Vitamin D is a fat-soluble vitamin that is essential for maintaining the mineral balance in the body. Its most active form in humans, vitamin D3 (‘cholecalciferol’), can be synthesized in the skin upon exposure to ultraviolet-B (UVB) radiation from sunlight (1). When exposure to UVB radiation is insufficient, adequate intake of vitamin D from the diet is essential for health.

 

Plants can synthesize ergosterol, which is converted to vitamin D2 (ergocalciferol), the less active form of vitamin D (less than 30% of D3), by ultraviolet light (2).

 

After it is consumed in the diet or synthesized in the skin, the biologically inactive form of vitamin D enters the circulation and is transported to the liver, where 25-hydroxyvitamin D [25(OH)D] is formed, the major circulating form of vitamin D and the indicator of vitamin D status

 

Increased exposure to sunlight or increased dietary intake of vitamin D increases blood levels of 25(OH)D, making the blood 25(OH)D concentration a useful indicator of vitamin D nutritional status.

 

In the kidney, 25(OH)D is converted to 1alpha,25-dihydroxyvitamin D [1,25(OH)2D], the most potent form of vitamin D. Most of the physiological effects of vitamin D in the body are related to the activity of 1,25(OH)2D (3). Its effects are mediated through the vitamin D receptor (VDR) (4). More than 200 genes in tissues throughout the body are known to be regulated by 1,25(OH)2D (5).