Why the weight comes back

After weight loss, resting metabolism falls by more than the loss of body mass predicts — the extra, unexplained slowdown is called adaptive thermogenesis. It is real and detected in the large majority of studies. It is leptin-mediated: restoring leptin to pre-diet levels in the lab reverses much of the drop, evidence the body is actively defending its former weight.

For a typical moderate weight loss the extra slowdown is modest — on the order of 50–180 kcal/day (~6–7% below predicted), largest during active dieting and often shrinking once weight stabilises. The famous ~500 kcal/day figure comes from the Biggest Loser follow-up (n=14, extreme rapid loss) and is an outlier, not the norm. Better long-term studies find it does not reliably predict who regains. Evidence ●●●○○ on magnitude

In a landmark study (Sumithran 2011), people lost ~13.5 kg on a 10-week diet. Their satiety signals (leptin, peptide YY, CCK, amylin, insulin) fell and hunger signal ghrelin rose — a coordinated ‘eat more’ state. Crucially, at 62 weeks these changes had still not normalised, and subjective hunger remained significantly above baseline. A year on, people were hungrier than before they started, despite being lighter. Evidence ●●●●○

Leptin, made by fat cells, is the brain’s ‘fuel-gauge’. When you lose fat, leptin falls, and the hypothalamus reads falling leptin as a starvation threat — a system evolution built far more machinery to defend against than against overeating. It responds by ramping hunger, raising the reward value of food, and trimming energy spent. This is physiology, and it operates below conscious awareness.

Whether the body defends a fixed ‘set-point’ weight is genuinely contested. The set-point view (active feedback toward a target) explains why dieting triggers hunger and slows metabolism. The settling-point view says weight simply lands wherever intake and expenditure balance, with no target — which better explains decades of population-wide weight gain. A middle model (dual-intervention-point) proposes the body strongly defends a lower and an upper boundary but drifts freely between them.

You do not need to resolve the debate. Both camps agree on the actionable part: the body resists loss more than it resists gain, that resistance persists, and it is countered by ongoing habits (activity, protein, monitoring) rather than a one-off ‘diet’ with an end date. Evidence ●●●○○ (model is debated; the defended-loss phenomenon is not)