Study objectivesObesity is often associated with lower lung function; however, the interaction of lung function with central obesity and physical inactivity is less clear. As such, we investigated the effect on lung function of body size (body mass index (BMI)), central obesity (waist circumference (WC)) and self-reported physical activity.MethodsLung function, height, weight and WC were measured in 22 743 participants (12 791 women), aged 45–75 years, from the EpiHealth cohort study. Physical activity, gender and educational level were assessed using a questionnaire.ResultsObesity, central obesity and physical inactivity were all associated with lower forced expiratory volume in 1 s (FEV₁) and forced vital capacity (FVC). However, in participants without central obesity there was an increase in both FEV₁ and FVC by BMI (% predicted FVC increasing from median 98%, interquartile range (IQR) 89–110% in underweight participants (BMI <20) to 103%, IQR 94–113% in obese participants (BMI ≥30)). In contrast, there was a decrease in % predicted FVC in participants with central obesity (from 98%, IQR 89–109% in the normal weight group to 95%, IQR 85–105% in the obese weight group). We further found a negative association between physical activity and lung function among those with low and high levels of physical activity (% predicted FEV₁ 97%, IQR 86–107% versus 103%, IQR 94–113%, respectively and % predicted FVC 96%, IQR 85–106% versus 103%, IQR 94–113%, respectively). All results remained when calculated by z-scores.ConclusionsThe association between BMI and lung function is dependent on the presence of central obesity. Independent of obesity, there is an association between physical activity and lung function.