Synthesis of vitamin D from the sunlight

The major source of vitamin D for most land vertebrates, including humans, comes from exposure to sunlight.

Vitamin D in general refers to both vitamin D2 and vitamin D3, however vitamin D2 or ergocalciferol is the naturally occurring form in plants and vitamin D3 or cholecalciferol is the form synthesized by vertebrates.

During exposure to sunlight, the ultraviolet B photons enter the skin and photolyze 7-dehydrocholesterol (provitamin D3) to previtamin D3 which in turn is isomerizes by the body’s temperature to vitamin D3. 

Ultraviolet B that is penetrating through ozone layer is with energies 290-315 nm. As a result of the energy of this irradiation, the sterol ring structure is split to form previtamin D. This photolysis of 7-dehydrocholesterol to previtamin D3 occurs in the plasma membrane of skin cells.

No more than 15% of the initial provitamin D3 concentration ends up as previtamin D3.

Previtamin D3 then is ejected out of the plasma membrane into extracellular space where it enters the dermal capillary bed bound to the vitamin D binding protein.

Approximately , 50% of previtamin D3 is converted to vitamin D3 in 2 hours and as vitamin D3 is formed in the membrane, its’ open flexible structure is thought to escape into extracellular space.

Next previtamin D3 is converted to vitamin D3 or cholecalciferol, which then diffuses in to the blood and circulates to the liver.

In the liver, cholecalciferol are converted into calcidiol and then sent to kidneys. The kidneys perform the finale step – the formation of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] which also called calcitriol. Calcitriol is the active form of vitamin D.
Synthesis of vitamin D from the sunlight