Evolutionary Biology and Pathology of Vitamin D

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There is mounting evidence that essentially all fungi, plants and animals living on earth produce provitamin D. It is likely that once exposed to sunlight, these provitamins are converted to previtamin D. It is unclear why fungi, phytoplankton, zooplankton and plants have the capacity to produce such large quantities of provitamin D. It is likely, however, that provitamin D and possibly vitamin D play an important biologic role in these organisms. Buchala and Schmid [20] found, for example, that vitamin D₃ promoted adventitious root development. It may be that provitamin D has a more fundamental function in lower life forms. Provitamin D and its photoproducts have UV absorption spectra that overlap with the ultraviolet absorption spectra from ultraviolet radiation-sensitive macromolecules including DNA, RNA and proteins. Thus, provitamin D and photoisomers could serve as a photon sink, and therefore, act as a natural sunscreen to protect lower life forms from the damaging effects of the high energy ultraviolet radiation that they are exposed to.
It is more clear, however, that amphibians, reptiles, birds, mammals and humans all require vitamin D and that the vitamin D must be metabolized to 1, 25 (OH)₂D₃ before it can carry-out its physiologic functions on calcium and bone metabolism. The intense research activities during the past decade on the antiproliferative and differentiation activities of 1, 25(OH)₂D₃ has opened a new chapter for this vitamin/hormone [21-23]. 1, 25(OH)₂D₃ and its analogs are being developed for the treatment of psoriasis, breast cancer, and leukemia. As we approach the 21st century, it is likely that vitamin D compounds will be of clinical value not only for the treatment acquired disorders of 25-OH-D metabolism but also for the treatment of such diverse diseases as osteoporosis, arthritis, psoriasis, atopic dermatitis, cancer and as an agent to enhance wound healing.