Discussion
This study is the first to assess the prevalence of overweight and obesity in a large population-based sample of schoolchildren aged 9–11 years and living in Metropolitan France, and the association of high BMI with allergic diseases (asthma, AR and eczema), SPT positivity and EIA. High BMI in children was positively associated with lifetime asthma in children with no wheezing symptoms ever. Furthermore, positive associations were also found in wheezing children between high BMI and lifetime and past-year AR.
The association between high BMI and lifetime asthma in non-wheezing children is consistent with previous studies.20–24 In addition, obesity and overweight as assessed by waist circumference, waist-to-height ratio and BMI were found to be associated with a diagnosis of asthma in children aged 5–11 years.25 Therefore, children with lifetime asthma, but without current wheezing, might have a high BMI because of insufficient physical activity, although this hypothesis cannot be ascertained since we did not collect data on physical activity.
The positive associations between high BMI and AR in wheezing children, thus atopic children, are inconsistent with previous studies that found no association between overweight and obesity and AR.26 ,27 This discrepancy might be due to the differences in the prevalence of rhinitis in the populations studied and to the fluctuation in the size of our sample. Moreover, we considered wheezing children with AR as allergic and not as asthmatic,28 unlike other authors.29 ,30
The absence of a significant association between high BMI and SPT positivity is in accordance with results from the National Health and Nutrition Examination Study III.9 In addition, the absence of a significant association between high BMI and EIA is consistent with data from seven epidemiological studies performed in Australia on Caucasian children and a cohort study conducted on asthmatic adults in Korea.31 ,32
Several of our findings about the risk factors associated with allergic diseases have already been demonstrated in other studies: the association of gender with the development of asthma in children is in agreement with other studies showing that male sex is a risk factor for respiratory symptoms in childhood,33 ,34 especially wheezing, which was found to be more prevalent in overweight children, especially boys.31 ,35 Furthermore, the inverse association between fruit consumption and allergies is consistent with previous studies concluding in the protective effect of fruits and antioxidants against allergies in children.36 ,37 There were also differences between children from the north and south of France. Therefore, children living in the south of France were more affected by atopy and EIA than those living in the north. This is consistent with a study conducted in children in China living in different geographical areas.38 These disparities might be due to differences in lifestyle and environment between residential areas.39 ,40 Moreover, passive smoking was positively associated with lifetime asthma in non-wheezing children, which is in accordance with several studies that have also shown the risks of passive smoking on respiratory health in children.41
The strengths of the current study include the large number of participants, its multicentre design and the detailed health outcome assessment including information on atopic sensitisation assessed by SPT which was performed in a large number of children aged 9–11 years. Furthermore, the use of an internationally validated questionnaire, filled out by the parents of the children who are very likely the people who are most aware of their children's health and lifestyle, and indicators to evaluate respiratory manifestations constitute strengths.15 ,42 ,43
Limitations of the study
The cross-sectional design is a major limitation since the same biases may arise as found in all observational studies, such as a recall bias and not being able to demonstrate causal relationships that could have affected the results. In addition, the time factor should be taken into account: this survey was conducted 14 years ago at a time when the epidemiological situation regarding allergic diseases and obesity status varied greatly. Therefore, these retrospective results need to be confirmed by future prospective studies and/or interventional trials. Furthermore, physical activity status was not assessed owing to the lack of information about it and the difficulty of assessing it in epidemiology. However, the multivariate analysis decreased the probability of confounding and an effort was made to correct for the following potential confounders: sex, passive smoking, parental education, parental ethnic origins, breastfeeding, day care outside the home and family history of allergic diseases.44–47 An underestimation of asthma and wheezing prevalence might also affect our results. Asthma and wheezing were reported subjectively by parents, without a doctor's diagnosis, as well as the identification of tobacco use and smoking. Furthermore, the prevalence of atopic dermatitis may have been overestimated in this study compared to other parts of Europe,48 ,49 owing to the subjective nature of reporting by parents.50 However, the internationally validated indicators we used to evaluate respiratory symptoms decrease the risk of having a differential bias.15 ,42 ,43