Metabolic adaptationAdaptive thermogenesis

Metabolic adaptation is the reduction in energy expenditure that occurs during prolonged dieting and weight loss — partly from being smaller, and partly from the body defending its weight by lowering metabolic rate and spontaneous movement more than size alone predicts.

It matters because it helps explain why fat loss slows over time and why a deficit that once worked stops producing results, without anyone 'cheating'. It also underlies the weight regain many experience after aggressive diets.

In practice, it's managed with sensible (not extreme) deficits, adequate protein, resistance training to preserve muscle, and periodic breaks at maintenance — and by re-estimating your TDEE as your weight changes.