Vitamin D Biology: Immune Modulation and Chronic Diseases

Vitamin D is increasingly recognized as a multifunctional secosteroid hormone that integrates endocrine regulation with immune modulation and chronic disease pathophysiology. Beyond its classical role in calcium and phosphate homeostasis, vitamin D undergoes tightly controlled metabolic activation that enables systemic effects through the vitamin D receptor (VDR) in diverse tissues. Its endocrine actions coordinate mineral metabolism and skeletal integrity, while its extra-skeletal functions influence cardiovascular health, insulin sensitivity, adipose tissue biology, and cellular proliferation. Emerging evidence highlights vitamin D as a pro-survival and immunoregulatory molecule, shaping innate and adaptive immune responses and modulating inflammatory pathways implicated in obesity, cancer, and metabolic dysfunction. This book examines vitamin D sources, metabolic processing, phosphate interactions, and mechanistic links to cardiovascular disease, insulin resistance, breast cancer, and musculoskeletal health, offering an integrated framework for understanding vitamin D biology in health and chronic disease.