Despite all the lecturing, the research doesn't really agree. Crash(extreme) diets work just as well as slower diets and the people gain back weight at roughly the same rate.

Contestants on Alone loose like a lb a day. Loosing weight is so easy if you get used to feeling hungry and eat like a bird. Throw in cardio and you will end up looking cut. Gaining a pound a day of muscle, on the other hand, is probably impossible, even gaining a pound of any kind of mass a day is dubious and will lead to very unpleasant bowel movements at the very least.

It doesn't help that BMI is still used even though it is such a crude measure as to be useless. Stuff like body fat% measurements and waist measurements are more direct indicators of losing fat.

We use PI/CI (Ponderal Index AKA Corpulence Index) in my lab and it's a huge improvement over BMI. Especially for modern heights in men and women. Still crude, but easy to apply. Wide margins.