Evidence is accumulating to support the effects of specific nutritional provision on both physical and mental performance, with implications to enhance performance. However, to date such investigations have been carried out independently. As deterioration to either physical or mental performance can have a negative impact on overall endurance performance, the purpose of this thesis was to investigate the combined effects of dietary manipulation on physiological and psychological parameters. At the outset, a qualitative study was conducted to explore the eating patterns and food choices of a group of endurance athletes, and to investigate attitudes towards nutritional management for performance enhancement. The analysis revealed higher order order themes of 'somatic complaints',' performance', 'trust',"'preferences' and 'routine'. It was found, for example, that food choices, especially those of more competitive athletes, were made in order to maximise performance where such choices were based on past experience and 'trial and error' rather than specialist advice. Interestingly the issue of 'trust' emerged which requires further investigation as reluctance to utilise nutritional advice may be a limiting factor when promoting the enhancement of performance. Laboratory experiments were then undertaken to investigate the effects of acute and chronic dietary manipulation on physiological and psychological parameters, A low glycaemic response (GR), generated via acute manipulations, was found to enhance cognition, improve mood and satisfy appetite when compared with a high GR. Following a chronic manipulation, the beneficial effects of a low GR were extended to include reduced heart rate and respiratory exchange ratio and lower ratings of perceived exertion during submaximal exercise. As the findings from the laboratory experiments confirmed benefits of a low GR, the results were extrapolated to a 'real world' setting.