Evolutionary Explanation

Neural and Hormonal Mechanisms in Eating Behaviour

Eating behaviour is regulated by a complex interplay of neural structures — particularly the hypothalamus — and peripheral hormonal signals from the gut, adipose tissue, and pancreas. These systems work together to maintain energy homeostasis: matching food intake to the body's current energy needs and long-term energy stores.

The Hypothalamus: Dual-Centre Model

The dual-centre model (Stellar, 1954) proposed that eating is controlled by two antagonistic hypothalamic centres:

• Lateral Hypothalamus (LH) — Hunger Centre: stimulation of the LH in animals causes eating (hyperphagia); destruction (lesioning) of the LH produces aphagia — the animal stops eating entirely, even to the point of starvation, unless force-fed.
• Ventromedial Hypothalamus (VMH) — Satiety Centre: stimulation of the VMH inhibits eating; lesioning the VMH produces hyperphagia — the animal overeats dramatically and becomes obese.

The dual-centre model has since been refined: the hypothalamus does not contain simple on/off switches but integrates multiple hormonal, neural, and cognitive signals. The arcuate nucleus (ARC) within the hypothalamus is now understood as a key integration site for hunger and satiety signals.

Hormonal Signals: Hunger

Ghrelin is the primary peripheral hunger hormone, produced mainly by the stomach when it is empty. Blood ghrelin levels rise sharply before meals and fall rapidly after eating. Ghrelin acts on the ARC and LH to stimulate appetite and initiate eating. Ghrelin also promotes fat storage. Cummings et al. (2004) demonstrated that ghrelin levels are chronically elevated in people who have lost weight through dieting — a biological mechanism contributing to weight regain.

Neuropeptide Y (NPY) is one of the most potent appetite-stimulating signals in the brain, produced in the arcuate nucleus. NPY release is triggered by low blood glucose and by ghrelin, and it acts on multiple hypothalamic sites to increase hunger, reduce metabolic rate, and promote fat storage. Chronic elevated NPY is associated with obesity.

Hormonal Signals: Satiety

Leptin is produced by adipose (fat) tissue and provides the hypothalamus with information about long-term fat stores. High leptin levels signal sufficient energy reserves and suppress appetite (via the ARC → inhibition of NPY → reduction of LH activity). In leptin deficiency (e.g. ob/ob mice lacking the leptin gene), animals become severely obese; exogenous leptin administration restores normal weight. In humans, leptin resistance — where the hypothalamus fails to respond normally to leptin — is associated with obesity.

Cholecystokinin (CCK) is released by the small intestine in response to fat and protein content, producing a short-term satiety signal that stops the current meal. It acts via the vagus nerve and directly on CCK receptors in the hypothalamus. Insulin, produced by the pancreatic beta cells in response to blood glucose, also acts on the hypothalamus to reduce appetite and promote glucose uptake by cells.

Serotonin (5-HT) in the brain promotes satiety — drugs that increase serotonin activity (e.g. sibutramine) reduce appetite. Serotonin's role in eating behaviour connects to its function in mood regulation, which may partly explain appetite changes in depression and the eating-related side effects of antidepressants.