In utero exposure to 25-hydroxyvitamin D and risk of childhood asthma, wheeze, and respiratory tract infections: A meta-analysis of birth cohort studies.

BACKGROUND: Studies of the associations between in utero 25-hydroxyvitamin D (25[OH]D) exposure and risk of childhood asthma, wheeze, and respiratory tract infections are inconsistent and inconclusive. OBJECTIVES: We sought to assess associations between 25(OH)D levels in cord blood or maternal venous blood and risk of offspring's asthma, wheeze, and respiratory tract infections. METHODS: Data were derived from PubMed, Embase, Google Scholar, references from relevant articles, and de novo results from published studies until December 2015. A random-effects meta-analysis was conducted among 16 birth cohort studies. RESULTS: Comparing the highest with the lowest category of 25(OH)D levels, the pooled odds ratios were 0.84 (95% CI, 0.70-1.01; P = .064) for asthma, 0.77 (95% CI, 0.58-1.03; P = .083) for wheeze, and 0.85 (95% CI, 0.66-1.09; P = .187) for respiratory tract infections. The observed inverse association for wheeze was more pronounced and became statistically significant in the studies that measured 25(OH)D levels in cord blood (0.43; 95% CI, 0.29-0.62; P < .001). CONCLUSIONS: Accumulated evidence generated from this meta-analysis suggests that increased in utero exposure to 25(OH)D is inversely associated with the risk of asthma and wheeze during childhood. These findings are in keeping with the results of 2 recently published randomized clinical trials of vitamin D supplementation during pregnancy.