What causes hunger?

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

How does my body know when it is full?

After you reacted by gorging on your breakfast you start to experience an opposing force: fullness. But how does your body actually know when you are 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. In general foods with more fiber protein and water tend to keep hunger at bay for longer.

As the hormones seep out they are swept up by the blood and eventually reach the hypothalamus in the brain.

Over 20 gastrointestinal hormones are involved in moderating the appetite.

One example is the cholecystokinin, which is produced in response to food by the cells in the upper small bowel. When it reaches the hypothalamus it causes a reduction in the feeling of reward you get when eating 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 are 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 the neuron population 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 stimulate the neurons that suppress it.

By this point your body has reached peak of fullness through the constant exchange of information between hormones, the vagus nerve, the brainstem and the different portions of the hypothalamus your brain gets the signal that you have eaten enough.