Can an apple a day keep the psychologist away? The role of fruit and vegetable intake in mental well-being.

‘You are what you eat’ is a well-known adage that is supported by evidence linking healthy diets to greater physical well-being. The cornerstone of a healthy diet is a high intake fruit and vegetables, containing a variety of micronutrients critical for optimal physical and mental functioning. Given the associated physical benefits, there has been an increasing level of interest in the potential role of fruit and vegetable intake (FVI) in mental health. However, the literature linking diet to mental health is limited and reflects a number of methodological issues that preclude researchers from understanding the full extent that FVI can play in mental health. These include: a predominant focus on factors relating to mental ill-being (such as depression), issues with control variables and specificity of mental health measures, limited investigation into plausible mechanistic pathways, and most importantly, a paucity of experimental research that speaks to the causal nature of this relationship. The aim of this thesis was to provide a comprehensive, multi-method approach to examining the observational and causal relationships between FVI and a range of mental health outcomes in young adults; guided by the World Health Organisation definition that mental health encompasses not only the absence of illness (e.g., depression, anxiety), but the presence of well-being (e.g., flourishing, vitality). First, I provide an overview of the literature, highlighting the methodological gaps which provide the rationale for the subsequent empirical studies. These include: a correlational study exploring the influence of FVI on a range of mental well-being outcomes (e.g., flourishing); a randomised controlled trial exploring whether the relationship between FVI and mental health is causal and whether key micronutrients mediate this link; and finally, a large observational survey study exploring the differential effects of raw versus cooked/canned/processed FVI on mental health. Additionally, secondary aims explored the development and execution of a mobile phone-based Ecological Momentary Intervention (EMI) to increase FVI in low-consuming young adults. Collectively, this body of work provides insights into the nuances of the relationship between FVI and mental health. Overall, daily fruit and vegetables, especially those consumed raw, appear to have significant links with mental well-being outcomes, such as vitality, creativity, curiosity, motivation, and socio-emotional flourishing. While this thesis provided some support for the role of FVI in buffering against mental ill-being such as depressive symptoms, the links with well-being were consistently stronger. Most importantly, this thesis addresses the major limitation highlighted in the literature – causality – by providing the first evidence that the relationship between FVI and mental well-being is causal, and can occur relatively rapidly in day-to-day life. These findings suggest that we eat has a powerful effect on how we feel, and that policy makers and clinicians can harness nutritional psychiatry strategies as a promising route of mental health improvement