The Science Behind Feeling Full and Hungry

What causes hunger?

Being hungry creates a powerful, often unpleasant physical sensation that is almost impossible to ignore. What causes this sensation? It's something called Ghrelin, a hormone produced by enteroendocrine cells in the gastrointestinal tract, especially the stomach, and is often called the "hunger hormone."

How does my body know when it is full?

After your breakfast gorge, you begin to experience an opposing force: fullness. But how does your body actually know when you're full?

The feeling of fullness is triggered as food moves from your mouth into your esophagus. Once it hits your stomach, it gradually fills the space, causing the surrounding muscle wall to stretch, slowly expanding like a balloon. A multitude of nerves wrapped intricately around the stomach wall sense the stretch. They communicate with the vagus nerve, all the way to the brain stem 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. In general, foods with more fiber, protein, and water tend to keep hunger at bay for longer.

As the hormones leak out, they are carried by the blood and eventually reach the hypothalamus in the brain.

More than 20 gastrointestinal hormones are involved in appetite moderation.

An example is cholecystokinin, which is produced in response to food by cells in the upper small intestine. When it reaches the hypothalamus, it causes a reduction in the feeling of reward you get from eating food. When that happens, the feeling of satiety begins to decline and you stop eating. Cholecystokinin also slows the movement of food from the stomach into the intestines.

This causes your stomach to stretch more over a period of time, allowing your body to register that it is getting full. This seems to be the reason why when you eat slowly you 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 is 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 produce another hormone called leptin. Leptin reacts with receptors in the population of neurons in the hypothalamus.

The hypothalamus has two sets of neurons important for our sense of hunger. One set produces the sensation of hunger by producing and releasing certain proteins.

The other set inhibits hunger through its own set of compounds. Leptin inhibits neurons in the hypothalamus that drive food intake and stimulates neurons that suppress it.

At this point, your body has reached peak fullness and through the constant exchange of information between hormones, the vagus nerve, the brain stem and the different parts of the hypothalamus, your brain receives the signal that you have eaten enough.

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