A comparison of free and total 25-hydroxyvitamin D levels as functional indicators of bone health in healthy children

<b>Context: </b>The “free hormone” hypothesis suggests that the free 25-hydroxyvitamin D (25OHD_Free) level may usefully indicate bone health.<b></b> <b>Objective: </b>To determine which vitamin D measure is optimally correlated with clinical and bone parameters in healthy children. <b></b> <b>Design and Participants: </b>A cross-sectional study including 146 healthy children (71 boys, 9.5±1.9 years) at a tertiary medical center. <b>Main Outcome Measures: </b>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. <b></b> <b>Results: </b>The concentrations of total 25OHD (25OHD_Total), the three forms of 25OHD_Free, and 24,25-dihydroxyvitamin D₃ correlated with parathyroid hormone levels (all p&lt;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&lt;0.05). The con-25OHD_Freelevelbetter reflected puberty and daylight hours than did the spe-25OHD_Freelevel(both p&lt;0.01). The association between the 25OHD_Total level and bone parameters varied according to the BMI (interaction p&lt;0.05). In 109 normal-weight children, the con-25OHD_Freelevel correlated with BMC and BMD (both p&lt;0.05), but the 25OHD_Total and 24,25-dihydroxyvitamin D₃ levels were associated with BMC (both p&lt;0.05). No association was found in overweight or obese children. <b></b> <b>Conclusions: </b>In healthy children,<b> </b>total and free 25OHDlevels comparably reflected lifestyle factors. In normal-weight children, the con-25OHD_Free level reflected BMC and BMD, whereas the 25OHD_Totallevel was associated with BMC_.