Crash diets can work, at least at first. Think juice cleanses, cabbage soup diets, extremely low-calorie plans, cutting out nearly all carbs, “detoxes,” or basically surviving on protein shakes and 800 calories a day. The scale may drop quickly, but your body doesn’t necessarily celebrate with you.

Significant calorie restriction and weight loss can trigger biological changes that increase hunger and make maintaining that lower weight harder.

Hunger signals such as ghrelin may increase, while signals involved in fullness and energy stores, including leptin, can decrease.

So when someone finishes a crash diet and suddenly feels ravenous, starts thinking about food constantly, or regains weight, it isn’t simply that they “lost their willpower.” Their body may be actively pushing them to eat and restore energy stores. That’s one reason the restrict → lose weight → get extremely hungry → regain → restrict again cycle can be so difficult to break.