Abstract
Nutritional psychology is a young but rapidly expanding field that has provided evidence that diet influences mental health. For example, fruit and vegetable intake has shown a positive dose-dependent relationship with mental health. However, fruit and vegetables contribute many vitamins and minerals to the diet, and precisely which micronutrients from fruit and vegetable intake are responsible for mental health benefits is not understood. One micronutrient worthy of further investigation is vitamin C, which is involved in the synthesis of numerous neurotransmitters and hormones and may therefore influence psychological processes.
This thesis aimed to provide insight into the relationship between vitamin C and mental health and determine if vitamin C drives mental health improvements derived from fruit and vegetable consumption. Firstly, I reviewed the literature on fruit- and vegetable-related mental health improvements and the link with vitamin C, detailing vitamin C mechanisms that may mediate this relationship. I also reviewed vitamin C-specific mental health research, which is mostly limited to homogenous samples with various pathologies. Consequently, the potential mental health benefits of vitamin C are largely unknown in a healthy heterogeneous population.
To determine the relationship between vitamin C and mental health, I conducted three empirical studies in healthy young adults (ages 18 - 35). Firstly, I conducted a placebo-controlled intervention trial comparing vitamin C-rich kiwifruit (whole-food) with a vitamin C tablet or placebo tablet to determine if vitamin C drove mental health improvements. Results from this trial are presented as Studies 1a and 1b. Study 1a presents the primary results with respect to mood and vitality assessment over the period of the lead-in, 4-week intervention and washout period. Study 1b presents ecological momentary assessments (EMA) used to determine the number of days of supplementation required to improve mental health. Second, I conducted a cross-sectional study assessing the relationship between plasma vitamin C levels and mental health in a heterogeneous sample (Study 2). Finally, I conducted a case study, in which a participant with clinically low vitamin C levels was supplemented with vitamin C (Study 3). In all three studies, I measured mental health outcomes that reflected aspects of ill-being (such as depressed and anxious moods), well-being (flourishing), and physical symptoms of fatigue.
Study 1a and 1b showed that kiwifruit significantly improved fatigue and well-being, and some negative mood states like anger. Vitamin C tablets showed comparable results, but only in participants with low vitamin C at baseline. No changes were observed in the placebo condition. EMA data (Study 1b) showed that mental health improvements occurred within four days of supplementation with kiwifruit and within 12 days of supplementation with vitamin C tablets. Interestingly, ethnicity moderated these results, which supported Study 2, where higher vitamin C levels were associated with better mental health among Māori, but worse mental health among Southeast Asians. The case study mirrored this association, in which the Southeast Asian participant’s mood was relatively good regardless of low vitamin C levels and appeared to worsen following vitamin C supplementation.
These studies provide novel insight into the role of vitamin C in mental health and possible variations in the relationship between vitamin C and mental health. While it is likely that vitamin C drives some mental health benefits observed from fruit and vegetable consumption, a whole food source such as kiwifruit can result in greater mental health benefits than vitamin C alone. Furthermore, the relationship between vitamin C and mental health may be moderated by ethnicity. These findings may have considerable implications in how nutritional psychology is approached and highlights that dietary needs may vary between individuals for obtaining optimal mental health outcomes.