Because research on this complex process has been a long- but not yet conclusive-exploration of many theories, we'll tell the story in small steps, analyzing each aspect of the process separately and then bringing it all together in a useful model of the hunger drive. We'll begin by describing a theory about the role of glucose, then we'll turn to a theory focusing on the amount of fat in the body. We'll then discuss hunger's psychological triggers, which help explain why physiological factors alone don't tell the whole story. Remember that no single theory explains the hunger drive completely; so don't form any conclusions until you reach the end of this module.

Set Point

The two physiological theories we're about to discuss are based on the idea that all bodily activities are geared to maintain homeostasis-a balanced or constant internal state. Set point refers to the point of stable internal balance for each organ system in our body: If some component of our internal state falls below its set point, we are motivated to engage in behaviors that will bring that component back to its normal level. Our internal mechanisms tell us when we are above or below the set point for each component. For example, when the amount of fluid in our body falls below its set point, we are motivated to search for something to drink, and the kidneys reduce the production of urine to conserve body fluids. But when our body has plenty of fluid, we are less motivated to drink, and the kidneys increase urine production to remove excess fluids. Of course, if we persist in drinking too much or too little, or eating too much or too little, our body may adjust to the new rate of intake, and the set point may move to a different level. Using a regulatory mechanism similar to a home heating system, the body attempts to maintain homeostasis through negative feedback (feedback from changes in one direction produces changes in the opposite direction to com pensate for the original changes).

Glucose Set-Point Theory: The Food Consumption Cycle

Most people eat substantial amounts of food three or four times a day; the body then distributes the energy obtained during a meal and stores the excess energy for later use. The regular pattern of obtaining energy from food, storing the energy, and then retrieving and using the energy is called the food consumption cycle or energy metabolism cycle. During a typical day, your body goes through several of these cycles. The following screens illustrate this cycle. It is the basis for an early theory-glucose set-point theory-that proposed that the trigger for hunger is the amount of glucose in the blood. When the glucose level is high, people feel satisfied; when the glucose level drops, people feel hungry and are motivated to eat until the glucose level is restored to its normal range, its set point.