Our results indicate that obese three-year old children living in urban environments are significantly more likely to have asthma compared with non-obese children; however, this association is not attributable to home social and environmental factors. Gender differences were noted in that, overweight boys had increased odds of asthma compared with normal weight boys, however this association was not noted among overweight girls.
The association between asthma and obesity could be explained by one condition leading to the other. The most accepted notion is that obesity could facilitate asthma. Possible mechanisms include mechanical alterations in the respiratory system, chronic systemic inflammation, alteration of energy-regulating hormones (e.g. leptin) or comorbid diseases. 39
It is also conceivable, however, that asthma could result in obesity, for example through low levels of physical activity. Given the cross-sectional design of most analyses conducted to date, including ours, it is hard to determine the direction of the association.
Alternatively, the association between asthma and obesity could be the result of risk factors shared by both conditions. One of the main goals of our analysis was to understand if shared risk factors between asthma and obesity present in the home could potentially explain the association between the two conditions. We found no evidence that the association between obesity and asthma is simply a result of shared social and physical characteristics of the home environment. Although other studies have investigated relevant shared risk factors before, such as maternal smoking 18
, we are unaware of any other studies which have included a wide range of potential home factors from IPV to child neglect. We also found no evidence that indicators of maternal asthma or obesity would explain the association between asthma and obesity among 3 year old children.
In this US sample of 3 year old children, gender differences in the association between asthma and obesity were not observed, however, gender differences in the association between being overweight and asthma were observed, but only among boys. Disagreement characterizes gender findings from prior studies which included children and pre-adolescents, with some studies reporting significant associations between obesity and asthma among both preschool boys and girls 5
, and others among school-aged girls only 7
or among preschool boys only. 6, 40
It is possible that differences in population and, in particular, age, contribute to the difference in results. Our results are consistent with the recent Australian study among preschool children (4–5 years of age) where a positive relationship between asthma and obesity among both boys and girls was noted.5
Other studies of preschool children in the US did not report an association between obese girls and asthma as we found in our study.6, 40
Our sample mainly comprised of children of low socioeconomic status with a higher prevalence of obesity than reported in other studies which could have contributed to the different findings.
Study limitations need to be noted. As is typical with longitudinal studies, there was a reduction in the sample available from the original cohort over time. Although there was a difference based on race/ethnicity between those who completed the 36-month assessment and those who did not, there were no differences based on maternal education, smoking status, low birthweight, or maternal IPV at baseline. Even though the Fragile Families Study included longitudinal information, the central study hypothesis (association between asthma and obesity) was tested cross-sectionally due mostly to incomplete data in the subsequent wave, which if used, would have restricted the sample size even more. By characterizing asthma as physician-diagnosed asthma by three years of age that has been active in the past year, we aimed to reduce the potential for misclassification of what is truly asthma. Our assumption is that children previously diagnosed with asthma without an attack in the past year may represent children with less severe underlying disease. Although we adjusted for a number of social and environmental factors not previously considered in the asthma and obesity association, it is possible the associations found could be partially attributed to unmeasured confounding. For example, even though we adjusted for housing features, we did not have information on presence of mold, indoor or outdoor air quality. However these factors are not directly related to obesity making it unlikely to be responsible for the associations noted. Lastly, there are characteristics of the neighborhood environment related to both asthma and obesity that were not considered in these analyses, such as neighborhood poverty and land use that should be considered in future studies 41–42
. Despite these limitations our study has a number of strengths. Our analyses are based on a national sample of children mainly of low socioeconomic status, with objectively measured height and weight. We explored a variety of home social and environmental factors not previously considered in the relationship between asthma and obesity, and furthermore many of the factors considered were measured repeatedly throughout follow-up (e.g., maternal depression, intimate partner violence).
In this study, we examined the physical and behavioral landscape of households to better understand salient factors surrounding sex differences in the co-occurrence between obesity and asthma early in life. Our findings suggest that the obesity-asthma association is not attributable to social or physical factors in the home environment however, given limitations with the study, these results should be interpreted with caution. Future studies should explore longitudinally the role of the home environment as a modifier of the obesity and asthma association to examine whether specific factors exacerbate the development of asthma among obese children. In addition, other social factors, such as the neighborhood environment, not considered in these analyses should be studied. Furthermore a more detailed evaluation of the role genetics (besides crude indicators of presence of maternal asthma or obesity) and gene-environment interactions play in this association should also be studied. Despite our findings that the social and physical environment does not explain the relationship between asthma and obesity in early childhood, these findings should not be used as evidence to discontinue intervention efforts that address children’s social and physical environment to improve children’s health, as the home social and environmental factors considered in this study have been previously related to the risk of asthma and obesity in children.