Appetite is often discussed as a matter of discipline. It is generated by a signalling system with identifiable components, operating on timescales from minutes to months.
The control centre sits in the hypothalamus
A small region of the hypothalamus receives inputs about nutritional state and adjusts hunger and energy expenditure in response.
It contains populations of neurons with opposing effects, one group promoting food intake and another suppressing it.
Circulating signals act on those neurons, which is how information from the gut and from fat tissue reaches the part of the brain that generates the sensation.
Short-term signals come from the digestive tract
Ghrelin is released by the stomach and rises before an expected meal, then falls after eating. It is the main circulating signal that promotes hunger.
Several gut hormones move in the opposite direction, released as nutrients arrive in the intestine, among them GLP-1, peptide YY and cholecystokinin.
Stretch receptors in the stomach wall contribute a separate mechanical signal, which is why volume affects fullness independently of energy content.
Long-term signals come from fat tissue
Leptin is produced by fat cells in proportion to how much fat tissue is present, so its level reports stored energy rather than the last meal.
Falling leptin is a powerful signal of energy deficit, increasing hunger and reducing energy expenditure.
The system responds much more strongly to a fall than to a rise, which is consistent with an evolutionary history in which scarcity was the recurring threat.
The response to deficit is defensive
When stored energy falls, several signals change together in the direction of restoring it, and those changes persist rather than resolving quickly.
Hunger signalling increases while the energy cost of activity declines somewhat, and both effects can outlast the period of restriction.
This is measured physiology rather than a description of willpower, and it is why maintaining a reduced body weight is generally reported as harder than reaching it.
Palatability enters through a different route
Reward pathways involving dopamine operate alongside the hypothalamic system and respond to taste, variety and expectation rather than to nutritional need.
This is why eating can continue past fullness when food is highly palatable, and why the two systems can issue conflicting instructions.
Appetite changes that are sudden, extreme or accompanied by other symptoms have medical causes worth investigating, and questions about weight and appetite belong with a doctor who knows the wider picture.