INTRODUCTION
Crohn's disease (CD) and ulcerative colitis (UC) are the 2 main subtypes of inflammatory bowel disease (IBD), a condition characterized by inflammation in the gastrointestinal tract because of an inappropriate immune response (1). The incidence of IBD increased in the Western nations during the 20th century, and the rates of pediatric-onset IBD continue to increase globally (2,3). Environmental factors related to Western culture and/or rapid socioeconomic development may be implicated in IBD pathogenesis (4).
The health effects of exposure to features of natural vegetation, often referred to as greenspace, is an emerging area of interest in environmental epidemiology. Urbanization has led to a decrease in the population's exposures to plants and the natural environment (5). Despite this, greenspace has not been considered in any published analysis of IBD incidence. Currently, residential greenspace has been shown to negatively correlate with other immune-mediated conditions such as atopic sensitization and asthma (6–8). Greenspace may improve health through many pathways, including the promotion of increased physical activity, psychosocial effects, decreased exposure to air pollution and urban noise, or direct exposure to a wider range of biodiversity (9,10).
The objective of this study was to investigate the potential independent effects of residential greenspace on the risk of developing pediatric-onset IBD.
DISCUSSION
In this retrospective population-based cohort study, we found that higher exposure to residential greenspace during childhood was associated with a reduced risk of pediatric-onset IBD. This relationship was observed for both disease subtypes and was stronger for very early onset IBD (diagnosed before 10 years of age). By contrast, greenspace during pregnancy did not show a consistent association with overall IBD or CD and UC subtypes. Future studies are necessary to confirm whether children playing in parks may serve as an approach to prevent some cases of pediatric-onset IBD.
This is the first study to demonstrate higher exposure to greenspace was associated childhood-onset IBD. The protective benefit of interactivity with greenspaces such as parks is consistent with other studies showing that healthy living lifestyle behaviours reduced the risk of IBD including low psychological stress (22), never smoking (23), and increased physical activity (24,25). In addition, greenspace may in part be an important mediator of the consistent finding that living in a rural area is protective of IBD (17,26,27). Similarly, exposures to greenspace may be a residual confounder that explains the inconsistent findings of an association between air pollution and IBD (28,29).
The observed protective association between residential greenspace and pediatric IBD may be explained by a variety of factors, including directly through increased exposure to biodiversity and indirectly through facilitating other healthy lifestyle factors such as physical activity and reduced stress. Exposure to microbes as a protective factor for IBD concurs with the widely supported “hygiene hypothesis,” which theorizes that the reduced microbial exposure and sterile environments that humans experience in industrialized nations may negatively affect the immune system. This reduced microbial load has been previously suggested to be responsible for the global increases in IBD incidence (30). Physical activity in children promoted by proximity and access to greenspace such as by parks and playgrounds may also be exerting influence on IBD risk (10,24). This could be because of changes in the composition of the gut microbiota caused by recreational exercise that has been reported in several animal studies (31).
This study is strengthened by its population-based design, which tend to be more robust to selection bias. In addition, we were able to consider the effects of other environmental exposures that may be spatially correlated with greenspace, such as urban/rural residence and exposures to air pollutants. Another strength is that we were able to assess exposures throughout the entire life-course starting from the estimated date of conception. This allowed us to characterize both prenatal and postnatal effects.
Several limitations to this study should be considered. First, exposure to greenspace may be spatially correlated with other unconsidered environmental risk factors or unmeasured characteristics of the population. For instance, those that reside in urban areas with high greenspace tend to be wealthier are less likely to be ethnic minorities and have better health literacy and behaviours (32). To help control for this possible confounding, competing environmental factors (rurality and air pollution), neighborhood income, and random effects representing geographical variables were added in the regression models. Another limitation of the study includes the use of secondary data that were not collected for research purposes. This may mean that there may be errors in diagnostic coding or changes in coding practice over the course of the study that are not accounted for, although as long-term estimates of greenspace were used, this would be unlikely to greatly affect our results. A small amount of misclassification of IBD diagnosis based on the accuracy of the algorithms is present as well, but this would not differ greatly across greenspace levels, limiting the impact on the observed associations. Residual confounding may be caused by confounders that were not available in the health administrative data sources, such as ethnicity or genetics. However, recent immigrants (that typically have lower rates of IBD) have lower average NDVI values in Canadian urban cities (33), meaning that the strength of our associations may have been underestimated. Finally, most of Ontario's population live in urban areas and the vegetation and other environmental exposures may be unique to this population, so caution should be exercised when interpreting the results of this study and extrapolating to other geographic regions.
Greenspace is an emerging area in environmental health research, and this study is the first to investigate and report its protective effect with IBD. Future studies should look at using alternative measures to NDVI to evaluate the specific features of greenspace responsible for this finding (such as type or access to vegetation) and look for effects on the gut microbiome, immune dysregulation, and genetic predisposition in IBD. Mediation analyses involving these factors are needed to identify possible causal mechanisms. Intervention studies designed to target specific environmental risk factors to reduce the risk of IBD should include greenspace in study design. It is possible that by changing our immediate environment, we could prevent childhood-onset IBD.
Full research: Residential Greenspace in Childhood Reduces Risk of Pediatric Inflammatory Bowel Disease: A Population-Based Cohort Study
