Signals for starting a meal | Signals for stopping food intake – Meal size | Signals for controlling food intake – Body weight | Pinel (2007)In Western countries, people usually eat to a set schedule, and the digestive system starts to prepare itself by releasing saliva and enzyme just before meal time. This represents a learned response, anticipating the presence of food. | Role of Cholecystokinin CCKThe hormone CCK has an opposite effect to ghrelin. Studies have shown that injection of the CCK in animals and humans causes a reduce meal size (Smith, Gibbs and Kulkosky, 1982), which animals with a genetic mutation eliminating the CCK system become obese. | Lipostatic Theory | The role of GhrelinGhrelin is secreted from the stomach, and the amount released is directly proportional to the emptiness of the stomach, i.e. as the time from the last meal increases and we feel hungrier, so ghrelin secretion is increased. | Glucostat theoryThis proposed that blood glucose levels were a key signal to the brain, falling when we were hunger and rising as we eat. Blood glucose levels do respond rapidly to food intake, but it was quickly shown that there were some weaknesses to this theory. Glucose levels do not vary much under normal circumstances and not enough to be an effective signal of hunger and satiety. Additionally, people with diabetes often have long-term high levels of blood glucose, but still attain their normal appetites. | Carlson (2007)Studied Ob Mice who didn’t produce leptin, which made them continuously eat and thus them becoming obese. By injecting leptin into the Ob mice prevented them from eating as much and their weight significantly reduced to normal.*Leptin = A hormone released from the adipose tissue. It acts as an indicator of body weight.*Ob mice = Genetically obese mice. | Neural Mechanisms controlling eating and satiation: