The toxicity of Detox diets.
Take a look at this blog entry on detox diets.
Specifically, it outlines how these extreme diets result in vitamin deficiency, a depressed immune system and muscle tissue breakdown, among other things.
This is in addition to the fact that the more you diet, the more your metabolism slows down and makes it easier to put on weight, harder to take it off. This can become a destructive spiral of dieting and putting the weight back on. Diets are rarely a long term fix.