The brain
The brain acts as the central processing unit for all these various forces and influences. Different parts of the brain control responses to different stimuli.
Perhaps of greatest interest in a discussion of eating is the hypothalamus. This cluster of nerves, located near the front of the brain and surrounded by the hemispheres of “gray matter,” is astonishingly powerful given its tiny size. The hypothalamus, serving as a kind of gatekeeper between the brain and the rest of the body, coordinates many activities, including the central nervous system, the endocrine system, and the autonomic nervous system that governs, among other things, breathing and digestion.
The hypothalamus receives signals from the body about the state of energy supplies, decides what has to be done, and issues orders accordingly. For example, “hunger” signals sent from one part of the brain to other areas propel us toward the refrigerator. During the meal, various feedback loops change the concentration of certain brain chemicals. The hypothalamus records all of these changes and signals other parts of the brain to stop the eating. You put your fork down and push yourself away from the table. This is a “satiety” response. Without it we would continue eating to the point of physical pain.
Although small, the hypothalamus is divided into several discrete areas. Since the 1940s, we have known that damage to one area can disturb metabolism, resulting in overeating and obesity. Such results led to the early conclusion that the hypothalamus housed the central mechanisms in charge of hunger and satiety. We have since learned, however, that the eating process is much more complex than this simplistic explanation would allow. Hunger signals are now thought of as “decentralized,” traveling not just from the brain to the body, but from the body to the brain as well.
Research has also revealed that the hypothalamus, like the rest of the brain, is awash in a biochemical bath. The different areas of the hypothalamus are studded with specific receptors geared to respond only to certain neurotransmitters.
Other organs
The brain is just one of the links in the chain of the eating process. Other links include the nose, mouth, esophagus, the digestive tract, the bowels, and the kidneys.
Until recently, people tended to look on the stomach as just a passive bag that received food, broke it down, and passed it on. New discoveries indicate that the gastrointestinal system plays an active role in regulating the intake of food.
Food landing in the stomach sets in motion a series of events. The stomach and small intestine react to the size of the meal and the type of nutrients contained in the foods-fats, carbohydrates, and so on-by releasing the right mix of acids and other compounds necessary for digestion. Various gastrointestinal peptides are released in response to feeding, which in turn signal the hypothalamus to “terminate eating.” (A peptide is a compound of two or more amino acids; proteins are long sequences of amino acids linked together.)
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