Definition
Adaptive thermogenesis refers to a drop in energy expenditure during weight loss that goes beyond what would be expected from simply being a smaller body. It is your body's survival response to sustained calorie restriction.
What drives it
Several systems shift at once during a prolonged deficit: thyroid hormone output typically declines, leptin (a satiety hormone tied to fat stores) drops, and non-exercise activity thermogenesis (NEAT) -- the unconscious fidgeting and movement that can burn hundreds of calories a day -- tends to quietly decrease.
How big is the effect
Estimates across clinical literature generally place the extra efficiency gain at 5-15% beyond mass-based predictions, with more pronounced effects after larger or more prolonged weight loss.
Planning around it
Because adaptive thermogenesis compounds slowly, it rarely shows up as an obvious event -- it shows up as an unexplained plateau weeks after the fact. Building in periodic recalculation of your calorie targets, and occasional maintenance-calorie "diet breaks," are two of the most commonly cited strategies for managing it.