A School-Based Behavioural Change Intervention Study Protocol for Adolescents in Sri Lanka to Prevent Obesity and Non-Communicable Diseases: Study Design, Implementation and Evaluation Plan

<ns5:p>Background Sri Lanka has made progress in communicable diseases and poverty reduction, however malnutrition in all forms remains a public health challenge. Previous research has focused primarily on short-term interventions towards primary school aged children, in high-income countries, often targeting narrow outcomes, such as fruit and vegetable consumption. This study aims to address some of these limitations by assessing the long-term effectiveness of a school-based behavioural intervention among adolescents through holistic dietary changes to help prevent obesity, type 2 diabetes, and cardiovascular disease. Methods This study will involve 11–14 year old adolescents attending public schools within Colombo and Gampaha districts of Western Province, Sri Lanka. It is a three-arm cluster randomised controlled trial, with one class per school being selected to participate. Based on sample size/power calculations, the trial will involve 244 schools/classes. Schools will be randomly allocated in a 1:1:1 ratio to two intervention arms (Arms A and B) and one control arm (Arm C). Arm A consists of a non-financial reward; Arm B consists of non-financial reward &amp; pre-commitment device and Arm C will be the control. A difference-in-differences approach will estimate the average treatment effect, adjusting for clustering, fixed effects, and relevant baseline covariates including but not limited to age, gender, and waist circumference. Subgroup analyses will also be conducted. Discussion This study aims to assess the feasibility and effectiveness of a holistic dietary behavioural interventions for adolescents aged 11–14. As the trial is school-based, participants and intervention staff cannot be blinded, but it will adhere to established procedures to maintain separation between staff that deliver the intervention and staff that take outcome measurements. As a minor percentage of children in this age group either study in private schools or drop out of school, we will not be missing a large percentage of children from our sampling framework. Registration Registered at ISRCTN (registration number: ISRCTN11340383) on 29 August 2025.</ns5:p>