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INSERM U472-IFR 69, Epidemiology and Biostatistics, Villejuif
Grenoble CHU, Grenoble, France
ABSTRACT
Asthma severity in relation to body mass index (BMI) has rarely been studied. The relation between BMI and asthma severity was studied by sex in 366 adults with asthma from the Epidemiological Study on the Genetics and Environment of Asthma, a case-control and family study on asthma. Factors related to asthma severity and BMI such as smoking, FEV1, bronchial hyperresponsiveness, and dyspnea were taken into account. The influence of early menarche was studied to assess the potential role of hormonal factors. Clinical asthma severity in the last 12 months was assessed by a score (0eC7) based on the frequency of asthma attacks, persisting symptoms between attacks, and hospitalization. Asthma severity, which was unrelated to sex, increased with BMI in women (p = 0.0001) but not in men (p = 0.3). In women, the association remained after adjustment for age, FEV1, smoking habits, and BMI-adjusted dyspnea and taking into account familial dependence (p = 0.0001). The association between BMI and severity was stronger in women with early menarche than in women without early menarche (p interaction = 0.02). Findings support the hypothesis of hormonal factors involved in the severity of asthma.
Key Words: asthma severity body mass index menarche sex
A significant and concomitant increase in the prevalence of both asthma and obesity has occurred worldwide. Association of asthma incidence with body mass index (BMI) and weight gain has been reported in women (1eC4) but not in men in most studies (2, 4), but not in all (5eC7). Decrease in physical activity because of asthma does not seem to explain the association (2). It is well known that asthma incidence is greater in boys than in girls (8, 9). With the onset of puberty, asthma incidence and frequency of hospital admissions for asthma are higher among women compared with men and remain higher throughout the reproductive years (10). Sex differences may depend of hormonal factors (sex-related) as well as sex-related behaviors (11eC13). Early onset of puberty and obesity independently favor the persistence of asthma (14).
Women have more severe asthma than men (15), report more dyspnea (8), and have more bronchial hyperresponsiveness (BHR) (16), associations only partly understood. Obesity may directly affect the asthma phenotype by mechanical effects including airway latching and increasing in airway resistance and responsiveness to methacholine (17). It is known that the perception of airflow limitation is different according to sex and it has been suggested that obesity could be more related to dyspnea than to airflow limitation (18). Therefore, sex-related patterns of asthma may depend on interrelationships of BMI, lung function, BHR, and dyspnea. Factors related to asthma severity may be similar or different from those related to asthma incidence. Only one study reported an association of BMI with asthma severity in women, but not in men (19). Until now, no epidemiologic study has included hormone levels (leptin and sexual hormones) to establish their role in sex-related patterns of asthma and related phenotypes. The cumulative frequency of ovulatory menstrual cycles is a critical determinant of a hormone-dependent disease, such as breast cancer (20), and results on breast cancer provide a research frame regarding the role of hormone-related events for asthma.
The purpose of this study was to determine the relation between BMI and severity of asthma by sex, taking into account factors related to asthma severity and BMI such as smoking, FEV1, BHR, and dyspnea, and to investigate the potential role of hormonal factors by studying early menarche. Some of the results have been previously reported in the form of an abstract (21).
METHODS
The design of the Epidemiological study on the Genetics and Environment of Asthma (EGEA) combines a case-control study and a family study of asthma cases. The protocol and descriptive characteristics have been described elsewhere (22eC25) (see the online supplement). Of the 368 subjects without missing data on BMI and FEV1, 2 outliers with morbid obesity were excluded (see the online supplement). The analysis was performed on 366 adults (age, 16 years or older) with asthma (211 cases and 155 relatives of cases).
Subjects answered a questionnaire regarding respiratory symptoms, environment, and treatment (including inhaled and oral steroids) based on international standardized questionnaires (25). For relatives, asthma status was based on a positive answer to either "Have you ever had attacks of breathlessness at rest with wheezing" or "Have you ever had asthma attacks" Dyspnea was evaluated according to five-point grading scale. For all subjects with asthma, the severity was assessed on the basis of international guidelines (26), similarly as in the study of the familial resemblance of asthma severity (27). Three criteria of severity were assessed (see Table E1 in the online supplement): the clinical asthma severity score in the last 12 months, hospitalization for asthma during life, and the use of inhaled steroids in the last 12 months. The clinical score was the primary outcome and was evaluated as a continuous variable. It varied between 0 and 7 and was based on frequency of asthma attacks (from 0 for less than once a month to 3 for at least once a day), persisting symptoms between attacks (from 0 for none to 3 for limiting activities), and hospitalization in the past 12 months (0 for none, 1 for more than 0).
A three-class BHR score was defined to correspond to the cumulative dose of methacholine producing a decline of 20% or more in FEV1: none (dose greater than 4 mg), mild (dose greater than 0.25 mg but no more than 4 mg), and severe (dose of 0.25 mg or less). The analysis using BHR did not include 184 subjects corresponding to 77 with baseline FEV1 of 80% or less, 16 with postdiluent decline, 16 contraindications, 3 refusals, 9 technical problems, and 63 for whom the test was not completed to 4 mg despite a decrease in FEV1 to less than 20%. BMI (weight per height2) was calculated and classified according to sex-specific quintiles. Analyses were also performed with BMI as a continuous variable and the hypothesis of a U- or J-shaped relation was tested using quadratic models (BMI2). Early menarche was defined as menarche at 11 years or earlier. Irregular cycles were defined by at least 5 days of difference between two cycles.
Univariate relationships between variables were explored with 2 tests, correlation, and analysis of variance. Multivariate linear, logistic, and ordinal logistic regressions were also performed. As there is no simple cause-and-effect relationship between dyspnea and the clinical severity score, models have been run with residuals of dyspnea grades a priori adjusted on BMI in a linear regression model. All analyses were stratified by sex. Familial dependence between observations was taken into account by using generalized estimated equations (Genmod and Mixed procedures in the SAS statistical software package; SAS Institute, Cary, NC).
RESULTS
Population Characteristics
The subjects were on average 36.8 years old, and 42% of men and 60% of women were never-smokers. About 20% of the women had early menarche (Table 1) and 13% were menopaused. Age of menarche was negatively related with BMI (r = eC0.20, p = 0.009). No relation was found between irregular cycles and age at menarche. Age at the time of study was unrelated to early menarche (mean ± SD: 34.5 ± 13.7 versus 35.4 ± 12.1 years, p = 0.7, in women with and without early menarche, respectively).
Relationship of BMI to Potential Confounders
FEV1 % predicted and BHR were unrelated to BMI. Women had more severe BHR than men, but it was on the borderline of significance (p = 0.09) (Table 2), a relation similar after adjustment for age, BMI, smoking habits, and FEV1 % predicted (odds ratio [OR], 1.71; 95% confidence interval [95% CI], 0.87 to 3.36; p = 0.12).
Dyspnea was positively and linearly related to BMI after adjustment for sex (OR, 1.20; 95% CI, 1.11 to 1.28), a relation not modified after further adjustment for age, smoking habits, FEV1 % predicted, and BHR. Women reported significantly more dyspnea than men (OR, 2.24; 95% CI, 1.47 to 3.43) and in particular more severe dyspnea (Grade 4 or more: OR, 2.58; 95% CI, 1.32 to 5.02). Taking into account age, BMI, smoking habits, FEV1 % predicted, and BHR even increased the strength of the association between dyspnea and sex (OR, 3.71; 95% CI, 1.79 to 7.68). In men, as in women, dyspnea was strongly related to the asthma severity clinical score (r = 0.28, p = 0.0006 in men; r = 0.44, p < 0.0001 in women).
Univariate Relationships with Asthma Severity
Asthma severity was unrelated to sex when considering clinical score, inhaled steroids, or hospitalizations during life (Table 2). Of 81 subjects with asthma who reported any hospitalization during life, only 28 had one in the last 12 months. Women with early menarche had a more severe clinical score than the others (mean ± SD: 2.00 ± 2.27 versus 1.37 ± 1.51, respectively; p = 0.06). The clinical score was unrelated to age at menopause or irregular cycles.
Multivariate Relationships with Asthma Severity
The clinical asthma severity score was related to BMI in women, but not in men (Table 3). A borderline interaction was found between sex and body mass index when considering the clinical asthma severity score (p = 0.09), but it became significant after adjustment for age, smoking habits, FEV1 % predicted, and BMI-adjusted grade of dyspnea (p = 0.02). The association between BMI and clinical asthma severity score remained for women after taking into account age, smoking habits, and FEV1 % predicted, and no interaction was found between smoking habits and BMI when considering asthma severity in women. Further adjustment for BHR marginally decreased the association. To assess the relationship of BMI to severity unrelated to dyspnea, a model including dyspnea was run. Because of the strong correlation between BMI and dyspnea, the included indicator of dyspnea was BMI-adjusted dyspnea grade. After adjustment for BMI-adjusted dyspnea, BMI remained related to the clinical asthma severity score in women but not in men (p = 0.0001 and p = 0.7, respectively). Taking into account familial dependence led to similar figures and ordinal logistic regression models confirmed these findings (see the online supplement). The exclusion of menopaused women, and taking into account steroid use (inhaled or per os), gave the same results. Other aspects of severity (treatment with inhaled steroids and hospitalization) were unrelated to BMI.
Role of Early Menarche
Early menarche modified the relationship of BMI quintiles (Figure 1) to clinical score (p interaction = 0.02). Adding the two obese women (one without and one with early menarche) to those in the highest quintiles led to similar results, with a p value for trend to borderline significance in women without early menarche (p = 0.09) and significant in women with early menarche (p = 0.007; p interaction = 0.11).
Results were confirmed by considering BMI as a continuous variable. Clinical severity score increased with the increase in BMI among women with early menarche (r = 0.49, p = 0.005) and to a much lesser extent among those without early menarche (r = 0.20, p = 0.03; p interaction = 0.03). Including BMI2 in addition to BMI in the model even increased the interaction (p = 0.001).
DISCUSSION
Among the 366 subjects with asthma from the EGEA study, the severity of asthma as assessed by clinical score increased with BMI in women but not in men. In women, the association remained after adjustment for confounders. Furthermore, the association between BMI and asthma severity was stronger in women with early menarche than in women without early menarche. Results support the role of hormonal factors in the severity of asthma.
Findings extend the observations of the relationship between asthma and BMI, already found in children and adults (1eC7, 14, 17, 28eC30). In adults, the relationship has been consistently observed in women (1eC4, 7, 17, 28). Whereas no association has been observed in men in most studies (2, 4, 17, 28), it remains unclear whether there is any relationship between obesity and asthma in men. In children, positive associations have been reported between BMI and an increased risk of new-onset asthma (30). In adults, one study found a positive association between BMI and asthma in men from minority groups (7) and some have reported U-shaped associations (5, 6), a result consistent with the observed U-shaped association of BMI and BHR in men (31). The proportion of obese subjects as well as nutrition patterns were variable in previous studies (5, 6), with a survey from China with 95% of the subjects under 25 kg/m2 (5) and one from the United States with a high proportion of overweight and obese subjects (46.9% were more than 25 kg/m2) (6). We did not observe an association of BMI with BHR in our data, but our sample with information on BHR was limited. Conflicting results have been observed regarding the association of BHR with BMI, with a U-shaped curve (31) and a paradoxical association with decreased BHR with increased BMI in children (32) that disappeared after adjustment for FEV1 level. Consistent with our observations, no association of BMI with BHR, despite an association with high BMI with recent asthma, was observed in a study of 1,900 adults (33). A single study of 321 patients with asthma had already reported an association of BMI with asthma severity, assessed by the dose of inhaled steroids and exacerbation in the previous year (19). Women with severe asthma had a BMI greater by 1.6 kg/m2 than those with controlled asthma, whereas no association was observed in men. No association of obesity with health care use was observed in 572 patients with asthma admitted to emergency departments (34). In the EGEA study, no association between asthma severity and BMI was observed in men, even when testing a U-shaped relation by considering models with a quadratic term.
Our study has several limitations. The number of obese subjects was too small to represent obesity per se in the population, and thus precluded the provision of a definite answer regarding obese subjects. The lack of information about the dose of inhaled steroids prevented us from distinguishing between asthma severity and asthma control. Asthma clinical severity in the EGEA study has been standardized and already used to assess the role of various factors, and as evidence of its familial resemblance (27). Previously, we reported in the EGEA study that active smoking increased asthma severity (35). In the present analysis, smoking did not modify the association between BMI and asthma severity in women. The definition of asthma severity in epidemiology is difficult, an aspect that we have reviewed (36). To approach the new Global Initiative for Asthma guidelines, which include therapy, we have performed analyses adjusted for therapy and results are unchanged. Another limitation of the study is that few of the subjects with asthma had high severity scores, which limits the interpretation of the severity score, analyzed as a continuous variable. However, similar conclusions were obtained when using ordinal logistic regressions, which do not assume the hypothesis of linearity for the score. Finally, data should be interpreted with caution because of the cross-sectional nature of the study.
Dyspnea is a dimension of asthma severity and was strongly related to BMI. The basis of the five-grade dyspnea scale is the magnitude of physical activity needed to cause breathlessness (37). By taking into account residuals of grades of dyspnea adjusted on BMI instead of dyspnea, we were able to study the part of dyspnea not related to BMI, which may be considered a proxy for decreased physical activity. Our result is consistent with the hypothesis that the relation of asthma severity to obesity is not a reflection of a diminution of physical activity leading to weight gain, but studies with a more direct assessment of physical activity are warranted before making any conclusions on that point. Longitudinal studies, which indicate that obesity took place before asthma (1, 2, 4), also support the hypothesis that the association of asthma (or asthma severity) with BMI is not mediated by a decrease in physical activity. Dyspnea is also a key component of quality of life, in particular in asthma-specific scales. Of all respiratory symptoms (cough, phlegm, wheezing, and dyspnea), dyspnea is the typical female symptom for reasons still not fully understood, although its occurrence is probably due to more than just adiposity-related factors. Therefore the relationship of dyspnea to asthma severity in women may be different from that in men. It is therefore of interest that the association of BMI with the severity score observed only in women remained after adjusted for this symptom. BHR, like dyspnea, is more prevalent in women than in men (16). The sex difference for dyspnea was not explained by BHR, a result described here for the first time. Further studies in men and women with adjustments for BMI, quality-of-life scales, and BHR are necessary to disentangle the sex-specific factors of asthma severity.
Several arguments support a role for hormones in asthma and asthma severity in women. Decreased incidence at menopause (10) and variation in visits to emergency departments according to menstrual cycle (38) have been reported, although the effects of menstrual cycles on asthma are not well understood (39) and there is limited evidence of the association of hormone levels with asthma-related traits (40, 41).
In general, limited information is available on the relationship of hormone-related events to asthma, persistence or severity of asthma, and asthma-related traits. In the EGEA study, it was shown that total IgE and atopy decreased with menopause, and that eosinophils were related to perimenstrual asthma and more strongly related to persistent asthma in women than in men (42). Increase in body silhouettes since menarche was evidenced as a strong factor of asthma incidence in a large prospective cohort study of women (3). The strong association of BMI with asthma severity in women with early menarche supports the hypothesis of the role of hormonal factors. It is unlikely that the effect relates to some recall bias as women who reported early menarche were the same age as the others, but the small number of women with early menarche and the cross-sectional nature of the study are limitations. It is well known that BMI is related to rapid sexual maturation, increases estrogens, and leads to early menarche (43). In the longitudinal Tucson Children's Respiratory Study, obesity was shown to relate to the incidence of wheezing in girls with early menarche (29), and obesity and early onset of puberty were shown to be independent risk factors for persistence of asthma after the onset of puberty in both boys and girls (14). Early menarche is a strong factor in breast cancer, a hormone-related disease (20). Earlier onset of regular menstrual cycles is associated with early regular ovulatory menstrual cycles, a risk factor for the disease. Two explanations have been proposed: first, that it is related to a long duration of ovarian activity; and second, that it corresponds to a long-lasting effect of age at menarche on estrogen concentrations during adulthood (44). Estrogen and progesterone, at pregnancy levels, favor the development of a helper T cell Type 2 (Th2) profile, a necessary condition for a successful pregnancy. Mechanisms involved may include inhibition of helper T cell Type 1 (Th1) inflammatory response through modulation both of quantity (decreasing recruitment of the inflammatory cells) and quality by the establishment of a Th2 type, a feedback of the effect of estrogen on the function of antigen-presenting cells (45) and a decrease in apoptosis of Th2 cells by progesterone (46). Limited information is available regarding the potential effect of physiological levels of hormones on Th1/Th2 balance.
Besides the role of estrogens, leptin could play a key related role. Among the numerous proteins associated with obesity, leptin plays the central role. Leptin is a 146-amino acid protein, encoded by the obesity (ob) gene and a member of the interleukin-6 family of cytokines (proinflammatory cytokines). Secreted by adipocytes, the circulating level of leptin correlates with the amount of body fat and BMI and is a permissive factor for the initiation of pubertal events in both boys and girls (47). The role of leptin in the association between BMI and asthma remains poorly understood. In a population-based study, FEV1 was shown to decrease with an increase in leptin levels in nonobese subjects, which could reflect the proinflammatory role of leptin (48), but no information about asthma status was available in that study. Recently, leptin has been shown to be a predictive factor for asthma in prepubertal children, especially in boys (49). Studies on the relationships of lung function, asthma, and asthma severity with sexual hormones and leptin are warranted.
In summary, we found that severity of asthma increased with BMI only in women and that this relationship was stronger among women with early menarche. These data suggest that factors of asthma severity are sex dependent and that hormonal factors could be involved in the severity of asthma.
This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org
Epidemiological Study on the Genetics and Environment of Asthma Cooperative Group: Respiratory epidemiology—INSERM U472, Villejuif: I. Annesi-Maesano, F. Kauffmann (coordinator), and M. P. Oryszczyn; INSERM U408, Paris: M. Korobaeff and F. Neukirch; Genetics—INSERM EMI 00-06, Evry: F. Demenais; INSERM U535, Villejuif: M. H. Dizier; INSERM U393, Paris: J. Feingold; CNG, Evry: M. Lathrop; Clinical centers—Grenoble: I. Pin and C. Pison; Lyon: D. Ecochard (deceased), F. Gormand, and Y. Pacheco; Marseille: D. Charpin and D. Vervloet; Montpellier: J. Bousquet; Paris Cochin: A. Lockhart and R. Matran (now in Lille); Paris Necker: E. Paty and P. Scheinmann; Paris Trousseau: A. Grimfeld; Data management—INSERM ex-U155: J. Hochez; INSERM U472: N. Le Moual.
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