The Underlying Mechanisms that Drive Positive Mood Improvement Following Diet Change. Investigating the Casual Role of Diet via Psychological Factors and Biological Processes in Mental Health Disorders

<p dir="ltr">Dietary improvements, such as adopting a modified Mediterranean diet, have been shown to significantly improve mood and reduce clinical depression within three weeks—effects comparable to antidepressants. While these outcomes are often attributed to physiological mechanisms affecting brain function, the potential role of psychological factors, such as self-efficacy (belief in one’s ability to succeed) and expectancy bias (belief in anticipated outcomes), remains underexplored. This study examines whether mood benefits from dietary interventions can, in part, be attributed to psychological factors (alongside biological explanations), particularly self-efficacy and expectancy effects. These questions extend to lifestyle intervention research more broadly, exploring experimental control of psychological variables. This thesis addresses three key aims: (1) to review randomised control trials (RCTs) of lifestyle interventions for inclusion and control of psychological factors, (2) to explore consumer beliefs and biases regarding canned and frozen fruit and vegetables as an indirect measure of expectancies and thus possible techniques for mitigating expectancy biases in food-based interventions, and (3) to investigate whether psychological factors contribute to mood improvements alongside or independent of physiological effects. An initial narrative literature review examines the control of psychological factors in lifestyle interventions from 2014–2025. Finding that while counselling research is better equipped to control psychological factors through the use of antidepressants and equivalent therapeutic interventions, diet and exercise interventions often face practical constraints. These constraints typically limit control conditions to waitlist or befriending groups. Studies 1 and 2 assessed consumer biases toward canned and frozen fruits and vegetables (F&V), which are nutritionally equivalent to fresh produce but underutilised. Implicit and explicit bias assessments revealed persistent negative perceptions regarding health, appeal, and mental health benefits. These results were used in Study 3 as an experimental manipulation for expectancy effects. Study 3, a RCT with participants experiencing low mood and poor diet, assigned individuals to one of four conditions: fresh food, canned/frozen food, cooking, or wait-list control. Despite dietary change occurring in only two intervention groups, all three showed significant mood improvements, suggesting psychological factors played a role. Regression analysis identified that self-efficacy and motivation are key psychological factors that relate to mood change when engaging in dietary intervention. These findings highlight the need to consider psychological influences in lifestyle interventions, as they may contribute to benefits traditionally attributed to physiological changes.</p>