Vitamin D Metabolism and Function

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Abstract
From intensive work carried out during the past decade, it is clear that vitamin D must be metabolized before it can function.1 One of the central metabolites in this phenomenon is 25-hydroxyvitamin D3 (25OHD3), to which this publication is devoted. It is also known that 25OHD3 is metabolized further to an active hormone, 1,25-dihydroxyvitamin D3 (1,25[OH]2D3). This report will review the physiology and biochemistry underlying the use of calcifediol (25OHD3) in the treatment of disease even in circumstances where there is a lack of 1,25(OH)2D3. FUNCTIONS OF VITAMIN D Vitamin D is unique among the vitamins in the sense that it is a precursor of at least one hormone and, hence, vitamin D3 must be regarded as a prohormone1-5 (Fig 1). The numbering system in the molecule is confusing because it is derived from cholesterol, which is