Longitudinal changes in motivational determinants of sustainable diets during a blended digital behavior change intervention: a mixed methods study

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. This mixed methods study explored longitudinal motivational determinants change during a year-long blended digital intervention aimed at promoting sustainable diet change in healthy omnivorous adults. The 22-week intervention comprised app-delivered motivational and informational text messages, as well as 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 across the year and were analyzed with linear mixed models and within-person network analysis. Qualitative data were collected through semi-structured interviews at three times: baseline, post-intervention, and six-month follow-up, and were analyzed by using framework analysis. The quantitative and qualitative findings were integrated using joint display techniques. Quantitative analyses at the group level demonstrated 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 change was found in health motivation and self-efficacy. The qualitative findings provided an in-depth understanding of participants evolving awareness, gradual adoption of new eating habits, and varying levels of 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 the use of mixed methods designs to track the dynamic process of dietary behavior change.