Where does the second hydroxylation of vitamin D take place in the body?

The second hydroxylation of vitamin D occurs in the kidneys. After vitamin D is initially synthesized in the skin upon exposure to sunlight or absorbed from dietary sources, it undergoes an initial hydroxylation in the liver, converting it to calcidiol (25-hydroxyvitamin D). The kidneys then perform the second hydroxylation, converting calcidiol into calcitriol (1,25-dihydroxyvitamin D), which is the active form of vitamin D that plays a crucial role in calcium homeostasis and bone metabolism.

The kidneys have specialized enzymes, particularly 1-alpha-hydroxylase, that facilitate this conversion. This active form of vitamin D then exerts its effects on various tissues, including the intestines, enhancing the absorption of calcium and phosphate, and is important for maintaining bone health. Understanding this pathway is essential for grasping how vitamin D is metabolized in the body and its physiological effects.