In this study we found self-reported childhood adversities to be associated with register-verified asthma diagnoses. Our results suggest that about half of this association is mediated by several factors. Psychiatric morbidity attenuated the relative risk most, almost a quarter, and demographic factors by about a fifth, whereas asthma risk factors accounted for a smaller portion of the attenuation of the risk after adjustment. Our analyses suggest that psychiatric disorder, having no relationship, being female, belonging to an older age group, low level of education, having allergy or atopia, health behaviours contribute to the excess risk of adult onset asthma associated with childhood adversity. The fact that circa half of this association was not accounted for by the variables measured in this study may reflect an independent relation between childhood adversities and adult asthma or some other unaccounted factors.
This is to our knowledge the first large-scale prospective, population-based study using various forms of childhood adversity as the exposure and comprehensive measures of asthma as the outcome, while controlling for sociodemographic factors, common risk factors, psychological distress and psychiatric morbidity. Our findings are consistent with previous longitudinal studies that reported an association between adverse childhood events and asthma.20
However, the EPIC-Norfolk study used incident hospitalisation as outcome, but did not adjust for mental disorders.36
The adversities measured in our study covered equally heterogeneous20
or a more comprehensive36
set of childhood adversities and the outcome measures of the present study were derived from reliable national registers comprehensively covering new asthma cases.
The study by Bartley et al38
focused on lung function and found that financial adversity was associated with poor lung function partly through poor housing and partly through pathways involving continuities in social disadvantage and the associated environmental exposures and behaviours. In our cohort roughly a fifth of the association between adversities and asthma was mediated by sociodemographic factors. Additionally, socioeconomic status (SES) has been found to associate with the risk of mental disorders and poor health behaviours.47
Hence, socioeconomic factors may operate on the micro level through these risk factors.
Likewise, our study is in line with the 10 cross-sectional population surveys conducted as part of the World Mental Health (WMH) surveys. In that particular study childhood adversities predicted adult-onset asthma with risk increasing with the number of adversities experienced (HR 1.49–1.71). However, the researchers of the WMH surveys also found that early-onset depression and anxiety disorders and childhood adversities both predicted adult-onset asthma after mutual adjustment.37
In our study, psychiatric disorder was only used as a covariate. It was not studied as a risk factor for asthma.
Interestingly, low maternal childhood SES was found to associate with increased cord blood immunoglobulin E (IgE) levels and repeated wheeze through both direct and indirect effects.49
Additionally, maternal cumulative interpersonal trauma was also associated with increased cord blood IgE levels. Psychoneuroimmunological pathway may be one key mechanism between adversity and asthma. Parenting difficulties have, likewise, been linked to childhood asthma and the risk seemed to be highest among those, who also had elevated IgE levels.32
Adults who have exposed to childhood maltreatment have been found to have elevated levels of inflammation biomarkers.10
Also anxiety disorders have been linked to inflammation.51
It has been suggested that stress-related elevation in clinically relevant inflammation proteins could contribute to the biological embedding of childhood stress.50