Change in lung volume in asthma with particular reference to obesity

Over the last 20 years both asthma and obesity have increased in prevalence. What is the link? There are data to suggest that increasing obesity is a risk for the increase in prevalence of asthma. A number of mechanisms have been postulated including the effects of reduced lung volume on bronchial reactivity and mechanical changes with lower lung volumes. Other possibilities include other obesity-induced co-morbidities including gastro-oesophageal reflux. The aim of this thesis was to evaluate the link between asthma and obesity in both adult and childhood populations and to undertake experimental studies to examine the effects of changes in lung volume on bronchial reactivity. In chapter 1, the literature is reviewed. The current literature suggests that there is a link between diagnosis of asthma, new onset of asthma, symptoms of shortness of breath and wheeze. In chapter 2, data on 1997 adults in 3 population studies were analysed and the association between body mass index (BMI) and symptoms of shortness of breath and wheeze, diagnosis of asthma, medication usage for asthma, lung function and bronchial responsiveness were studied. This study showed that obesity was a risk for recent asthma (OR 2.04; 95%CI 1.02-3.76, p=0.048), symptoms of shortness of breath and wheeze (OR 2.6; 95%CI 1.46- 4.70, p=0.001), and medication usage for asthma (OR 2.53; 95%CI 1.36-4.70, p=0.003). There was a reduction in lung volume as measured by forced vital capacity (FVC), but there was no increase in bronchial hyperresponsiveness (BHR) (OR 0.87; 95% CI 0.35-2.21, p=0.78). Thus although the symptoms of asthma are increased there were no increases in BHR, despite significantly reduced lung volumes. The increase the medication usage is unlikely to have normalised the BHR, as there were ongoing symptoms suggestive of asthma. In chapter 3, data on 5993 children in 7 population studies were analysed and the association between BMI percentile and symptoms of cough, wheeze, ix diagnosis of asthma, medication usage for asthma, atopy, lung function and bronchial responsiveness was studied. After adjusting for atopy, sex, age, smoking and family history, BMI was a significant risk factor for wheeze ever (OR=1.06; 95%CI 1.01-1.10, p=0.008) and cough (OR=1.09; 95%CI 1.05-1.14, p=0.001) but not for recent asthma (OR=1.02; 95%CI 0.98-1.07 p=0.43), or bronchial hyperresponsiveness (OR=0.97; 95%CI 0.95-1.04 p=0.77). In girls, a higher BMI was significantly associated with higher prevalence of atopy (x2 trend 7.9, p=0.005), wheeze ever (x2 trend 10.4, p=0.001), and cough (x2 trend 12.3, p