PHYSICAL SIDE OF EATING: CCK AND BIOLOGICAL THERMOSTAT

April 22nd, 2009

THE CCK STORY

One of the best-studied gastrointestinal peptides is cholecystokinin, or CCK. After you eat, the small intestine releases this hormone, which stimulates pancreatic secretion and gallbladder contraction. It also sends satiety signals up to the hypothalamus. Women who have bulimia may have a malfunction in their CCK system. In one study, bulimic women tested before a meal had the same amount of CCK as normal women. Yet after eating they had a significantly smaller increase in CCK than did normal women. They also reported fewer feelings of fullness than the normal women did. Treatment with tricyclic antidepressants not only reduced their bingeing, but also helped their CCK levels return to normal.

This important new research doesn’t necessarily mean that a faulty CCK system causes bulimia. But decreased CCK, and thus the decrease in satiety signals, may reinforce a woman’s urge to keep eating.

Such findings have led to a revolution in the way we think about the digestive system. As one expert put it, rather than being a passive system, the gut now appears to be a “great sensory sheet extending from the mouth to the small intestine.” This sheet is exquisitely sensitive not only to the presence of food, but to its exact chemical composition as well.

THE BIOLOGICAL THERMOSTAT

Think of the thermostat in your home. You set the dial to maintain a constant temperature of, say, seventy-two degrees. When the furnace has put out enough heat, the thermostat switches it off for a while. If someone leaves the front door open and cold air rushes in, the thermostat kicks on again, staying on until the temperature returns to the desired level.

Your body works in a similar way. Earlier, I used the term homeostatic to describe how your body tries to maintain its equilibrium. The biological “homeostat” responds to changes in the environment to keep your metabolism working on an even keel.

Currently, one of the most exciting fields in medicine involves research into the fascinating and complex ways the body responds to such signals. We are beginning to learn that a number of subsystems work together to control eating as well as other types of behavior.

These subsystems operate on the feedback principle: Signal A activates process B, which in turn sends signal Ñ to shut off signal A. These signals are carried by hormones or other chemical messengers that activate nerves or stimulate other responses. Many of these processes are triggered, not just internally, but by physical and social cues from the outside world. Just about anything, from contact with other people to the amount of daylight you receive in a twenty-four-hour period, can affect your body’s function—your homeostasis.

The concept of feedback is important to eating disorders for several reasons. For example, your body tries to keep weight at a constant. If you start weighing too much, your metabolism speeds up to burn off the excess pounds. Conversely, should your weight drop, so will your metabolism, to conserve dwindling energy supplies.

Another series of feedback loops involves your body’s cravings for certain nutrients and its response to the nutritional content of the food you eat. Studies on animals (and simple observation of humans) show that at times we prefer to eat carbohydrates, for example, and at other times we choose protein-rich foods. The menus we choose can have a tremendous impact on our moods and our behavior.

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