Vitamin D can be synthesized in the skin from a precursor, 7 dehydrocholesterol
after exposure to ultraviolet light or can be consumed in the diet.
McNeill, "Photometabolism of 7 dehydrocholesterol
to previtamin [D.sub.3] in skin," Biochemical and Biophysical Research Communications, vol.
Vitamin Dis photosynthesized in the skin of vertebrates by the action of solar ultraviolet (UV) B radiation on 7-3 dehydrocholesterol
Hydroxylation of 7, 8 dehydrocholesterol
to 25(OH) [D.sub.3] in the liver and subsequently to 1, 25 [(OH).sub.2] [D.sub.3] in the proximal tubular cells of the kidneys, and several other vitamin D responsive organ systems are well known facts of vitamin D physiology.