set-point theory

Set-point theory says your body behaves as if it has a target weight it tries to maintain, much like a thermostat defends a target temperature. Push the weight below that target by dieting and the body fights back with more hunger and a slower metabolism; push it above and (more weakly) it tries to come back down.

The biological machinery behind this idea is the energy-homeostasis system: leptin from fat reports how big the stores are, the hypothalamus compares this to its internal reference, and it adjusts appetite and energy expenditure to defend the level. When weight drops, leptin falls, hunger-driving AgRP neurons become more active, and adaptive thermogenesis lowers calorie burn — all pushing weight back up.

The honest caveat is that the “set point” is not fixed in stone. It can drift upward over years with chronic overeating and obesity, and the defense is asymmetric — strong against loss, weak against gain. Some researchers prefer the term “settling point” to stress that the defended weight is shaped by environment and behavior, not a single hard-wired number.