The effect of endogenous and exogenous carbohydrate availability on exercise-induced skeletal muscle signalling

Whilst pre-exercise muscle glycogen concentrations appear to regulate mitochondrial-related cell signalling responses to exercise, the absolute concentrations of muscle glycogen required to potentiate such responses are currently unknown. With this in mind, chapter 4 and 5 of this thesis were designed to systematically titrate muscle glycogen concentrations and determine subsequent mitochondrial-related signalling responses to endurance exercise in an attempt to elucidate the potential existence of a glycogen threshold and provide a better definition of its upper and lower limits. Collectively, using a sleep-low, train-low model, these studies demonstrate that reducing pre-exercise muscle glycogen prior to high-intensity endurance exercise does not confer any benefit to mitochondrial-related signalling and, in some cases, may impair exercise capacity. Nonetheless, these data raise the possibility that obtaining low post-exercise glycogen concentrations (regardless of pre-exercise status) may also be an important factor in regulating mitochondrial-related signalling. Accordingly, given the potential for exogenous CHO to modulate post-exercise muscle glycogen concentrations, chapter 6 examined the effects of exogenous carbohydrate (CHO) feeding during exercise on mitochondrial-related signalling. However, exogenous CHO feeding did not alter muscle glycogen utilisation during exercise and resulted in comparable increases in post-exercise mitochondrial-related signalling. In conclusion, this thesis provides novel data demonstrating that reducing pre-exercise muscle glycogen and/or exogenous CHO restriction during exercise does not confer any additional benefit to exercise-induced mitochondrial related signalling. Practically, these data suggest that the additional stress of CHO restriction may not be required within skeletal muscle that is already subjected to a sufficient metabolic challenge and may be better suited during conditions that do not elicit such cellular perturbations (e.g. prolonged low-intensity exercise completed below lactate threshold)