The goal of this study is to evaluate the effectiveness of a school-based weight management intervention delivered through a digital platform (WeChat) in preventing excessive weight gain and improving health behaviors in school-aged children in Fangshan District, Beijing. The main questions it aims to answer are:

Does the intervention group show a significantly lower mean increase in BMI, body fat percentage, and waist circumference compared to the control group? Does the intervention group show a significantly lower obesity rate and new obesity incidence compared to the control group? Does the intervention group show significant improvements in healthy eating, sedentary behavior, sleep, physical activity, and physical fitness test scores compared to the control group? Researchers will compare children in intervention schools (who receive the multi-level intervention targeting students, families, and schools, supported by a digital platform for behavior monitoring and feedback) to children in control schools (who do not receive the intervention during the study period).

Participants in the intervention group will: (1) attend 10 student health education sessions; (2) have their physical activity at school increased to 60 minutes/day of moderate-to-vigorous activity; (3) have parents attend health education lectures and use a WeChat platform to receive health information, log child health data, and receive automated feedback; (4) have their height, weight, waist circumference, and body composition measured monthly by school health staff; and (5) be followed with assessments at 6, 12, and 24 months.

Study Design This is a two-arm, cluster-randomized controlled trial (cRCT) designed to evaluate the implementation and effectiveness of a multi-level, digital platform-based weight management intervention for school-aged children in Fangshan District, Beijing. The study consists of a 12-month active intervention period followed by an additional 12 months of follow-up, totaling 24 months of observation post-baseline.

Setting and Participants The study will be conducted in 10 primary schools in Fangshan District, Beijing, recruited through the Fangshan District School Health Care Center. Schools will be randomized (1:1 ratio) to either the intervention or control arm. Participants will be third-grade students (aged 8-9 years) at baseline.

Inclusion Criteria: Eligible students must provide signed informed consent from a parent or guardian and their own assent.

Exclusion Criteria: Students with pre-existing medical conditions that contraindicate physical activity or secondary obesity of known endocrine origin will be excluded from the analytic sample but may participate in the general program activities to avoid exclusion-based stigma.

InterventionThe intervention targets three levels-students, families, and schools-over one academic year. A digital platform embedded within WeChat serves as the central hub for knowledge dissemination, behavior monitoring, and automated feedback.

1. Student-level Interventions Health Education: Ten 40-minute classroom sessions delivered every 2-3 weeks, focusing on healthy weight maintenance, reducing sugar-sweetened beverages (SSBs), limiting sedentary behavior, and increasing physical activity. Sessions include interactive "hand-in-hand" homework assignments to be completed with parents.

   Physical Activity Promotion: Targeted daily routines to achieve at least 60 minutes of moderate-to-vigorous physical activity (MVPA) at school, emphasizing outdoor activities and muscle/bone-strengthening exercises.

   Monitoring: Monthly anthropometric measurements (height, weight, waist circumference, and body composition) conducted by trained school health staff, supplemented by weekly student self-weighing using classroom scales.

2. Family-level Interventions Parental Education: Two health education seminars per academic year (30-40 minutes each) and optional parent discussion forums to address implementation challenges.

   Digital Engagement: Parents use the WeChat platform to: (a) receive regular push notifications of health education materials, (b) complete monthly behavioral questionnaires (diet, activity, sleep), and (c) view individualized monthly feedback reports comparing their child's BMI and behavioral progress against health standards.

   Home Environment: Encouragement of home-based active play and parental modeling of healthy habits.

3. School-level Interventions Coordination: Designation of a school-level project coordinator to oversee logistics.

Environmental Changes: Integration of health sessions into the formal timetable and implementation of policies to limit the availability and consumption of SSBs and unhealthy snacks on campus.

Atmosphere: Classroom-based health promotion materials (e.g., posters, food environment labeling) and professional development for involved teachers.

Control Group Control schools will maintain their usual curriculum during the 12-month intervention period. Following the 24-month follow-up assessment, control schools will be provided with the intervention curriculum and relevant health education resources as an ethical cross-over.

Follow-Up and Data Collection Assessments will be conducted at baseline, 6 months, 12 months, and 24 months. Anthropometric & Clinical: Height, weight, waist/hip circumference, blood pressure, and body fat percentage (via bioelectrical impedance analysis).

Physical Fitness: 1-minute rope skipping, 1-minute sit-ups, standing long jump, and endurance shuttle runs.

Questionnaires: Validated tools for students and parents assessing dietary habits, physical activity, sedentary time, and sleep quality.

Statistical Analysis and Sample Size Sample Size: Assuming a mean BMI reduction of 0.50 kg/m2 (SD: 1.40kg/m2), an intra-cluster correlation coefficient (ICC) of 0.03, and a two-sided significance level of 0.05. Accounting for a 10% attrition rate, a total of 135 students per school is required. With 10 schools (4 classes per school, approximately 35 students per class), the study targets a total sample of approximately 1,400 students, achieving 80% power.

Evaluation: Mixed-effects models will be used to analyze primary and secondary outcomes, accounting for school-level clustering and repeated measures.

Ethical Considerations The protocol and all materials have been approved by the Institutional Review Board of Peking University Health Science Center. Participation is strictly voluntary. Personal data will be anonymized, and protocols include specific measures to avoid weight-based stigma and ensure privacy during physical assessments.

Study Timeline Recruitment & Baseline: March 2026 Active Intervention: March 2026 - March 2027 Follow-up Assessments: September 2026 (6m), March 2027 (12m), and March 2028 (24m) Analysis & Publication: March 2027 - June 2028

Inclusion Criteria:

• Schools will be selected based on the consent of primary leadership, demonstrated cooperation, and availability of necessary personnel (e.g., school health professionals).
• Classes will be limited to grades 2 and 3, with teachers showing strong willingness to collaborate.
• Both parental informed consent and child assent are required for participation.

Exclusion Criteria:

• Schools will be excluded if they cater to special populations (e.g., schools for children with disabilities), are involved in other obesity-related interventions within the specified timeframe, or plan to close or relocate within two years.
• Children will be excluded if they have histories of major organ diseases (e.g., cardiac, pulmonary, hepatic, or renal conditions), special diets (e.g., vegetarian), pathological eating disorders, physical limitations affecting activity, or obesity due to endocrine disorders or medication side effects.