The term “vitamin D” denotes a group of steroid, fat-soluble compounds that are essential for the regulation of calcium and phosphorous metabolism, mediated mainly through intestinal absorption. The two most important isoforms, referred to cumulatively as “native vitamin D”, are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Ergocalciferol, which is only synthesised by plants and fungi, is introduced through the diet, whereas cholecalciferol, on the other hand, is both exogenously and endogenously synthesised and is derived from the photolysis of 7-dehydrocholesterol, mediated by UVB radiation affecting the skin.