Our results suggest strong associations that differ by gender between BMI percentile and current asthma among Black and Hispanic children 2–11 years of age in Central Harlem. More specifically, a U-shaped curve was observed for boys and a linear trend was observed for girls when examining BMI percentile versus the probability of having asthma. There also appear to be smaller differences in these observed associations by age group and race/ethnicity. Among girls, the increased risk of asthma with greater BMI percentile was found among 6–11 year olds but not among 2–5 year olds. Among 6–11 year old children, Hispanic girls accounted for a greater proportion of the asthma cases among those at risk for overweight than Black, non-Hispanic girls, while Hispanic boys accounted for a greater proportion of the asthma cases among those who were overweight than Black, non-Hispanic boys. However, these sub-group analyses by age and race/ethnicity involved small numbers and should be interpreted with caution. Additionally, in order to gain a better understanding of the impact of race/ethnicity in our study, we would need more complete information on other factors such as barriers to health care access and quality of care, language spoken at home, and discrimination based on skin color.
Although recent studies of asthma and BMI percentiles among children have tended to focus solely on the upper range of BMI percentile, our findings are consistent with reports of a U-shaped association between asthma and BMI among men and a less dramatic (J-shaped) or linear association observed among women.27–29
A similar pattern has been observed among children 6–14 years of age.18
In addition, our analyses presented here utilized both a continuous measure of BMI percentile, which strengthened our statistical analysis, and the CDC-defined categories for underweight, normal weight, at risk for overweight, and overweight, which facilitated interpretation of our results. We observed a two-fold risk of asthma among overweight boys and girls and a three-fold increased risk of asthma among underweight boys compared with children of normal weight.
Previous investigations have found associations between underweight and asthma in adults and adolescents.30,31
A prospective study of children recruited during 1974–1979 in six U.S. cities demonstrated an increased risk of developing asthma among the leanest boys; however, their analysis did not include children younger than 6 years of age.18
Our analyses of a more recent cohort of children assessed in 2002–2004 reveal that an association between underweight and asthma appears to occur early on in the life course, and was even stronger among 2–5 year old boys than among 6–11 year old boys. Furthermore, studies such as those demonstrating a U-shaped association between ponderal index at birth (birth weight divided by length cubed) and adult asthma and atopy32
and increased risk of asthma among low-normal gestational age boys 33
point to the role of perinatal and early life factors in the development of asthma and atopy. It is possible that among some of the children in our sample, nutritional and growth restriction occurring during these periods could have contributed to abnormal lung growth and asthma. The gender difference in risk associated with underweight that we observed may reflect, in part, smaller airways for lung size in boys who are more susceptible to asthma at early ages compared to girls.34,35
The U-shaped association between asthma and BMI among boys may be due to different mechanisms underlying the relationship among underweight boys and overweight boys. Further research is needed to elucidate these potential pathways.
We found an increased risk of asthma associated with being at risk for overweight among 6–11 year old girls but not among younger girls or among boys. The importance of intervening among pre-adolescent girls who are at risk for overweight was also suggested by a longitudinal study of children in the Tucson Children’s Respiratory Study, which demonstrated that girls, but not boys, with BMIs greater than or equal to the 85th percentile at age 11 were more likely to experience wheezing at 11 and 13 years of age than those with BMIs less than the 85th percentile.11
In addition, girls in the Tucson study who became overweight or at risk for overweight between ages 6–11 years were seven times more likely to develop new asthma symptoms at 11 or 13 years of age than girls who did not become overweight or at risk for overweight during this period of development.
Our report has a number of important limitations. The cross-sectional nature of our data precludes our ability to draw conclusions about causation or the directionality of the BMI-asthma association or to distinguish age from cohort effects in our study sample. Sub-group analyses by age and race/ethnicity involved relatively small numbers of children, as did our analyses of underweight children. While resource limitations influenced the eventual number of children we were able to screen for heights and weights, the sensitivity analysis we conducted demonstrated that lower rates of physical screening in more recent years does not seem to have biased our overall results. In addition, while BMI percentile was assessed from objectively measured heights and weights, our assessment of asthma was based upon parent/guardian-reported asthma diagnosis and asthma-related symptoms or emergency care in the previous 12 months, although self-reported asthma was associated with objective measures of lung function in our study. We cannot exclude the possibility of bias associated with differential diagnosis of asthma or contact with the health care system with overweight or underweight status. Our analysis relies upon baseline data that did not include a measure of asthma severity, clinical management, or symptom control. During follow up, children enrolled in the asthma intervention are being assessed for asthma management, including inhaled corticosteroid use.22,36
We did not find a significant difference in prevalence of asthma diagnosis or symptoms with younger age; however, it is still possible that differential diagnosis by age could have influenced our findings by age group. Finally, our method of recruitment was intended for reaching as many diagnosed and undiagnosed asthmatic children as possible within the 60-block area of the HCZ and was not designed to achieve a representative sample. Nonetheless, the proportion of Black and Hispanic children in our study reflects the racial and ethnic profile of Central Harlem from the 2000 U.S. Census, and the high rates of current asthma and overweight observed in our dataset are similar to those in published reports of other less affluent, urban communities in New York City.4,37,38
In short, our results are consistent with findings from other cross-sectional and prospective studies in children and adults18,27–31
and address the unresolved issue of whether gender modifies the putative association between obesity and asthma.39
On the basis of our findings, we are in the process of implementing an intervention for overweight and at risk for overweight children with asthma enrolled in the HCZAI, which will include physical activity and nutrition sessions and an asthma educational intervention.
In summary, we were able to demonstrate an association between BMI percentile and asthma among children 2–11 years in Central Harlem. Different shapes of the curves were observed by gender for this association along the entire range of the BMI percentile, including an increased risk of asthma among underweight boys and overweight boys and girls. Pediatricians should be aware of a gender difference in the relationship between BMI percentile and the probability of asthma. The association we observed between underweight and asthma among boys needs to be further explored with a large, prospective study, which utilizes both continuous and categorical measures of BMI and includes assessments of gestational age and weight at birth along with a substantial number of very young children. Finally, interventions that aim to prevent excessive weight gain in the pre-school and elementary school years are especially important to healthy development and may also help to reduce asthma- and obesity-related morbidity in school-aged children, adolescents, and adults.