NOT ALL HEROS SMELL NICE: SELENIUM

Elements that are essential for living things and whose daily dose is found in milligrams per kilogram (mg/kg)in biological tissues are called trace or microelements. Since micro elements do not constitute the basic structure of tissues such as muscle, bone, and connective tissue, they do not have to be in absolute majority or abundance in kilograms or grams. Yet this does not change the fact that they are still as important as microelements. Inorganic elements take place as cofactors in the structure of most of the enzymes that work to provide the necessary driving force for both the formation of these basic tissues and the continuity of metabolic reactions. Selenium, which can be considered a “newbie or newly discovered element”, is among the essential trace elements that contain elements such as iron, copper, zinc, cobalt, manganese, molybdenum, chromium, boron, fluorine, and iodine, which are the main cofactors for enzymes.

Selenium is a nonmetal in group 6ain the 4th column of the periodic table and has four different oxidation statuses in nature. These are the elemental state of selenium in which it is neutral; the -2 charged selenide form;  the +4 charged selenite state and the +6 charged selenate state. Especially when selenium and dimethyl selenide are heated, they are very special with their unique, garlic or garlic-breath-like odor. The charges of selenium in nature also determine the relationship of selenium with organic molecules and water. Inorganic selenate and selenite are found in water with moderate bioavailability, while the organic selenium compounds selenomethionine form the primary selenium in grains and vegetables with higher bioavailability in the form of selenocysteine.

Selenium, which is frequently found in foods rich in sulfur-containing amino acids methionine, cysteine, cystine, or γ-glutamyl methyl selenocysteine, cannot be directly absorbed from the intestine due to its complex structure. Since it is needed to be absorbed by being released from amino acids during digestion, its absorption is almost reduced by 50% of the digested amount. However, in fortified foods and supplements, an inorganic form of selenium (sodium selenite) is used. Selenium, of which 85-95%of its absorption takes place in the intestine, is transported to the liver by albumin, by selenium carrier protein 1 (SEPP1). Then glutathione peroxidase-3 releases the transported selenium to the liver. It is recommended that the daily intake of selenium, which can be stored in all tissues, especially in the liver, kidney, testis, thyroid, and pancreas tissues, should not exceed 70 μg/day, except for adipose tissue. Studies have claimed that selenoproteins protect the organism against endothelial damage by reducing oxidative stress, endoplasmic reticulum stress, and inflammation; It has been suggested that the ability of selenoproteins to control eicosanoid production and to regulate vascular tone can also be utilized in preeclampsia.

The biological importance of selenium is not limited to its incorporation into the main selenoprotein structure. Its prominent features are that it is a component of glutathione peroxidase, which regulates redox homeostasis, as a cofactor, and that it is the prosthetic group of the iodothyronine deiodinase enzyme. The relationship of selenium with thyroid hormone is based on the fact that iodine units can only be activated by removal in the presence of selenium. Since iodothyronine deiodinase enzyme activity will decrease in selenium deficiency, one of the 4 iodine atoms of the thyroxine hormone (T4) is removed and its conversion to the functional triiodothyronine (T3) is interrupted when selenium is not available. Since this will reduce the T3 effect, negative feeding cannot be provided, which explains the reason for the high thyroid stimulating hormone (TSH) seen in selenium deficiency. In erythrocytes, which are responsible for the transport and recovery of oxygen and carbon dioxide in the circulation and lose their nuclei when they reach their mature state, damage occurs in the lipid membrane, especially when passing through the capillaries, due to the effect of friction, and this damage is counted as the reason why the lifespan of erythrocytes is approximately 120 days. In their limited life, the lipid membranes of erythrocytes, which carry oxygen-carbon dioxide transport, react with oxygen-derived superoxides, causing an increase in oxidative damage to the erythrocyte membrane. Since there is no nuclear DNA, and there is no division signaling to repair the membrane, a repair mechanism is required to protect the membrane to ensure its integrity in erythrocytes. Selenium, which is in the selenoprotein structure, is located in the active center of the glutathione peroxidase enzyme. This structure helps the enzyme form an important line of defense against oxygen radicals, hydrogen peroxide, and free radicals that are effective against lipid peroxidation. Similarly, there are studies claiming that 100 μg daily selenium supplementation provides significant benefits, particularly in allergic asthma, by promoting differentiation of the primary immune cell CD4+ into T-helper-1 (Th1).

No more or less…
In the Turkey-Specific Dietary Guidelines (TSDG) of 2022 reports, the daily selenium intake of adult men and women was determined as 55 µg, and this amount was regulated as 70 µg regardless of gender in the Turkish Dietary Guidelines 2016. In studies conducted in Turkey, daily selenium intake levels were measured below their commended daily intake value of 30-44 µg/day on average. Selenium is a toxic element ( Well, like everything when exceeds its limits! Smirk!).  

Especially in the case of inhalation overdose, acute toxicity can be characterized by garlic odor on breath and sweat, respiratory tract irritation, metallic taste in the mouth, dermatitis, and digestive disorders. Chronic high selenium doses are related to dental caries. The relationship between selenium and cancer, in a nutshell, is "complicated". Selenium, which was included in the list of carcinogens by the FDA in 1958 due to causing adenoma and carcinoma, kept this bad reputation until the late 1975’s. As a result of the studies carried out by the International Cancer Research Institute, selenium was finally removed from the list because there was not sufficient evidence to support the view that it is carcinogenic. In studies conducted in the early 1980s, it was found that high-dose selenium could benefit from its anticarcinogenic properties in studies performed on both viral oncogenes and transplantable tumors. Although the known diseases related to selenium deficiency are Kashin-Beck and Keshan’s syndromes, there are publications related to the use of selenium in reducing the development of myocarditis and acute kidney injury with the red-striped heart muscle, which is particularly affected by oxygen. Foods high in selenium include oil seeds (Brazil nut, cashew), nuts, chicken, fish, turkey, seafood, cereals, and selenium-enriched eggs. Considering the amount of selenium found in high concentration in seafood in fish in Turkey, the highest Se concentration is found in sea bass with 0.294 µg/g; the lowest Se concentration was observed with a value of 0.047 µg/g.

Lec. Eda Merve Kurtuluş