Vitamin D Status: Measurement, Interpretation, and Clinical Application
Introduction
The association of sunlight and vitamin D for the promotion of bone health began with the industrialization of northern Europe. The lack of adequate sun exposure resulted in an epidemic of children with severe growth retardation and bony deformities that was commonly known as rickets (1). In 1919, Huldschinsky et al. (2) reported that exposure to ultraviolet radiation cured rickets. This was followed by Hess and Unger in 1921 (3), who observed that exposure to sunlight cured rickets.
In the 1930s, it was appreciated that ultraviolet irradiation of yeast extract was effective in producing an antirachitic substance known as vitamin D. This vitamin D was structurally identified and called vitamin D2. Vitamin D3 was identified by the irradiation of 7-dehydocholesterol. Because vitamin D2 was inexpensive to produce, vitamin D2 was used widely for the fortification of foods, including milk and bread in the United States and Europe. When 7-dehydrocholesterol was easily extracted from lanolin from sheep's wool, vitamin D3 was inexpensively made and was used in food fortification and for supplements.
In the early 1950s, there was an outbreak of hypercalcemia, caused by what was thought to be the overfortification of milk with vitamin D and, as a result, most European countries forbid the fortification of milk and other dairy products with vitamin D. In the United States, milk and orange juice are fortified with vitamin D3, whereas a majority of multivitamin supplements and pharmaceutical preparations contain vitamin D21, 4.
The appreciation that vitamin D (D represents either D2 or D3) required a hepatic hydroxylation on carbon 25 to produce 25-hydroxyvitamin D [25(OH)D] (Fig. 1) led to the development of a binding protein assay using the vitamin D binding protein (DBP) to measure circulating levels of 25(OH)D in the circulation 5, 6, 7. The identification of 1,25-hydroxyvitamin D as being the biologically active form of vitamin D led to the development of a binding protein assay using the vitamin D receptor as the binder to measure circulating levels of 1,25(OH)2D 8, 9, 10.
Section snippets
25(OH)D Assays
The first assays for 25(OH)D used the competitive protein binding format with the vitamin D binding protein (DBP) as the binder. The advantage of this assay was that DBP recognized 25(OH)D2 equally as well as 25(OH)D3. The major limitation of this assay was that the assay measured 25(OH)D in a serum sample that contained other vitamin D metabolites, including 24,25-dihydroxyvitamin D [24,25(OH)2D], 25,26-dihydroxyvitamin D, and the 25,26-dihydroxyvitamin D-26, 23-lactone (11). However,
1,25-Dihydroxyvitamin D Assays
Once 1,25(OH)2D was identified, an assay using the chicken intestinal vitamin D receptor was developed as a competitive protein binding assay to measure circulating levels of 1, 25(OH)D (8). It was observed that the bovine thymus was an excellent source for the VDR and competitive binding protein assay for 1, 25(OH)2D was developed using bovine VDR as the binding protein 9, 11. Radioimmunoassays were later developed to measure 1,25(OH)D. The Diasorin assay was reported to measure 1,25(OH)D3
Determination of Vitamin D Status
25(OH)D is the only vitamin D metabolite that is used to determine whether a patient is vitamin D deficient, sufficient or intoxicated 1, 4, 16, 17. 25(OH)D is the major circulating form of vitamin D that has a half life of approximately 2–3 weeks. 25(OH)D is a summation of both vitamin D intake and vitamin D that is produced from sun exposure 1, 4.
Although 1,25(OH)D3 is the biologically active form of vitamin D and, thus, would be thought to be the ideal measure for vitamin D status, it is
Definition of Vitamin D Insufficiency and Deficiency
There is no absolute consensus as to what a normal range for 25(OH)D should be. Part of the difficulty is how a normal range is determined, i.e., typically it is done by obtaining blood from several hundred volunteers and deeming them to be normal and to perform the measurement of the analyte and do a distribution with a mean ± 2SD as the normal range. However, since it is now recognized that 30–50% of both the European and US population are vitamin D insufficient or deficient, the previously
Conclusion
The only way to determine whether a person is vitamin D deficient or sufficient is to measure their circulating level of 25(OH)D. There are a variety of assays used to measure 25(OH)D. The radioimmunoassays and competitive protein binding assays for 25(OH)D are useful in detecting vitamin D deficiency and sufficiency. However, these assays are fraught with technical difficulties, especially if they are not run routinely (Fig. 4) (33). Several reference laboratories have now switched to LC-MS
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Editor's note: With this issue, we initiate a symposium on the epidemiology of vitamin D. Our Guest Editor, Dr. Cedric Garland, has overseen the project and will provide an overview and commentary at its completion. The current contribution deal with measurement and clinical interpretation of Vitamin D, and with some aspects of the relationship of Vitamin D and cancer. Subsequent issues will continue the discussion of cancer, with attention to the risks of vitamin D and perspectives on its use for cancer prevention.
This work was supported in part by NIH grants M01RR00533 and AR36963 and the UV Foundation.