Association between childhood obesity and vitamin D: a Mendelian randomization study

Background: Previous randomized controlled trial studies have confirmed that obesity can cause changes in serum vitamin D levels, but these changes has not been studied in children. This study evaluated the causal relationship between childhood obesity and vitamin D levels by performing a Mendelian randomization analysis using publicly available genome-wide association study (GWAS) summary statistics. Methods: Vitamin D is present in the blood in the form of 25-hydroxyvitamin D (25(OH)D), childhood obesity and 25(OH)D levels data were obtained from the IEU open GWAS project, which were subjected to Mendelian randomization analyses. In this study, the inverse variance weighting (IVW) method was used as the predominant analysis method and was complemented by Mendelian randomization-Egger regression (MR-Egger), simple mode, weighted median and weighted mode methods. The Mendelian randomization pleiotropy residual sum and outlier（MR-PRESSO）method was utilized to identify horizontal pleiotropy and potential outliers. Results: This study indicated that childhood obesity cases the serum of 25(OH)D, which was significant in the IVW [OR (95%CI), 0.977 (0.966–0.989), P = 0.0001], and weighted-median [OR (95%CI), 0.983(0.969–0.997), P = 0.015] analyses, but nonsignificant in the MR-Egger [OR (95%CI), 0.985 (0.897–1.082), P = 0.784] , simple mode [OR (95%CI), 0.985 (0.965–1.005), P = 0.233] and weighted mode [OR (95%CI), 0.985 (0.967–1.004), P = 0.214] analyses. No significant heterogeneity or potential pleiotropy was detected, and the possibility of weak instrumental variables (IVs) was also excluded. Conclusion: In summary, we found a potential inverse association between elevated childhood obesity and 25(OH)D levels, which suggested that obese children need timely vitamin D supplementation.