title: Vitamin D and Respiratory Tract Infections: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.
abstract: Low levels of 25-OH vitamin D are associated with respiratory tract infection (RTI). However, results from randomized controlled trials are inconclusive. Therefore, we performed a systematic review and meta-analysis to assess the preventive effect of vitamin D supplementation on RTI.
  
  
  
  Randomized, controlled trials of vitamin D for prevention of RTI were used for the analysis. The risks of within-trial and publication bias were assessed. Odds ratios of RTI were pooled using a random-effects model. Heterogeneity was assessed using Cochran's Q and I(2). Meta-regressions and subgroup analyses were used to assess the influence of various factors on trial outcome. The pre-defined review protocol was registered at the PROSPERO international prospective register of systematic reviews, registration number CRD42013003530.
  
  
  
  Of 1137 citations retrieved, 11 placebo-controlled studies of 5660 patients were included in the meta-analysis. Overall, vitamin D showed a protective effect against RTI (OR, 0.64; 95% CI, 0.49 to 0.84). There was significant heterogeneity among studies (Cohran's Q p<0.0001, I(2) = 72%). The protective effect was larger in studies using once-daily dosing compared to bolus doses (OR = 0.51 vs OR = 0.86, p = 0.01). There was some evidence that results may have been influenced by publication bias.
  
  
  
  Results indicate that vitamin D has a protective effect against RTI, and dosing once-daily seems most effective. Due to heterogeneity of included studies and possible publication bias in the field, these results should be interpreted with caution.
authors.affiliation: Department of Laboratory Medicine, Division of Clinical Microbiology, Karolinska Institutet, Stockholm, Sweden ; Department of Medicine, Center for Infectious Medicine (CIM), Karolinska Institutet, Stockholm, Sweden.
authors.forename: Peter
authors.initials: P
authors.lastname: Bergman
authors.forename: Asa U
authors.initials: AU
authors.lastname: Lindh
authors.forename: Linda
authors.initials: L
authors.lastname: Björkhem-Bergman
authors.forename: Jonatan D
authors.initials: JD
authors.lastname: Lindh
mesh_terms: D019587:Dietary Supplements; D005260:Female; D006801:Humans; D008297:Male; D016017:Odds Ratio; D011446:Prospective Studies; D016032:Randomized Controlled Trials as Topic; D012141:Respiratory Tract Infections / Q000453:epidemiology / Q000517:prevention & control*; D016896:Treatment Outcome; D014807:Vitamin D / Q000008:administration & dosage*
chemical_list: D014807:Vitamin D