It is largely through historical accident in the interval of 1920-1940 that vitamin D became classified as a vitamin rather than as a steroid hormone. The formal definition of a vitamin is that it is a trace dietary constituent required to produce the normal function of a physiological process or processes. The emphasis here is on trace and the fact that the vitamin must be supplied regularly in the diet; this implies that the body is unable to metabolically synthesize the vitamin in question. However, the ultraviolet exposure of 7-dehydrocholesterol present in the skin results in the photochemical production of vitamin D(3).
Thus, vitamin D(3) becomes a true vitamin only when the animal or human does not have regular access to sunlight or ultraviolet light. Under normal physiological circumstances, all mammals, including humans, can generate, via ultraviolet exposure of 7-dehydrocholesterol present in the skin, adequate quantities of vitamin D(3) to meet their nutritionally defined requirements.
There is a vibrant historical record beginning in 1650 and culminating in 1963 concerned with the determination of the chemical structures of vitamin D(3) and vitamin D(2). A surprising aspect concerning vitamin D(3) is that it is itself biologically inert. There are no known essential biological actions or contributions that rely specifically on the molecule vitamin D(3).
While chemists had certainly appreciated the strong structural similarity between the vitamins D and other steroids, this correlation was never widely acknowledged in the biological, clinical, or nutritional sciences until 1965-1970.
The biological role of vitamin D(3) is to serve as a substrate for the liver 25-hydroxylase which produces 25-hydroxyvitamin D(3) [25(OH)D(3)]. 25(OH)D(3) in turn serves as the substrate for the kidney proximal tubule 25(OH)D(3)-1?-hydroxylase enzyme which produces the steroid hormone 1?,25(OH)(2)-vitamin D(3) [1?,25(OH)(2)D(3)].
Ann Nutr Metab. 2012;61(3):199-206. doi: 10.1159/000343104. Epub 2012 Nov 26.
The history of the discovery of vitamin D and its daughter steroid hormone.