Appetite is regulated by a set of signals travelling between the gut, fat tissue and the brain. Two partly independent systems are involved, and the difference between them explains a great deal about eating behaviour.
The gut reports on a meal as it arrives
Stretch receptors in the stomach wall respond to volume, sending signals through the vagus nerve that contribute to the sensation of fullness.
As nutrients reach the small intestine, cells there release hormones including GLP-1 and peptide YY, which slow stomach emptying and reduce further intake.
These signals rise and fall over a meal and the hours after it, making them short-term regulators rather than controllers of body weight.
Fat tissue reports on stores
Leptin is produced roughly in proportion to fat mass and acts on the hypothalamus as a longer-term signal about energy reserves.
Falling leptin during weight loss increases hunger and reduces energy expenditure, which is the mechanism behind much of the resistance to maintaining a lower weight.
Rising leptin at higher fat mass produces a weaker corresponding response, an asymmetry that limits how well the system defends against gain.
Ghrelin rises before expected meals
Ghrelin is released from the stomach and increases before habitual mealtimes, falling after eating, which links appetite to routine as well as to depletion.
Because the rise is partly learned, hunger arrives at accustomed times even when energy is not required, and shifts as eating patterns change over several days.
This anticipatory element is why altered meal schedules feel difficult initially and become comfortable once the pattern resets.
A reward system runs alongside the regulatory one
Dopamine-based circuits respond to the palatability of food independently of energy need, which is why appetite for a particular food can appear after a full meal.
Foods combining fat, sugar, salt and refined texture activate these circuits more strongly than the whole foods from which they were derived.
The regulatory system can be overridden by the reward system, which is a substantial part of why energy balance is not maintained automatically in a food environment designed for palatability.
Why these mechanisms became treatment targets
Medicines based on gut hormone signalling extend an effect that normally lasts minutes into one lasting much longer, reducing appetite substantially.
They act on the same pathway the body uses rather than introducing a new one, which is why their effects on hunger are so pronounced.
Whether such treatment is appropriate depends on individual circumstances and carries side effects and monitoring requirements, making it a decision for a clinician rather than a general recommendation.