Regardless of how you obtain vitamin D either by sun, food or supplements it is required to under go two hydroxylations before it is biologicaly activated.
In the Skin
The process of making endogenous vitamin D begins with:
- 7-dehydrocholesterol converted to pre-vitamin D by UVB light and is spontaneously isomerized to cholecalciferol (D3 supplement)
In the Liver
- Cholecalciferol is hydroxylated by the enzyme Vitamin D-25-hydroxylase to calcidiol (25-hydroxycholecalciferol) by the genes CYP27A1 and CYP2R1.
In the Kidneys
The second hydroxylation reaction occurs when calcidiol (25-hydroxycholecalciferol) to calcitriol (1,25-dihydroxycholecalciferol) the biologically active form by the genes CYP27B1 and CYP24A1. Calcitriol circulates throughout the blood as a hormone.