Sources of Vitamin D

Vitamin D is a potent steroid hormone with a wide distribution of receptors suggesting diverse physiological roles.1'2 Despite its name, relatively little vitamin D comes from dietary sources (apart from fortified foods or supplements). In healthy people of normal mobility under most climatic conditions, the majority of their vitamin D requirements are produced through the sunlight-mediated (via ultraviolet, UV B) photochemical conversion of 7-dehy-drocholesterol in the skin to cholecalciferol (vitamin D3).3 The rate of this conversion is reduced by increased skin pigmentation, by covering up when outdoors or by staying indoors.3"5 Cholecalciferol is hydroxylated in the liver to 25-hydroxycholecalciferol (25(OH)D3) by the enzyme 25-hydroxylase. In turn, 25(OH)D3 is hydroxylated (principally though not exclusively in the kidney) to many metabolites (Fig. 1). The main ones are the biologically active metabolite 1,25-dihydroxyvitamin D (l,25(OH)2D3), via the enzyme 1-a-hydroxylase, and 24,25-dihydroxyvitamin D (24,25(OH)2D3). Hepatic synthesis of 25(OH)D3 is only loosely regulated, so blood levels reflect the amount of vitamin D produced in the skin or ingested. Its half-life is several weeks, whereas that of l,25(OH)2D3 is a few hours, and its synthesis is tighdy regulated.6

1-a-hydroxylase is found in decidual and trophoblastic cells in the placenta, with substantially higher levels in early versus late gestation, suggesting the potential importance of l,25(OH)2D3 during early pregnancy.7 l,25(OH)2D3 regulates genes associated with implantation, such as HOXAIO, whereas its immunosuppressive effects may have a role in

'Corresponding Author: Marianne Tare—Department of Physiology, Monash University, Clayton, Victoria, 3800, Australia. Email: [email protected]


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