Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality and is associated with several systemic diseases, such as type 2 diabetes. It has been suggested that comorbidity between COPD and type 2 diabetes is due to shared genetic factors.

To examine the relationship between type 2 diabetes and chronic bronchitis and COPD in adult twins, and to examine to what extent comorbidity between these diseases is explained by shared genetic or environmental factors.

Questionnaire data on chronic bronchitis and hospital discharge data on diagnosed COPD in 13,649 twins, aged 50–71 years, from the Danish Twin Registry were cross-linked with hospital discharge diagnosis data on type 2 diabetes from the Danish National Patient Registry.

The risk of type 2 diabetes was higher in persons with symptoms of chronic bronchitis than in those without symptoms (3.5 vs. 2.3%), OR = 1.57 (1.10–2.26), p = 0.014, and in individuals with diagnosed COPD than in those without the diagnosis (6.6 vs. 2.3%), OR = 2.62 (1.63–4.2), p < 0.001. The results were significant after adjusting for age, sex, smoking, and BMI. Correlations between genetic effects on chronic bronchitis and type 2 diabetes, and between genetic effects on diagnosed COPD and type 2 diabetes, respectively, were 0.33 (0.00–0.79), p = 0.103, and 0.43 (0.00–0.98), p = 0.154. Non-shared environmental correlations between chronic bronchitis and type 2 diabetes were −0.13 (−0.43 to 0), p = 0.498 and diagnosed COPD and type 2 diabetes −0.12 (−0.48 to 0), p = 0.665.

Patients with chronic bronchitis or COPD have an increased risk of type 2 diabetes independent of sex, age, smoking and BMI. The genetic correlation between type 2 diabetes and chronic bronchitis was 33% and type 2 diabetes and COPD was 43%, however neither were statistically significant. The increased risk of type 2 diabetes should be accommodated in the management of patients with chronic bronchitis or COPD.