What is metabolic adaptation and is it permanent?

A smaller body needs less energy, which is simple arithmetic. On top of that, hormones shift — leptin falls, thyroid hormone drops somewhat, and unconscious movement decreases — so expenditure falls further than size alone would account for.

At the same time appetite signalling increases. You are hungrier and burning slightly less, which is a difficult combination and the reason weight regain is so common. None of it is a malfunction; it is a system defending its reserves.

Typically in the range of 50 to 150 calories a day beyond prediction, though it varies and can be larger after very aggressive loss. Meaningful, but not the thousand-calorie collapse it is sometimes described as.

Most of it resolves when energy intake returns to maintenance. The portion that persists longest is the part attributable to lost muscle, which is a composition problem rather than a metabolic one — and it is the part you can prevent on the way down.

Lose at a moderate rate, keep protein high and train against resistance, which protects the muscle that determines the lasting portion. Take deliberate periods at maintenance rather than dieting continuously for a year.

And when you reach your target, eat at maintenance on purpose rather than drifting. The failure mode is staying in a permanent half-deficit, which is both unpleasant and ineffective.