Relation of body mass index to asthma and atopy in children: the National Health and Nutrition Examination Study III

Background-An increase in the prevalence of obesity and asthma over recent decades has been reported in affluent societies. Both overweight and obesit y have been shown to be inversely related to having been breastfed, which i s also a potential protective factor against childhood atopic diseases. The aim of this analysis was to explore the relation of body mass index (BMI) to asthma and atopy in a large representative sample of the United States p opulation. Methods-Children aged 4-17 years were included in the NHANES III survey. Pr evalences of atopic diseases and potential confounding factors such as expo sure to environmental tobacco smoke, birth weight, breast feeding, and hous ehold size were assessed using structured interviews with parents. Height a nd weight were measured, and BMI was calculated as kg/m(2) and transformed into Z scores. Children underwent skin prick tests for atopy to a battery o f food and inhalant allergens. Results-The prevalence of asthma (8.7% v 9.3% v 10.3% v 14.9%, p=0.0001) an d atopy (48.6% v 50.5% v 53.0% v 53.2%, p=0.05) rose significantly with inc reasing quartiles of BMI. After adjustment for confounders, a significant p ositive association between BMI and asthma remained (adjusted OR 1.77, 95% confidence interval 1.44 to 2.19 between the highest and lowest quartiles o f BMI), whereas no independent relation between BMI and atopy was evident. No effect modification by sex or ethnic group was seen. Conclusions-The effects of increased BMI on asthma may be mediated by mecha nical properties of the respiratory system associated with obesity or by up regulation of inflammatory mechanisms rather than by allergic eosinophilic inflammation of the airway epithelium.