The most classic extreme diets are those very low in calories, those that are based on eating few types of food for a week, almost total fasts, those that only allow vegetable smoothies for days, those that totally eliminate certain types of foods, some called «detox», those that promise to lose 5-10kg in a week, and so on.
Although you lose a lot of weight in a few days, the truth is that these diets not only eliminate fat, but a lot of lean mass, which is what helps us have a good metabolism, be more toned and burn more calories at rest.
Therefore, you lose some fat but also muscle, which means that when you return to eating in a «normal» way, you gain weight immediately, and frequently take on a few extra pounds, the well-known rebound effects.
In conclusion: Do not do extreme diets because you annihilate your metabolism, they involve a sacrifice that will make you suffer, and you will feel even worse when you regain the previous weight after so much effort plus some extra.
If you want to lose weight without rebound effect, without starving and in a healthy way, I recommend taking your free nutritional test.