So if a person takes in fewer calories than they use up over a longer period of time, then the body switches to a kind of emergency mode in order not to waste energy and not to starve for as long as possible.
Slow Down Hunger Metabolism
With a diet, where the goal is to lose weight, you have to avoid this starvation metabolism in order to be able to achieve the goals of the diet. Normally one can assume that the organism then switches to emergency mode if it consumes around 500 fewer calories a day than it uses up for three to five days.