Stopping eating is not a decision made in one place. Several signals arrive at different points during and after a meal, and together they assemble the sensation described as fullness.
Stretch reports volume before content
The stomach wall contains receptors that detect distension, and they begin reporting as soon as the stomach fills, regardless of what it has filled with.
This is the fastest signal available, which is why bulky low-energy foods produce fullness quickly while energy-dense foods of small volume do not.
Because it responds to volume rather than energy, stretch alone is a poor regulator, and it is easily overridden by foods engineered to be easy to swallow.
Gut hormones report what arrived
As nutrients pass into the small intestine, cells lining it release hormones in response to the specific fats, proteins and carbohydrates detected.
These slow stomach emptying, promote digestive secretion and signal to brain regions governing appetite, producing fullness that builds over the minutes after eating rather than during it.
The delay is why eating quickly allows a large quantity to be consumed before the signal registers, and why the same meal eaten slowly can feel sufficient sooner.
A hunger signal rises between meals
One gut hormone works in the opposite direction, rising before an expected meal and falling once food arrives, which makes it the main driver of anticipatory hunger.
Its rise is partly learned, following habitual meal times, so hunger tends to appear on schedule rather than strictly in response to depletion.
This is why changing meal timing produces hunger at the old times for a period before the pattern resets.
Long-term signals set the background
Separately from meal-by-meal signals, hormones released in proportion to fat stores inform the brain about longer-term energy availability.
These do not stop a meal directly. They adjust how strongly the meal-related signals are heard, raising or lowering the threshold at which fullness registers.
Which is why appetite after a period of restriction is stronger than the same person's appetite before it, even with identical meals.
Palatability competes with all of it
Reward pathways respond to taste, texture and variety independently of nutritional need, and they can sustain eating well past the point at which fullness signals have arrived.
Variety is particularly effective at this, since fullness is partly specific to the food just eaten, and a new flavour restores appetite that the previous one had reduced.
Persistent absence of fullness, or appetite changes alongside unexplained weight change, is a reason to see a doctor rather than to adjust meals.