Everyone must eat to live: to sustain the human body and supply energy for bodily processes. This is a fairly simple concept, yet many people find themselves living with an eating disorder that threatens their life, health, or quality of life. In America, more than half of the adult population is classified as clinically obese; adversely, there has been a similar rise in eating disorders related to not eating enough: roughly 3% of the teen and adolescent populations are diagnosed as anorexic (Pinel, 2007). These are staggering statistics, but what are the implications? At some point in your life, you or someone close to you will develop an eating disorder. At its simplest, there are two primary factors in obesity: energy input and energy output. That said, there are several factors that can contribute to the possibility of obesity. Though not frequently the case, genetics can contribute to obesity; however, more likely influences involve cultural influences, strong personal preference for certain high-calorie foods such as fatty or sweet tastes, familial influence that promotes excess eating, and abnormally strong responses to the thought, sight, or smell of food. Energy output is different for everyone; the most obvious thought that comes to mind is in regard to the amount of exercise one gets. However, there are other factors that affect energy output, including: the basal metabolic rate, diet-induced thermogenesis (reaction to fat increases), and non-exercise activity thermogenesis (fidgeting, maintaining posture, or degree of muscle tone). (Pinel, 2007) The prevailing belief about eating and hunger is that the body’s eating system is designed to trigger hunger when energy levels are low, and once one has eaten and energy levels return to normal, one’s hunger is satiated. This is the “set point” theory that proposes the body works very much like a heating system in which an internal “thermostat” controls hunger and satiety. However, if the set point theory were true, eating disorders would be nearly obsolete; instead, the number of those suffering from eating disorders continues to grow. (Pinel, 2007) One major contradiction to the set point theory is supplied by looking at human evolution. Our ancestors faced some significant issues when it came to eating; in particular, food supplies were not abundant, as they are today, and they were far from predictable. To survive, our ancestors had to eat as much as possible when food was available so that extra calories could be stored as fat to ensure survival until the next meal was procured, in much the same way as bears, groundhogs, and other hibernating creatures store fat to survive the winter. (Pinel, 2007) Another contradiction to the set point theory is that it does not account for the myriad variety of factors that contribute to hunger and eating, such as taste preferences, environment, and learned behaviors. These issues are addressed by the theory of positive-incentive perspective. Positive-incentive theory basically states that eating is triggered by the anticipated pleasure the eating provides, and that the level of hunger felt is based on the interaction of a variety of factors, including: flavors, length of time between meals, amount and composition of food previously eaten, blood glucose levels, environment, and previous experiences with the food. (Pinel, 2007) One primary selling point for the positive-incentive perspective is its comparison to sexual behavior: one does not have a set point for sexual release or sexual expression that must be made up when a deficit occurs. Instead humans have evolved to desire sexual release and expression simply for the pleasure derived from such activities. The argument here is that humans have evolved to desire food and eating for the pleasure derived from the tastes, textures, and flavors of food, among other factors. The positive-incentive perspective, at its most basic, means that eating is triggered by the anticipation of the pleasure that eating provides. It is believed that persons who suffer from anorexia have experienced the opposite effect. In other words, for anorexics, the positive-incentive value of food has dropped or is non-existent, even though the person may seem to be obsessed with food. (Pinel, 2007) Sweet, salty, and high fat foods have become preferences for many through evolution, since these foods are associated with high-calorie and sodium-rich foods. The types of foods we choose are also based on learned taste preferences or aversions. Some taste preferences are learned through the flavor of milk excretions during breastfeeding and culturally specific foods. Equally important are factors that influence how often one eats. The frequency that one eats is affected by one’s socio-economic situation, cultural norms, schedules and routines, habits, and personal preferences to name a few. (Pinel, 2007) There are also factors at work that influence how much one eats; primary of these are the satiety mechanism which sends signals based on the volume and calorie content of food consumed that causes one to stop eating. Appetizers or small amounts of food consumed prior to a meal increase hunger. One’s environment also plays a part in how long it takes one to feel satiated; studies have found that 60% more is consumed when eating with others; this is not taking into account the excess of calories one consumes when distracted by activities such as watching television. Particularly in humans, social environments can also have the opposite effect, in that often humans will not eat as much as they are accustomed to in social settings. (Pinel, 2007) Sensory-specific satiety is the taste-specific effect that positive-incentive food offers. A variety of tastes during a meal (the cafeteria diet) influences how much is eaten. Cafeteria diets are, for the most part, taste-specific. When one food is eaten, positive-incentive for that food bottoms out, while the positive-incentive for other foods also drops. Yet, cafeteria diets can lead to an increase in caloric consumption through the effect of the variety of foods that offer differing tastes. (Pinel, 2007) Some research that has been conducted by Woods points to the aversive physiological effects that meals can have, which may be more powerful in individuals who have not been eating very much. The physiological effects of meals are the disruption of the homeostatic balance in the body when it receives a sudden influx of calories. Additionally, society’s belief that meals, versus small snacks throughout the day, are the “normal and healthy” way of eating leads to many anorexics being forced to eat, which causes conditioned taste aversions. Repeated conditioned taste aversions only serve to strengthen the anorexic’s desire to not eat. (Pinel, 2007) There are many factors that can contribute to the incidence of obesity, anorexia nervosa, and other eating disorders, and there are numerous factors that influence hunger and satiety. Learning about the physiological aspects behind hunger and satiety and the factors that contribute to eating disorders may help increase your understanding of eating disorders and perhaps which direction future treatment will take. Reference Pinel, J. P. J. (2007). Basics of biopsychology. Boston, MA: Allyn and Bacon.