Total, bioavailable and free 25-hydroxyvitamin D levels as functional indicators for bone parameters in healthy children

OBJECTIVES: Vitamin D is essential for bone health. Not only total but also free 25-hydroxyvitamin D (25OHD) may contribute to bone mass. We sought to determine which vitamin D measure best reflected clinical and bone parameters in healthy children. METHODS: A cross-sectional study including 146 healthy children (71 boys, 9.5 ± 1.9 years) conducted at a tertiary medical center. We used a multiplex liquid chromatography-tandem mass spectrometry-based assay to simultaneously measure vitamin D metabolites. The bioavailable and free 25OHD (25OHD(BioA) and 25OHD(Free)) levels were calculated using the genotype-specific or genotype-constant affinity coefficients of vitamin D-binding proteins (yielding spe-25OHD(BioA), spe-25OHD(Free) and con-25OHD(BioA), con-25OHD(Free) respectively). The 25OHD(Free) level was directly measured (m-25OHD(Free)). Bone mineral content (BMC) and bone mineral density (BMD) were assessed via dual-energy X-ray absorptiometry. RESULTS: The total 25OHD (25OHD(Total)), the two forms of 25OHD(BioA), the three forms of 25OHD(Free), and 24,25-dihydroxyvitamin D(3) levels correlated with parathyroid hormone level (all p < 0.01). Serum 25OHD(Total) and m-25OHD(Free) levels were influenced by age, pubertal status, season, body mass index (BMI), daylight hours, and vitamin D intake (all p < 0.05). The con-25OHD(BioA) and con-25OHD(Free) levels better reflected pubertal status and daylight hours than did the spe-25OHD(BioA) and spe-25OHD(Free) levels (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(BioA) and con-25OHD(Free) levels correlated with total body BMC and BMD (both p < 0.05), whereas the 25OHD(Total) and 24,25-dihydroxyvitamin D(3) levels were associated with total body BMC (both p < 0.05). No such association was found in overweight or obese children. CONCLUSIONS: In healthy children, total, bioavailable, and free 25OHD levels comparably reflected lifestyle factors. In normal-weight children, the con-25OHD(BioA) and con-25OHD(Free), but not m-25OHD(Free) levels, reflected bone mass, as did the 25OHD(Total) level.