Effect of energy restriction on appetite regulation and metabolism at rest and during exercise

Current methods of energy restriction are not successful for achieving long-term weight loss and maintenance for the majority of individuals. As a result, the prevalence of obesity and obesity related diseases continue to increase. This calls for the development of novel lifestyle interventions to combat the obesity epidemic. Hunger has been highlighted as a major factor influencing the long-term success of weight management methods and therefore how a given dietary intervention affects the appetite regulatory system may dictate the success of the diet by augmenting long-term adherence. In addition, the effect of a given dietary intervention on exercise may determine its suitability for exercising individuals and may influence the energy deficit that can be achieved by the diet. This thesis investigated the acute effects of two novel methods of dietary restriction; breakfast omission and severe energy restriction. The main aims for this thesis were to determine the effect of these methods of energy restriction on ad-libitum energy intake, subjective appetite sensations, and peripheral concentrations of hormones involved in appetite regulation. In addition, this thesis also investigated the effects of these methods of energy restriction on metabolism and glycaemic control at rest, and performance and perceived exertion during exercise. This work found that moderate and severe energy deficits induced by breakfast omission and 24 h of severe energy restriction, respectively, resulted in either no (Chapter VIII) or partial (Chapters IV and VII) energy intake compensation over the subsequent 24-48 h. Subjective appetite was increased during (Chapters IV, V, VII and VIII) and shortly after (Chapter VII) energy restriction, but this effect was transient and was offset after an ad-libitum (Chapters IV and VII) or standardised (Chapters V and VIII) meal. In addition, none of the work presented in this thesis demonstrated an appetite hormone response to energy restriction that was indicative of compensatory eating behaviour. Compared to breakfast omission, breakfast consumption resulted in an increased in resting energy expenditure and carbohydrate oxidation, with a concurrent reduction in fat oxidation during the morning. However, there were no differences after lunch (Chapter V). In response to a standardised breakfast, resting energy expenditure was suppressed (Chapter VII) or not different (Chapter VIII) the following morning, after 24 h severe energy restriction compared to energy balance. Plasma NEFA and fat oxidation was greater, carbohydrate oxidation was reduced, and postprandial insulin sensitivity was impaired in the after 24 h severe energy restriction (Chapter VI, VII and VIII). In Chapter IV, omission of breakfast in the morning was shown to reduce exercise performance in evening, even after provision of an ad-libitum lunch 4 h before. However, there was no difference in perception of effort during steady state exercise, independent of breakfast consumption or omission in the morning (Chapters IV and V). Collectively, breakfast omission and 24 h severe energy restriction reduce energy intake and promote an appetite regulatory response conducive to maintenance of a negative energy balance. Chronic intervention studies are now required to confirm whether these effects persist after long-term practice of these dietary interventions.