Selenium - characteristics, requirement, excess, deficiency, and source products

Selenium constitutes an essential trace element with a total body content in humans ranging from 10 to 30 milligrams, exhibiting a highly specific tissue distribution: approximately one-third of the total pool accumulates in hepatic tissue, 15 percent is localized within renal structures, an additional 30 percent is concentrated in muscular mass, and the remaining 10 percent circulates in blood plasma. This micronutrient plays a pivotal role in regulating cellular metabolic pathways and serves as a critical coenzyme for numerous enzymatic reactions, including redox processes catalyzed by glutathione peroxidase—a protein that protects cellular membranes and erythrocytes from oxidative damage induced by reactive oxygen species. Furthermore, selenium participates in the regeneration of ascorbic acid via thioredoxin reductase and is indispensable for thyroid hormone synthesis due to its incorporation into deiodinase, the enzyme responsible for converting thyroxine (T4) into its active form, triiodothyronine (T3). Additionally, this trace element mediates lipid metabolism, influences nucleic acid replication (DNA/RNA), and accumulates at high concentrations in male gonads, where it fulfills a vital function in spermatogenesis through the selenoprotein PHGPx, which acts protectively and regulatively during the early stages of sperm maturation and subsequently becomes a structural component of mitochondrial sheaths in mature gametes, thereby ensuring their viability and motility. It is crucial to emphasize that the selenium atom itself lacks inherent biological activity *in vivo*; its functional properties derive from integration into organic compounds—primarily selenocysteine and selenomethionine—which are incorporated into peptide chains.

Selenium, an essential trace element with potent antioxidant capabilities, plays a pivotal role in safeguarding cellular integrity—particularly that of nucleic acids—by mitigating damage induced by free radicals and oxidative stress. By enhancing immune defense mechanisms, it significantly reduces susceptibility to inflammatory and degenerative diseases. Due to its capacity to neutralize reactive oxygen species, selenium is frequently referred to as the "youth-preserving element," as it decelerates age-related cellular degradation. One of its most critical functions lies in immune modulation: it stimulates the production of lymphocytes, macrophages, and antibodies, thereby bolstering the body’s resistance to bacterial, viral, and even fungal pathogens (including toxic mycotoxins such as aflatoxins). Research further substantiates its anticancer potential, as selenium inhibits uncontrolled cellular proliferation and may lower the risk of certain malignancies. Additionally, it exerts cardioprotective effects by preventing atherosclerosis and preserving vascular integrity. Its endocrinological significance is equally notable: selenium supports thyroid function by optimizing iodine utilization and shielding the gland from oxidative damage, thereby mitigating hypothyroidism. It also facilitates the detoxification of heavy metals (e.g., lead, cadmium, mercury) by forming insoluble complexes and accelerating their excretion. Within the nervous system, selenium contributes to impulse transmission regulation and neurogenesis, rendering it potentially beneficial in managing specific neurological disorders. Furthermore, it correlates positively with male fertility (by enhancing sperm quality) and alleviates menopausal symptoms in women. However, interactions with other nutrients must be considered: excessive vitamin C may impair its absorption, whereas vitamin E acts synergistically, amplifying its antioxidant efficacy.

Scientific guidelines indicate that the optimal mean daily selenium intake for adults is established at 45 micrograms. Exceeding this threshold to levels as high as 900 micrograms may induce toxicological symptoms. Notably, physiological demand for this trace element rises substantially during pregnancy—reaching up to 400 micrograms per day—and remains elevated throughout lactation, where requirements peak at 360 micrograms. Blood selenium concentrations exhibit a direct correlation with dietary intake, though genetic predispositions and environmental variables—such as soil composition and dietary patterns—also exert influence. Meanwhile, selenium levels in hair are determined not solely by supplementation methods but also by methionine consumption, inherent hair pigmentation, and even the presence of selenium in hair care products like shampoos.

Selenium represents a distinctive trace element whose optimal concentration within the human body demands meticulous equilibrium—both insufficient and excessive levels can provoke significant health complications. Contemporary observations indicate a systematic decline in selenium content within food sources, attributable to the depletion of this mineral in soils as a consequence of prolonged erosive processes. Consequently, distinct geographical regions exhibit either pronounced deficiencies or excessive selenium availability, which correlates with population-specific pathological conditions. A paradigmatic example of severe deficiency is observed in endemic diseases prevalent in China, such as Keshan disease (a juvenile cardiomyopathy) and Kashin-Beck disease (degenerative articular cartilage dystrophy). Areas characterized by low selenium levels demonstrate elevated incidence rates of cardiovascular disorders and malignancies. Selenium deficiency disproportionately affects individuals diagnosed with conditions including AIDS, cystic fibrosis, phenylketonuria, rheumatoid arthritis, circulatory diseases, renal failure, immunological disorders, and depression. A robust association also exists between inadequate selenium intake and thyroid dysfunction—children born to mothers with concurrent iodine and selenium deficiencies face an elevated risk of developing cretinism. Non-specific manifestations of selenium insufficiency encompass chronic fatigue, myalgia, generalized weakness, recurrent infections, progressive hepatic and pancreatic damage, and male infertility. It is critical to emphasize that exceeding the recommended daily selenium intake poses a risk of acute toxicity. Chronic selenosis (selenium poisoning) is characterized by symptoms such as brittle and shedding nails, alopecia, persistent nausea and vomiting, a distinctive garlic-like breath odor, hyperhidrosis, neurological impairments, depressive episodes, and heightened irritability. Therefore, uncontrolled selenium supplementation may yield adverse outcomes contrary to intended benefits, and any additional dosage should be preceded by consultation with a healthcare professional.

Selenium in foodstuffs is found predominantly in organic forms—namely selenomethionine and selenocysteine—which exhibit superior bioavailability compared to inorganic variants. The three inorganic forms of this trace element (with oxidation states of –II, +IV, and +VI) include selenides and selenates, though their absorption by the human body is markedly lower. The richest dietary sources of selenium include offal, particularly kidneys, as well as seafood such as marine fish and shellfish, where selenium concentrations range from 20 to 60 micrograms per 100 grams, depending on the species. In dairy products like milk and cheese, selenium content is directly correlated with its presence in animal feed. Among plant-based foods, legumes—especially lentils and peas—as well as garlic stand out, though Brazil nuts are unparalleled, containing an extraordinary 1,917 µg/100 g. Most other vegetables and fruits contain negligible amounts of selenium, rarely exceeding 2 µg/100 g, with Brussels sprouts being a notable exception at approximately 102 µg/100 g. Edible mushrooms also provide a valuable source (6–14 µg/100 g), while minimally processed cereal products such as whole-grain wheat flour (70 µg/100 g) and oatmeal (45 µg/100 g) contain selenium primarily as selenomethionine—the most efficiently absorbed form. It is critical to note that selenium is an unstable trace element: its absorption is compromised by excessive intake of simple carbohydrates, leading to a loss of its bioactive properties. A balanced diet incorporating diverse selenium-rich foods ensures optimal intake of this essential micronutrient, thereby supporting long-term health and vitality across all life stages.