Crash diets still as popular as ever. If people want to lose weight quickly, maybe for a big upcoming event, or perhaps for a quick recording of self-esteem, they often take drastic measures, described by you on a crash diet, also common as a starvation diet. There are different types of crash or starvation diets, but how do I tell from the name that most of these diets involve eating very little food.
The terrible irony of this kindof diets is that they trick your body (and not in a good way) by integrating them into starvation mode. When a human body is in starvation mode, the metabolic rate (or velocity, with the calories burned) are slowed way, way down to accommodate the lack of food / fuel.
Not only that, but with such a drastic diets, it's not just fat, lose the people. You lose water weight and muscle mass. The loss of muscle mass is the exact opposite of what you want to do, because leanMuscle mass is what you can burn more calories not only when you are active and exercising, but if you are at rest.
When a lot of people lose weight very quickly because they put on a crash diet, this is not the kind of weight that they tend to keep away from the long term. It is better to eat healthy and exercise regularly to lose the weight gradually over the course of several weeks or months, and keep it off.