The burden of neurocognitive diseases, such as Alzheimer’s, is of global concern and points to an urgent need for accurate detection and prevention strategies. This thesis investigated dietary components, in particular those that affect vitamin B levels, such as white fish consumption and veganism, which have the potential to alter cognitive outcomes in the population. Also investigated were external factors, significantly caffeine consumption, time of day, and quantity of sleep, not associated with age-related impairment, that can alter neuropsychological test performance. The literature reveals that short, multi-domain cognitive tests are commonly used for dementia screening; this study has focused exclusively on cognitive domains associated with the early stages of Alzheimer’s disease. Cognitively healthy adults were assessed using a cognitive test battery that focused on memory and processing speed. Memory test scores improved by 0.03 points for every gram of white fish consumed, whilst participants following a vegan diet scored significantly worse on tests of composite memory than their omnivorous counterparts. Interaction tests showed that prior caffeine consumption improved short-term memory test scores up to age 70. Participants of all ages performed best at their age-optimal times and further improved for immediate word recall tests with prior caffeine consumption. Processing speed scores increased linearly with hours of sleep. The results of this thesis suggest that nutrition and external factors have a significant impact on cognitive test performance. These novel findings have implications for research, diagnosis and prevention of chronic neurological disease, and public health guidance relating to cognitive function, for all ages.