Vitamin D as a multisystem regulatory hormone: Molecular pathways, physiological integration, and implications for...

Vitamin D is a pleiotropic secosteroid hormone whose biological actions extend far beyond calcium- phosphate homeostasis. Emerging experimental evidence highlights the regulatory role of this vitamin across multiple organ systems and signaling pathways involved in endocrine, musculoskeletal, immune, metabolic, and neurobiological functions. Concurrently, growing recognition of the high prevalence and multisystem implications of vitamin D deficiency has led to increasing incorporation of vitamin D screening into routine clinical and hospital practice. Despite the emerging evidence that excessive vitamin D exposure or supplementation can produce adverse physiological effects, clinical focus has largely centered on deficiency. Accordingly, this review synthesizes current evidence on vitamin D metabolism, its physiological roles across major organ systems, the molecular pathways and systemic effects associated with altered vitamin D status, both deficiency and excess, and the resulting risk factors and clinical consequences, with an emphasis on mechanistic integration rather than organ-specific compartmentalization. By positioning vitamin D as a regulator of interconnected molecular networks, this review provides a systems-level framework linking vitamin D signaling to metabolic efficiency, mitochondrial function, and musculoskeletal health. These mechanisms have direct implications for exercise performance, recovery, and injury risk, highlighting the importance of maintaining optimal vitamin D status in athletic and physically active populations.