Background: Prevalence of obesity among adolescents is increasing at an alarming rate. Currently, 34% of all 12-19 year olds are either at risk for overweight or are overweight. Without a comprehensive understanding of the myriad of environmental factors that influence physical activity, we cannot effectively curb the rising rates.

Methods: The objective of this study is to evaluate the direct and indirect effects of individual, built, and psychosocial environmental factors on the body mass index (BMI) of African American adolescents, and to assess how the relationships may be mediated by physical activity (PA) levels in an urban setting. A cross-sectional sample of 350 Students (grades 9-12) was drawn from two urban magnet high schools in Baltimore City. The outcome measurements cover a broad array of variables, including BMI, walking behavior, and overall PA. The independent variables are measures of three environments and individual socio-demographic status. The hierarchical linear model analysis was employed. In addition, the mediation analysis was conducted to examine the mediating effect of PA levels.

Results: The majority of the sample was African American (69%) and female (58%) with 40% at risk of overweight or overweight. After adjustment for individual socio-demographic factors, living in more hazardous neighborhoods was positively associated with moderate-to-vigorous physical activity (MVPA) and walking for transportation but not with BMI. Densities and proximities of destinations were not associated with BMI. Access to school and museums within 0.25 mile, and grocery stores, retail and shopping, post offices and mix of destinations within 0.5 mile and beyond was positively associated with participation in walking. In addition, densities of fast food restaurants, parks and recreation within 1 mile and 1.5 mile were associated with increased walking. Closer proximity to schools and museums increased both student's MVPA level and their walking trips. Green space coverage within 0.25 mile distance was associated with decreased BMI. In addition, the percentage of green space coverage around homes was not associated with physical activity. Physical activity did not mediate either the destination densities and proximity or the greenness on BMI.

Conclusion: The findings support the importance of focusing on places rather than the individuals who live in those places. Increasing the diversity of walkable destinations may contribute to adolescents doing more transport-related walking and achieving recommended levels of physical activity. In general, ecologic approaches to obesity prevention need to focus on urban design and food availability.