Dense, walkable spaces will not make young adults less obese, although such environments may attract individuals who already live more active lifestyles.
Those are the findings of Shima Hamidi and Reid Ewing in their article "Compact Development and BMI for Young Adults" published in the Journal of the American Planning Association (Vol. 86, No. 3).
Environment vs. Self-selection in BMI
Following young adults aged 20–31 from 2002 to 2011, Hamidi and Ewing explore which is more salient in determining young adults' body mass index (BMI) scores: their environment or their decisions to live in that environment.
Initially, the authors predict the built environment is a key factor in predicting one's BMI. Reflective of environmental determinism, this suggests that by holding other factors constant, one's BMI will shift with changes in one's environment. If true, this has promising implications for planners. It reaffirms decisions to design communities that encourage physical activity as they can reduce obesity.
However, there remains the contrasting perspective of self-selection. Here, the BMI of young adults is not a byproduct of their environment but rather the result of attitudes and preferences compelling young adults to also live in communities that encourage physical activity.
Figure 1: A simplified conceptual framework representing two causal pathways between self-selection, the built environment, and BMI.
Compactness Affects BMI for Movers
This article combines both cross-sectional and longitudinal analyses. Using a cross-sectional study design, Hamidi and Ewing analyze the relationship between young adults' BMIs in 2011 and the compactness of their environments. As with other studies on the subject, they too confirm there is a significant relationship between the built environment and BMI. Yet, they find differences when assessing the effect of moving in the period 2008–2011.
Among young adults who moved at least once, compact spaces were associated with lower young adults' BMIs. For individuals who stayed in place, no relationship was found.
If the theory of environmental determinism were true, regardless of one's decision to move, compactness would have a discernible effect on young adults' BMIs.
They continue their analysis with a longitudinal study. Contrary to their cross-sectional results, their analysis suggests there is no relationship between changes in compactness and young adults' BMIs. Young adults who moved to more compact communities did not experience a decrease in their BMIs. But then what explains differences initially found among young adult movers?
Data reveal that young adults who were not overweight strongly preferred to relocate to areas with greater compactness. The opposite was not true for young adults who were overweight: they were nearly indifferent about the compactness of their communities.
Ultimately, this confirms self-selection, not environmental determinism, and provides a better understanding of young adults' BMIs.
These results imply that planners have a rather limited ability to design out obesity. However, health is greater than BMI. One's BMI cannot account for differences among genders or body compositions, or even gauge one's true physical activity. Therefore, this should not discourage planners from creating compact, active spaces. In these spaces there may be greater opportunities for social interaction, and perhaps through these interactions the opportunities to encourage positive health behaviors.
Hamidi and Ewing ultimately acknowledge this: "These findings do not suggest that place characteristics are unimportant, as planners need to meet the growing demand for walkable, compact and connected places, which are currently undersupplied."
Top image: Public domain photo of young adults walking.
About the Author
Kyle Miller is a joint Master in Urban Planning and Master of Public Health candidate at Harvard University.