These genes encode for proteins mentioned above: 7-dehydrocholesterol reductase, vitamin D binding protein (DBP), 25-hydroxylase, 1-alpha-hydroxylase, and 24-hydroxylase, respectively. Other genes, whose variants may lead to altered availability and metabolism of vitamin D are: DHCR7, GC, CYP2R1, CYP27B1, CYP24A1. The best characterized are the four single-nucleotide polymorphic (SNP) variants of the VDR gene (ApaI, BsmI, FokI, and TaqI) that have been associated with several pathological situations, including autoimmune disorders or cancers. Several polymorphic variants of the genes involved in metabolism, transport, and activity of vitamin D have been described in the last years. Also, a form of a membrane-bound vitamin D receptor has been hypothesized, which would mediate non-genomic, rapid effects of calcitriol. This complex interacts with vitamin D responsive elements of target genes to exert its effects. The active form of vitamin D binds nuclear vitamin D receptor (VDR) and heterodimerizes with retinoic acid. Calctriol is inactivated by the action of the 24-hydroxylase. Calcitriol formation is down-regulated via a negative feedback by calcitriol concentrations and by the fibroblast growth factor 23 (FGF23). This protein, encoded by the CYP27B1 gene and expressed mainly in the kidney, determines the hydroxylation of calcidiol to 1,25-(OH) 2D3 (calcitriol). The vitamin D active form is produced by the 1-α-hydroxylase protein.

Evaluation of serum 25-(OH)-D3 is considered to provide a reliable evaluation of the vitamin D status. This is the major circulating and storage form of vitamin D. Both forms are transferred to the liver, were they are hydroxylated to 25-hydroxyvitamin D (25-OH-D3, or calcidiol). Ergocalciferol represents the dietary source of vitamin D and it is synthesized by plants and fungi. In addition, it can be introduced with the diet from few dietary sources (i.e. Vitamin D3 is normally synthetized in the skin upon exposure to ultraviolet B (UVB) radiation by the action of the 7-dehydrocholesterol reductase.

The term ‘vitamin D’ generally indicates two different compounds, the cholecalciferol (or vitamin D3) and the ergocalciferol (vitamin D2).

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