Cutaneous Biosynthesis of Vitamin D

Vitamin D in the form of vitamin D3 is made in the skin when ultraviolet radiation (UVR) penetrates the skin and is absorbed by 7-dehydrocholesterol (7-DHC) in the epidermis and dermis to form pre-vitamin D3. This reaction only occurs at the Earth’s surface in the presence of wavelengths of 290-315 nm in the UVB range, with peak conversion occurring at 295-297 nm. Photosynthesis of vitamin D3 in the skin depends upon season and latitude, time of day, and on the amount pigment and thickness of the skin. In order to become biologically active, vitamin D3 must undergo a two successive hydroxylation steps, first in the liver to 25(OH)D3 and then in the kidney under the influence of parathyroid hormone (PTH) into the active metabolite, 1,25(OH)2D3. The active metabolite acts as a steroid hormone through binding to its specific intranuclear receptor, the vitamin D-receptor (VDR). This modulates the transcription of responsive genes such as that of calcium binding protein, which regulates mineral ion (especially calcium) homeostasis in the body. The most obvious function of vitamin D in humans is in the building and maintenance of the bony skeleton. The essential connection between vitamin D status and bone health was established because serious vitamin D deficiency was linked to the highly visible and disfiguring bone disease, nutritional rickets. The discovery of VDRs in tissues of the brain, heart, stomach, pancreas, skin, gonads, in the activated T and B lymphocytes of the immune system, and in 28 other tissues has led to a growing appreciation of the varied and important roles vitamin D plays in the body.