The biological responses your body deploys to resist losing weight.
What the evidence says
- Hunger increases — ghrelin rises, appetite intensifies[1],[2]
- Metabolic rate decreases — resting energy expenditure drops beyond what’s explained by smaller body size[1],[3]
- Energy efficiency improves — less NEAT (fidgeting, movement)[1]
- Food reward increases — brain regions mediating hedonic response to food show increased activation after weight loss[4]
- Psychological symptoms appear, and they are physiological — food preoccupation, depressed mood and irritability follow sustained energy restriction. In the semi-starvation work these emerged in previously healthy men and were responses to the energy deficit itself rather than pre-existing traits — true of the cause, not of the variation: Keys adds that men with “latent personality weaknesses developed more severe symptoms” than clinically stable men, and that the authors could not have predicted which[5],[6]
- Sympathetic nervous system drops ~40% and parasympathetic tone rises ~80% after a 10% weight reduction, reducing calorie burn at rest and during activity[1]
- Skeletal muscle becomes ~20% more efficient at low-level exercise — the body extracts more work per calorie, making weight maintenance harder[1]
- A formerly obese person needs ~300-400 fewer kcal/day than a never-obese person of the same weight; this penalty persists for years[1]
- Leptin repletion to pre-weight-loss levels reverses the declines in TEE, NREE, T3, T4, SNS tone, gonadotropins and skeletal muscle efficiency, and decreases energy intake — `clr03`’s own list. *That leptin therefore mediates the defense is the inference the reversal supports, not a sentence this record states*[1]