The broad framework of the mechanisms controlling food intake involves peripheral nutritional cues, including hormones or nutrients such as glucose, amino-acids or fatty acids, which circulate in the blood and signal to the brain. Leptin for example, is secreted from fat, with its circulating levels directly related to the amount of fat in the body, thereby signalling to the brain the status of ‘long-term’ energy stores and thus how long we would survive without food. Other hormones are produced from the gastrointestinal tract, and informing our brain not only where in the body the food is (is it in the oesophagus, stomach, small or large intestine?), but also the proportion of protein, carbohydrate and fat, as well as the caloric content of the meal. These gut and stomach signals can be considered ‘short-term’ signals, reflecting meal-to-meal variation. The brain integrates these short term and long term signals in order to control food intake.
The system our brain uses to control food intake consists of two parts; a fuel sensor, which calculates how many calories we have burnt during the day, and therefore how many calories we would need to consume to meet this need. However, given the importance that eating enough has on keeping us alive, our brain has also evolved mechanisms to make sure that it also feels ‘good’ or rewarding to eat….the oooo factor in food. Certain foods, such as energy dense, sweet and sticky desserts, trigger the rewarding feeling better than others, giving us the motivation, making sure we store all the extra energy we can, to give us enough fuel to chase down the next antelope. Having evolved over 10s of thousands of years to stay alive through multiple famines, any increase in motivation, however small, to continue to search for food was an evolutionary advantage.