Obesity and asthma are both prevalent in the U.S. The a few studies that have examined the differences in association between obesity and asthma by sex provided mixed results. Reason for the sex differences is not well understood.

Using U.S. nationally representative data we aimed to examine the association between obesity and asthma and potential sex differences.

Data were obtained from the 2012 National Health Interview Survey (n = 33,153 adults, 4197 had asthma). Asthma was determined by a positive response to the question “Has a doctor or health care professional ever told you had asthma?” Obesity was determined as a Body Mass Index (BMI)≥30. Obese status was categorized as BMI = 30–34.9 (Class I obesity), BMI = 35–39.9 (Class II obesity), and BMI≥40 (Class III obesity).

The prevalence of asthma was 12.6% (11.0% in men, 14.2% in women), and was higher in the obese than non-obese individuals (16.6% vs. 11.1%, p < 0.0001). After controlling for potential confounders, Class I obesity (OR = 1.27, 95% CI = 1.11–1.44), Class II obesity (OR = 1.55, 95% CI = 1.31–1.84), Class III obesity (OR = 1.85, 95% CI = 1.54–2.21) were positively associated with asthma. The association between Class III obesity and asthma was stronger in women than men (2.11 (1.70–2.63) vs. 1.40 (1.01–1.96), p < 0.05), although the sex difference in the association between BMI and asthma was not significant. Positive associations between class II and class III obesity and asthma were observed among young and middle-aged women compared to young and middle-aged men. Stratified by allergic status, obesity remained being positively associated with asthma.

The prevalence of asthma was higher in women than men. Obesity (and BMI) was positively associated with asthma, overall or stratified by allergic status. The association between Class III obesity and asthma was stronger in women. Obesity and sex may be taken into consideration for the management of asthmatic patients.