Look at what season one winner Ryan Benson wrote on his personal blog:
I wanted to win so bad that the last ten days before the final weigh-in I didn't eat one piece of solid food! If you've heard of "The Master Cleanse" that's what I did. Its basically drinking lemonade made with water, lemon juice, maple syrup, and cayenne pepper. The rules of the show said we couldn't use any weight-loss drugs, well I didn't take any drugs, I just starved myself! Twenty-four hours before the final weigh-in I stopped putting ANYTHING in my body, liquid or solid, then I started using some old high school wrestling tricks. I wore a rubber suit while jogging on the treadmill, and then spent a lot of time in the steam room. In the final 24 hours I probably dropped 10-13 lbs in just pure water weight. By the time of the final weigh-in I was peeing blood.
Was this healthy? Heck no! My wife wanted to kill me if I didn't do it to myself first. But I was in a different place, I knew winning the show could put us in a better place financially and I was willing to do some crazy stuff. All this torture I put myself through has had no lasting effects on me (that I know of) and at the time it was sort of a fun adventure for me -- but I am sure it wreaked havoc on my system.
In the five days after the show was over I gained about 32lbs. Not from eating, just from getting my system back to normal (mostly re-hydrating myself). So in five days I was back up to 240 - crazy!
Or consider this from Kai Hibberd of season three:
I dehydrated off 19 pounds in the last two weeks before the BIG weigh in. I stopped eating solid food after eating only protein and asparagus (a diuretic) then I had two colonics and spent the night before the weigh in and out of a sauna. There really was no "diet" the day of the weigh in, we weigh in as dehydrated as possible on empty stomachs after 2 hour workouts in the morning.