Nutritional Manipulation Strategies to Enhance Endurance Exercise Performance
Athletes manipulate training volume and intensity to maximise training adaptations and performance. As performance levels continue to rise, athletes often seek to incorporate novel strategies to optimise their training response beyond the simple levers of training volume and intensity. Nutritional stimuli can undoubtedly alter the adaptive response to training. Recent scientific advances have shaped our understanding of how the presence or absence of specific nutrients during training can synergistically modify the adaptive training response and influence exercise performance. In this regard, intentional manipulation of carbohydrate (CHO) intake to modify glycogen availability before and during training sessions is a strategy that may present opportunities to promote favourable endurance adaptations. This is a practice adopted amongst elite and recreational athletes; however, currently there is limited convincing evidence that such strategies produce improved performance in well-trained athletes. Furthermore, mechanisms that link CHO manipulation-induced adaptations and performance are not fully understood. This thesis aimed to determine the acute and chronic effects of CHO manipulation on high intensity exercise performance and various physiological performance determinants in well-trained athletes.
In the first part of the thesis, the literature review outlines the impact of reduced glycogen availability on various duration endurance performance tasks. It highlights physiological and metabolic targets that may be affected when an exercise bout is commenced with reduced muscle glycogen. Several of these performance-related factors were addressed in the subsequent studies of this thesis. Study 1 (Chapter 3) explored the relationship between acute muscle glycogen manipulation and high intensity exercise performance and physiological responses that could explain performance outcomes. The following study (Study 2, Chapter 4) evaluated to what extent CHO intake during prolonged and high intensity exercise with low muscle glycogen could alter performance and other performance-related factors such as exercise economy and substrate metabolism. Finally, Study 3 (Chapter 5) explored the practical application of periodised CHO manipulation during a 4-week training intervention period using high intensity interval training (HIIT). This study was key to understanding the dynamic changes in substrate metabolism and economy during the training intervention and determining if periodised CHO manipulation could improve performance in well-trained endurance athletes.
The results from these studies indicate that well-trained athletes can maintain high intensity exercise performance even under low muscle glycogen conditions, albeit at higher perceived effort in some instances. Importantly, CHO intake may provide performance benefits when high intensity exercise is commenced with reduced muscle glycogen. However, the combination of HIIT and periodised CHO restriction did not enhance performance, compared to training under high CHO availability, despite rapid yet temporary alterations in substrate metabolism during the training period. Overall, these results provide new knowledge of the acute and chronic effects of periodised CHO restriction on high intensity exercise performance. The findings have practical relevance and may assist athletes, practitioners, and coaches with designing programs that incorporate CHO manipulation strategies as part of training.