Does the Intensity, Mode, and Timing of Exercise Affect Postprandial Gastrointestinal Function, Metabolic Responses and Energy Intake in Healthy Men

The global prevalence of obesity has dramatically increased and has become a major economic burden for western countries; therefore, health professionals are looking at strategies to control this increase in body weight. Despite the well-established physiological effects of exercise, such as increased muscle oxygenic capacity and fuel utilisation being well documented, there is limited research available investigating how the human body responds to a change in exercise characteristics. Manipulating exercise intensity, mode, or timing has become a popular strategy for controlling metabolic health and might support improved weight management programs. The ability of the human body to respond to nutritional intervention after exercise may determine how fat stores are regulated and in what manner the body responds postprandially. This may hold the key to how the body recovers and adapts after exercise, however, this area of research remains ambiguous.

Through a series of studies on human volunteers this thesis is aimed at enhancing our understanding of how different exercise characteristics (intensity, mode, or timing) might affect gastrointestinal function, metabolic responses, appetite, and energy intake (EI), and as a result support the development of novel non-pharmacological interventions for weight management.

The main findings of this thesis were as follows. Gastric emptying rate (GER) is similar 30 min after continuous and intermittent exercise at a low intensity of 40% V̇O2peak. Although, at a moderate intensity of 60% V̇O2peak the volume of the stomach empties faster after continuous exercise and is reduced after intermittent. Multiple bouts of continuous exercise cause food within the stomach to empty faster when compared to a one-off exercise bout matched at a high intensity of 70% V̇O2peak. Intermittent exercise > 40% V̇O2peak and continuous exercise > 60% V̇O2peak reduces subjective feelings of hunger immediately post-exercise. Although, continuous exercise < 50% V̇O2peak has no effect. A subsequent meal following both intermittent and continuous exercise at various intensities abolishes any compensatory effects in subjective feelings of hunger. Acylated ghrelin increases immediately after continuous exercise < 50% V̇O2peak whereas continuous exercise >70% V̇O2peak and intermittent exercise at peak power output (PPO) decreases acylated ghrelin. Furthermore, postprandial acylated ghrelin increases after multiple exercise bouts compared to a one-off continuous bout at the same intensity, although this also leads to an increase in EI within the first 24-h. EI was also found to be increased after moderate intense intermittent exercise. Intermittent exercise >40% V̇O2peak increases blood glucose during and immediately after exercise. Conflicting evidence revealed continuous exercise triggered a spike in blood glucose after a calorific meal more so than intermittent exercise causing blood glucose to remain elevated during recovery periods. Substrate utilisation shifts to predominantly fat oxidation after continuous and intermittent exercise at various intensities between 40- 70% V̇O2peak while a small calorific meal diminishes this increase in the postprandial period.

The role of manipulating exercise characteristics through intensity, mode, or timing may hold positive implications for weight management practices in healthy and overweight populations. Future work is warranted to investigate the influence of ingesting whole foods/meals after exercise to better recognise the changes during the postprandial period; on GER, appetite, and appetite regulatory hormones over an extended duration to explore the effects on energy balance and metabolic health in the long-term.