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

ABSTRACT: 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. 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-energy X-ray absorptiometry. Clinical, diet, and laboratory information also was gathered at this time. We determined free and bioavailable (free + albumin-bound) 25(OH)D using previously validated formulas and examined their associations with BMD. BMD was not associated with total 25(OH)D levels (r = 0.172, p = .236). In contrast, free and bioavailable 25(OH)D levels were positively correlated with BMD (r = 0.413, p = .003 for free, r = 0.441, p = .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 = .03). It is concluded that 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.

SUBMITTER: Powe CE

PROVIDER: S-EPMC3351032 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundThere are several mechanisms via which increased protein intake might maintain or improve bone mineral density (BMD), but current evidence for an association or effect is inconclusive. The objectives of this study were to investigate the association between dietary protein intake (total, plant and animal) with BMD (spine and total body) and the effects of protein supplementation on BMD.MethodsIndividual data from four trials that included either (pre-)frail, undernourished or healthy older adults (aged ≥65 years) were combined. Dietary intake was assessed with food records (2, 3 or 7 days) and BMD with dual-energy X-ray absorptiometry (DXA). Associations and effects were assessed by adjusted linear mixed models.ResultsA total of 1570 participants [57% women, median (inter-quartile range): age 71 (68-75) years] for which at least total protein intake and total body BMD were known were included in cross-sectional analyses. In fully adjusted models, total protein intake was associated with higher total body and spine BMD [beta (95% confidence interval): 0.0011 (0.0006-0.0015) and 0.0015 (0.0007-0.0023) g/cm2 , respectively]. Animal protein intake was associated with higher total body and spine BMD as well [0.0011 (0.0007-0.0016) and 0.0017 (0.0010-0.0024) g/cm2 , respectively]. Plant protein intake was associated with a lower total body and spine BMD [-0.0010 (-0.0020 to -0.0001) and -0.0019 (-0.0034 to -0.0004) g/cm2 , respectively]. Associations were similar between sexes. Participants with a high ratio of animal to plant protein intake had higher BMD. In participants with an adequate calcium intake and sufficient serum 25(OH)D concentrations, the association between total protein intake with total body and spine BMD became stronger. Likewise, the association between animal protein intake with total body BMD was stronger. In the longitudinal analyses, 340 participants [58% women, median (inter-quartile range): age 75 (70-81) years] were included. Interventions of 12 or 24 weeks with protein supplementation or protein supplementation combined with resistance exercise did not lead to significant improvements in BMD.ConclusionsAn association between total and animal protein intake with higher BMD was found. In contrast, plant protein intake was associated with lower BMD. Research is warranted to further investigate the added value of dietary protein alongside calcium and vitamin D for BMD improvement, especially in osteopenic or osteoporotic individuals. Moreover, more research on the impact of a plant-based diet on bone health is needed.

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

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