Hi Master, glad to hear from you again 
Right, vitamin D can be taken orally or it can be activated under
exposure to the sun (sunbathing).
Vitamin D or better known as calciferol starts from its root of
synthesis in the skin. The resulting metabolite known as
cholecalciferol will make it's way to the liver to get conjugated
(become water soluble). The liver will add a hydroxyl group to its
25th position making it a 25-hydroxycholecalciferol (this is
basically chemistry).
Once this happen, the 25-HCC will make its way to the kidneys, and
the kidneys will decide to either activate it by adding another
hydroxy group to its first position, making it 1,25 -
dihydroxycholecalciferol (1,25-DHCC) or making it inactive by
adding it to the 24th position making it 24,25-DHCC.
The activated vitamin D will then continue on with it's function
after it leaves the kidneys. One of it being aiding calcium
absorption from the gut...
Once the kidney goes conked out, the ability to activate 25-HCC
will not be able to work properly or might fail altogether. That is
the sole reason of taking Rocaltrol (1,25-DHCC synthetic) or
1-alpha (1-HCC) for patients who have kidney failure...
