Asthma and type 2 diabetes are two important causes of morbidity globally. We examined both the association of type 2 diabetes with asthma in Swedish adults and the familial co-aggregation of the diseases.
We conducted a cross-sectional study of all adults aged 25-85 in Sweden between 2009 and 2013. Asthma and type 2 diabetes status were ascertained from the health registers. Models were adjusted for sex, age, education level, income and country of birth and in a subset, for body mass index (BMI). We further conducted a familial coaggregation analysis to determine if shared familial factors could explain any observed findings.
The study included 5 299 245 participants, 25 292 (0.5%) had both asthma and type 2 diabetes. In the total population, the OR for the association between type 2 diabetes and asthma was 1.47 (95% CI 1.45 to 1.49); in the population of men (1.30 (95% CI 1.27 to 1.32)) and women (1.63 (95% CI 1.60 to 1.66)). The ORs were slightly higher among men (1.51 (95% CI 1.45 to 1.56)) and women (2.04 (95% CI 1.96 to 2.11)) for whom BMI measurements were available but attenuated with adjustment for BMI (1.45 (95% CI 1.40 to 1.51)) and (1.76 (95% CI 1.68 to 1.84)). Diabetes was more likely if a full sibling had asthma than if the sibling did not (1.13 (95% CI 1.10 to 1.15)).
We found an association between asthma and type 2 diabetes that was sustained after adjusting for BMI, indicating that BMI alone does not explain this relationship. We also found that the two conditions coaggregate in siblings, indicating that the association is partly due to shared familial genetic and environmental risk factors.

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