Figure 1. Metabolism of vitamin D. Vitamin D3 is synthesized in the skin from provitamin D3 (7-dehydrocholesterol) under the influence of UV light. Vitamin D2 (ergocalciferol) is obtained from vegetable dietary sources where it derives from the plant sterol ergosterol. Vitamin D is metabolized first to calcidiol (25(OH)D), and later to the active form calcitriol (1,25(OH)2D3). Interaction of 1,25(OH)2D3 with the vitamin D receptor (VDR), which is an intracellular transcription factor, facilitates its binding to DNA sequences. The binding of the complex VDR/1,25(OH)2D3 to these regulatory sequences (vitamin D response elements (VDREs)) regulate transcription of genes involved in many different cellular homeostatic functions.