Abstract
Vitamin D has a well-established role in physical health, specifically bone formation and mineralization through its role in calcium homeostasis. Recently, new evidence suggests that vitamin D may play a role in mental health as well. For example, observational studies have shown that low serum 25-hydroxyvitamin D, the stable circulating form of vitamin D used as the marker of vitamin D status, is associated with a greater prevalence of depression, presence of depressive symptoms, and incidence of seasonal affective disorder. There is also some evidence from intervention studies, showing that vitamin D supplementation improves depressive symptoms, but the findings are conflicting. The aim of this PhD thesis was to investigate the role of vitamin D in mental health, focusing not only on depressive symptoms, but also expanding to a broader range of well-being measures, including some close-to-real time measures of well-being. To do this, I conducted an observational study followed by a randomised double blind placebo controlled trial. The observational study aimed to replicate previous findings linking serum 25-hydroxyvitamin D and depressive symptoms in a young adult sample, and to extend previous findings by testing novel links between 25-hydroxyvitamin D and a range of psychological well-being measures. University of Otago students (n=615) in Dunedin, New Zealand (45°52′S) completed the Center for Epidemiologic Studies Depression Scale (CES-D) and provided a non-fasting venous blood sample for serum 25-hydroxyvitamin D analysis. Even after controlling for age, gender, ethnicity, BMI, and time spent outdoors during two weeks prior to the blood sample collection, there was a strong association between serum 25- hydroxyvitamin D and the total CES-D score in line with previous research. Next, I examined the association of serum 25-hydroxyvitamin D with a broader range of well being outcomes, including positive and negative mood, subjective well-being, and flourishing in the same sample. Participants reported their well-being using global and daily reporting formats (“How they felt typically”, and “How they felt today” aggregated across 13 days, respectively). Interestingly, I found that higher serum 25-hydroxyvitamin D was associated with greater global positive mood, global subjective well-being, and global flourishing, but there was no association between vitamin D and the daily measures. The randomised double blind placebo controlled trial aimed to test the causal effects of vitamin D3 supplementation on depression and well-being. A sample of premenopausal women (n=152) provided a baseline blood sample for serum 25-hydroxyvitamin D analysis and took a monthly dose of either 50,000IU vitamin D3 or placebo over 6 months during the winter period in Dunedin, New Zealand. They completed monthly reports of depressive symptoms, anxiety, and global flourishing. Additionally, they reported on their daily positive and negative mood for three consecutive days every second month. At the end of the follow up period, participants provided another blood sample. The participants in the active ingredient group maintained their serum 25-hydroxyvitamin D at near summer concentration, while the concentration decreased in the placebo group. Nevertheless, there were no group differences in depression, anxiety, or any of the well-being measures, suggesting that supplementation with vitamin D3 was not effective at improving these outcomes. The mixed results across the two studies indicate that while vitamin D is associated with depressive symptoms and psychological well-being, it is unlikely to be the causal agent. Even though low 25-hydroxyvitamin D is seen in depression and other mental health issues, there could be other factors influencing both mental health and vitamin D status. Such factors could include wavelengths of light other than the range required for vitamin D production, nitric oxide effects on cerebral vasculature, or the presence of inflammation. Future studies should investigate other factors involved in depression and psychological well-being, taking into account vitamin D status.