Vitamin D3 (VD3, cholecalciferol) is a steroid hormone that principally plays roles in regulating intestinal calcium absorption and in bone metabolism. It is obtained from the diet and produced in the skin by photolysis of 7-dehydrocholesterol and released into the bloodstream. Only a few food sources have significant amounts of vitamins D2 and D3 but many foodstuffs nowadays are fortified with vitamin D. The metabolites of vitamin D3 are carried in the circulation bound to a plasma protein called vitamin D binding protein (GC) (for review see Delanghe et al. 2015, Chun 2012). VD3 undergoes two subsequent hydroxylations to form the active form of the vitamin, 1,25(OH)2 vitamin D3 (CTL, calcitriol). The first hydroxylation takes place in the liver followed by subsequent transport to the kidney where the second hydroxylation takes place. CTL acts by binding to nuclear vitamin D receptors and regulates over 60 genes involved in calcium homeostasis, immune responses, cellular growth, differentiation and apoptosis. Inactivation of CTL occurs via C23/C24 oxidation catalysed by cytochrome CYP24A1 enzyme (Christakos et al. 2016).