Effect of Vitamin D or Activated Vitamin D on Circulating 1,25-Dihydroxyvitamin D Concentrations: A Systematic Review and Metaanalysis of Randomized Controlled Trials

BACKGROUND: Evidence is accumulating that circulating 1,25-dihydroxyvitamin D [1,25(OH)(2)D] concentrations are inversely related to overall mortality. METHODS: We searched PubMed, Embase and ISI Web of Science for randomized controlled trials with a control group receiving a placebo instead of vitamin D/acdvated vitamin D and performed a metaanalysis to evaluate the effect of oral vitamin D/acdvated vitamin D on circulating 1,25(OH)(2)D concentrations using a random effects model. RESULTS: We included 52 vitamin D intervention groups (4796 individuals) and 14 intervention groups with activated vitamin D (668 individuals). Vitamin D supplements increased circulating 1,25(OH)(2)D by 12.2 pmol/L (95% CI, 7.8-16.5 pmol/L) and 18.8 pmol/L (95% CI, 9.2-28.4 pmol/L) if only studies with a low risk of bias in study design and reporting were considered (n = 18). There was significant heterogeneity among studies (Cohran's Q P < 0.001, I-2 = 91%). The incremental effect was larger in studies using vitamin D alone compared with coadministration of calcium supplements (18.6 pmol/L; 95% CI, 12.7-24.4 pmol/L vs 4.9 pmol/L; 95% CI, -0.4 to 10.2 pmol/L; P = 0.001), and if quantification was performed with RIA vs other methods (17.1 pmol/L; 95% CI, 11.1-23.1 pmol/L vs 6.9 pmol/L; 95% CI, 1.0-12.8 pmol/L; P = 0.02). Activated vitamin D increased - the mean circulating 1,25(OH)(2)D by 20.5 pmol/L (95% CI, 8.3-32.7 pmol/L; P = 0.04). Again, there was evidence for significant heterogeneity among studies (Cochran Q = 85.4; P < 0.001; I-2 = 87%), but subgroup analysis did not identify parameters significantly influencing the increment in 1,25 (OH)(2)D concentrations. CONCLUSIONS: Both vitamin D and activated vitamin D significantly increase circulating 1,25(OH)(2)D concentrations, but in vitamin D users this increase is suppressed by calcium coadministration. (C) 2015 American Association for Clinical Chemistry