| Summary: | Background: Vitamin D status is associated with muscle strength and maintenance of muscle fibers. However, which serum vitamin D biomarker better reflects sarcopenia remains unclear. The aim of this study was to investigate associations between various serum vitamin D biomarkers (total 25-hydroxy vitamin D [25(OH)D], bioavailable 25(OH)D, 24,25-dihydroxyvitamin D [24,25(OH)2D], and vitamin D metabolite ratio [VMR]) and sarcopenia. Methods: The data for 83 hip fracture patients were finally included in the analysis. Sarcopenia was defined according to the Asia Working Group for Sarcopenia (AWGS) criteria. Measurements of 24,25(OH)2D and 25(OH)D were made using solid-phase extraction (SPE) and subsequent liquid chromatography-tandem mass spectrometry (LC-MS/MS). Vitamin D binding protein (VDBP) concentration was measured using an enzyme-linked immunosorbent assay. The VMR was calculated by dividing serum 24,25(OH)2D by serum 25(OH)D and then multiplying by 100. Based on total 25(OH)D, VDBP, and albumin concentrations, bioavailable 25(OH)D concentrations were calculated using the equations from the other previous studies. Results: Bioavailable 25(OH)D levels were significantly (p = 0.030) decreased in the sarcopenia group compared with the non-sarcopenia group. Results of ROC analysis for the diagnosis of sarcopenia using serum level of bioavailable of 25(OH)D revealed that the cutoff point for bioavailable 25(OH)D was 1.70 ng/ml (AUC = 0.649, p < 0.001). In the group with a bioavailable 25(OH)D less than 1.70 ng/ml, the incidence of sarcopenia increased by 3.3 times (odds ratio: 3.33, p = 0.013). Conclusion: We demonstrated that bioavailable 25(OH)D was associated with sarcopenia among the various serum vitamin D biomarkers. Bioavailable vitamin D might be helpful for assessing the risk of sarcopenia. © 2021 The Authors. Journal of Clinical Laboratory Analysis published by Wiley Periodicals LLC
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