Gastric emptying
How fast food leaves the stomach: slowed markedly by GLP-1 medicines.
Gastric emptying is the rate at which the stomach passes its contents into the small intestine. It is regulated rather than passive, and the stomach releases food gradually so that digestion and absorption downstream are not overwhelmed.
Composition changes the rate substantially. Liquids leave faster than solids, and fat and protein slow emptying more than carbohydrate does, which is part of why a fatty meal feels heavy for longer and why a liquid meal is less filling than the same calories eaten as food.
It matters for appetite because a fuller stomach for longer means more satiety signalling. Slowing emptying is one of the main mechanisms by which GLP-1 reduces appetite, both the natural hormone and the medicines that mimic it.
That mechanism is also the source of their most common side effects. Nausea, early fullness, bloating and reflux are the expected consequences of a stomach that empties more slowly, and they are the usual reason people stop treatment.
It has become a genuine clinical consideration for procedures. Delayed emptying is why anaesthetic and endoscopy teams now ask about GLP-1 medicines beforehand, since the stomach may not be empty when standard fasting rules assume it is.
For endurance athletes the same physiology sets a different limit. Emptying rate and intestinal absorption together cap how much carbohydrate can be taken in per hour during exercise, and gut tolerance is trainable with practice.
In practice, this is the mechanism behind several things that otherwise look unrelated: why liquid calories are easy to over-consume, why GLP-1 medicines cause nausea, and why race fuelling has to be rehearsed.