Two meals containing the same energy can leave a person full for very different lengths of time. Fullness is generated by several distinct signals, and energy content is only loosely connected to them.
Stretch is the first signal
The stomach wall contains receptors that report distension, and this is the earliest fullness signal to arrive during a meal.
Distension responds to volume rather than to energy, which is why watery and airy foods produce a sense of fullness disproportionate to what they contain.
The signal fades as the stomach empties, so it governs the end of a meal more than the interval before the next one.
The intestine senses what arrives
As food moves into the small intestine, its contents are detected and hormones are released that slow gastric emptying and register at the brain.
Protein and fat trigger this signalling strongly, which is one reason protein consistently ranks as the most satiating macronutrient in controlled comparisons.
Because these signals depend on digestion progressing, they build over the period after a meal rather than during it, which is why eating quickly overshoots.
Physical form changes the response
The same food eaten whole, puréed or drunk as a liquid produces measurably different fullness, with whole forms generally the most satiating.
Structure slows both eating rate and digestion, so nutrients are released over a longer period and the signalling is spread out rather than compressed.
Which is why fruit juice and whole fruit behave differently despite similar composition on paper, and why blending is not a neutral operation.
Fibre acts on several of these at once
Fibre adds volume without energy, so it contributes to distension, and viscous fibres slow gastric emptying directly.
Fermentation further along produces compounds that appear to influence appetite signalling over subsequent hours.
The combined effect is why high-fibre meals tend to be reported as more filling than their energy content predicts, though the size of the effect varies between individuals.
Learning and context modify the signals
Expected fullness, based on prior experience with a food, influences reported satiety independently of what the food contains.
Portion cues, plate size and eating while distracted all shift how much is eaten before the signals are noticed at all.
Which is why satiety is best understood as a system with physiological and learned inputs, and why appetite changes that are persistent and unexplained are worth raising with a clinician rather than managed by food choice alone.