In this study, we examined the relationship between overweight in pre-school children and several built environmental factors. Our main findings indicated no significant associations between built environmental factors (walkability index, availability of playgrounds and availability of greenspace) and children’s weight status. However, our results suggested an interaction of individual SES (parental educational level) and the greenspace availability while not for area-level SES: For children with lower compared to higher educated parents, a higher spatial availability to greenspace was significantly associated with reduced body weight.
Our results resonate with previous findings of the literature suggesting that individual SES factors are strongly associated for childhood BMI. To be noted, parental educational level is the only available SES factor in our study. Previous literatures suggest that children from families with low SES are at higher risk of becoming overweight or obese by assessing other SES factors. According to Saelens et al. (34), children from families with low incomes had higher risk of being obese. However, social gradient in the overweight prevalence cannot be fully explained by individual factors only. With the emergence of social ecological theory, the area-level SES as a predictor has been investigated. Prior studies indicate that adolescents who lived in deprived areas were more likely to be overweight and had higher levels of body fat (8, 35). However, the findings of area-level SES are inconsistent. A cross-sectional study authors found that a disparity in income among families affected the occurrence of childhood obesity, irrespective of neighbourhood SES (36). In our findings, area-level factors (unemployment rates and percentage of migrants) did not provide significant associations which could partially due to the reason of that there is no universal area effect on health outcome at present.
Factors of the built environment, such as greenspace availability, playground availability, or walkability, did not show any association with overweight and obesity on the aggregated level of analysis in pre-school children in Hannover. Similar studies targeting German population were not able to identify a significant association neither (31, 37), except a study based on data from the city of Munich (23). This study identified that lack of greenspace, low/middle playground space and low park space are associated with higher BMI although only in the bivariate analyses (23). However, the evidence of an association between built environment and physical activity was solid. Buck et al. (38, 39) found a strong variation in this association between physical activity and built environment using several variables including features of the walkability concept and the availability of recreational facilities like playgrounds and greenspace.
For greenspace availability specifically, while most studies showed a mixed or weak evidence of a relationship between greenspace and BMI, several reports have indicated a positive relationship (i.e., reduced BMI) between greenspace and BMI. Liu et al. (16) found that increased greenspace availability was associated with reduced weight among children living in areas with a high population density, while Petraviciene et al. (40) reported that less greenness exposure was associated with higher odds of being overweight and obese. All these studies highlighted the potential effect of SES on weight status change. Since more affluent parents tend to live in more salubrious areas, the effect of the environment may be partly driven by the parental SES (15).
Moreover, the environmental context may matter more for those otherwise not being able to take advantage of it. An interaction relationship between the SES and the environmental context on the children weight status change was explored in this study. We were able to demonstrate that the associations between the environment and childhood overweight/obesity were moderated by the educational level of the parents. At the same time, two area-level SES variables failed to provide a significant association. According to our findings, higher neighbourhood greenspace availability was associated with a lower BMI percentile, while the effect was stronger for children growing up in less-educated families compared to children from higher-educated families. As a frequently used indicator of SES in health behaviour surveys, parental educational level is believed to reflect the lifestyle among parents, which consequently has an influence on lifestyle among children (41). Our results are consistent with the findings of Lovasi et al. (42, 43) who found that children in lower income families had a reduced risk of obesity if they lived in an area with a higher density of trees. Less affluent families might be more restricted to their immediate surrounding and thus benefit more from greenspace availability (14, 44).
Physical activity is a potential mechanism through which built environments may influence obesity. Among youth, various elements of the built environment have been linked to increased physical activity. Children with access to recreational facilities, usually around their neighbourhoods, are more active than those without such access (42). A large body of literature has found associations between neighbourhood walkability and physical activity (45, 46). Some studies identified physical activity to be a mediator of the neighbourhood environment - body mass index (BMI) association (47, 48).
In addition to the complex mechanism related to physical activity, many other factors could confound the association between built environment and BMI. ‘Residential self-selection’ has been put forward as a possibly important confounder of the positive associations between walkability and physical activity. Residential self-selection implies that families are likely to select their neighbourhood according to their culture, lifestyle and personal preferences, and consequently those who are already active or who wish to be active may choose to live in a high-walkable neighbourhood and vice versa (49). Many studies on physical activity have controlled for residential self-selection in their analyses, resulting in mixed findings ranging from attenuation of the associations between walkability and PA to minimal effects on the associations (45, 50, 51). Some residents may choose to live in neighbourhoods that support their activity preferences in some cases. In another situation, residents may prefer to live in neighbourhoods with fewer recreational facilities because of low cost housing (48, 50). Although those analyses assume that children have little choice in their residential location (mostly depends on family selection), residential self-selection remains a significant factor (52). Overall, without including the residential selection factor, the association of built environment features and children’s BMI might be overestimated. Definitive evidence of the presence or absence of residential selection confounding still awaits further exploration.
The strength of our study is the large sample size (n = 22,678), which allowed us to conduct multi-level analyses to explore how the association between neighbourhood environment and childhood overweight and obesity varies by sex and to create maps illustrating the spatial patterns of overweight across the city of Hannover. Moreover, we were able to obtain objective measures assessing built environment in this study. Built environment features can be collected using either subjective or objective ways (46, 53). Many studies had applied subjective methods (54, 55) placing considerable value in subject's judgment of its own neighbourhood and the factors that contribute to it. Subjective tools can relate to self-reported perceptions of the environment, including self-evaluations of the subject’s familiarity to the surroundings. However, studies showed a mismatch between objectively and subjectively measured built environment features suggesting that environmental perceptions are stronger correlates of activity among children than objective measures in specific situations (56). Future research could consider combining these two measurements in order to produce a more complete perspective.
This study has several limitations. First, due to the cross-sectional design, causality cannot be attributed to the observed findings. Second, although we captured an important outcome (BMI percentile) objectively for children in the study, several other unmeasured variables, such as physical activity, may be key mediators or confounding factors in the built environment-obesity relationship (57, 58). Objective measuring of physical activity for over 22 thousands children are challenging to collect, but additional research should include multiple health behaviours and outcome measures to better explicate the relationship between key environmental features and obesity. At the same time, there is a potential for residual confounding secondary to unmeasured aspects of the area or individual-level SES measures. Many important SES variables from previous literature, including household income, were not included due to data availability. Moreover, we used administrative boundaries as a proxy for the neighbourhood environment that may have induced a misclassification. Our environmental measures were conducted at the area-level because individual home addresses were not available. Area-level built environment measurement can be coarse, and variation at a finer or coarser scale (zip-code, home address) may be crucial to influencing physical activity (59). Hence, it was unable to assess the sensitivity of our results to different spatial scales (59). The influence of scale on matched exposure-response relations in the built environment related literature needs for further investigation.