energy homeostasis

Energy homeostasis is the body's long-term budgeting of calories — the running tally of energy coming in (food) against energy going out (metabolism, heat, and activity). Like a thermostat for fuel, the system continuously nudges hunger and energy use to keep body energy stores remarkably steady over months and years, even though daily intake varies wildly.

The balancing is done by a network of hormones and brain circuits. Adipose tissue reports its size via leptin; the pancreas reports via insulin; the gut reports meal-by-meal via ghrelin, GLP-1, PYY, and CCK. The hypothalamus integrates these inputs and adjusts both sides of the ledger — appetite (intake) and energy expenditure including basal metabolism and thermogenesis (output). Thyroid hormones and catecholamines fine-tune how fast fuel is burned.

Crucially, the system is biased to defend against energy deficit far more aggressively than against surplus. When you lose weight, hunger rises and metabolism slows to push you back up; when you overeat, the counter-defenses are comparatively weak. This asymmetry is a core reason sustained weight loss is so difficult.