Hunger claws at your grumbling belly. It tugs at your intestines, which begin to writhe, aching to be fed. Being hungry generates a powerful, often unpleasant physical sensation that’s almost impossible to ignore. After you’ve reacted by gorging on your morning pancakes, you start to experience an opposing force, fullness, but how does your body actually know when you’re full? The sensation of fullness is set in motion as food moves from your mouth down your esophagus. Once it hits your stomach, it gradually fills the space. That causes the surrounding muscular wall to stretch, expanding slowly like a balloon. A multitude of nerves wrapped intricately around the stomach wall sense the stretching.
They communicate with the vagus nerve up to the brainstem and hypothalamus, the main parts of the brain that control food intake. But that’s just one input your brain uses to sense fullness. After all, if you fill your stomach with water, you won’t feel full for long. Your brain also takes into account chemical messengers in the form of hormones produced by endocrine cells throughout your digestive system. These respond to the presence of specific nutrients in your gut and bloodstream, which gradually increase as you digest your food.
As the hormones seep out, they’re swept up by the blood and eventually reach the hypothalamus in the brain. Over 20 gastrointestinal hormones are involved in moderating our appetites. One example is cholecystokinin, which is produced in response to food by cells in the upper small bowel. When it reached the hypothalamus, it causes a reduction in the feeling of reward you get when you eat food. When that occurs, the sense of being satiated starts to sink in and you stop eating. Cholecystokinin also slows down the movement of food from the stomach into the intestines. That makes your stomach stretch more over a period of time, allowing your body to register that you’re filling up. This seems to be why when you eat slowly, you actually feel fuller compared to when you consume your food at lightning speed.
When you eat quickly, your body doesn’t have time to recognize the state it’s in. Once nutrients and gastrointestinal hormones are present in the blood, they trigger the pancreas to release insulin. Insulin stimulates the body’s fat cells to make another hormone called leptin. Leptin reacts with receptors on neuron populations in the hypothalamus. The hypothalamus has two sets of neurons important for our feeling of hunger.
One set produces the sensation of hunger by making and releasing certain proteins. The other set inhibits hunger through its own set of compounds. Leptin inhibits the hypothalamus neurons that drive food intake and stimulates the neurons that suppress it. By this point, your body has reached peak fullness. Through the constant exchange of information between hormones, the vagus nerve, the brainstem, and the different portions of hypothalamus, your brain gets the signal that you’ve eaten enough. Researchers have discovered that some foods produce more long-lasting fullness than others. For instance, boiled potatoes are ranked as some of the most hunger-satisfying foods, while croissants are particularly unsatisfying. In general, foods with more protein, fiber, and water tend to keep hunger at bay for longer. But feeling full won’t last forever. After a few hours, your gut and brain begin their conversation again. Your empty stomach produces other hormones, such as ghrelin, that increase the activity of the hunger-causing nerve cells in the hypothalamus.
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