Purpose: First, we sought to estimate the magnitude of the cross-sectional associations between overweight/obesity and asthma stratified by gender and by self-reported hay fever and second we sought to assess both directions of causality in longitudinal analyses. Methods: We used cross-sectional and longitudinal data from a population-based cohort study (n = 5114, ages 40-65 at baseline). After 8.5 years, 4010 adults were followed-up by questionnaires. Self-reported height and weight were used to calculate body mass index categories. Multivariate adjusted prevalence ratios (PRs), relative risks (RRs), and 95% confidence intervals (95% CI) were calculated using Poisson regression. Results: In cross-sectional analyses, adjusted PRs were comparable for overweight women and men but differed between obese women and men (PR 1.93, 95% CI 1.19-3.14 and PR 0.98, 95% CI 0.56-1.72). PRs were similar when stratified by hay fever. Longitudinal analyses suggested that overweight/obesity did not increase asthma risk substantially (RR 1.02, 95% CI 0.50-2.06), but a relation between asthma and subsequent weight gain could not be excluded (RR 1.34, 95% CI 1.01-1.77). Conclusions: The prevalence of asthma is almost twice as high in obese versus normal weight women, but not in obese men. The association between overweight/obesity and asthma does not vary by hay fever. A causal relationship between asthma and incident weight gain cannot be excluded. © 2008 Elsevier Inc. All rights reserved.
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