• asthma;
  • environment;
  • genetics;
  • obesity;
  • twin study

Background:  Obesity is linked to asthma in a yet poorly understood manner. We examined the relationship between obesity and asthma in a population-based sample of twins.

Methods:  From the cohorts born between 1953 and 1982, who were enrolled in The Danish Twin Registry, a total of 29 183 twin individuals participated in a nationwide questionnaire study, where data on height, weight and asthma were collected. Latent factor models of genetic and environmental effects were fitted using maximum likelihood methods.

Results:  The age-adjusted risk of asthma was increased both in obese females, OR = 1.96 (1.45–2.64), P ≤ 0.001 and in obese males, OR = 1.59 (1.08–2.33), P = 0.02. According to best-fitting models, the heritability for obesity was 81% in males and 92% in females, whereas the heritability for asthma was 78% and 68% in males and females respectively. The age-adjusted genetic liabilities to obesity and asthma were significantly correlated only in females, r = 0.28 (0.16–0.38).

Conclusions:  Obese subjects have an increased risk for asthma, which in females seems partly because of common genes.