Vitamin D: Role, Sources, Mechanism of Action and Significance for Human Health

Introduction.Vitamin D is a key micronutrient regulating calcium–phosphate homeostasis, immune responses, and several extra-skeletal functions. Its active form, 1,25-dihydroxyvitamin D, acts via the vitamin D receptor present in numerous tissues. Materials and methods.A literature review was conducted using PubMed, Dove Press Medical, and Google Scholar. Studies concerning the sources, metabolism, mechanisms of action, and health effects of vitamin D were analyzed.Results.Vitamin D supports calcium absorption and bone mineralization, and supplementation of 800–1000 IU/day may reduce fracture risk in deficient individuals. It modulates innate and adaptive immunity by influencing cytokine production and the activity of T and B lymphocytes and macrophages. Deficiency is linked to increased susceptibility to infections, including tuberculosis and viral respiratory illnesses. Research also suggests associations between low vitamin D levels and autoimmune diseases, depression, and cancer, although intervention studies show inconsistent outcomes.Discussion.The reviewed evidence confirms the broad physiological effects of vitamin D; however, uncertainties remain regarding optimal dosage and its extra-skeletal benefits. Variability in clinical results may reflect differences in baseline 25(OH)D levels, study populations, and supplementation regimens.Conclusions.Vitamin D supplementation appears most beneficial for individuals with documented deficiency, whereas its preventive or therapeutic impact in the general population is less clear. Further high-quality studies are needed to establish optimal dosing and clarify its role in chronic and immune-related diseases.