The most extreme diet I’ve ever heard of is of course that of an eating disorder.  But some of those diets out there I would swear are equal to an eating disorder in so many ways.  For example, I know of one that actually requires you to eat nothing but protein shakes and eggs, that’s it.  You can eat until you are full, but seriously?  You really can’t get effective results in the way you want.  To be sure, you will be losing weight, but you’ll gain it all back because it’s not sustainable, and it is by no means actually healthy, which presents a huge problem to say the least. 

Either way, it would be highly inadvisable to actually use this kind of diet.  I compare it to the Atkins diet in some ways.  But it’s even more unrealistic and unattainable to a larger degree in general terms.  It doesn’t give you the results you want to be quite sure, and it can do quite a bit of damage in various ways, especially in terms of the possible vitamin deficiencies and otherwise.  And frankly, you shouldn’t be replacing food sources with a multivitamin, let’s be completely honest, that’s a really bad sign.