In a sample of predominantly African-American, economically disadvantaged children and youth, a significant inverse association was apparent between neighborhood greenness and BMI z-scores at Time 2, controlling for the child’s BMI z-score at baseline and other important covariates. It also was found that children and youth in greener settings were less likely to increase their BMI z-scores over 2 years compared to their counterparts in less-green neighborhoods. These findings add to prior cross-sectional studies demonstrating associations between greenness and weight24
and are distinguished by modeling density and greenness together and by longitudinal measures of BMI.
These findings demonstrate that the effect of greenness on weight status is independent of residential density. In contrast, density modeled without greenness had no effect on BMI z-scores, while marginally protective effects of higher density were apparent only when greenness was controlled. Assuming that the inverse relationships between density and BMI in the adult literature are driven by enhanced walkability—as defined by street characteristics, commercial–residential land-use mix, and population density—the current results suggest that such conceptualizations may have less relevance to children and youth in urban environments unless greenness is taken into account. Even then, the magnitude of the association with residential density was small.
Associations between greenness and BMI z-scores were significant and similar when NDVI was measured within radial versus network buffers. There was no evidence that network buffers better depict than radial buffers regions of access and exposure in this sample of children and youth. In contrast to prior cross-sectional work demonstrating differences in NDVI–BMI relationships by population density,24
no effect modification of associations was found between neighborhood NDVI and BMI z-scores by density, perhaps due to different specification of the dependent variable or areal units used to measure density.
Interestingly, non-Hispanic black children and youth had significantly higher BMI z-scores at Time 2 only when greenness was controlled. This finding suggests that the advantageous BMI z-scores in , Model 2, similar to those of non-Hispanic white children and youth, may have been conferred, at least in part, by greener neighborhood environs.
The magnitude and significance of the NDVI estimates reported here are clinically meaningful and biologically plausible. Holding height constant and comparing to a national reference group,28
a BMI z-score lower at Time 2 by 0.05 SDs equates to lower weight of approximately 1.6 kg for girls aged 4 years (2 kg for boys) and 5.1 kg for girls aged 16 years (5.9 kg for boys). This study’s findings align with previous research linking exposure to green landscapes with health improvements. Among adults, greenness is associated with less stress and lower BMI,36
improved self-reported health,37
and shorter post-operative recovery periods.38
Among children and youth, the positive health effects of green landscapes include improved cognitive functioning39
and reduced attention deficit hyperactivity disorder symptoms.40
Physical activity is one obvious pathway through which urban vegetation might influence the BMI of children and youth. Unlike adults, children and youth in urban environments may be active in a wider variety of open spaces (e.g., yards, parks, vacant lots) and less likely to constrain activity to streets and sidewalks. Consistent with other work,17–19,21,22,24
this study posited that greenness might indicate proximity to parks, playfields, or other open spaces that promote either physical activity or increased time spent outdoors in active play. The findings are consistent with these claims and supported by prior literature. For instance, in inner-city neighborhoods, outdoor spaces with trees are used with higher frequency than treeless outdoor spaces, and the more trees, the more simultaneous users.41,42
Children and youth are twice as likely to be supervised by adults in green urban spaces compared to barren but otherwise similar spaces.43
Tree shade and scenery are associated with reports of increased walking.44
Green landscaping might also influence indirectly the physical activity of children and youth as an indicator of territorial personalization, implying that inhabitants actively care about their homes.45
In one study, greener apartment-building surroundings were associated with reports of fewer crimes.46
It is reasonable to conjecture that territorial personalization could be associated with increased community surveillance that deters crime and thereby increases parents’ willingness to encourage outdoor physical activity as well as perceptions of safety by children and youth.
Finally, if improved mental health or self-esteem influence health behavior, greenness could indirectly influence the BMI of children and youth by providing access to places for mental and sensory stimulation, privacy, or opportunities for creative play.40,47,48
Future research is required to test these potential pathways and mechanisms with direct measures of child behavior in prospective studies, ideally those designed to take advantage of natural experiments in which green space in urban settings is radically increased or decreased.
The results are subject to limitations. The study region, geographic scale, and sample limit generalizability. As an observational study, results may reflect selection bias if families exhibiting healthier behaviors related to diet or physical activity also choose to live in greener neighborhoods. Omitted variables, including more-robust measures of SES and neighborhood attributes such as crime and the presence of resources and amenities, may also influence the findings. Additionally, physical activity is not available in medical records, precluding the examination of the full sequence of environmental factors influencing behavior and, ultimately, weight status. The temporal mismatch between the clinical and greenness measures limits the ability to assess the impact of changing greenness exposure. Residential density in the study region may have had insufficient variability to measure its effects. Future research is warranted within or across settings with wider-ranging measures of density.
The results may reflect unmeasured confounding by SES. In Denver, the NDVI was associated with census tract–level socioeconomic advantage.49
In Denmark, access to green areas was associated with lower self-reported stress and obesity; however, the number of trips to green areas did not predict either outcome, raising the question of whether it is greenness per se that matters to health, or other associated factors.36
In this study, most subjects were Medicaid enrollees, presumably from economically disadvantaged families. Notably, these estimates were of similar direction and significance in high- and low-SES strata; only the magnitude of the estimate differed. It is unlikely that the general findings were due solely to unmeasured confounding by SES.