Background The association between asthma and obesity in children, and the effect modification of allergy on this association have not been fully established.
Aims The objective of the study was to know the effect modification of the severity of asthma and of the coexistence of rhinoconjunctivitis (RC) in the relationship between obesity and asthma.
Methods A cross-sectional study of 17 145 schoolchildren 6–7 years old from eight Spanish cities who had completed information on height and weight of the ISAAC phase III questionnaire, which also included questions about asthma and RC symptoms and on various risk factors. Body mass index (BMI) was used to define obesity according to international standards. Two different logistic regressions, using current occasional asthma (COA) and current severe asthma (CSA) as dependent variables, were made stratifying for gender and for the coexistence of RC and controlling for age, older and younger siblings, exercise, mother's education, truck traffic, cat/dog during the first year of life and smoking father or mother.
Results Obesity was a risk factor of CSA without RC, both for boys (1.92, CI 95% 1.13–3.25) and for girls (2.99, CI 95% 1.68–5.32). Every BMI unit increment increased by 6.7% the risk of CSA without RC in boys and by 12.4% in girls. Obesity was not a risk factor for CSA with RC. The association between COA and obesity was weaker and the coexistence of RC did not modify it greatly.
Conclusions Obese schoolchildren are more at risk of suffering from non-allergic asthma than the non-obese subjects.