Vitamin D

Vitamin D refers to a group of fat-soluble compounds that function mainly as precursors to a hormone involved in calcium and phosphate regulation. Its two principal forms are vitamin D3, or cholecalciferol, and vitamin D2, or ergocalciferol. Vitamin D3 can be produced in the skin when ultraviolet B radiation acts on 7-dehydrocholesterol, while both forms can also be obtained from dietary sources .^1,2

Vitamin D itself has relatively little biological activity and undergoes metabolic activation before producing most of its physiological effects. It is first converted mainly in the liver to 25-hydroxyvitamin D, or 25(OH)D, the major circulating form. It is then converted primarily in the kidneys to 1,25-dihydroxyvitamin D, or calcitriol, which is the principal hormonal form. Calcitriol binds to the vitamin D receptor, a nuclear receptor that regulates gene expression in responsive cells .^1,2

A central function of the vitamin D system is maintaining calcium and phosphate homeostasis. Calcitriol increases intestinal absorption of these minerals and participates with other hormones, including parathyroid hormone, in regulating their handling by the kidneys and skeleton. These actions help maintain mineral availability for normal bone mineralization .^1,2

Vitamin D, 25(OH)D, and calcitriol are related but distinct substances. In particular, 25(OH)D is commonly measured to assess vitamin D status because it is the major circulating form and reflects vitamin D obtained from skin synthesis and external sources. Calcitriol, by contrast, is a tightly regulated active hormone responsible for many of the classical biological effects of the vitamin D system .^1,2