In a 36‐month randomized controlled trial examining the effect of high‐dose vitamin D3 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)D3; 24,25‐(OH)2D3; 1,25‐(OH)2D3; and 1,24,25‐(OH)3D3]. 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)D3, 24,25‐(OH)2D3 and 1,24,25‐(OH)3D3, but no dose‐related change in plasma 1,25‐(OH)2D3 was observed. There was a significant negative slope for radius TtBMD and 1,24,25‐(OH)3D3 (−0.05, 95% confidence interval [CI] −0.08, −0.03, p < 0.001) after controlling for sex. A significant interaction between TtBMD and sex was seen for 25‐(OH)D3 (female: −0.01, 95% CI −0.12, −0.07; male: −0.04, 95% CI −0.06, −0.01, p = 0.001) and 24,25‐(OH)2D3 (female: −0.75, 95% CI −0.98, −0.52; male: −0.35, 95% CI −0.59, −0.11, p < 0.001). For the tibia there was a significant negative slope for 25‐(OH)D3 (−0.03, 95% CI −0.05, −0.01, p < 0.001), 24,25‐(OH)2D3 (−0.30, 95% CI −0.44, −0.16, p < 0.001), and 1,24,25‐(OH)3D3 (−0.03, 95% CI −0.05, −0.01, p = 0.01) after controlling for sex. These results suggest vitamin D metabolites other than 1,25‐(OH)2D3 may be responsible for the bone loss seen in the Calgary Vitamin D Study. Although plasma 1,25‐(OH)2D3 did not change with vitamin D dose, it is possible rapid catabolism to 1,24,25‐(OH)3D3 prevented the detection of a dose‐related rise in plasma 1,25‐(OH)2D3. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).
