Association of vitamin D with risk of type 1 2 diabetes: a Mendelian 2 randomisation study in European and Chinese adults

Background: Observational studies have reported that higher plasma 25-hydroxy vitamin D (25[OH]D) concentrations are associated with lower risks of diabetes, but it is unclear if these associations are causal. The aim of this study was to test the relevance of 25(OH)D for type 2 diabetes using genetically-instrumented differences in plasma 25(OH)D concentrations. Methods and findings: Data were available on four 25(OH)D single nucleotide polymorphisms (SNPs: n=82,464), plasma 25(OH)D concentrations (n=13,565) and cases with diabetes (n=5565) in China Kadoorie Biobank (CKB). The effects on risk of diabetes were assessed by a genetic score using two 25(OH)D synthesis SNPs (DHCR7-rs12785878 and CYP2R1-rs10741657), or the addition of SNPs affecting transport (GC/DBP-rs2282679) and catabolism (CYP24A1-rs6013897) of 25(OH)D. The CKB results were combined in a meta-analysis of 10 studies for the two synthesis SNPs (n=58,312 cases) and 7 studies for four SNPs (n=32,796 cases). Mean (SD) 25(OH)D concentration was 62 (20) nmol/L in CKB and the per allele effects of genetic scores on 25(OH)D were 2.87 (SE 0.39) for the synthesis SNPs and 3.54 (SE 0.32) for all SNPs. A 25 nmol/L higher biochemically measured 25(OH)D was associated with 9% (95%CI: 0-18%) lower risk of diabetes in CKB. In a meta-analysis of all studies, a 25 nmol/L higher genetically-instrumented 25(OH)D concentration was associated with a 14% (95%CI: 3-23%) lower risk of diabetes (p=0.01) using the two synthesis SNPs. An equivalent difference in 25(OH)D using a genetic score with four SNPs was not significantly associated with diabetes (OR 8%, 95% CI: -1-16% lower risk, p=0.07), but had some evidence of pleiotropy. A limitation of the meta-analysis was the access only to study level rather than individual level data. Conclusions: The concordant risks of diabetes for biochemically-measured and genetically-instrumented differences in 25(OH)D using synthesis SNPs provide evidence for a causal effect of higher 25(OH)D for prevention of diabetes.