Metabolic Adaptation
Metabolic adaptation is the fall in energy expenditure after weight loss that exceeds what the smaller body size and composition alone would predict.
Losing weight lowers energy expenditure for the obvious reason: less tissue to fuel and less mass to move. Metabolic adaptation, or adaptive thermogenesis, is the residual, the part of the fall that remains after expenditure is adjusted for the new fat-free and fat mass. Isolating it requires a prediction equation or a regression against measured composition, so the number depends on how expenditure was measured and what it was adjusted for. Falling leptin and thyroid signalling, reduced sympathetic tone and improved muscle efficiency are the proposed mechanisms.
The classic experiments held participants at ten percent below their usual weight and found total expenditure a few hundred kilocalories per day below prediction, persisting while the reduced weight was maintained. A long-term follow-up of contestants from a televised weight-loss competition reported resting expenditure still below predicted years afterwards. Other cohorts, particularly after bariatric surgery, find little residual adaptation once composition is measured carefully, so magnitude and durability are contested.
What survives the disagreement is direction and scale. Adaptation is an offset of a few hundred kilocalories at most: enough to put maintenance intake below what a textbook equation predicts and to explain part of the pull toward regain, nowhere near enough to make weight loss impossible or to hold weight stable in a real deficit.
Starvation mode, in its popular form, claims eating too little stops weight loss altogether. Nothing here supports that. Two confusions do most of the damage: treating the whole drop in expenditure as adaptation when most of it is simply a smaller body, and blaming a plateau on metabolism when unintentional drift in intake, which self-report systematically understates, dominates in almost every controlled comparison.