Invited Commentary: Obesity and Asthma—New Perspectives, Research Needs, and Implications for Control Programs

Stephen C. Redd1 and Ali H. Mokdad2

1 Air Pollution and Respiratory Health Branch, Division of Environmental Hazards and Health Effects, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA.
2 Assessment Branch, Data Management Division, National Immunization Program, Centers for Disease Control and Prevention, Atlanta, GA.


    INTRODUCTION
 TOP
 INTRODUCTION
 REFERENCES
 
The prevalences of obesity and of asthma in the United States have increased sharply in recent years (1Go, 2Go). Furthermore, obesity and asthma are responsible for a tremendous economic burden (3Go, 4Go). Although obesity increases the risk of heart disease, diabetes mellitus, and hypertension, and it affects quality of life (5Go), its possible association with asthma has been documented only recently, in both cross-sectional studies (6GoGo–8Go) and a few prospective studies (9GoGo–11Go). In the context of a paper in the current issue of the Journal (12Go), this commentary addresses the status of overweight and obesity as possible causes of asthma.

Studies designed to determine whether being overweight causes asthma must confront such issues as ensuring that identified cases of asthma were truly incident cases, understanding the relation of obesity onset with asthma onset, and, pending additional confirmation that being overweight predisposes to asthma, determining whether other risk factors for asthma, such as specific allergies, are synergistic or unrelated.

The study by Chen et al. in this issue of the Journal (12Go) extends observations of previous studies to compare the different risks for adult men and women that being overweight contributes to developing incident asthma. This study is based on a 2-year follow-up of 4,266 men and 4,883 women participating in Canadian National Population Health Surveys. During the 2-year observation period, 1.6 percent of the men and 2.9 percent of the women developed asthma. Obesity was associated with the development of asthma in women but not in men.

When interpreting the results of Chen et al.'s study (12Go), readers should consider several issues. The first concerns the case definition for incident asthma. The current study identified incident asthma cases by using a two-step process. In the first round of interviews, respondents answered a single question about whether they had asthma that had been diagnosed by a health professional. Asthma cases so identified in the first interview were ineligible to be incident asthma cases; among the remainder, incident asthma was defined as occurring in persons who answered the same question affirmatively in the second interview.

The problem of defining asthma cases has challenged asthma epidemiologists for decades (13Go). Although groups in the United States have developed definitions of asthma for clinical or surveillance purposes (14Go, 15Go), these definitions do not fully address the need for a definition of asthma in a follow-up study such as the one published in this issue of the Journal (12Go). The definition developed and adopted by the Council of State and Territorial Epidemiologists classifies cases according to the certainty of diagnosis (14Go). In the current study, cases met the Council of State and Territorial Epidemiologists' definition for probable asthma, implying a greater likelihood for misclassification than if they had met the definition for confirmed asthma. Confirmed cases of asthma require an objective measure of reversible airways obstruction. The definition that the National Asthma Education and Prevention Program developed requires the exclusion of alternative diagnoses, and observation of symptoms and measurement of lung function over a several-week period are suggested to aid in the diagnosis of difficult cases (15Go). With the data collected in the Canadian survey, the extent to which clinicians confirmed cases with objective measures or excluded alternative diagnoses is impossible to gauge. The definition of incident cases poses additional challenges. In the study by Camargo et al. (11Go), study subjects with potentially new-onset asthma were recontacted and asked specifically about symptom onset and medication use.

The ability to distinguish true incident cases from cases that were quiescent for years or decades is particularly difficult and important. Better knowledge of the natural history of asthma is needed to determine whether adults with new-onset asthma actually have incident asthma rather than cases of the disease that have relapsed after an extended symptom-free period. Adults who had asthma as children but in whom asthma resolved might be at especially high risk of obesity-associated asthma compared with adults who never had asthma. The approach that the authors of the current study (12Go) took to identify incident asthma cases raises the possibility of misclassification and limits any conclusions about the natural history of obesity and asthma.

The second issue is the short duration of follow-up and the relatively high incidence rate. Some of the incident cases may have been prevalent cases at baseline. One way to have addressed this concern would have been to extend the follow-up period and exclude the cases identified in the first 2 years. In Camargo et al.'s study (11Go), differences were minimal in the strength of the association between obesity and asthma during the first and second 2-year follow-up periods of the total 4-year follow-up. Whether this finding would hold true in the current study (12Go) cannot be determined. The high incidence rate also suggests that respondents may have given affirmative answers to the question about asthma at the first interview only when the disease was active, thereby underestimating prevalent, but asymptomatic asthma cases.

The third issue is the effect of adjustment for variables associated with obesity and asthma. Two factors—dietary intake and hormone replacement therapy in women—that might be related to obesity and asthma were not adjusted for in the current study (12Go). The definitions used to classify smoking status may have caused misclassification between former and current smokers. Moreover, the limited data available to classify physical activity and alcohol consumption restricted the authors' ability to determine the extent to which these behaviors could have explained the association.

The fourth issue in the current study (12Go) is, compared with men, the elevated risk of asthma for all women, regardless of body mass index category (normal, overweight, and obese). Moreover, women who lost weight or gained weight had a higher risk of asthma. In fact, in the analysis of asthma incidence by weight change, women who lost 2 kg or more had the highest incidence. These findings suggest a more complex relation between weight change and asthma in women than others have reported. Alternatively, weight loss may be a marker for some other medical problem that could lead to accessing medical care and an increased likelihood of an asthma diagnosis.

This report (12Go) contributes to the expanding literature associating being overweight or obese with the risk of asthma, particularly the effect of sex on the association. Additional study is needed to confirm the association between body mass index and asthma. In the authors' study, better defining incident cases and ensuring that questionnaire responses indicated true incident asthma cases, having better data on other possible risk factors for asthma, and including other data that might help explain the association (such as on hormone replacement therapy and allergies) would all have been important. With the evidence of an association between obesity and asthma in this study and others, clinical trials of weight reduction interventions should be considered. Such trials should assess the efficacy of weight reduction to reduce the incidence or severity of asthma or to cure the disease. Ongoing clinical trials of weight reduction strategies for other illnesses could perhaps be modified to include asthma outcomes.

From a population standpoint, the implications of these studies are substantial. If asthma were added to the list of conditions related to obesity, then reducing the prevalence of obesity could be expected to produce even greater public health benefits than are estimated currently. In addition, evidence that weight loss benefits persons with asthma would add to the medical and environmental modification tools available to manage asthma. Compliance with programs to achieve weight loss might be greater among overweight or obese persons with asthma because asthma symptoms might serve as reminders of the need for persistence in the program. However, it is too early to incorporate weight reduction strategies in efforts to reduce the health and economic burden of asthma.

The broad national effort targeting asthma should continue to emphasize implementation of proven programs, improved systems to track the disease, and research to improve the tools to manage and prevent it (16Go). Just as a national agenda for asthma control has been developed, so has the time come to develop a national, comprehensive plan to prevent and treat the obesity epidemic (17Go). The goals of such a program would be to prevent further weight gain in persons of normal weight or who are overweight, appropriately reduce body weight among obese and overweight persons, and maintain a lower body weight over the long term. Such intervention programs should not be delayed by efforts to determine the exact relation between asthma and obesity.


    NOTES
 
Reprint requests to Dr. Stephen C. Redd, National Center for Environmental Health, Centers for Disease Control and Prevention, 1600 Clifton Road, Mailstop E-17, Atlanta, GA 30333 (e-mail: scr1{at}cdc.gov).


    REFERENCES
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Received for publication September 27, 2001. Accepted for publication October 17, 2001.


Related articles in Am. J. Epidemiol.:

Obesity May Increase the Incidence of Asthma in Women but Not in Men: Longitudinal Observations from the Canadian National Population Health Surveys
Yue Chen, Robert Dales, Mei Tang, and Daniel Krewski
Am. J. Epidemiol. 2002 155: 191-197. [Abstract] [FREE Full Text]