Background
The prevalence of obesity has increased rapidly worldwide, particularly in low and middle income countries like China [
1]. A number of diseases are related to obesity [
2,
3]. There is a current debate on the association between obesity and atopic diseases in adults. It is still not clear if the recent epidemic of obesity has contributed to the rise in the incidence of atopic diseases. Some studies showed a positive association [
4‐
6] while others showed no association between obesity and atopic diseases in adults [
7,
8]. The incidence rates of atopic diseases, including atopic dermatitis, allergic rhinitis, allergic asthma, and food allergy, have increased in China recently [
9]. Understanding the associations of obesity with those atopic diseases has important public health implications to elucidate the causal link and the importance of weight control in preventing atopic diseases.
Common allergens vary among different geographic regions and cultures [
10], while the manifestations of atopy also vary among different populations [
11‐
13]. Those variations may have contributed to the inconsistent findings in different study populations. The findings from other populations may not apply to Chinese adults. As obesity has become a major health concern in China, its associations with atopic diseases in this population are still not clear. In this individually matched case–control study of Chinese adults, we assessed the associations of obesity with four atopic diseases.
Discussion
In this matched case–control study, we found that obesity was significantly associated with atopic diseases in Chinese adults. Specifically, obesity was significantly associated with atopic dermatitis and atopic rhinitis, but the association between obesity and atopic food allergy was weak and not statistically significant.
In the current debate on the association between obesity and atopy in adults, some studies support the presence of a positive association [
4‐
6]. In a study of 1997 Canadian adults, Chen et al. found a significant association between obesity and atopy with an adjusted odds ratio 1.33 (1.04, 1.71). In another study of 2090 American adults, Silverberg et al. reported positive associations of obesity with atopic dermatitis and atopic asthma [
5]. On the other hand, several studies have failed to support the association between obesity and atopy in adults [
7,
8]. The data from Germany suggest no association between obesity and atopy (OR = 1.03, 95% CI: 0.70, 1.50) [
7]. The data from Australian adults showed no association between BMI and atopy [
26]. A multicentre cross-sectional survey of young adults in Europe showed that there was a positive association between high BMI and the risk of asthma attacks in women but there was no association between BMI and sensitization to any of allergens tested in the study [
8]. Leung et al. showed that obesity was not associated with atopy in Chinese children [
27]. Little data are available from Chinese adults. In our study, we examined the associations of obesity with four atopic diseases and our findings confirmed significant associations of obesity with atopic dermatitis and atopic rhinitis in Chinese adults.
The association between obesity and asthma was inconclusive. The sample size for assessing the association between obesity and atopic asthma in our study was too small to provide a conclusive association, as indicated by the wide 95% confidence intervals even though the point estimate of OR was as high as those for atopic dermatitis and atopic rhinitis. Our data do not support an association between obesity and food allergy.
Although our findings support a positive association between obesity and atopic diseases such as atopic dermatitis and atopic rhinitis, we are unable to establish a cause-effect relationship. Due to the nature of case–control design, all cases in this study had previously diagnosed symptomatic atopic diseases which were confirmed by positive allergen-specific IgE tests during the study period. It is possible that the observed association was due to the fact that atopic diseases increased the risk of obesity. Those with atopic dermatitis or atopic rhinitis might be more likely to experience weight gain because of restricted physical activities, increased energy intake and side effects of some treatments. Nevertheless, the positive and significant association between obesity and atopic diseases has important public health implications for control and management of both atopic diseases and obesity related chronic diseases. Our findings warrant further investigation on the causal relationship between obesity and atopic diseases and the effect of weight reduction on preventing atopic dermatitis and rhinitis in adults. As there is an increasing trend in both obesity and atopic disease in China [
1,
9], further understanding the underlying mechanism is important for planning intervention strategies for both obesity and atopic disorders.
There are several strengths in this study. First, all cases and controls were confirmed by allergen-specific tests to sixteen common allergens in the region to minimise potential misclassification. Second, we used an individually matched design according to multiple factors of age, sex and residential regions to improve the comparability between cases and controls. Third, data on weight and height of both cases and controls were obtained through direct physical measurements which should be more accurate and reliable than self-reported values.
There are some limitations in this study. First, because BMI was used to define obesity, we were not able to distinguish central obesity from peripheral obesity in this study. Nevertheless, several studies have consistently documented strong correlations of BMI with waist circumference and the body fat measured by dual-energy X-ray absorptiometry [
28,
29]. Second, the sample size was relatively small for some atopic disorders. For example, we had only 27 cases and 54 controls for assessing the association between obesity and atopic asthma. It is possible that obesity is truly associated with atopic asthma but our data had a low statistical power to detect such an association. Therefore, further studies are needed to investigate the association between obesity and asthma. Also, even though we were able to establish a positive association for some atopic disorders, due to the small sample sizes, the effect estimates were imprecise as reflected by the wide 95% confidence intervals. Further research with a large sample size is needed to more accurately quantify the association between obesity and atopic disorders in Chinese adults. Third, although we confirmed the associations of obesity with two atopic diseases (atopic dermatitis and rhinitis), we were not be able to establish the time sequence of which conditions had occurred first. Nevertheless, our findings of the associations of obesity with atopic diseases warrant further cohort studies in this area.
Competing interest
The authors declare that they have no competing interests.
Authors’ contributions
XL, ML and ZW conceived the study idea and participated in its design. XL, JX, XD, FC, ML and JS participated in the study design and conduct of data collection. XL, ML and ZW carried out statistical analysis. All authors contributed to the writing of the manuscript and critically reviewed the final version submitted for publication. All authors read and approved the final manuscript.