Vitamin D is generally accepted as being essential for life in higher animals (278). It is one of the most important biological regulators of calcium and phosphorus (406) metabolism. Along with the two peptide hormones PTH and calcitonin, vitamin D is responsible for the minute-to-minute as well as the day-to-day establishment and maintenance of calcium homeostasis.

The substance vitamin D, or cholecalciferol, was first identified through the nutritional studies of Sir Edward Mellanby (243), which identified rickets as a disease state resulting from a deficiency of vitamin D, as well as through the biochemical/chemical studies of McCollum and co-workers (242), who demonstrated that there was more than one fat-soluble vitamin. Together these studies established a deficiency state, namely rickets, which was attributable to the newly discovered, nutritionally important fat-soluble component, vitamin D.

In retrospect, however, the most important discovery related to the appreciation by Huldschinsky (167, 168) and Hess and Gutman (148) of the importance of ultraviolet light or radiant energy to the cure and etiology of rickets; they showed that the UV rays from a mercury vapor lamp were quite effective in increasing the degree of calcification of the epiphyses of rachitic infants.

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