Credit: Michele Blackwell on Unsplash, CC0 Genetically determined vitamin D levels do not have a large effect on risk of type 1 diabetes in Europeans, according to a study published 25th February 2021 in the open-access journal PLOS Medicine by Despoina Manousaki from the CHU Sainte Justine and the University of Montreal, Canada and colleagues. Type 1 diabetes is a relatively common autoimmune disease that inflicts substantial lifelong illness and significant economic burden. Its incidence is increasing worldwide, and there are no known interventions that can be used to prevent the disease. Vitamin D deficiency has been associated with type 1 diabetes in observational studies, but evidence of a causal effect from randomized controlled trials is lacking. In the new study, the researchers used a Mendelian randomization design to test whether genetically decreased vitamin D levels increase risk of type 1 diabetes. Mendelian randomization is a method of using measured variation in disease-related genes to examine the causal effect of an exposure on a disease. The two-sample Mendelian randomization study involved a vitamin D genome-wide association study (GWAS) including 443,734 Europeans, and a type 1 diabetes GWAS including 9,358 cases and 15,705 controls.