Frankly, the type of people who go on the show would never lose the weight. They're already in their late 30s usually and have set themselves to being fat. I'm not sure how much damage losing the weight would do but seemingly it would be less than the continued damage that these morbidly obese people would have continued to do just by being so fat.  On the other hand if you balloon back up you're getting the worst of both worlds. Effects from losing weight so dramatically and the effects from the fat. They have their cake and ate it too.