Incidence of adult-onset asthma after hypothetical interventions on body mass index and physical activity: an application of the parametric g-formula.

High body mass index (BMI) (calculated as weight (kg)/height (m)(2)) is associated with increased asthma risk, but uncertainty persists about the role of physical activity. We estimated the independent and joint associations of hypothetical interventions on BMI and physical activity with the risk of adult-onset asthma in 76,470 asthma-free women from the Nurses' Health Study who were followed between 1988 and 1998. Information about asthma, BMI, physical activity, and other factors was updated every 2 years. We used the parametric g-formula to estimate the 10-year asthma risk in the following 4 scenarios: no intervention, 5% BMI reduction in a 2-year period for those who were overweight or obese, at least 2.5 hours/week of moderate-to-vigorous physical activity, and both of the previous 2 interventions. At baseline, women had a mean age of 55 (standard deviation, 7) years and a mean BMI of 25.4 (standard deviation, 4.8). Median time spent in physical activity was 0.7 hours/week. During follow-up, 1,146 women developed asthma. The 10-year asthma risk under no intervention was 1.5%. Compared with no intervention, the population risk ratios were 0.96 (95% confidence interval (CI): 0.93, 0.99) under the BMI intervention, 0.96 (95% CI: 0.81, 1.10) under the physical activity intervention, and 0.92 (95% CI: 0.78, 1.06) under the joint intervention. Interventions on BMI and physical activity may have a modest impact on the risk of adult-onset asthma in this population of US women.