appetite regulation

Appetite regulation is the body's way of deciding when, how much, and how eagerly to eat. Imagine a control room in the brain receiving a constant stream of reports — from the stomach, gut, fat tissue, and bloodstream — and weighing them against memory, habit, and emotion before issuing the order to start or stop eating.

The central hub is the hypothalamus, where two opposing neuron populations integrate signals. AgRP neurons drive hunger; POMC neurons promote fullness. They respond to long-term signals of energy stores, such as leptin and insulin, and to short-term meal-related signals from the gut, such as ghrelin (hunger), and CCK, GLP-1, and PYY (fullness). The brainstem and reward circuits add information about taste, palatability, and pleasure.

This system is layered and redundant, which is why it is so hard to override with willpower or single drugs. It also tilts toward defending against weight loss more strongly than against weight gain, an asymmetry that made sense in environments of food scarcity but contributes to obesity in environments of constant food abundance.