Vitamin D3 from sun-derived sources is converted to different forms in the body. These processes are affect by several factors:
Factors affecting D3 availability and Conversion in the Body
- 7-dehydrocholesterol (7-DHC) absorbs UV-B in the lower epidermis and dermis layers of the skin to produce previtamin D3 (preD3) - and then vitamin D3 (aka provitamin D) (PreD3 is swiftly converted to D3 by a heat dependent process).D3 then enters the extracellular space and on into the dermal capillary bed, where it is bound to vitamin D binding protein for transport in the circulation.
-Overexposure to sunlight degradespreD3 and D3 making them inactive -providing a protective mechanism against overproduction of D3 from too much sun exposure.
CALCIDIOL (25 [OH] D)S (Biologically INACTIVE form of D)
Circulating Vitamin D is converted by the liver to CALCIDIOL. Vitamin D binding protein transports circulating vitamin D to the liver, which converts it to CALCIDIOL , via the vitamin D-25-hydroxylase enzyme (25-OHase); CALCIDIOL is a biologically inactive form, and is usually the form that is measured to determine vitamin D status (ideally should be 50-65 ng/mL).
CALCITRIOL (1,25(OH)2D (Biologically ACTIVE form of D)
The conversion of CALCIDIOL -▲ CALCITRIOL requires hydroxylation by the enzyme 25-hydroxyvitamin D-1α-hydroxylase (1-CYP27B1; 1-OHase); production is tightly controlled by parathyroid hormone, calcium, phosphorus and fiberblast growth factor (FGF-23). CALCITRIOL regulates its own production in a negative feedback loop which decreases synthesis /secretion of parathyroid hormone (PTH) in the parathyroid glands;
The main production site for CALCITRIOL is in the kidneys, but it is believed that many cells can also make CALCITRIOL from CALCIDIOL - several cell cultures have recently been found to express 1-OHase, including colon, prostate, breast and lung; although many cells can convert CALCIDIOL to CALCITRIOL, they use it themselves, and only the kidneys can make enough for distribution in the body.
