Abstract

The adoption of sustainable diets in high-income countries is an urgent environmental and health priority. Understanding motivational determinants in behavior change interventions can contribute to clarifying the main pathways to effective dietary transformation. Yet, longitudinal mixed-methods evidence on motivational shifts during digital behavior change interventions remains scarce. This mixed methods study explored longitudinal motivational determinants change during a year-long blended digital intervention promoting sustainable diet change in healthy omnivorous adults. The 22-week intervention comprised app-delivered motivational and informational text messages (focusing on self-monitoring, information about health and environmental consequences and instructions on how to perform a behavior), and tailored online feedback sessions. Quantitative data were collected through repeated self-reports of diet and motivational determinants (e.g., health, environmental, ethical, price, self-efficacy, and habit strength) over 15 weeks and analyzed with linear mixed models and within-person network analysis. Qualitative data from semi-structured interviews at baseline, post-intervention, and six-month follow-up were analyzed using framework analysis. Quantitative and qualitative findings were integrated using joint display techniques. Group-level quantitative analyses demonstrated small (0.02-0.04 points/week on a 0-10 scale) but significant increases in environmental and ethical motivations, habit strength, and adherence to a more sustainable diet, alongside a decrease in price as motivation for food choice. No significant change was found in health motivation and self-efficacy. These quantitative shifts were complemented by qualitative findings providing an in-depth understanding of participants' evolving awareness, gradual adoption of new eating habits, and varying confidence in maintaining behavior changes despite contextual barriers. Individual-level results showed important heterogeneity of both motives and behaviors, and within-person network analysis showed no covariation between motivational determinant change and diet change. This study demonstrates the potential for blended digital interventions to influence motivational pathways related to sustainable diets and supports using mixed methods designs to track the dynamic dietary behavior change process.