Blood vitamin D status and metabolic syndrome in the general adult population: A dose-response meta-analysis

Context: Increasing evidence has suggested an association between blood vitamin D levels and metabolic syndrome. Objective: Our objective was to determine the relationship between blood vitamin D status and metabolic syndrome in the general adult population, using a dose-response meta-analysis. Data Source: We searched the PubMed, EMBASE, Web of Science, and Cochrane Library databases through July 2013 to identify relevant studies. Study Selection: Observational studies, reporting risk ratios with a 95% confidence interval (CI) for metabolic syndrome in ≥3 categories of blood 25-hydroxyvitamin D [25(OH)D] levels, were selected. Data Extraction: Data extraction was performed independently by 2 authors, and the quality of the studies was evaluated using the risk of bias assessment tool for nonrandomized studies. Data Synthesis: The pooled odds ratio of metabolic syndrome per 25 nmol/L increment in the serum/plasma 25(OH)D concentration was 0.87 (95% CI = 0.83-0.92, I² = 85%), based on 16 "cross-sectional studies" and 1.00 (95% CI = 0.98-1.02, I² = 0%) for 2 "cohort and nested casecontrol studies." The dose-response meta-analysis showed a generally linear, inverse relationship between 25(OH)D levels and metabolic syndrome in the cross-sectional studies (P for linear trend < .001). Conclusions: Blood vitamin D levels were associated with a risk of metabolic syndrome in crosssectional studies but not in longitudinal studies. Randomized, clinical trials will be necessary to address the issue of causality and to determine whether vitamin D supplementation is effective for the prevention of metabolic syndrome.