Background: The comorbidity between allergic diseases (asthma, allergic rhinitis, eczema) is well described, but less is known on their relationship to type 1 diabetes. Aims and objectives:
To improve the aetiological understanding of the comorbidity between allergic diseases and type 1 diabetes. Methods: In this population-based cohort study, 3 million Swedish children born 1987?2017 were linked to nationwide registers for healthcare- and sociodemographic data. Logistic regression estimated associations between each allergic disease and type 1 diabetes within individuals and the familial co-aggregation between their relatives (parents, siblings). To explore a genetic overlap, we used linkage disequilibrium- and polygenic risk score (LDSC, PRS) regression in 30,880 twins from the Swedish Twin Registry. Results: Individuals with an allergic disease were at an increased risk of type 1 diabetes (odds ratio [OR] 1.11 [95% CI 1.07?1.15] for asthma, 1.23 [1.19?1.27] for allergic rhinitis, 1.31 [1.26?1.35] for eczema). Type 1 diabetes co-aggregated in parents and full siblings to children with asthma or allergic rhinitis (OR ranging from 1.07 to 1.22) but not for eczema (OR ranging from 0.98 to 1.02). LDSC and PRS regression displayed limited or no evidence for a genetic overlap. Conclusions: Each allergic disease and type 1 diabetes co-occur within children, but only asthma and allergic rhinitis co-aggregate with type 1 diabetes in families. Although this familial co-aggregation indicates a shared genetic or environmental aetiology, we found little other evidence for a genetic overlap in molecular genetic analyses. Awareness of the comorbidity is pertinent to the management of these children.