What makes us hungry or full? Hunger is driven by signals such as ghrelin, while fullness comes from a mix of stomach expansion, nerve signals, and hormones released by the digestive system.
Ghrelin is known as the “hunger hormone” because it is released when the stomach is getting empty and it is one of the main signals that tells the brain food is needed. It is produced mostly in the stomach, although the brain, small intestine, and pancreas produce small amounts of it as well. Ghrelin rises as the level of food in the stomach drops. There is no single trigger point. It just gradually increases. The hormone travels to the hypothalamus in the brain, where it stimulates Agouti-related peptide (AgRP) neurons. These are a specialized group of nerve cells that are major drivers of hunger and food-seeking behavior. The hormone reaches the brain and helps create the feeling of hunger. It also increases the desire to go and find food. It affects brain areas involved in motivation and reward as well, making high-calorie, energy-dense foods seem especially appealing.
Ghrelin does other things too. It helps the body manage how it uses and stores energy. It affects how quickly food moves through the digestive system. It also interacts with blood sugar control, insulin, and glucagon, showing that hunger is not just a simple feeling in the stomach but part of a much wider system that helps regulate energy in the body.
Ghrelin can also rise when people are stressed or sleep deprived, which is one reason stressed and tired people are more likely to snack. Things become more complicated in obesity and in dieting. People with obesity often have lower baseline ghrelin levels than leaner people, but the body’s hunger and reward systems can still become dysregulated. On the other hand, people who restrict calories often have higher ghrelin levels, which is one reason dieting can make hunger feel stronger rather than weaker. This is one reason it is often easier to say that appetite regulation has become disrupted than to point to one single hormone and blame that alone.
A feeling of fullness after eating comes from several different sources. The first is that ghrelin declines. Because ghrelin helps drive the sensation of hunger, that feeling starts to ease as its level falls. The second is the size of the stomach. The stomach has layers of tissue and muscle surrounding it. These muscles are folded into ridges called rugae. As food goes into the stomach, the brain signals the muscles to relax and the ridges slowly unfold. The stomach expands to accommodate what has been eaten. As it expands, nerves in the stomach wall send signals up the vagus nerve to the brain. This gives the brain information about how full the stomach is. As the stomach fills, the pressure increases and the brain sends stronger and stronger signals to stop eating. Anyone who has been at an all-you-can-eat buffet knows these sensations well.
There are several chemicals released by the digestive system that also tell the brain the body is full. Cholecystokinin is released by the small intestine when it comes into contact with fats and proteins. It slows the rate at which the stomach empties and strengthens the signals traveling through the vagus nerve. This is one of the first signs the brain gets that fullness is beginning. Then there are hormones called GLP-1 and PYY. They are released further down the digestive tract and they help suppress appetite after eating. GIP also plays a role, especially in insulin release and the body’s response to nutrients. Bile acids, which help break down food, can also stimulate the production of some of these hormones and signals. Finally, receptors in the digestive system and elsewhere help inform the brain about how many nutrients have been taken in and how much energy is now available.
The important thing is that hunger and fullness are not controlled by a single on-off switch. They are produced by a whole network of signals involving the stomach, the intestines, the brain, hormones, and nerves. If these systems are working well, the body will generally make us hungry when food is needed and make us feel full when enough has been eaten. The problem is that modern life can interfere with these systems. Stress, lack of sleep, constant access to food, and ultra-processed foods can all play havoc with the body’s normal signals. That may be one reason many of us now eat more than we used to, and more than we really need. And this is what I learned today.
Sources
https://www.hopkinsmedicine.org/health/wellness-and-prevention/hunger-and-fullness-awareness
https://en.wikipedia.org/wiki/Ghrelin
https://my.clevelandclinic.org/health/body/22804-ghrelin
https://primalpictures.com/blogs/overeating-anatomy-physiology-stomach
Photo by Jonathan Borba: https://www.pexels.com/photo/assorted-mini-slider-platter-for-party-event-35247169/