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Bhan, Ishir

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Bhan

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Ishir

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Bhan, Ishir
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    Vitamin D Binding Protein and Bone Health
    (Hindawi Publishing Corporation, 2014) Bhan, Ishir
    Vitamin D binding protein (DBP) is the major carrier protein of 25-hydroxyvitamin D (25(OH) D) in the circulation, where it may serve roles in maintaining stable levels during times of decreased 25(OH) availability and in regulating delivery of 25(OH) D to target tissues. Several genetic polymorphisms of DBP have been described that lead to phenotypic changes in the protein that may affect affinity, activity, and concentration. These polymorphisms have been linked with alterations in bone density in several populations. One of the mechanisms by which DBP may alter bone health involves regulating vitamin D bioavailability. DBP-bound vitamin is thought to be relatively unavailable to target tissues, and thus alterations in DBP levels or affinity could lead to changes in vitamin D bioactivity. As a result, functional vitamin D status may differ greatly between individuals with similar total 25(OH) D levels. Additionally, DBP may have independent roles on macrophage and osteoclast activation. This review will summarize recent findings about DBP with respect to measures of bone density and health.
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    Bioavailable Vitamin D Is More Tightly Linked to Mineral Metabolism than Total Vitamin D in Incident Hemodialysis Patients
    (Nature Publishing Group, 2012) Bhan, Ishir; Powe, Camille; Berg, Anders; Ankers, Elizabeth; Wenger, Julia; Karumanchi, Subbian; Thadhani, Ravi
    Prior studies showed conflicting results regarding the association between 25-hydroxyvitamin D (25(OH)D) levels and mineral metabolism in end-stage renal disease. In order to determine whether the bioavailable vitamin D (that fraction not bound to vitamin D binding protein) associates more strongly with measures of mineral metabolism than total levels, we identified 94 patients with previously measured 25(OH)D and 1,25-dihydroxyvitamin D \((1,25(OH)_2D)\) from a cohort of incident hemodialysis patients. Vitamin D binding protein was measured from stored serum samples. Bioavailable 25(OH)D and \(1,25(OH)_2D\) were determined using previously validated formulae. Associations with demographic factors and measures of mineral metabolism were examined. When compared with whites, black patients had lower levels of total, but not bioavailable, 25(OH)D. Bioavailable, but not total, 25(OH)D and \(1,25(OH)_2D\) were each significantly correlated with serum calcium. In univariate and multivariate regression analysis, only bioavailable 25(OH)D was significantly associated with parathyroid hormone levels. Hence, bioavailable vitamin D levels are better correlated with measures of mineral metabolism than total levels in patients on hemodialysis.