Murwan,
I found an article about provitamin D that might help answer your question and you can access it using this link, below.
http://www.jbc.org/content/191/2/765.full.pdf
The reaction involves the photochemical ring-opening of the steroidal precursor 7-dehydrocholesterol (provitamin D) to previtamin D and can be done in a laboratory. Are you asking if this reaction can occur in a human body without the help of sunlight? Through the diet, vitamin D can be obtained from oily fish and dairy; however, I believe UV light is necessary to convert provitamin D to 25(OH)D and the liver/kidneys further to the usable form.
Ultra violet B ( UVB ) in Sun light converts provitamin D in the skin to previtamin D, so the presence of sun Light is necessary, unless the skin is exposed to artificial UVB .
The production of provitamin D3 from 7-dehydrocholesterol in the skin is UV-B dependent (either from the sun or from a suitable UV lamp).
Conversion from provitamin D3 to vitamin D3 needs some body heat.
Alternatively, vitamin D3 can be produced using UV-B and some heat from fatcontaining slaughterhouse waste or from plant origin (in the latter case you will obtain vitamin D2).
Thank you, it is very important sun or UV light to convert pro vitamin D to vitamin D
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