For example, consider water and protein shake – both are liquids of similar viscosity and sensation. How does my stomach feel fuller when I drink the shake?
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It’s called the [gut-brain connection](https://www.health.harvard.edu/diseases-and-conditions/the-gut-brain-connection). Your liver has hormone receptors that can detect protein and fat. This causes your gut to shut down the production of hunger sensation-stimulating hormones like ghrelin, and this makes your brain tell you that you are satiated.
I answered this, then realized that I had misread the question.
Food, such as the protein shake, contains fat, carbs, and protein. These require digestion or they would just pass through the body. Your body detects that these are present and starts the digestion process, making the food able to be absorbed and its calories and nutrients utilized.
Water doesn’t trigger any of these reactions. Your body just absorbs it as it passes through the intestines. It doesn’t need to do anything special as the water requires no digestion beyond this. It has no nutrients that require breaking down, no fat, no carbs, and no protein.
A combination of physical sense and hormones.
Your gut senses different macronutrients like fats, carbs, etc. It has chemical receptors that detect these and then release other hormones into the body in relation. More of a certain thing in the gut means more of a certain hormone released into the body. These reach the brain, and the brain reacts accordingly, by feeling sated to different degrees.
Some food physically fills the stomach more effectively as well. Large amounts of insoluble fiber for instance doesn’t quickly break down, so your gut detects the persistent physical fullness and passes the message along to the brain accordingly.
These processes are based on millions of years of evolution. Your body is trying to give you the best chances of survival in a prehistoric, caveman type way of life.
Your description of stomach feeling fuller is very complicated. There are vague explanations with respect to the feeling of fullness due to stretching detecting cells in the stomach, residency time of foodstuff, various neurotransmitters, but I think something better understood and easier to comprehend is the way to go. Obviously this is going to be a simplistic take on things.
When you consume sugar your body needs to prepare for and engage in changes in order to break it down and obtain energy from it. Note that carbs in general are just sugar with an extra step. The chemical most involved with this is insulin.
When you have excess insulin you feel hungry. The insulin wants sugar to process. When you eat a big meal (especially one of pasta or bread which are all carbs) your insulin levels spike in response to dealing with all the sugar. They over cook it though and there’s left over insulin after all the sugar has been dealt with leading to being hungry just a couple hours after a huge pasta meal.
If instead you eat a meal high in fats and protein rather than carbs your insulin has no reason to spike to deal with sugar and you don’t feel as hungry again just a few hours later.
Satiating your hunger due to high insulin or low blood sugar also gets picked up on by your dopamine reward system which reinforces behaviours that result in feeling good. It’s an evolutionary drive to eat because if you don’t you starve. That’s a bit too far afield though I think. It’s touching on hunger being modulated by fats and proteins too.
Hopefully though this gives you an idea of how it isn’t just physical sensation that contributes to the feeling of fullness.
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