Photosynthesis of Cholecalciferol in the Skin

Sunlight is not strictly essential for cutaneous synthesis of cholecalciferol, because UV-B penetrates clouds reasonably well complete cloud cover reduces the available intensity by about 50%. It also penetrates light clothing. However, low-intensity irradiation (below 20 ml per cm in vitro) does not result In significant photolysis of 7-dehydrocholesterol to previtamin D. Acute whole-body exposure to UV-B irradiation below 18 ml per cm does not result in any detectable increase in plasma cholecalciferol or calcidiol. In temperate regions (beyond about 40°N or S), the intensity of UV-B is below this threshold in winter, so there is unlikely to be any significant cutaneous synthesis of the vitamin in winter, and plasma concentrations of calcidiol show a marked seasonal variation in temperate regions (Holick, 1995 see Table 3.2). 

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Synthetic ergocalciferol is used for enrichment and fortification of foods its metabolic fate is the same as that of dietary cholecalciferol. Except where there are known to be differences between the two vitamers, it is assumed that aU of the following discussion applies equally to ergocalciferol and cholecalciferol. There are few rich dietary sources of vitamin D, and the major source is usually photosynthesis in the skin. Dietary vitamin D is absorbed in chylomicrons and taken up rapidly by the liver as chylomicron remnants are cleared from... 

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