• Vitamin D is a steroid hormone that is important in regulating the body levels of calcium and phosphorus, and in the mineralization of bone.

• Receptors for vitamin D hav ealso been found in a wide variety of cells, and thus this hormone has biologic effects which extend far beyond control of mineral metabolism.

• The term vitamin D refers to a group of steroid molecules. Cholecalciferol, Vitamin D3, can be produced in the skin of animals when light energy is absorbed by a precursor molecule 7-dehydrocholesterol.

• Dietary sources of vitamin D, include egg yolk, fish oil and a number of plants. However, natural diets typically do not contain adequate quantities of vitamin D, and exposure to sunlight or consumption of foodstuffs purposefully supplemented with vitamin D are necessary to prevent deficiencies.

• Vitamin D3 (and the plant Vitamin D2) does not have significant biological activity. It is thus metabolized within the body to the hormonally-active form. This transformation occurs in two steps one within the liver where cholecalciferol is hydroxylated to 25-hydroxycholecalciferol by the enzyme 25-hydroxylase.

• The next step involves the kidney where 25-hydroxycholecalciferol is further hydroxylated to 1,25-dihydroxycholecalciferol, the biologically active form of vitamin D, by the enzyme 1-alpha-hydroxylase.

• All forms of vitamin D are hydrophobic, and are transported in blood bound to carrier proteins. The major carrier is called, appropriately, vitamin D-binding protein.

Physiologic Effects

• Similar to other steroid hormones, vitamin D binds to intracellular receptors that then function as transcription factors to modulate gene expression.

• Vitamin D is involved in mineral metabolism and bone growth.

• It facilitates intestinal absorption of calcium, phosphate and magnesium ions. In the absence of vitamin D, dietary calcium is not absorbed at all efficiently.

• Vitamin D acts as a transcriptional regulator of bone matrix proteins, it induces the expression of osteocalcin and suppresses synthesis of type I collagen.

• In cell cultures, vitamin D stimulates differentiation of osteoclasts.

• Vitamin D provides the proper balance of calcium and phosphorus to support mineralization.

Control of Vitamin D Synthesis

• Blood levels of 25-hydroxycholecalciferol largely reflect the amount of amount of vitamin D produced in the skin or ingested. Ont the other hand, the activity of 1-alpha-hydroxylase in the kidney is tightly regulated and serves as the major control point in production of the active hormone.

• Major inducers of 1-alpha-hydroxylase is parathyroid hormone and low blood levels of phosphate.