Citizen science for healthier food environments: a realist-informed process evaluation of the big food map app in Flanders, Belgium.

Introduction: Food environments, the physical, economic, political, and sociocultural contexts shaping food availability, affordability, marketing, and social norms, strongly influence dietary behaviors and population health. The Big Food Map ( Methods: The Big Food Map smartphone application was co-created with citizens, particularly adolescents and adults on lower incomes, as well as with actors, such as local governments, retailers, and short-chain initiatives. Co-creation involved Photovoice workshops with adolescents and adults on lower incomes and Group Model Building sessions with actors to explore how they understand food environments and identify key food environment dimensions and settings. Citizens and actors also tested and refined the app. The app was launched in May 2025 until August 2025 to crowdsource and provide feedback on citizen perceptions and behaviors related to neighborhood food environments. A brief evaluation questionnaire assessed the app's usefulness and potential behavioral impact among citizens, while a more detailed survey explored stakeholder perspectives on the co-creation process and policy relevance. Results: In total 4,707 citizens contributed data across 1,320 neighborhoods (48% of all Flemish neighborhoods) and evaluated 11,313 food outlets. Participants appreciated the opportunity to visualize and discuss their local food environments. Approximately half liked the app's design, 24% reported increased awareness of their food environment, and 56% said they would participate again. Both citizens and stakeholders highlighted challenges around long-term engagement, technical usability, and inclusion of vulnerable groups. The co-creation process strengthened partnerships between research, policy, and practice and underscored the value of tailoring citizen science tools to local contexts and communication channels. Discussion: The Big Food Map demonstrates the feasibility and potential of citizen science to enrich food environment monitoring and stimulate local dialog and action. To sustain engagement and policy relevance, future efforts should strengthen collaboration with local governments and media, diversify participation strategies, and embed citizen science tools into municipal food policy strategies or frameworks.