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GSE GSE Vitamin D - Sunshine Vitamin / Antioxidant against health problems - Forms in Body

Vitamin D3 converted to different forms in the Body

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

Vitamin D3 Conversions in THE body

Step 1

UV-B rays on vitamin D3 precursor in skin convert:

7-dehydro-

Cholesterol

(7-DHC)

▲ 

UV-B

CHOLECALCIFEROL

D3

D3 is also called Provitamin D

FAT STORAGE FORM

Step 2

Liver converts:

D3

▲ 

25-OHase

CALCIDIOL25(OH)D

MAIN CIRCULATING FORM of D - Water soluble, bound to vitamin D binding protein;

Step 3

Kidneys (mainly; but all cellsto some extent)convert:

25(OH)D

▲ 

1-OHase

CALCITRIOL 1,25(OH)2D

Hormonal, ACTIVE form of D

Previtamin D3 (preD3) / vitamin D3

–   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.

 

Vitamin D (D2 or D3) can be stored in and released from fat cells;

CALCIDIOL (25 [OH] 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 vitamin 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.

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