Leptin
A satiety hormone released by fat tissue; it falls as you lose fat, weakening fullness signals.
Leptin is a hormone secreted by fat cells that signals energy sufficiency to the brain. As you lose fat, leptin falls in proportion, which the brain reads as a signal to eat more and conserve energy. GLP-1 medications mute hunger despite low leptin; once they stop, that protection is gone and leptin recovers only slowly, which is why regain is protracted rather than sudden. This is part of why a reduced weight is biologically defended.