This is the first longitudinal study assessing the relation between the regular practice of physical activity and risk of asthma exacerbations. In our population of female U.S. health professionals, with an average low-risk lifestyle (20
) (not smoking, exercising daily, and eating a healthy diet), we found that a higher level of regular physical activity is associated with a lower risk of an asthma exacerbation. This association was independent of asthma severity and other covariates, and was consistent after different stratifications and sensitivity analyses, although statistical significance was lost in some models, most likely due to a reduction in statistical power.
The association between physical activity and respiratory symptoms in patients with asthma has been examined in several cross-sectional studies (3
). Those including adult subjects yielded an association between higher physical activity level and lower asthma symptoms (3
), as we found. The studies with null results were conducted in children (6
), and/or used less reliable measures of asthma (defined as wheezing) (7
) or physical activity (defined from single questions about TV watching  or sports playing ), suggesting that differences in subjects' age or study design may be responsible for the seemingly discrepant results. The possibility that sex plays any role in these inconsistent results is difficult to ascertain: whereas one of the previously mentioned studies yielded similar associations between physical activity and asthma symptoms in men and women (3
), the other found statistically significant associations only in men (4
). Finally, the cross-sectional design of all previous studies also makes their interpretation and comparisons with current results difficult. Data from the population-based cross-sectional European Community Respiratory Health Survey (28 to 57 yr old; 51% men) had shown reduced bronchial hyperresponsiveness among the most physically active subjects (22
), which is in agreement with our findings given that bronchial hyperresponsiveness is a feature of asthma exacerbations (23
). In addition, because bronchial hyperresponsiveness has been considered an epiphenomenon of airway inflammation (24
), it is likely that the effect of physical activity in asthma exacerbations is partly mediated by inflammation, as has been suggested for many chronic diseases (25
). Finally, the follow-up of the population-based Copenhagen City Heart Study (mean age, 52 yr; 43% male) showed that regular physical activity may attenuate lung function decline, an association that was stronger in subjects with asthma than in the general population (26
), pointing to the possibility of patients with asthma as a specific subgroup being more sensitive to the effects of physical activity. A systematic review of physical training for patients with asthma found that physical training programs can improve cardiopulmonary fitness (maximal oxygen uptake and maximal expiratory ventilation) even without changing lung function (9
). It should be noted that the mechanisms underlying short-term results obtained by rehabilitation programs may differ from the mechanisms underlying the long-term effects of regular physical activity, both of them deserving further specific research that is beyond the scope of epidemiological studies.
Despite the physical activity benefits for asthma that can be inferred from existing data, and despite the mention of the importance of physical activity both in the National Asthma Education and Prevention Program (27
) and in the Global Initiative for Asthma (23
) 2007 guidelines, the potential of physical activity as a treatment to improve evolution of the disease is not addressed in any of these guidelines. The present study provides additional evidence in support of the recommendation of regular practice of physical activity in individuals with asthma, a recommendation that could also have benefits for many other health outcomes (1
). The dose (duration, frequency, intensity, or type) of physical activity to be recommended cannot be derived from a single study. Our results suggest an inverse and linear relationship for asthma exacerbations, as has been found for all-cause mortality or total cardiovascular disease (28
). However, the evaluation of dose–response relationships between physical activity and health outcomes is a complex issue that has not yet been solved for diseases such as cancer, hypertension, or hypercholesterolemia (28
). It is especially worth mentioning that fear of an exercise-induced asthma attack may lead to the avoidance of physical activities by individuals with asthma. Unfortunately, our data did not include information about exercise-induced asthma exacerbations, thus not allowing us to study the effects of physical activity in this specific subgroup of patients. However, despite the potential difference in the physiological response to a strenuous exercise challenge in comparison with the regular practice of moderate physical activity, our results show that physical activity even of high intensity appears to have pulmonary benefits.
Potential limitations of the current study include likely misclassification of the disease, the outcome, or the exposure; selection bias; and residual confounding. The asthma diagnosis was self-reported by registered nurses, and an earlier validation study confirmed a high percentage of confirmed diagnoses (95%) (15
). Nevertheless, we specifically excluded all subjects with a concomitant diagnosis of emphysema, chronic bronchitis, or COPD to minimize misclassification. Exacerbations were also self-reported, and therefore subject to potential misclassification. A key strength of studying registered nurses is that they have a higher level of education and demonstrated health interest, thus improving the quality of data obtained from questionnaires. Differences in the effect of physical activity among the several types of exacerbation were not addressed because of the reduction in statistical power and potential misclassification. The validity of self-reported physical activity by the NHS questionnaire had been previously assessed against activity diaries with correlation coefficients ranging from 0.41 to 0.79 (16
). Indeed, the NHS questionnaire has been used to study the effects of regular physical activity on several health outcomes (29
). Overall, nondifferential misclassification of the disease, the outcome, or the exposure in this prospective study would bias the estimates toward a lack of association between physical activity and exacerbations, thus underestimating the true association. Although women who died during follow-up or did not fill out the follow-up questionnaire (year 2000) had reported a lower level of physical activity at baseline (year 1998), the inclusion of respiratory deaths in the exacerbated group did not yield differences in the association between physical activity and asthma exacerbation, thus suggesting that survival bias did not play a relevant role in the present analysis. Finally, a certain degree of residual confounding cannot be ruled out, specifically due to misperceptions in the asthma severity.
In summary, we found that regular physical activity was associated with reduced risk of asthma exacerbations in a well-characterized cohort of older U.S. women. This finding helps to address ongoing uncertainty about the respiratory benefits of physical activity, in comparison with the abundant literature on the health benefits of exercise for other chronic diseases (1
). The results support the inclusion of physical activity advice in asthma guidelines as well as the promotion of research about the effects of exercise training programs on the clinical course of asthma.