Vitamin D in health and disease
Vitamin D is the precursor of the steroid hormones 1,25-dihydroxy-vitamin D (1,25(OH)2D). Most of the biological effects of 1,25(OH)2D derive from its interaction with vitamin D receptor (VDR), an intracellular transcription receptor. VDR regulates genes that influence bone mineral homeostasis, immunomodulation, insulin secretion, cell cycle and the detoxification of exogenous and endogenous compounds. The most investigated function of 1,25(OH)2D is the regulation of calcium and phosphate homeostasis and, in turn, the bone turnover rate in skeletal tissue. Recently, also the extra-osseous activity of 1.25 (OH) 2D has received increasing attention. Nevertheless, the lack of definition of optimal status and dosage of vitamin D limits the knowledge derived from available studies and highlights the need for further studies on these topics.
Vitamin D is a group of fat-soluble seco-sterols of which the most common in foods are cholecalciferol or vitamin D3 and ergocalciferol or vitamin D2. Nutritionally, the two forms are similarly metabolized in humans and for a long time were considered equivalent, however, recent studies have revealed a different half-life of resulting metabolites (see below).
Cholecalciferol is produced in the skin of animals, including humans, when light energy is absorbed by a precursor molecule, the 7-dehydrocholesterol, however, because any excess previtamin D3 or vitamin D3 is destroyed by sunlight, excessive exposure to sunlight does not cause vitamin D3 intoxication (1). Ergocalciferol is the vegetable form of the vitamin, is obtained also from the UV