This large population-based study provides evidence that several respiratory disorders, including asthma, are linked with gastroesophageal reflux symptoms on a population-based level. Strong or moderate associations between a variety of respiratory symptoms and symptoms of reflux were identified, with breathlessness showing a particularly strong and dose-response association with reflux symptoms. The single most important advantage of this study compared to previous research is the population-based design, with a high participation rate, that acted against selection bias. Selection bias has been an important source of error in many previous studies addressing reflux in relation to respiratory conditions, where the participants had been referred and therefore selected with a special hypothesis in mind. so
Further advantages of the present study are the ability to adjust the results for all biologically plausible confounding variables, and the large sample size. Limitations of our study include the crosssectional assessment of respiratory symptoms and reflux symptoms, which prevented us from addressing the temporal, and hence also causal, relation between these conditions. The symptoms used for case classification, heartburn and acid regurgitation, are well validated as representative of true reflux, but we did not use a validated questionnaire to assess reflux symptoms. For this latter reason, a separate validation study was performed and showed that the questions used in the HUNT-2 survey were highly specific for reflux as compared to a more detailed and previously used reflux questionnaire. However, some data indicate poor correlation between esophageal pH monitoring of acid reflux and respiratory symptoms of reflux. Moreover, we excluded survey participants reporting minor reflux symptoms, since this group was more heterogeneous than the group with severe symptoms. Another potential limitation of our study was the assessment of the respiratory disorders.