| Summary: | Background: Based on the non-linear relationship between energy intake and physical activity level demonstrated by Mayer et al. (1956), it has been proposed that there is a dysregulation of appetite at lower levels of physical activity leading to overconsumption, whereas at higher levels of physical activity, appetite control is more sensitive. However, the mechanisms underlying this relationship are not well understood. Objective: This thesis aimed to shed light on the impact of habitual physical activity level in lean individuals and exercise training in individuals with overweight and obesity on homeostatic (physiological) and non-homeostatic (hedonic and eating behaviour traits) appetite processes, including those that initiate and terminate feeding (satiation) and suppress inter-meal hunger (satiety), as well as passive overconsumption (unintentional increase in energy intake with high-energy-density/high-fat foods). Methods: A systematic review was conducted, followed by four experimental studies. The studies employed a multi-level experimental platform that included biological, behavioural and psychological aspects of energy balance including free-living physical activity, food intake, diet composition, food reward (liking and wanting), body composition, energy expenditure and fasting appetite-related peptides. Results: The systematic review revealed a J-shaped relationship between physical activity level and energy intake, corroborating previous findings. Data from the experimental studies indicated that in lean individuals, physical activity did not influence satiation at meals varying in dietary fat content, but moderate to high active individuals showed enhanced satiety with better ability to adjust intake following preloads varying in energy content. Exercise training (12 weeks) in inactive individuals with overweight and obesity improved both homeostatic and non-homeostatic appetite control, which may be mediated by exercise-induced fat loss. Across all studies, physical activity was associated with lower body fat and greater daily energy expenditure, and energy density was positively associated with energy intake and passive overconsumption. Conclusions: This thesis provides confirmatory evidence that physical activity impacts appetite control through a dual-process action expressed through an increased drive to eat from greater energy expenditure, together with enhanced satiety response to food in both lean and overweight/obese individuals. These processes may allow for more accurate matching of energy intake to requirements and a reduction in the risk of overconsumption at higher levels of physical activity.
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