Killjoy, I don't think for most people losing this fast is healthy or safe. That is my dilemma with the show. We have seen illnesses and injuries in every season, and already (was it Neil) has become ill and taken to the hospital. In the "real" world uncontrollable vomitting would be a big hint to cut back on the extreme exercising and dieting. If he weren't sharp enough to pick this up on his own, his doctor would have suggested he make changes more gradually.
This show has more in common with "iron man competitions" and extreme sports than it does real world dieting. People are taking real risks, and experiencing real pain, and that's why the prize money has to be there I think. Without it, very few people would be willing to suppress their natural instincts for self-preservation and pain avoidance. Blisters on the bottom of the feet from suddenly doing a lot of walking or running, aren't tiny "new shoe" blisters. They can cover the whole foot, and are extremely painful. I got them on my heels and balls of both feet my first year in college (we pretty much walked everywhere, very few people had cars, or would use them if they did because they didn't want to lose their parking space), and again in graduate school. The blisters were each about 2 - 4 inches across and it was like walking on fire. It isn't just being fat (though friction AND weight do make them worse) as they're also common to military marches, long sight-seeing tours, and marathons and other foot races. The horrible part was that when you have to keep walking on them, they take forever to heal, and you get blisters on top of blisters, and some develop callouses and more blisters develop underneath the callous. At the end of the day, my bandage and likely my sock would be bloody. Sometimes I had to soak my foot in warm water with my bandage and/or sock still on, so I could get them off without tearing up the wound. Really gross.
Having had them before, I'm not sure a guaranteed $250,000 would get me onto a treadmill with those kind of blisters on my feet. I did wonder why Lynn and Jenni didn't switch to a water workout with the blisters, but maybe the campus doesn't have a pool this year, but I thought I saw them preview water challenges. Then again, that would be an open sore, wouldn't it?