Screen-based sedentary behavior (SB) is associated with elevated BMI; however, the underlying mechanisms remain poorly understood. This study investigates the mediating role of weight concern in the SB-BMI relationship and examines how body image perception and gender moderate this mediation. By focusing on body image as a critical contextual factor, the study aims to inform targeted interventions for adolescent obesity and overweight. In 2016, self-report questionnaires assessing screen-based SB, weight concern, and body image perception were collected from 508 Chinese adolescents (grades 7–9), yielding 438 valid responses (response rate: 86.22%). Path analysis and bootstrap procedures with bias-corrected 95% confidence intervals were used for data analysis. Screen-based SB was positively associated with weight concern (b = 1.115, p = 0.009) and BMI (b = 0.183, p < 0.001). The mediation analysis showed that weight concern partially mediated this relationship, with an indirect effect of 0.056 (95% CI = [0.006, 0.114]). This mediation was moderated by body image perception and gender. Specifically, the interaction between BMI overestimation and weight concern was negatively associated with BMI (b=-0.157, p = 0.011), whereas the three-way interaction of gender, weight concern, and overestimated body image perception showed a positive association with BMI (b = 0.080, p = 0.025). Screen-based SB influences BMI, with weight concern as a mediator and body image perception and gender as moderators. These findings highlight the importance of targeting sedentary behavior and body image concerns in gender-sensitive interventions for adolescents.

Objectives: Family functioning (FF), physical activity (PA), and high-fat food consumption (HF) are associated with adolescents being overweight and obese; however, little is known about their interactions. Therefore, this study aimed to examine how they work jointly on adolescent obesity with BMI as the outcome variable. Methods: A cross-sectional survey utilizing a cluster sampling design was conducted. Multinomial logistic regressions, multiplication interaction (MI), and marginal effects (MEs) were tested. Results: Active PA (non-overweight vs. obesity: OR = 2.260, 95% CI [1.318, 3.874]; overweight vs. obesity: OR = 2.096, 95% CI [1.167, 3.766]), healthy HF (non-overweight vs. obesity: OR = 2.048, 95% CI [1.105, 3.796]) and healthy FF (overweight vs. obesity: OR = 2.084, 95% CI [1.099, 3.952]) reduced obesity risk. Overweight students with healthy FF were less likely to become obese regardless of PA (inactive: OR = 2.181, 95% CI [1.114, 4.272]; active: OR = 3.870, 95% CI [1.719, 8.713]) or HF (unhealthy: OR = 4.615, 95% CI [1.049, 20.306]; healthy: OR = 5.116, 95% CI [1.352, 19.362]). The MEs of inactive PA and unhealthy FF were −0.071, 0.035, and 0.036 for non-overweight, overweight, and obese individuals, respectively (p < 0.05); the MEs of HF and healthy FF individuals were −0.267 and 0.198 for non-overweight and obese individuals, respectively (p < 0.05). Conclusions: Unhealthy FF regulated the influence of inactive PA or unhealthy HF on adolescent obesity, altogether leading to a higher risk of obesity.