Vitamins are chemically unrelated families of organic compounds that are essential in small amounts for normal metabolism. Because vitamins (with the exception of vitamin D) cannot be synthesized by humans, they need to be ingested in the diet to prevent disorders of metabolism. They are divided into water-soluble and fat-soluble (table 1). Vitamin D is a fat-soluble vitamin.
Deficiency of vitamin D (commonly referred to as "rickets" when it occurs in children) is of unique historical value. Rickets was first described in the mid 1600s by Whistler and Glisson  but for decades thereafter, no progress was made in identifying the cause. In 1918, Sir Edward Mellanby described the deficiency of a fat-soluble nutrient as the cause for rickets . Shortly thereafter, Goldblatt and Soames demonstrated that skin exposed to sunlight or ultraviolet light produced a substance with similar properties to this fat-soluble nutrient . This ultimately led to the discovery of the chemical structure of vitamin D by Windaus .
Vitamin D and its metabolites have a significant clinical role because of their interrelationship with calcium homeostasis and bone metabolism. Rickets due to vitamin D deficiency is now rare except in populations with unusually low sun exposure and lack of vitamin D in fortified foods. Subclinical vitamin D deficiency, as measured by low levels of 25OHD, has been described among adolescents [5,6] and the elderly [7,8], and may contribute to the development of osteoporosis and an increased risk of fractures and falls in the elderly. In addition to its role in calcium and bone homeostasis, vitamin D potentially regulates many other cellular functions.
This topic review provides an overview of vitamin D. Other reviews discuss specific issues related to vitamin D. (See "Metabolism of vitamin D" and "Causes of vitamin D deficiency and resistance" and "Etiology and treatment of calcipenic rickets in children" and "Overview of rickets in children" and "Calcium and vitamin D supplementation in osteoporosis" and "Vitamin D and extraskeletal health" and "Vitamin D deficiency in adults: Definition, clinical manifestations, and treatment".)
Vitamin D, or calciferol, is a generic term and refers to a group of lipid soluble compounds with a four-ringed cholesterol backbone.