Higher population density protects from type 1 diabetes? Dissecting the literature through meta-analysis.

Even though urbanization has been repeatedly established in relation to risk of type 1 diabetes in scientific research, meta-analyses or systematic reviews revealing overall trends in the literature have not been published. We performed a meta-analysis to investigate the association between population density and risk of type 1 diabetes and aimed to include all available literature up to February 22, 2025. Eighteen studies in 14 countries with 82,493 type 1 diabetes cases were included in the meta-analysis. We found evidence of an inverse association between population density and risk of type 1 diabetes (risk ratio = 0.86 [95 % CI: 0.81, 0.92], P < 0.001, N = 18, adjusted for all available confounders). However, variation in the association between studies and geographic areas was observed. Risk of bias due to uncontrolled confounding in observational studies can never be excluded. In addition, we found high unexplained effect size variation among the included studies in our meta-analysis. Our results are not in line with the view that urbanization would systematically increase the risk of type 1 diabetes and highlight the variability of the association among geographic regions. More studies outside Europe are needed to further establish the potential association.