What makes you feel full when you're eating?

The short answer

Feelings of fullness are triggered when your stomach physically stretches and hormones signal to your brain that you’ve eaten enough. The food's nutritional content, your body fat and gut bacteria, as well as the variety of food on your plate all affect how strong those signals are.

The long answer

Your body is constantly sending you signals. You get ​itch signals​ to stop touching a suspicious plant, ​emotional signals​ to survive as a social animal, and fullness signals to stop eating when you've had enough.

Satiation—the feeling of fullness while you're eating—is a surprisingly complex process. But I've boiled it down to five key factors explaining what makes you feel full when you're eating.

Factor #1: The volume of food

The physical volume of food you eat plays a key role in your body letting you know you've had enough.

Medical image and 3D-model view of an empty, half-full, and full stomach.

Image by ​Radiotherapy & Oncology​ is licensed under ​CC BY 4.0​

The amount of food you eat physically stretches your stomach. This stretching triggers nerves around the stomach that signal to the brain you're nearing capacity.

Factor #2: The nutritional content of food

What you're eating also impacts which appetite-controlling hormones get released.

• Carbohydrates: Carbohydrates raise blood glucose levels which triggers glucagon-like peptide-1 (GLP-1) to release insulin and reduce appetite. This mechanism is the basis for how weight-loss drugs like Ozempic work.

The many, many functions of GLP-1.

"​FunctionsOfGLP-1​" by ​Lthoms11​ is licensed under ​CC BY-SA 4.0​.

• Protein: Protein stimulates the release of several hormones including cholecystokinin (CCK), peptide YY (PYY) and GLP-1, all of which signal to the brain that you're approaching satiation. Eating protein also decreases ghrelin, known as the "hunger hormone."

• Fats: Fats also trigger the release of CCK, as well as another appetite-controlling hormone called oleoyl ethanolamide (OEA).

As these (and other) hormones travel to your brain, you receive a signal that you're getting full. It's important to note that there's a time delay of ~20-30 minutes for these hormones to build up and get detected by your brain. This is why it's common advice to eat slowly so you can detect you're full before you're unbutton-your-pants full.

Factor #3: Your fat levels

Leptin, a hormone responsible for long-term energy regulation, is proportional to the amount of fat tissue you have — more fat you have, the higher your leptin levels.

Leptin works by modulating the effectiveness of short-term satiation hormones like CCK, GLP-1, and PYY. So, generally speaking, higher leptin levels should lead to faster feelings of fullness and decreased appetite.

The mouse on the left has a genetic mutation resulting in it being unable to produce leptin. Since leptin isn't there to modulate its hunger, it eats excessively and becomes obese.

"​Fatmouse​" by Oak Ridge National Laboratory is part of the public domain.

In many cases of obesity, leptin levels are elevated to such high levels that the body develops leptin resistance. This means that people with obesity often feel more hungry and less full because their brains have become less responsive to leptin's signals. However, exercise has been found to lower leptin levels and counteract leptin resistance, which helps the brain respond better to fullness signals.

Factor #4: Your gut health

The health of your gut microbiome also affects how you feel full. While there's no set definition for good gut health, it generally means that there's more "good" bacteria than "bad" in your system

Good bacteria like Lactobacillus, commonly found in fermented foods, help your gut release appetite-suppressing hormones like CCK, GLP-1, and PYY, which tell your brain that you're full. These bacteria also produce short-chain fatty acids (SCFAs), which help signal fullness.

"​Lactobacillus paracasei​" by Dr. Horst Neve, Max Rubner-Institut is licensed under ​CC BY-SA 3.0​.

If you have "bad" gut health, it results in a slower triggering of those hormones and fewer SCFAs. An imbalance of gut bacteria also affects the hunger hormone ghrelin, potentially making you feel less full and more hungry.

Factor #5: The variety of food

Eating a variety of flavors and textures makes it harder to feel full. This sensory-specific satiety phenomenon explains why you always seem to have room for dessert even after eating a full dinner.

Just seeing this spread could make you go from "I couldn't eat another bite!" to "One bite couldn't hurt."

"​Dessert Table​" by ​Travis Wise​ is licensed under ​CC BY 2.0​.

The introduction of new flavors can reignite your appetite, making it easier to ignore fullness cues. There's a simple evolutionary reason for this. Humans need a wide range of nutrients, and different flavors often signal different nutrient profiles. By making "room" for new foods, your body increases the odds of meeting its nutritional needs.

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Sources

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