Body fat set point theory suggests the body defends a fat range, and dropping below it causes negative effects like increased hunger and decreased BMR.

“body fat set point theory is essentially that you kind of have this relatively tight range of homeostatic body fat that your body will defend — and once you start to drop below that range uh it's really you really start to feel the the negative effects of caloric restriction in terms of — your hunger goes up your bmr drops your non-exercise activity thermogenesis drops your libido drops you start to have those negative side effects”