Received on August 23, 2005
Accepted on November 17, 2005

Endocrinology and Metabolism

Assessment of Vitamin D Status in Male Osteoporosis

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

^* To whom correspondence should be addressed. 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); 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.