Vitamin D is a crucial micronutrient that plays a vital role in human health, influencing various physiological processes. It is primarily obtained through sunlight exposure, dietary sources, and supplements. Vitamin D undergoes metabolic processes in the body to form its most active form, calcitriol, which regulates calcium and phosphorus metabolism and controls blood ionized calcium concentrations. Beyond its classical roles, vitamin D has been linked to the prevention and reduction of cardiovascular and metabolic diseases, autoimmune disorders, infections, and cancer. Peripheral target cells, such as immune cells, obtain vitamin D and its precursor, 25(OH)D, through concentration-dependent diffusion from the circulation. Calcitriol is synthesized intracellularly in these cells, which is crucial for their protective physiological actions. Deficiency in vitamin D exacerbates inflammation, oxidative stress, and increases susceptibility to metabolic disorders and infections, leading to premature deaths. Maintaining optimal serum levels above 40 ng/mL is essential for health and disease prevention, but achieving this requires several times more than the government's recommended doses. Recent research suggests that maintaining 25(OH)D concentrations between 40 and 80 ng/mL is optimal for disease prevention and reducing morbidities and mortality without adverse effects. The recommendation for individuals is to maintain serum 25(OH)D concentrations above 50 ng/mL for optimal clinical outcomes. Integrating vitamin D assessment and supplementation into clinical practice can significantly benefit public health, including reduced healthcare costs. This review synthesizes recent findings on the physiology of vitamin D and its implications for disease prevention.Vitamin D is a crucial micronutrient that plays a vital role in human health, influencing various physiological processes. It is primarily obtained through sunlight exposure, dietary sources, and supplements. Vitamin D undergoes metabolic processes in the body to form its most active form, calcitriol, which regulates calcium and phosphorus metabolism and controls blood ionized calcium concentrations. Beyond its classical roles, vitamin D has been linked to the prevention and reduction of cardiovascular and metabolic diseases, autoimmune disorders, infections, and cancer. Peripheral target cells, such as immune cells, obtain vitamin D and its precursor, 25(OH)D, through concentration-dependent diffusion from the circulation. Calcitriol is synthesized intracellularly in these cells, which is crucial for their protective physiological actions. Deficiency in vitamin D exacerbates inflammation, oxidative stress, and increases susceptibility to metabolic disorders and infections, leading to premature deaths. Maintaining optimal serum levels above 40 ng/mL is essential for health and disease prevention, but achieving this requires several times more than the government's recommended doses. Recent research suggests that maintaining 25(OH)D concentrations between 40 and 80 ng/mL is optimal for disease prevention and reducing morbidities and mortality without adverse effects. The recommendation for individuals is to maintain serum 25(OH)D concentrations above 50 ng/mL for optimal clinical outcomes. Integrating vitamin D assessment and supplementation into clinical practice can significantly benefit public health, including reduced healthcare costs. This review synthesizes recent findings on the physiology of vitamin D and its implications for disease prevention.