What it means
When you lose weight, your body needs fewer calories simply because there is less of you to move and maintain. Researchers have found that calorie burn often falls a little further than that, and they call the extra drop adaptive thermogenesis or metabolic adaptation.
The best-known example comes from an NIH follow-up of contestants from a televised weight-loss competition. Six years later, their resting metabolism was still lower than expected for their size. That was a small, unusual group, and later analyses suggest changes in physical activity may explain part of the effect.
For most people the extra slowdown is modest, often in the range of tens to a few hundred calories a day, and it varies widely. It can help explain why a plateau arrives even when habits have not changed.
Keeping up protein, strength training and daily movement are the habits most often suggested to limit the gap, though none of them removes it entirely.
Why it matters
It explains why calorie targets usually need to be recalculated as you lose weight, including on a GLP-1.