A Comparison of Free and Total 25-hydroxyvitamin D Levels as Functional Indicators of Bone Health in Healthy Children

Abstract Context: The “free hormone” hypothesis suggests that the free 25-hydroxyvitamin D (25OHD(Free)) level may usefully indicate bone health. Objective: To determine which vitamin D measure is optimally correlated with clinical and bone parameters in healthy children. Design and Participants: A cross-sectional study including 146 healthy children (71 boys, 9.5±1.9 years) at a tertiary medical center. Main Outcome Measures: We used a multiplex liquid chromatography-tandem mass spectrometry-based assay to simultaneously measure vitamin D metabolites. The 25OHD(Free) level was directly measured (m-25OHD(Free)) or calculated using genotype-constant or genotype-specific affinity coefficients of vitamin D-binding proteins (con-25OHD(Free) or spe-25OHD(Free)). Bone mineral content (BMC) and density (BMD) were assessed via dual-energy X-ray absorptiometry. Results: The concentrations of total 25OHD (25OHD(Total)), the three forms of 25OHD(Free), and 24,25-dihydroxyvitamin D(3) correlated with parathyroid hormone levels (all p<0.01). Serum 25OHD(Total) and m-25OHD(Free) levels reflected age, puberty, season, body mass index (BMI), daylight hours, and vitamin D intake (all p<0.05). The con-25OHD(Free) level better reflected puberty and daylight hours than did the spe-25OHD(Free) level (both p<0.01). The association between the 25OHD(Total) level and bone parameters varied according to the BMI (interaction p<0.05). In 109 normal-weight children, the con-25OHD(Free) level correlated with BMC and BMD (both p<0.05), but the 25OHD(Total) and 24,25-dihydroxyvitamin D(3) levels were associated with BMC (both p<0.05). No association was found in overweight or obese children. Conclusions: In healthy children, total and free 25OHD levels comparably reflected lifestyle factors. In normal-weight children, the con-25OHD(Free) level reflected BMC and BMD, whereas the 25OHD(Total) level was associated with BMC(.)