Asthma Risk: Cohort Studies

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Assessing incident risk requires prospectively following an affected cohort (obese) and an unaffected cohort (non-obese) for the development of the condition in question (asthma), and cannot rely solely on cross-sectional or case-control studies.  Several high-quality cohort studies have been conducted among diverse populations assessing the risk of incident asthma among lean and obese children [12-18].  These studies involve children of varying ages, socioeconomic status and race/ethnicity.  These studies followed children without asthma and compared the risk of developing new asthma in overweight versus normal weight children.  Overweight status appears to increase asthma risk across the complete pediatric age spectrum.  Overweight non-asthmatic infants, toddlers, pre-pubescent school-age children, and adolescents have all been shown to display increased risk for incident asthma.  Though risk ratios vary by study, obesity appears to roughly double the incident risk for asthma in children [13, 15, 16].  Though obesity-related asthma risk has been reported to be stronger among adult females, there is not a clear and consistent link with either gender among pediatric studies.  Obesity-related asthma risk also does not appear to be related to greater atopy [12-14, 17].  Rapid change in adiposity or body habitus, distinct from obesity status itself, may also influence asthma risk in children.  Gold found that extremes in body mass index (BMI) change and low baseline BMI were also associated with asthma risk [15]. 

 

Asthma lacks a precise set of diagnostic criteria, and evidence exists for over-diagnosis of asthma [25, 26] in some groups.   A rationale hypothesis is that obesity alters the perception of perturbations in airway function, leading to a lower threshold to symptom reporting.    Greater rates of false diagnosis of asthma could contribute to reported asthma risk among cohort studies using ‘physician-diagnosis’ as the sole outcome measure.  Obesity-related false asthma diagnosis is unlikely to explain the increased asthma risk.  Castro-Rodriguez et al reported increased objective markers of true asthma (peak flow variability and responsiveness to bronchodilator) in overweight girls compared with lean girls.  

We evaluated lean and obese children referred to a pediatric asthma clinic in order to determine whether high BMI-percentile is associated with misdiagnosis of asthma or significantly different objective measures of asthma compared to lean referrals [25].   We found that the prevalence of high BMI-percentile was the same among the referral population and the cases confirmed by a specialist with accompanying objective data. Referring physician-diagnosed asthmatics did not have higher rates of obesity, and referring physician-diagnosed asthmatics had objective indicators of asthma that were the same as asthmatics diagnosed by a specialist. There was good diagnostic correlation between referring physicians and asthma specialists that was not affected by BMI. Among specialist-diagnosed asthmatics, increased BMI-percentile was associated with significantly reduced forced expiratory volume in 1 second (FEV1), forced expiratory flow during the middle half of the forced vital capacity (FEF25−75), and FEV1/forced vital capacity (FVC).  We concluded that referring physicians do not appear to overly or erroneously diagnose children with asthma due to overweight status. Our data confirm that overweight status is extremely high in children with true asthma and likely increases the risk for true asthma.