Abstract/Summary

Vitamin D was established for its skeletal function with less evidence available for its non-skeletal function, including cognitive function. Following recent interest in vitamin D supplementation to improve cognitive function perfonnance, this thesis aimed to identify the acute effects of 25-dihydroxyvitamin D3 on 25(OH)D levels and cognitive function, as well as other health outcomes such as mood and sleep. Two intervention studies; acute, randomized, double-blind, crossover intervention studies were conducted with the first study comparing the acute effect of 25- dihydroxyvitamin D3 and the usual form of vitamin D3 on plasma vitamin D levels and cognitive function at 8 and 24 hours post intervention. Results revealed that 25- dihydroxyvitamin D3 increased plasma vitamin D levels but not cognitive perfonnance relative to the Vitamin D3 formed at 24 hours, thus the second experiment was conducted to explore dose response effects on cognitive performance, as well as mood and sleep. Four doses of 0, 800, 2,000 and 4,000 IU of 25- dihydroxyvitamin D3 were used in this study. It was found that a low dose of 800 IU of 25-dihydroxyvitamin D3 revealed better perfonnance on executive function compared to higher doses of 2,000 and 4,000 IU. Conversely, quicker processing speed was observed after placebo and 2,000 IU. These findings suggested 25- dihydroxyvitamin D3 is most likely to benefit cognitive function at a low dose but not at a high dose. Finally, considering that the majority of vitamin Dis produced by skin exposure to the sun, this thesis also explored seasonal vitamin D levels and the effects on cognition and mood. It is evident that vitamin D level was significantly higher during summer with a higher score indicating more times was spent outside. This exploratory study also showed that participants completed the Stroop Task quicker with less errors during summer which suggested the positive effect of being outdoor under the sunshine. Overall, whilst evidence for acute effects of Vitamin D on cognition in the current thesis was mixed, future studies are recommended to includ