Supplementary Material for: Design, methods and pilot baseline results of the SEÍSMO study: a school-based cluster randomised controlled trial to prevent pediatric obesity among disadvantaged communities in Spain.

Introduction: Pediatric obesity is a major health issue with a growing worldwide trend since 1990. The present study aims to describe the design, methods, and pilot baseline results of a school-based obesity-prevention intervention implemented in disadvantaged communities. Methods: The SEÍSMO study integrates two stages: The first stage includes baseline and follow-up assessments for 11,139 children aged 6-12, evaluated at baseline between 2020 and 2024 in 120 Spanish schools. The second stage is a six-year cluster randomised controlled trial (RCT) that incorporated the learnings of the first stage to assess from 2024 to 2041 two primary outcomes (zBMI and WHtR) and several secondary outcomes (healthy lifestyles, quality of life, academic performance, psychosocial determinants, psychological resilience, and weight stigma) will be evaluated at 12, 24, 36, and 48 months. A sample size of 56 clusters and 4760 participants is needed, assuming an attrition rate of 35% by 36 months post-baseline, but an oversampling will result in 10,080 participants. In this stage will involve three recruitment editions (2024-2025, 2025-2026, 2026-2027) and will incorporate children aged 3-12 located in Catalonia, Spain. The intervention includes a total of 50 specific action groups. In both study stages, a follow-up will be conducted at 18 years of age, involving a total of 21,219 children. Results: SEÍSMO first stage participants present prevalences of severe obesity and abdominal obesity of 5.4% and 31.3%, respectively. Conclusions: There is an urgent need to have effective obesity-prevention with the capacity to reduce the current inequalities. Trial Registration: This study protocol is registered and approved in the ISRCTN registry (ISRCTN10335633) on the 19th of March 2025.