ABSTRACT Nearly one third of the world’s population lacks access to adequate food, and over three billion people cannot afford a healthy diet. Nutrition insecurity - the lack of consistent access, availability, and affordability of foods and beverages that promote well-being and prevent disease - leads to both undernutrition and obesity/overweight: a ‘dual burden’ associated with high rates of non-communicable disease, particularly in low- and middle-income countries (LMICs). This is especially true in informal urban settlements, where our work has shown that a lack of formal infrastructure and access to services exacerbates underlying social and economic barriers to eating well. Adolescence is an optimal time to intervene to reduce nutrition insecurity and malnutrition. However, interventions seldom target adolescents, nor are they tailored to the lived experiences of adolescents or to the broader systems that influence adolescent food choices and nutrition security. The overarching goal of this project is to reduce nutrition insecurity and malnutrition among adolescents in LMICs. To do this we employ an innovative combination of participatory ‘systems thinking’ methods, quantitative empirical data, and modeling. We will work in informal and formal settlements in Nairobi, Kenya so that we can characterize differences in the food systems between them, and understand how these differences, and other individual-, household- and community-level factors, influence adolescents’ food choices. We will (Aim 1) physically mapping the food environments of 10 schools (2 per neighborhood in each of 5 neighborhoods) and use a participatory systems science toolkit that includes cognitive mapping and group-based modeling to understand how adolescents make decisions around food. In parallel, we will quantify the influence of nutrition insecurity on dietary patterns, malnutrition, and food preferences (Aim 2) by conducting surveys (using standardized measures of food and water insecurity and the perceived food environment) with adolescents and their primary caregivers as well as repeated 24-hour dietary recalls, and physical measurements (body mass index, micronutrient status, and hemoglobin) with adolescents (n=700) at three time points. Food preferences will be assessed by conducting a discrete choice experiment with the adolescents included in our sample. The data from Aims 1 and 2 will then be used to co-design food systems interventions to improve adolescents’ food choices, nutrition security, and malnutrition outcomes and rank them using stakeholder-derived multi-criteria decision analysis (Aim 3). We will work with local policymakers and community leaders to prioritize interventions with the potential to transform LMIC food systems that are experiencing urban growth based on their feasibility and likely impact. This project will address an urgent need to develop double-duty interventions that address the root cause of undernutrition and overweight/obesity ...