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Endocrinology and Metabolism |
1 School of Clinical & Laboratory Sciences and School of Clinical Medical Sciences, The Medical School, University of Newcastle, Newcastle upon Tyne, United Kingdom.
2 Departments of Rheumatology and Biochemistry, Derbyshire Royal Infirmary, Derby, United Kingdom.
aAddress correspondence to this author at: School of Clinical & Laboratory Sciences, The Medical School, University of Newcastle, Framlington Place, Newcastle upon Tyne, NE2 4HH, United Kingdom. Fax 44-191-222-6227; e-mail h.k.datta{at}ncl.ac.uk.
Background: Clinical assessment of vitamin D status often relies on measuring total circulating 25-hydroxyvitamin D3 (25OHD3), but much of each vitamin D metabolite is bound to plasma vitamin D–binding protein (DBP), such that the percentage of free vitamin is very low. We hypothesized that measurement of free rather than total 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and 25OHD3 may provide better assessment of vitamin D status. We therefore aimed to assess vitamin D status in men with idiopathic osteoporosis, in whom possible secondary causes of osteoporosis had been excluded, and to determine the extent of change in biologically active "free" vitamin D caused by variation in plasma DBP concentrations.
Methods: We measured 1,25(OH)2D3 and 25OHD3 in plasma samples from 56 men with idiopathic osteoporosis [mean (SD) age, 59.6 (13.6) years; range, 21–86 years] and 114 male controls [62.4 (10.4) years; range, 44–82 years].
Results: Mean total plasma 25OHD3 in the 56 men with osteoporosis and the 114 controls was 44.7 (21) and 43.3 (17) nmol/L, respectively; total plasma 1,25(OH)2D3 measured in randomly selected men with osteoporosis (n = 50) and controls (n = 50) was 90 (37) and 103 (39) pmol/L, respectively. Mean plasma DBP was significantly higher (P <0.001) in men with osteoporosis [224 (62) mg/L; n = 56] than in the controls [143 (34) mg/L; n = 114], but calculated free plasma 25OHD3 and 1,25(OH)2D3 were significantly lower in the osteoporotic men than in controls [6.1 (3.1) vs 9.1 (4.4) pmol/L (P <0.00001) and 77 (37) vs 142 (58) fmol/L (P <0.00001), respectively].
Conclusions: Measurement of total vitamin D metabolites alone, although providing a crude assessment of vitamin D status, may not give an accurate indication of the free (biologically active) form of the vitamin. The ratio of total 25OHD3 and 1,25(OH)2D3 to plasma DBP, rather than total circulating vitamin D metabolites, may provide a more useful index of biological activity. Further studies are required to substantiate this hypothesis.
The following articles in journals at HighWire Press have cited this article:
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  K. E. Ensrud, B. C. Taylor, M. L. Paudel, J. A. Cauley, P. M. Cawthon, S. R. Cummings, H. A. Fink, E. Barrett-Connor, J. M. Zmuda, J. M. Shikany, et al. Serum 25-Hydroxyvitamin D Levels and Rate of Hip Bone Loss in Older Men J. Clin. Endocrinol. Metab., August 1, 2009; 94(8): 2773 - 2780. [Abstract] [Full Text] [PDF]
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 S. M Smith, K. K Gardner, J. Locke, and S. R Zwart Vitamin D supplementation during Antarctic winter Am. J. Clinical Nutrition, April 1, 2009; 89(4): 1092 - 1098. [Abstract] [Full Text] [PDF]
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A. Prentice, G. R Goldberg, and I. Schoenmakers Vitamin D across the lifecycle: physiology and biomarkers Am. J. Clinical Nutrition, August 1, 2008; 88(2): 500S - 506S. [Abstract] [Full Text] [PDF]
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J. E Zerwekh Blood biomarkers of vitamin D status Am. J. Clinical Nutrition, April 1, 2008; 87(4): 1087S - 1091S. [Abstract] [Full Text] [PDF]
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