If stored energy (fat) and leptin remain stable over time, we are more easily sated during and between meals. Smaller portions feel OK. And our metabolic rate stays high. If stored energy (fat) and leptin drop over time, it sends a message to the brain (mainly the hypothalamus, which links your nervous system with your endocrine system) that we need to start preventing starvation. The brain responds to lower leptin levels with several anti-starvation strategies:

We get hungry. Like real hungry. Like eat-your-own-arm hungry. We move around less. Our NEAT (non-exercise activity thermogenesis), or our daily movement like fidgeting, standing up, and anything other than purposeful exercise, goes down. The couch starts looking better and better. We burn fewer calories through movement as our skeletal muscles become more efficient. Our metabolic rate slows down significantly (as seen in the infamous ”Biggest Loser” study). It follows, then, that if stored energy (fat) and leptin go up over time, you’ll want to eat less… right? Unfortunately, you can’t always count on that response.