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Bone density correlates better with free D (calculated) rather than measured vitamin D – Jan 2014

Serum free and bio-available 25-hydroxyvitamin D correlate better with bone density than serum total 25-hydroxyvitamin D.

Scand J Clin Lab Invest. 2014 Jan 2.
Martin S. Johnsen 1, Guri Grimnes 1,2, Yngve Figenschau 3, Peter A. Torjesen 4, Bjørg Almås 5 & Rolf Jorde 1,2
1 Tromsø Endocrine Research Group, Department of Clinical Medicine, University of Tromsø,
2 Division of Internal Medicine, University Hospital of North Norway, Tromsø
3 Institute of Medical Biochemistry, University of Tromsø and Department of Laboratory Medicine, University Hospital of North Norway, Tromsø
4 Hormone Laboratory, Department of Endocrinology, Oslo University Hospital, Oslo
5 Hormone Laboratory, Haukeland University Hospital, Bergen , Norway
Correspondence: Rolf Jorde, Medical Department, University Hospital of North Norway,
9038 Tromsø , Norway. Fax: + 47 776 69730. E-mail: rolf.jorde at unn.no

In the circulation 25-hydroxyvitamin D (25(OH)D) is bound to vitamin D-binding protein (DBP) and albumin. Only a small fraction is in the unbound, free form. According to the 'free-hormone-hypothesis' only the free form is biologically active. Genetic differences in DBP may affect the binding to 25(OH)D and thereby the amount of free 25(OH)D. In the present study sera were obtained from 265 postmenopausal women with low bone mass density (BMD). Serum 25(OH)D, DBP and albumin were measured and the free and bio-available (free + albumin-bound) 25(OH)D calculated. Based on genotyping of the polymorphisms rs7041 and rs4588, the six common DBP phenotypes were identified and the free and bio-available 25(OH)D calculated according to the corresponding binding coefficients.
Relations between measures of 25(OH)D and PTH and BMD were evaluated with linear regression adjusted for age and BMI.
The calculated amount of free and bio-available 25(OH)D was 0.03% and 13.1%, respectively, of the measured total serum 25(OH)D.
Adjusting for DBP phenotype affected the calculated free and bio-available 25(OH)D levels up to 37.5%.
All measures of 25(OH)D correlated significantly with PTH, whereas a significant association with BMD was only seen for the free and bio-available 25(OH)D measures.

Adjusting for the DBP phenotypes improved the associations.

These relations were almost exclusively seen in subjects not using vitamin D and/or calcium supplements.

In conclusion, the free and bio-available forms of 25(OH)D may be a more informative measure of vitamin D status than total 25(OH)D. Adjustment for DBP phenotype may improve this further.

PMID: 24383929

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This paper seems to imply that at low levels of vitamin D that the Vitamin D Binding protein limits the vitamin D benefits to the bone. Wonder if this is the case for:

  1. higher levels of vitamin D
  2. the scores of vitamin D benefits other than bone

See also VitaminDWiki