Definitions and Nomenclature
Vitamin D is a fat-soluble substance required by most vertebrates, including humans, to keep blood calcium and phosphate levels within a narrow normal range and thereby maintain a normal skeleton and optimal cellular function. The term, vitamin D, is a misnomer. Vitamin D is not a vitamin. It is synthesized in the skin, and so, unlike other vitamins, which are essential dietary components, it does not satisfy the criteria for classification as a vitamin. Nor is it a hormone because it is biologically inactive and must be metabolized by the body into a multihydroxylated version, known as calcitriol, which is biologically active and the true hormonal form. Thus vitamin D is more accurately described as a prohormone. The natural form of the vitamin, known as vitamin D3, is a cholesterollike substance produced in the skin by a nonenzymatic process involving ultraviolet light and heat. An artificial form of the vitamin, with an altered side chain, known as vitamin D2, is derived from the plant sterol ergosterol and is often used instead of vitamin D3 as a dietary supplement.
Most of the complexity associated with the nomenclature in the vitamin D field stems from confusion surrounding its discovery during the period 1919 to 1922. Early research showed that the deficiency associated with lack of vitamin D (rickets in children or osteomalacia in adults) was cured by seemingly unrelated treatments: exposure to sunlight or ingestion of a fat-soluble substance. The early nutritional pioneers of that period, including Sir Edward Mellanby and Elmer V. McCollum, realized that several related factors would cure rickets and that one of these substances, vitamin D3, could be made in the skin. Students often ponder the fate of vitamin D1. It was a short-lived research entity comprising a mixture of vitamins D2 and D3, and the term has no value today. Vitamin D3 is sometimes referred to as cholecalciferol or, more recently, calciol; vitamin D2 is known as ergocalciferol or ercalciol. The discovery of the hydroxylated versions of vitamin D by Hector F. DeLuca and Egon Kodicek in the 1967 to 1971 period led to a major expansion of our knowledge of a number of biologically active compounds, but calcitriol is the singularly most important version of these. For purposes of discussing the history of foodstuffs, we shall use the term vitamin D to describe all substances that can be activated to produce biological effects on calcium and phosphate metabolism in humans.