Table 1.

Chemical and metabolic background of vitD.

Shortcut	Explanation
vitD	In this manuscript the shortcut vitD sums up vitaminD3 and any intermediate of vitaminD3
vitaminD3	Cholecalciferol (inactive)
vitaminD2	Ergocalciferol
7-DHC	7-Dehydrocholesterol
25OHD3	25-HydroxyvitaminD3
1,25(OH)2D3	1,25-DihydroxyvitaminD3, calcitriol (active)
VDR	vitD receptors

There are different chemical forms of vitD to be distinguished (in this work, the shortcut vitD sums up vitaminD3 and any intermediates). There are two sources of vitD; the majority is generated via the skin in a non-enzymatic process; the minor part is gained via food. The starting product 7-DHC is converted to Pre-vitD₃ via UV-B irradiation. This pre-vitamin isomerizes to vitD₃ in a thermo-sensible process. VitD₃ is converted to 25OHD₃ in the liver. The biologically active form of this vitamin is 1,25 (OH)₂D₃, generated via hydroxylases in the kidneys. This active form is able to bind to VDRs, transcription factors, present in nearly every tissue. Alternatively, 1,25 (OH)₂D₃ can be converted to the biologically inactive form calcitroic acid for storage (1, 8).