Integrating the theory of planned behavior and the self-determination theory to promote Mediterranean diet adherence: A randomized controlled trial

The Mediterranean diet (MD) is one of the healthiest and most sustainable food regimes. Nevertheless, MD diffusion is still limited, emphasizing the need to understand the psychosocial factors that could predict and promote its adoption. Starting from an integrated model of Theory of Planned Behavior (TPB) and Self-Determination Theory (SDT), the present randomized controlled trial investigated the effect of manipulating motivation (autonomous vs. controlled) on intention and MD adherence behavior. Participants included 726 Italian adults randomly allocated to one of three conditions: autonomous motivation manipulation, controlled motivation manipulation, and control group. TPB variables were measured immediately after manipulation (T1), while MD adherence was evaluated 2 weeks later (T2). Results from multivariate analyses of variance highlighted that participants in the autonomous motivation condition reported higher intention and a more favorable cognitive attitude than control group participants. However, no change in behavior was found. Moreover, a path analysis with mediation effect showed that the impact of autonomous motivation condition versus control group on intention was mediated by cognitive attitude. Findings support the integration of TPB and SDT to encourage intention to adhere to the MD, also suggesting that prompting autonomous motivation may help to promote a greater diffusion of this healthy and sustainable dietary pattern.