It's one of the most common fears in dieting, and one of the most overstated. Your metabolism adapts to dieting — but 'damaged' and 'starvation mode' overstate what's really a modest, reversible effect.
During a sustained calorie deficit, your resting metabolic rate does fall — partly because you weigh less, and partly through an extra adaptive drop beyond what weight loss alone predicts. This is real and well-documented. But the size of the extra adaptation is modest: on the order of 100–150 calories for a typical dieter, not the metabolic shutdown the word "damaged" implies.
The popular idea that eating too little makes you gain weight, or stops loss entirely, isn't supported. A 100–150 calorie adaptation cannot erase a genuine 400–500 calorie deficit. When people are convinced their metabolism has stopped, the far more common explanation is that actual intake has crept above what they think — un-weighed food, oils, drinks. See why weight loss stalls.
Adaptive thermogenesis isn't permanent damage. As you return to maintenance and your body stops perceiving a deficit, much of the adaptation reverses over weeks. That's the principle behind reverse dieting — gradually raising calories to restore your maintenance in a controlled way.
Keep deficits moderate rather than extreme, keep protein high, include resistance training to preserve muscle (muscle is metabolically active), and don't sit in a deep deficit indefinitely — periodic breaks at maintenance help. The figures behind the adaptation estimate are on the methodology page.
No. Metabolic rate adapts during dieting — modestly, around 100–150 kcal beyond weight-driven change — and largely reverses as you return to maintenance. It isn't permanent damage.
Adaptive thermogenesis is real but small. The idea that eating too little stops weight loss or causes gain isn't supported — a modest adaptation can't erase a real deficit. Mis-measured intake explains most apparent stalls.
Return to maintenance gradually (reverse dieting), keep protein high, train with resistance to preserve muscle, and avoid prolonged extreme deficits.