Background: A convincing body of literature links obesity with a higher risk for developing adult-onset asthma. The impact of obesity on asthma severity among adults with pre-existing asthma, however, is less clear.

Methods and Procedures: In a prospective cohort study of 843 adults with severe asthma, we studied the impact of BMI on asthma health status.

Results: The prevalence of obesity and overweight were 44% (95% confidence interval (CI) 41–47%) and 28% (95% CI 25–32%). The obese BMI group was associated with a higher risk for daily or near daily asthma symptoms than was the normal BMI group (odds ratio (OR) 1.81; 95% CI 1.10–2.96). Compared to the normal BMI group, generic physical health status was worse in the overweight (mean score decrement −2.42 points; 95% CI −4.39 to −0.45) and the obese groups (−6.31 points; 95% CI −8.14 to −4.49). Asthma-specific quality of life was worse in the underweight (mean score increment 8.66 points; 95% CI 2.53–14.8) and obese groups (4.51 points; 95% CI 2.21–6.81), compared to those with normal BMI. Obese persons also had a higher number of restricted activity days that past month (5.05 days; 95% CI 2.90–7.19 days).

Discussion: It appears that obesity has a substantive negative effect on health status among adults with asthma. Further work is needed to clarify the precise mechanisms. Clinicians should counsel dietary modification and weight loss for their overweight and obese patients with asthma.