This cross-sectional study was aimed to investigate the influence of asthma definition on the asthma-obesity association. We adopted two very common diagnostic criteria for asthma: 1) symptoms associated with a positive methacholine challenge test; and 2) self-report of physician-diagnosed asthma. We found that the relationship between asthma and obesity in epidemiological studies depends on the criteria adopted. An association was detected between physician-made diagnosis of asthma and obesity, but not between BHR-confirmed asthma and obesity. These findings did not change for any of the variables used to evaluate obesity. In addition, female gender, schooling and smoking were associated with asthma defined by BHR.
One probable interpretation of this observation is the overdiagnosis of asthma in obese people. Because people who are obese are more likely than nonobese people to report dyspnea [23
] or asthma-like symptoms, they may be more likely to be misdiagnosed by their physicians as having asthma. Thirty-seven percent of non-asthmatic obese women reported a higher degree of dyspnea during moderate exercise. This exercise-induced breathlessness seemed to be consequence of an elevated oxygen cost of breathing and may explain the overdiagnosis of asthma in obese patients [24
]. Obesity is also associated with mechanical respiratory disorders regarding functional residual capacity and tidal volume that could cause respiratory symptoms [25
Aaron et al. [27
] conducted a study to determine the proportion of obese Canadian adults who had an incorrect diagnosis of asthma. They found that one-third of the patients who had received a diagnosis of asthma by a physician had no evidence of asthma when their medication was tapered and when they underwent spirometry and bronchial challenge tests. In a recent study [28
], among 304 adults who reported physician-diagnosed asthma recruited directly from the community, 83 (27%) had negative methacholine challenges. Authors suggested that there was an incorrect diagnosis of asthma in many of these subjects. Further studies are necessary to evaluate the impact of respiratory symptoms on the overdiagnosis of asthma among obese people in different settings and countries.
Our results could also be interpreted as a consequence of the time interval between past symptoms and study measurements. Natural history of asthma shows variations of asthma severity in a same individual. Data on this subject indicate that a high proportion (up to 50%) of asthmatics experience a clinically defined remission from childhood to adulthood [29
]. Some of the volunteers in our study may have received an appropriate diagnosis and went into remission before our evaluation. Although such a remission could present with normalized lung function, several studies have documented persisting bronchial hyperresponsiveness in subjects in remission of asthma [30
]. Since BHR-confirmed asthma was based on both symptom and BHR, currently symptom-free patients with BHR would not be considered to have asthma in our protocol.
If the obesity influence on asthma is really dependent on the asthma definition, the evaluation of asthma components separately will reveal different associations with obesity. In fact, respiratory symptoms are clearly affected by obesity in most published studies [25
], whereas BHR and exercise-induced bronchospasm have not been showed to be under the influence of obesity or weight loss in previous studies [41
Schachter et al. reported an analysis of cross sectional data in 1971 Australian adults. Despite the fact that severe obesity was associated with a higher prevalence of wheeze, obese subjects did not have any increase in bronchial responsiveness to histamine [45
]. Aaron et al. analyzed spirometry and BHR of 58 obese asthmatic women before and after an intensive 6-month weight loss program. The results showed that FEV1
and FVC (forced vital capacity) increased in obese women who lost a significant amount of weight and did not improve in those who failed to lose weight. Findings did not demonstrate any significant effect of weight loss on bronchial responsiveness. The authors suggested that improvements in FEV1
and FVC occur due to a reduction in massloading on the respiratory system, rather than improvements in asthma per se [46
]. A recent article reported on an analysis of cross-sectional data from 717 Korean adolescents, Yoo et al. tested the effect of increased BMI on the prevalence of allergic diseases, atopy, BHR, and biomarkers of allergic inflammation (eosinophilic count and serum IgE levels). Overweight (BMI > 85th
percentile) was associated with an increased prevalence of wheezing and atopy, but it was not associated with augmented bronchial responsiveness, except among girls [47
]. An increase in serum inflammatory markers (including white blood cell counts) has been reported in this and in some case–control studies of obese children [47
]. Then, authors postulated that “BHR seems to represent a different pathway in the association between obesity and asthma” [47
]. Several of other studies tested the association between obesity and BHR in patients with asthma; they have not found significant association [50
Airway inflammation is another defining characteristic of asthma [58
]. The effect of obesity on asthma inflammation has also been studied by many authors. Although inflammatory pathways are thought to play a role, obesity is not associated with increased eosinophilic inflammation and there is not enough evidence in humans that systemic inflammation, heightened in obesity, regulates asthmatic airway response or symptoms in the obese. Also, the importance of non-eosinophilic airway inflammation needs better understanding. [13
There is little information about the trends of asthma in Latin America, although the prevalence of asthma symptoms has been shown to be high. Obesity has markedly increased in Latin America [61
]. A recent study with a protocol similar to the one used in our study looked at the association of asthma and obesity in Chile [51
]. In that study the authors found a high prevalence of obesity, especially in women, demonstrating that obesity is also a problem in some of the middle industrialized countries. There was a positive association between BMI and asthma symptoms. However, waist circumference was not associated with asthma symptoms and both BMI and waist circumference were negatively associated with BHR. Similar to our study, BHR did not explain the association between asthma and obesity.
BHR and respiratory symptoms, used herein to define asthma, may be found in other diseases than asthma, like chronic obstructive pulmonary disease (COPD). However, since the subjects in our study were young (ranging from 23 to 25 years old) and smoking prevalence was low in this cohort [62
], COPD does not seem to be a confounding diagnosis.
The increasing prevalence of asthma is still a matter of preoccupation worldwide [63
]. The need for explanation and prophylaxis makes our study relevant to contribute to the understanding of this phenomenon. It is interesting to know the role of obesity as a possible factor increasing asthma prevalence. Defining the component of asthma in which obesity has significant influence will improve the search for mechanisms of the asthma-obesity association. Our observations should lead to future studies to distinctly look for the effects of obesity upon specific characteristics of respiratory diseases, for instance, respiratory symptoms, airway inflammation, lung function and bronchial responsiveness.