Vitamin D metabolism and function.

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 D 3 (25OHD 3 ), to which this publication is devoted. It is also known that 25OHD 3 is metabolized further to an active hormone, 1,25-dihydroxyvitamin D 3 (1,25[OH] 2 D 3 ). This report will review the physiology and biochemistry underlying the use of calcifediol (25OHD 3 ) in the treatment of disease even in circumstances where there is a lack of 1,25(OH) 2 D 3 . 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 D 3 must be regarded as a prohormone 1-5 (Fig 1). The numbering system in the molecule is confusing because it is derived from cholesterol, which is