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BMD associated with free and bioavailable vitamin D – March 2011

Vitamin D binding protein modifies the vitamin D-bone mineral density relationship.

J Bone Miner Res. 2011 Mar 17. doi: 10.1002/jbmr.387.
Powe CE, Ricciardi C, Berg AH, Erdenesanaa D, Collerone G, Ankers E, Wenger J, Karumanchi SA, Thadhani R, Bhan I.
Division of Nephrology, Department of Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, MA.

BACKGROUND: Studies examining the relationship between total circulating 25-hydroxyvitamin D (25(OH)D) levels and bone mineral density (BMD) have yielded mixed results. Vitamin D binding protein (DBP), the major carrier protein for 25(OH)D, may alter the biologic activity of circulating vitamin D. We hypothesized that free and bioavailable 25(OH)D, calculated from total 25(OH)D, DBP and serum albumin levels, would be more strongly associated with BMD than levels of total 25(OH)D.

METHODS: We measured total 25(OH)D, DBP, and serum albumin levels in 49 healthy young adults enrolled in the Metabolic Abnormalities in College-Aged Students (MACS) study. Lumbar spine BMD was measured in all subjects using dual X-ray absorptiometry. Clinical, diet, and laboratory information was also gathered at this time. We determined free and bioavailable (free + albumin bound) 25(OH)D using previously validated formulae and examined their associations with BMD.

RESULTS: BMD was not associated with total 25(OH)D levels (r=0.172 p=0.236). In contrast, free and bioavailable 25(OH)D levels were positively correlated with BMD (r=0.413 p=0.003 for free, r=0.441 p=0.002 for bioavailable). Bioavailable 25(OH)D levels remained independently associated with BMD in multivariate regression models adjusting for age, sex, body mass index, and race (p=0.03).

CONCLUSION: Free and bioavailable 25(OH)D are more strongly correlated with BMD than total 25(OH)D. These findings have important implications for vitamin D supplementation in vitamin D deficient states. Future studies should continue to explore the relationship between free and bioavailable 25(OH)D and health outcomes. © 2011 American Society for Bone and Mineral Research.
Copyright © 2011 American Society for Bone and Mineral Research.

PMID: 21416506

See also VitaminDWiki