Vitamin D : Distribution

The D vitamins are a group of sterols that have a hormone-like function. The active molecule, 1,25-dihydroxycholecalciferol ([1,25-diOH-D3], or calcitriol), binds to intracellular receptor proteins. The 1,25-diOH-D3–receptor complex interacts with response elements in the nuclear DNA of target cells in a manner similar to that of vitamin A  and either selectively stimulates or represses gene transcription. The most prominent actions of calcitriol are to regulate the serum levels of calcium and phosphorus.
1. Endogenous vitamin precursor: 7-Dehydrocholesterol, an intermediate in cholesterol synthesis, is converted to cholecalciferol in the dermis and epidermis of humans exposed to sunlight and transported to liver bound to vitamin D–binding protein.
2. Diet: Ergocalciferol (vitamin D2), found in plants, and cholecalciferol (vitamin D3), found in animal tissues, are sources of preformed vitamin D activity (Fig. 1). Vitamin D2 and vitamin D3 differ chemically only in the presence of an additional double-bond and methyl group in the plant sterol. Dietary vitamin D is packaged into chylomicrons. [Note: Preformed vitamin D is a dietary requirement only in individuals with limited exposure to sunlight.]

Figure 1: Sources of vitamin D. Vitamins D2 and D3 are first converted to calcidiol and then to calcitriol (active vitamin D). [Note: 7-Dehydrocholesterol (provitamin D3) is decreased in the skin of older adults.]