Clarke SLN, Mitchell RE, Sharp GC, et al. - In view of a lack of a causal association between 25-(OH)D levels (the major circulating form of vitamin D) and JIA, it is unlikely that population level vitamin D supplementation will decrease the incidence of JIA.

• A two sample Mendelian randomization analysis was conducted in which data were used from the largest and most recent genome wide association study (GWAS) of 25-(OH)D levels (sample size 443,734) as well as from two JIA GWASs (sample sizes 15,872 and 12,501), all from European populations.

• Genetically predicted 25-(OH)D levels were not causally associated with JIA incidence (OR 1.00, 95% CI 0.76-1.33 per standard deviation increment in standardized natural-log transformed 25-(OH)D levels).

• Also, no causal impact of genetically predicted JIA on 25-(OH)D levels was found (-0.002 standard deviation alteration in standardized natural-log transformed 25-(OH)D levels per doubling odds in genetically predicted JIA, 95% CI -0.006-0.002).