Resting metabolic rate (RMR)
The calories your body burns at rest; muscle is its main driver, which is why preserving muscle matters.
Resting metabolic rate is the energy your body uses at rest to keep basic functions running, and it accounts for the majority of daily calorie burn. Muscle (fat-free mass) is the principal driver of RMR, so losing muscle lowers it and makes regain easier. After a GLP-1, protecting muscle through adequate protein and resistance training keeps RMR up exactly when appetite surges, which is why those two pillars carry the strongest evidence for maintenance.