Measurements of the Vitamin D Metabolome in the Calgary Vitamin D Study: Relationship of Vitamin D Metabolites to Bone Loss

In a 36-month randomized controlled trial examining the effect of high-dose vitamin D on radial and tibial total bone mineral density (TtBMD), measured by high-resolution peripheral quantitative tomography (HR-pQCT), participants (311 healthy males and females aged 55-70 years with dual-energy X-ray absorptiometry T-scores > -2.5 without vitamin D deficiency) were randomized to receive 400 IU (N = 109), 4000 IU (N = 100), or 10,000 IU (N = 102) daily. Participants had HR-pQCT radius and tibia scans and blood sampling at baseline, 6, 12, 24, and 36 months. This secondary analysis examined the effect of vitamin D dose on plasma measurements of the vitamin D metabolome by liquid chromatography-tandem mass spectrometry (LC-MS/MS), exploring whether the observed decline in TtBMD was associated with changes in four key metabolites [25-(OH)D ; 24,25-(OH) D ; 1,25-(OH) D ; and 1,24,25-(OH) D ]. The relationship between peak values in vitamin D metabolites and changes in TtBMD over 36 months was assessed using linear regression, controlling for sex. Increasing vitamin D dose was associated with a marked increase in 25-(OH)D , 24,25-(OH) D and 1,24,25-(OH) D , but no dose-related change in plasma 1,25-(OH) D was observed. There was a significant negative slope for radius TtBMD and 1,24,25-(OH) D (-0.05, 95% confidence interval [CI] -0.08, -0.03, p