Whilst exercise training can improve aspects of metabolic health (e.g. increasing insulin sensitivity), there can be some apparent variability between people in their response to supervised exercise training. Exercise studies typically do not control for the nutrient status of participants pre-exercise and this may, at least partially, explain this variable response. Performing a single bout of exercise before versus after eating breakfast can alter substrate metabolism during- and energy balance post-exercise. This includes increasing the utilisation of fat as a substrate during exercise, augmenting the activation of intramuscular signalling pathways relating to substrate metabolism and when breakfast is omitted, lowering energy intake on the day that the exercise is performed. These acute exercise-responses may be important for metabolic health when regularly accrued with training, especially for humans with obesity, for whom impairments relating to lipid metabolism and/or a low turnover of endogenous lipid pools (muscle and adipose tissue) may explain the association between obesity, physical inactivity and insulin resistance. The studies in this thesis first showed that when healthy men performed a bout of endurance-type exercise, substrate utilisation during exercise, plasma glucose flux at a meal consumed after the exercise, post-exercise intramuscular signalling responses and 24 h energy balance were dependant on whether breakfast was consumed before the exercise or omitted (Chapters 3 and 5). Nutrient-exercise timing was then shown to mediate training responsiveness in men classified as overweight or obese, with exercise before versus after breakfast increasing oral glucose insulin sensitivity, which was linked to increased lipid utilisation during exercise and augmented skeletal muscle adaptations (Chapter 6). These findings have implications for future research and clinical practice. For example, both acute and training exercise studies should control for nutrient-timing as a potential confounding variable when measures of postprandial metabolism are outcome measures and for populations at risk of metabolic diseases, endurance-type exercise should be recommended before versus after nutrient consumption (breakfast) to increase the health benefits acquired from training.
- Energy Balance
- Insulin Sensitivity