked, dry heat, 3 ounces	92	167
Halibut, cooked, dry heat, 3 ounces	47	85
Sardines, canned in oil, drained solids with bone, 3 ounces	45	82
Ham, roasted, 3 ounces	42	76
Shrimp, canned, 3 ounces	40	73
Macaroni, enriched, cooked, 1 cup	37	67
Beef steak, bottom round, roasted, 3 ounces	33	60
Turkey, boneless, roasted, 3 ounces	31	56
Beef liver, pan fried, 3 ounces	28	51
Chicken, light meat, roasted, 3 ounces	22	40
Cottage cheese, 1% milkfat, 1 cup	20	36
Rice, brown, long-grain, cooked, 1 cup	19	35
Beef, ground, 25% fat, broiled, 3 ounces	18	33
Egg, hard-boiled, 1 large	15	27
Bread, whole-wheat, 1 slice	13	24
Baked beans, canned, plain or vegetarian, 1 cup	13	24
Oatmeal, regular and quick, unenriched, cooked with water, 1 cup	13	24
Milk, 1% fat, 1 cup	8	15
Yogurt, plain, low fat, 1 cup	8	15
Lentils, boiled, 1 cup	6	11
Bread, white, 1 slice	6	11
Spinach, frozen, boiled, ½ cup	5	9
Spaghetti sauce, marinara, 1 cup	4	7
Cashew nuts, dry roasted, 1 ounce	3	5
Corn flakes, 1 cup	2	4
Green peas, frozen, boiled, ½ cup	1	2
Bananas, sliced, ½ cup	1	2
Potato, baked, flesh and skin, 1 potato	1	2
Peach, yellow, raw, 1 medium	0	0
Carrots, raw, ½ cup	0	0
Lettuce, iceberg, raw, 1 cup	0	0

*DV = Daily Value.

References

1. Health Benefits of Iodine (Travel and Health)
2. Beck MA. Antioxidants and viral infections: host immune response and viral pathogenicity.  J Am Coll Nutr 2001; 20 (5 Suppl): 384S-388S, discussion 396S-397S.
3. Garlic—a Vegetable, a Condiment, and a Medicine (Travel and Health)
4. Health Benefits of Carotenoids (Travel and Health)
5. What You Need to Know About Your Thyroid Health (Dr. Mercola)
6. Selenium (Harvard School of Public Health; good)
7. Selenium saves ferroptotic TFH cells to fortify the germinal center
8. Nutritional risk of vitamin D, vitamin C, zinc, and selenium deficiency on risk andclinical outcomes of COVID-19: a narrative review
9. Huang Z, Rose AH, Hoffmann PR. The role of selenium in inflammation and immunity: from molecular mechanisms to therapeutic opportunities. Antioxid Redox Signal 2012 Apr 1;16(7):705-43.
10. Zhang J, Saad R, Taylor EW, Rayman MP. Selenium and selenoproteins in viral infection with potential relevance to COVID-19. Redox Biol 2020 Oct;37:101715
11. Iodine (Linus Pauling Institute)
12. Selenium in Human Health and Gut Microflora: Bioavailability of Selenocompounds and Relationship With Diseases
13. The Role of Selenium in Inflammation and Immunity: From Molecular Mechanisms to Therapeutic Opportunities
14. Vitamin C and the risk of developing inflammatory polyarthritis: prospective nested case-control study
15. Yu L., Sun L., Nan Y., Zhu L.Y. Protection from H1N1 influenza virus infections in mice by supplementation with selenium: a comparison with selenium-deficient mice. Biol. Trace Elem. Res. 2011;141(1–3):254–261.
16. U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, 2019.
17. Selenium—Fact Sheet for Health Professionals (NIH)
18. Egg consumption improves vascular and gut microbiota function without increasing inflammatory, metabolic, and oxidative stress markers