When the brain detects the need for food, it presses the body’s hunger button. We respond by eating. The swelling stomach eventually sends a chemical message to the brain: “Enough already!” When this feedback message arrives, the hunger system shuts down until the process begins again a few hours later.
At one time, scientists believed that eating was a single process. Hunger and satiety were thought of as opposite sides of the same coin, just as inhaling and exhaling form one complete breathing cycle. In the past few years, however, we have come to look at eating as a series of processes that operate somewhat independently of one another. Hunger, in other words, is a somewhat separate system from satiety.
The discovery of these separate systems changed the way we think about eating disorders. Experts used to believe that anorexia involved a malfunction only in the hunger feedback loop. Although patients usually admit feeling hungry, they somehow condition themselves to ignore those signals. Perhaps the signals are faulty in some way—they may be too weak or are sent along the wrong pathway.
But we now have evidence that anorexia also involves malfunctioning satiety feedback loops. For example, an anorexic’s stomach will often be slow in passing food along—a condition known as delayed gastric emptying. It has been well demonstrated that the slowing of stomach emptying in anorexia increases the perception of fullness or satiety. Thus in these people even the presence of a small amount of food in the stomach may trigger satiety signals. Other studies confirm that an anorexic’s feelings of hunger and satiety do not necessarily correlate with the actual amount of food in her stomach. Bulimia, too, may arise from abnormalities in both the hunger and the satiety systems.
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