Vitamin D Pathway: Synthesis, Activation, and Function
Vitamin D is a fat-soluble vitamin essential for calcium balance, bone health, immune regulation, and many metabolic processes. Unlike most vitamins, vitamin D can be synthesized in the skin upon exposure to sunlight. The vitamin D pathway involves a multi-step process that includes synthesis, activation, transport, and action at target tissues. 1. Synthesis in the Skin The vitamin D pathway begins in the skin, where 7-dehydrocholesterol (a cholesterol derivative) absorbs ultraviolet B (UVB) radiation from sunlight. This leads to the production of cholecalciferol (vitamin D₃) . In people who consume a plant-based diet, ergocalciferol (vitamin D₂) from plant sources may also contribute. 2. Transport to the Liver Once synthesized in the skin or absorbed from the diet, vitamin D binds to vitamin D-binding protein (DBP) and is transported to the liver . In the liver, it undergoes its first hydroxylation by the enzyme 25-hydroxylase (CYP2R1) , forming 25-hydroxyvitamin D [25(OH)D] ,...