The epidemiology of adult-onset asthma is largely unknown. However, recent observations indicate a possible association between B2-adrenoceptor regulation and the disorder. There is an association between high BMI and asthma, and this association is most marked among sedentary women. Finally, it has been documented that those who possess the Cly16 allele have more severe adult-onset asthma symptoms (i.e., greater nocturnal symptoms, greater-than-expected frequency of steroid-dependent asthma, poorer response to therapy, and increased atopy).
The significance of recognizing patients with the Gly16 allele is that this subgroup of patients is distinctly different from the subpopulation that has exercise-induced asthma, and hence forms a separate subpopulation that might be at risk of asthma associated with a lack of physical activity, particularly among older subjects. Remarkably, women with the Glyl 6 genotype were found to have no increased risk of adult-onset asthma if they were physically active (walking at a brisk pace for 1 hr/ wk). We speculate that physical activity may act as primary prevention of adult asthma in patients with the Glyl 6 allele.
McFadden and Gilbert hypothesized that at low levels of physical activity, mechanisms related to lack of deep inspirations could promote bronchospasm. More studies are needed to better document the intensity and duration of physical activity needed to produce sufficient deep inspirations to inhibit bronchospasm. Presently, very little is known about the cellular and molecular mechanisms that are affected by physical activity to produce this dramatic change. Studies on how exercise regulates gene expression of the B2 adrenergic receptor and the interplay with the Glyl 6 allele are potentially fruitful avenues for further research.