Assessment of vitamin D status in male osteoporosis.
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.
Al-oanzi
Ziad H
ZH
School of Clinical & Laboratory Sciences, The Medical School, University of Newcastle, Newcastle upon Tyne, United Kingdom.
Tuck
Stephen P
SP
Raj
Nicholas
N
Harrop
John S
JS
Summers
Gregory D
GD
Cook
David B
DB
Francis
Roger M
RM
Datta
Harish K
HK
D015502:Absorptiometry, Photon; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015519:Bone Density / Q000502:physiology; D002112:Calcifediol / Q000097:blood; D002117:Calcitriol / Q000097:blood; D006801:Humans; D008297:Male; D008875:Middle Aged; D010024:Osteoporosis / Q000097:blood* / Q000378:metabolism / Q000503:physiopathology; D014807:Vitamin D / Q000097:blood* / Q000378:metabolism; D014809:Vitamin D-Binding Protein / Q000097:blood
D014809:Vitamin D-Binding Protein; D014807:Vitamin D; D002117:Calcitriol; D002112:Calcifediol