Study objective: To determine the modifying effects of sex and allergy history on the association between body mass index (BMI) and asthma prevalence.
Design: Cross-sectional study of 86,144 Canadians who were 20 to 64 years of age in 2000–2001.
Setting: A national survey.
Measurements and analysis: Self-reported asthma, allergy history, height, and weight. Logistic regression analysis was used to detect effect modification and to adjust for covariates. Population weight and design effects associated with complex survey design were taken into consideration.
Results: The adjusted odds ratios (ORs) for obesity associated with asthma was 1.85 (95% confidence interval [CI], 1.65 to 2.07) for women and 1.21 (95% CI, 1.05 to 1.40) for men. One unit of increased BMI was associated with an approximate 6% increase in asthma risk in women, and 3% in men. A stronger association between obesity and asthma was observed in nonallergic women than in allergic women, with the adjusted ORs being 2.53 (95% CI, 2.11 to 3.04) and 1.57 (95% CI, 1.36 to 1.82), respectively. For men, the corresponding ORs were 1.30 (95% CI, 1.05 to 1.62) and 1.18 (95% CI, 0.98 to 1.53), respectively.
Conclusions: Obesity is likely to have a larger effect on nonallergic asthma. The greater prevalence of nonallergic asthma in women may explain the stronger obesity-asthma association seen in women compared with men and children who have a greater prevalence of allergic asthma.