Abstract

Vitamin Ds are a group of secosteroids which are precursors to hormonal substances that control calcium and phosphorus homeostasis and participate in the process of regulation of gene transcription in animals. Animals convert 7-dehydrocholesterol stored in the skin to vitamin D₃ upon irradiation by sunlight or obtain supplemental supplies of ergocalciferol (vitamin D₂) or cholecalciferol (vitamin D₃) from dietary sources.

Recent research has elucidated the complicated pathway by which the vitamin D structure is converted to hydroxylated metabolites which serve to control the calcium intestinal absorption and bone mineralization or resorption to maintain calcium blood levels. The metabolites have been implicated in disease states relating to calcium homeostasis, such as rickets, osteomalacia, and other bone diseases. The metabolites have also been shown to function like other steroid hormones to support selective cell differentiation. They have thus been implicated in diseases such as cancer, psoriasis, diabetes, etc.

Vitamin D₃ is manufactured by the chemical conversion of cholesterol to 7-dehydrocholesterol. Ultraviolet irradiation of the 7-dehydrocholesterol produces a mixture of photoisomers. The two active isomers, previtamin D₃ and cis-vitamin D₃ form a 20:80 equilibrium mixture. Vitamin D is best assayed by high performance liquid chromatography.

Vitamin D₃ is active in all animals, whereas Vitamin D₂ is active in cattle, horses, swine, and particularly poultry. Pure vitamin D₃ has a potency of international units per gram. It is relatively active in most animals at levels of 40–400 IU per day. Vitamin D₃ is generally recommended as safe.