Obesity and associated conditions such as type 2 diabetes and cardiovascular disease are major global health concerns. A contributing factor is overconsumption of energy dense foods, rich in fat and sugar, driven by our hedonic and physiological desire to consume energy rich foods. One approach has been to reduce the energy content of foods whilst retaining the sensorial quality. The food industry has developed a wide range of healthier products. In order for these products to be successful, consumers must continue to purchase and consume them. However, recent studies have suggested that reducing the energy content of a food may have unintended consequences. Specifically, if a food looks, and tastes like it is rich in nutrients, the gut and brain systems prepare to expect a high energy intake. If this does not happen, as in the case of a reduced calorie food, the brain appears to induce hunger signals that drive the individual to overconsume at subsequent meals (rebound hunger), resulting in an increased calorie intake, thus negating the whole effect of consuming the reduced calorie food. This leads to consumer dissatisfaction with these products, and potentially to imbalances in appetite and hunger hormones that often cause rapid weight gain following periods of dieting. Therefore we need to rethink how reduced calorie foods can be used more effectively to control energy intake. To do this, we need to understand the relationship between how we sense foods, how we digest and absorb the nutrients and how the brain responds to these processes and controls subsequent appetite signalling. This Mouth-Gut-Brain system is key to controlling our appetite and energy intake. The aim of this project is to understand how the mismatch between sensory properties and nutrient intake of food controls our appetite and rebound hunger. The key question we aim to address is, by how much can the energy content of a food be reduced before rebound hunger and overconsumption occurs? The key impact will be our ability to modify reformulated foods to reduce the gap between sensory and nutrient signals in order for reduced-energy alternatives to satisfying and not result in subsequent over-consumption. This project will focus on fat, as fat has the highest energy content, and the sensory properties of low fat foods are challenging for both consumers and the food industry. We will investigate the impact of reducing the fat content of foods by determining how the sensory properties control consumer expectations of satiety (feeling of "fullness"), and measure how much we can reduce fat content before consumers develop rebound hunger and overconsume at a subsequent meal. Using this information we will design a more realistic food where the structure, physical behaviour and sensory properties are closely matched, but with a range of fat contents and fat type, to carefully control how much fat is "sensed" and how much is absorbed. We will measure how the appetite response of these foods controls the consumption of food in a following meal; and study differences between individuals who are sensitive or insensitive to fat content in foods. This will determine how much we can alter the sensory and fat content of food and still maintain an overall reduction in energy across subsequent meals. Results will provide valuable information on how mouth-gut-brain signalling fundamentally controls appetite, and begin to unravel why different individuals may be more susceptible to rebound hunger following the consumption of reduced calorie foods. The research will also enable us to define a broader research programme to investigate mouth-gut-brain interface that will study in more detail variations between individual responses, and the biological mechanisms behind our behavioural measures (such as gut hormone levels). Knowledge generated will enable better approaches to reduced calorie foods that are more effective at reducing energy intake in the longer term.