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.