Metabolic adaptationAdaptive thermogenesis
The body's tendency to burn fewer calories as you diet.
Metabolic adaptation, also called adaptive thermogenesis, is the fall in energy expenditure that comes with prolonged dieting and weight loss. Part of it is simple arithmetic, because a smaller body costs less to run. The part that earns the name is the extra: expenditure drops further than size alone predicts.
Three things do the dropping. Resting metabolic rate falls a little beyond what the new bodyweight explains, the energy cost of moving the smaller body falls, and spontaneous daily movement quietly declines, so you fidget less, take the lift, and sit down sooner without deciding to.
It matters because it explains why fat loss slows without anyone cheating, and why the deficit that produced a kilogram a week in month one produces half of that by month four. Treating that slowdown as a failure of discipline is the most common and least useful reading of it.
The scale of it, in the most extreme case measured: six years after The Biggest Loser competition, 14 of the original 16 contestants had regained 41 of the 58 kg they lost, and their resting metabolic rate sat about 704 kcal a day below baseline, with metabolic adaptation accounting for roughly 499 kcal a day of that. Persistent, and proportional to how hard they were fighting to stay light.
That study is the ceiling rather than the average. It followed people who lost extraordinary amounts extraordinarily fast under competition conditions, and the adaptation is generally smaller in ordinary dieting. It is, however, real enough that maintenance is a phase to plan for rather than a finish line.
What helps: deficits that are moderate rather than heroic, enough protein and resistance training so that the weight lost is mostly fat rather than muscle, deliberate attention to daily steps, and periods spent at maintenance rather than an endless push. None of these abolish it, and all of them soften it.
In practice, re-estimate your TDEE every few kilograms with our calculator rather than working from the number you started with. The old figure is not wrong because your metabolism is broken; it is wrong because you are a different size.