The Effects of Dietary Protein at Breakfast and Across the Day on Appetite Control & Satiety, Food Intake, and Sleep Quality

The movement to adopt healthful lifestyle practices to reduce the prevalence of obesity and associated chronic diseases has garnered global interest [1]. As such, the desire to consume more protein-rich foods is a commonly used strategy due to the documented improvements in weight management observed with higher-protein (HP) diets, ranging from 1.2-1.6 g protein•kg body weight-1•d-1[2-6]. One postulated mechanism through which HP diets elicit greater weight (and fat) losses includes the improvements in ingestive behavior which appear to be mediated by a number of satiety-stimulating, physiological and/or hedonic pathways, and lead to reductions in daily food intake [3]. The main objectives of this dissertation were to: 1) Test whether the consumption of higher-protein (HP) compared with normal-protein (NP) meals consumed at each eating occasion reduces free-living, daily carbohydrate and fat intakes in overweight women during energy balance conditions and whether the distribution of protein consumed throughout the day affects food intake outcomes. 2) Test whether the consumption of a HP energy-restriction diet reduces carbohydrate and fat intakes through improvements in daily appetite, satiety, and food cravings compared with NP versions and to test whether protein type within the NP diets alters protein-related satiety. 3) Test whether the consumption of breakfast vs. breakfast skipping improves subjective and hormonal markers of appetite, satiety, and sleep in combination with ad libitum food intake in young adults prone to subclinical sleep disturbances. This dissertation is organized into chapters which consist of published manuscripts or manuscripts formatted for submission to peer-reviewed journals. Details pertaining to the status of each manuscript are included at the beginning of each chapter. Chapter 2 includes a comprehensive review of the intervention-based evidence surrounding the consumption of breakfast vs. breakfast skipping with specific emphasis on appetite control and satiety, energy expenditure, and sleep and circadian health. Chapter 3 includes a randomized cross-over trial which examines the effects of increased dietary protein intake across the day on free-living ad libitum food intake in overweight women during weight maintenance conditions. As a secondary outcome, the effects of protein distribution across the day were tested. Chapter 4 includes a randomized cross-over trial which examines the effects of consuming a HP energy-restriction diet vs. NP energy-restriction diet on appetite control, food cravings, and free-living ad libitum food intake in overweight women. Secondary assessments examining whether protein type within the NP diets alters protein-related satiety were also included. Chapter 5 includes a randomized cross-over trial examines whether breakfast consumption compared to breakfast skipping improves subjective and hormonal markers of appetite, satiety, and sleep in young adults. Finally, Chapter 6 summarizes the main dissertation findings and presents future directions for research. Collectively, the findings from this dissertation demonstrated the following: 1) Providing 30 g protein/meal at each eating occasion throughout the day did not influence free-living, daily intake of highly palatable, carbohydrate and fat-rich foods in overweight women; 2) While appetite control, satiety, and food cravings were improved following a higher-protein, energy-restriction diet, increased protein consumption did not reduce free-living carbohydrate and fat intake throughout the free-living test day in overweight, healthy women exposed to highly palatable foods; and 3) the daily consumption of breakfast improved appetite control and diet quality, through reductions in unhealthy evening snacking, and may support improvements in some aspects of sleep health in healthy young professionals. These findings provide evidence which suggests that the inclusion of increased dietary protein at breakfast and across the day are dietary strategies for improving some aspects of appetite control. Notably, not all findings demonstrated consistent support of appetite control. Thus, this work highlights that the effectiveness of including dietary protein to support appetite control and improved eating behavior as contributing factors underlying weight management is contingent upon various physiological and behavioral circumstances