Nonlinear Associations Between Built Environment and Overweight: Gender and Marital Status Differences in Urban China

Overweight has become a major public health concern in China’s rapidly urbanizing cities. Patterns of environmental exposure differ notably between men and women, both before and after marriage. This study examines how built environment characteristics influence the risk of overweight, with particular attention to nonlinear associations as well as variations by marital status and gender. Drawing on survey data from 2634 Shanghai residents, we applied extreme gradient boosting to model complex environment–health relationships. The results indicate that greenness, park accessibility, population density, and transit conditions are associated with overweight through nonlinear pathways, with threshold and plateau effects suggesting that benefits taper off, or risks escalate, beyond certain levels. These optimal ranges differ across gender–marriage groups: moderate density and green exposure were generally protective, but the effective ranges were narrower for women and unmarried individuals. Married men benefited more consistently, likely supported by healthier routines reinforced through spousal support, whereas married women showed weaker or even adverse effects, potentially due to greater family responsibilities. Overall, the findings reveal that overweight is shaped by socially differentiated nonlinearities in environmental exposures. Urban planning and public health policies should therefore optimize built environment attributes within effective ranges and tailor interventions to diverse demographic groups.