By conducting a systematic review and meta‐analysis of the effect of high weight on future risk of asthma, we have obtained an estimate that suggests that high body weight among school aged children increases the risk of future asthma by approximately 50%. The effect of high birth weight appears to be less pronounced but still quite significant, with an RR of 1.2 compared to babies without high birth weight. The association between high weight and asthma remains significant when combining estimates of all age groups for analysis and also remains strong when results reported as ORs are analysed separately from those reported as RRs and when the definition of outcome is narrowed to include physician diagnosis only.
What is already known on this topic
- Some studies have found that high birth weight and high BMI in childhood are predictive of future asthma, while others have shown no association
These findings have important implications for public health. The past 20 years have seen a dramatic increase in both high BMI and asthma, and it seems possible that increasing weight may have contributed to increasing asthma. The prevalence of overweight among 6–11 year olds in the United States has recently been estimated to be 15.3%, while that among 12–19 year olds was estimated at 15.5%.43
The population attributable risk could therefore be estimated at 0.066, meaning that 6.6% of all cases of childhood asthma are due to overweight. The 2000 census counted 41.1 million Americans between the ages of 5 and 14 years. Given a prevalence rate for paediatric asthma of 57.8 per 1000,1
the results of this meta‐analysis suggest that over 100
000 American children in this age group suffer from asthma each year as a result of being overweight. The impact of high weight could increase further when high birth weight is also considered.
The purpose of this study was to evaluate the effect of high body weight on the development of asthma. This goal is complicated by the fact that weight is not one of the largest contributors to the development of asthma, and the effect of weight on asthma is likely to be relatively weak compared to other risk factors such as atopy, air pollution, and family history of asthma. It is further complicated by the facts that currently available cohort studies are diverse in both exposure measures and in outcome measures.
Several sources of bias should be considered in the interpretation of these results. First, misclassification of exposure is a potential source of bias in this systematic review, especially due to the acceptance of three different definitions of high body weight, the acceptance of high body weight at a variety of ages, and the possibility that BMI centile standards varied for different cohorts. However, significant misclassification of exposure would be expected to either bias findings towards the null result of no association between exposure and outcome or cause significant heterogeneity in the meta‐analysis model. Also, there is no indication that there is a threshold level for weight that sharply demarcates those at increased risk from those with average risk. The levels of homogeneity found in the significant results in the meta‐analysis suggest that the association found between high body weight and asthma is not due to misclassification of exposure.
Another potential source of bias is misclassification of outcome. This is a concern both for the meta‐analysis as a whole, which includes several different definitions of outcome and includes several different ages for the assessment of outcome, and within the individual studies, especially due to diagnostic bias. Regarding potential misclassification of outcome due to the acceptance of several different criteria for outcome in this meta‐analysis, it is important to note that the subset analysis examining the outcome of physician diagnosis of asthma shows results very similar to the other models. This argues against significant bias from the use of several different criteria for outcome. Misclassification of outcome from varying age at time of outcome remains a possible source of bias in this paper, although the relatively good homogeneity in the models would argue against a strong bias from this source.
What this study adds
- This paper provides a systematic review and meta‐analysis of the studies on this topic and shows that both high birth weight and high BMI during childhood are predictive of future asthma
- High birth weight is associated with a RR of 1.2 (95% CI 1.1 to 1.3) for future asthma, while high BMI during childhood is associated with a RR of 1.5 (95% CI 1.2 to 1.8) for future asthma. Over 100000 children in the United States may suffer from asthma due to childhood overweight
Regarding misclassification of outcome within the individual studies, the results of the analysis of effect of birth weight argue against significant diagnostic bias. Parents (and doctors) of larger babies do not view them as more sickly than smaller babies, and so it is unlikely that they would report increased rates of either physician diagnosis of asthma or asthma treatment or hospitalisation due to high birth weight. Nevertheless, the possibility of diagnostic bias cannot be completely eliminated in the meta‐analysis and remains a potential limitation. An additional source of potential systematic error is publication bias, which is especially concerning because some of the included papers are based on findings from cohorts designed for other purposes. However, the Begg and Egger tests did not offer any evidence for publication bias, and a review of the grey literature did not reveal any additional research meeting inclusion criteria.
This study offers strong evidence that high body weight during middle childhood increases the odds of future asthma by approximately 1.5‐fold, and that high birth weight increases the odds of asthma 1.2‐fold. However, an important limitation of our analysis is the inability to adjust for individual confounding variables, such as other types of atopic disease, environmental tobacco exposure, family history of asthma, and gender. Although the individual studies adjusted for variables as they reached significance in their population, the meta‐analysis cannot examine how these might modify the relation between weight and asthma. Also, several studies did report results differently for boys and girls, but there were not enough of these to allow for subset analysis by gender in this paper. This is an important limitation, as some researchers have found association to differ by gender, and others have not.32,33,44
In order to examine the effect of other predictor variables on the relation between body weight and risk of subsequent asthma, the ideal future study would follow a very large cohort of children from birth through adolescence. Simple information such as weight and height would need to be collected at regular intervals, and asthma status would be assessed annually. Such a study could also collect data on types of dietary intake, age of pubertal changes, and gastric and atopic symptoms, and might even be able to perform forced expiratory volume (FEV1) testing to reduce diagnostic bias. This type of research could examine the effects of additional predictor variables both singly and in combination on the relation between high body weight and risk of asthma, and might result in new knowledge about the causes of the disease. Improved knowledge about the causal pathways leading to an association between high body weight and asthma could result in improved understanding of the pathophysiology involved in the dramatic increase in prevalence of asthma, which could potentially lead to important knowledge about methods for prevention of this common childhood disease.