BACKGROUND: 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.

AIM: 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.

METHODS: 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.

RESULTS: 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.

CONCLUSIONS: 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.