This analysis from a representative survey of U.S. children shows that being overweight or obese is associated with an increased likelihood of reporting current asthma. The association with BMI was uniformly stronger for the obese category than for the overweight category. We also demonstrated that the continuous BMI z-score was associated in a linear fashion with the odds of reporting asthma. In addition we were able to objectively classify subjects in NHANES 2005-2006 as atopic or non-atopic based on allergen-specific IgE tests. This analysis revealed that the association of overweight and obesity with asthma was stronger among the non-atopic children.
These findings differ from those in one study using NHANES III data, which did not find a difference in the association between obesity and asthma between atopic and non-atopic children.(10
) That study differs from the present analysis in the definition of obesity (quartiles vs. the AMA-recommended ranges) and in the manner of assessing atopy (skin prick tests vs. specific IgE tests). The earlier study did find a strong relationship between obesity and asthma, especially for highest quartile of obesity, but reported that there was no effect modification by atopic status. Stratified data were not presented, however, and lack of effect modification was not defined. Another study of NHANES III data, using a slightly different age range and continuous percentiles of BMI, found that children with non-atopic asthma (but not atopic asthma) had higher BMIs than non-asthmatic children.(21
Our findings are similar to studies in adults that found obesity to be more strongly related to non-atopic than to atopic asthma. In an Australian study of 4,060 men and women, an increased risk for asthma was found for those with central obesity (measured by waist circumference and waist/hip ratio) who were non-atopic.(8
) In a cross-sectional survey of more than 86,000 Canadian adults, overweight was associated with asthma in both allergic and non-allergic women, but the odds ratio was stronger in non-allergic women, whereas in men only the relationship in non-allergic men was significant.(9
Several previous studies have found the effect of obesity on asthma to differ by gender in children. Gilliland et al. found a significant relationship between overweight and asthma among boys, but not girls,(22
) while Gold et al. found a relationship only among girls.(23
) Castro-Rodriguez et al. reported from the Tucson Chidren’s Respiratory Study that obesity among girls at age 11 was associated with frequent wheeze at age 13.(24
) The authors suggest that these gender differences may be due to female hormones. Indeed, the strongest effects of obesity on asthma were among girls that started puberty before age 11, compared to those with later pubertal development.(24
) Varraso et al. also found that BMI was more strongly related to asthma severity among girls with early menarche.(25
) The present analyses, with a large and representative sample of children and adolescents, did not find evidence of effect measure modification by gender.
Previous studies suggest that systemic inflammation is a potential mechanism behind the observed relationship between obesity and asthma.(26
) Our results demonstrated a relationship between CRP levels and asthma, and we therefore examined whether CRP could be on the pathway between obesity and asthma. It is inappropriate to control for such an intermediate variable as a confounder,(28
) and mediation models and other models that attempt to partition effects into direct and indirect effects rely on assumptions that are unlikely to hold in our data (e.g. no unmeasured common causes of CRP levels and asthma).(29
) The relationship of CRP to asthma in non-atopic children, and the confounding of this relationship by BMI, suggests that overweight may indeed lead to systemic inflammation that in turn leads to an increased risk of asthma in non-atopic individuals. There was no evidence of a relationship between systemic inflammation and asthma among atopic youth. These findings suggest that allergic and systemic inflammation may operate independently on the pathway to asthma.
One potential limitation of these findings is that the measure of obesity used, BMI, is not a direct measure of adiposity and cannot differentiate between lean and fat mass. Nevertheless the AMA recommends the use of the CDC 2000 BMI-for-age percentiles as used in these analyses, considering them to have good sensitivity and specificity for identifying the children at greatest health risk.(16
) Good age-specific reference data for other indices of adiposity in children, such as waist circumference, do not exist.
Because these NHANES data come from a cross-sectional survey the associations seen may not be causal. Confounding of the CRP-asthma relationship by BMI could represent systemic inflammation on the causal pathway to asthma, or could indicate that obesity is a proximate cause of both inflammation and asthma. Unmeasured confounding could affect these relationships. In order to understand the true causal pathways that may underlie the relationships between adiposity, atopy, and the development and manifestation of asthma, it will be necessary to look at these relationships prospectively.
We recently found childhood obesity to be associated with atopy,(13
) which seems at odds with the current finding of no relationship with asthma among atopic children. However, there was at least some evidence of an elevation of risk of asthma in these children (OR=1.34). This may mean that obesity may indeed be a risk factor for atopy, but once atopy is established, it overshadows the effect of weight. Again, because these data come from a cross-sectional survey, it is not possible to test this time sequence in this dataset
The NHANES is a large national dataset that uses standardized data collection procedures, contains an abundance of information regarding every study subject, and is generalizable to the U.S. population. Using an objective measure of atopy, we found that excess weight in children appears to be associated with higher rates of asthma in children, especially asthma that is not accompanied by allergic disease. Current efforts to decrease overweight and obesity among U.S. children could potentially have the added benefit of decreasing asthma as well.