The eNotes essays linked below provide good summaries of the distinctions between “appetite” and “hunger,” but the difference basically comes down to the fact that “hunger” is defined as the product of a number of physiological developments that occur in the body, especially in the digestive system, that indicate a need for food, while “appetite” is generally defined as a desire for food, especially for certain types of food (in other words, a craving). Living creatures, plants and animals, require sustenance to survive. Human beings have basic requirements for survival including shelter, clothing sleep and food. Appetite can exist absent genuine feelings of hunger, which is a major contributor to obesity, as unsuppressed appetites can leave one feeling the need for food irrespective of whether that feeling is a product of genuine hunger or simply a desire to consume foods that taste good. Appetite is function of the release within the digestive system of hormones that stimulate the brain into thinking that food is needed irrespective of whether the individual in question has already consumed sufficient quantities of calories to satisfy the survival mechanism involved in hunger. It is possible to have an appetite despite not feeling hungry, and it is possible to feel hungry without wanting to eat.
The human digestive system and is relationship to the brain is extremely complex. The function of the hypothalamus in the brain is to regulate food intake, and its proper functioning is crucial to the control of body weight. At the same time, hormones produced in the digestive system are processed through the brain and also regulate hunger, as well as appetite. A flaw in any part of this complex system can cause habitual overeating which leads to obesity. As mentioned, however, for purposes of defining the two terms, “appetite” refers to the desire for food, while “hunger” refers to the requirement for food.