Those people are seriously stupid. They lost all that weight and what, they regained it overnight? Did they not see it creeping it up over months and years?
You think you can control your hunger while maintaining a 40-50% calorie deficit? Good luck with that. It may work for a very short while and the dieter will lose some weight, but then he will come to a grinding halt as the metabolism goes into survival mode and slows down. And "doing without" that substantially for any length of time causes people to cheat, crash and burn. That's just reality, proven over and over again. Crash diets inevitably crash. The tortoise wins the race.
Some years ago, I had volumetric measurements done a few times and compared that to the results from using calipers and from using an impedance scale. By now, having gone through so many diet cycles, I kinda know what I look like at different levels of body fat. Just for the reference, I am 179cm and thin boned, so my BMI fluctuates safely between 21 and 19, depending on the point of my training cycle.
I was on a 40% calorie deficit diet for at least 2 months. No issues with hunger WHATSOEVER. I lost so much fat that it was freaky. But i had tons of social pressure to stop. When you become leaner and leaner, (fatter) people around you think you have cancer. Metabolism doesn't go into survival mode till about 800 calories a day energy intake. Btw, a 40% deficit for me means an intake of about 1200. What actually happens is that the body dumps muscle, thereby lowering the rate of metabolism. So you will still lose fat, but at a much reduced pace.
I would have thought that was evident. This is my take on these biggest losers people. They initially used dieting and probably cardio to lose weight. Meant that yes, they lost a load of fat but also burned off a load of muscle. Their metabolism rate drops (the less you weigh, the less your metabolism rate, basic physics). They are then released back into society and they go back to eating the same or perhaps slightly less calories than before they went on the show. So obviously with a lower metabolic rate and increased calories in their diet, of course they are going to (re)gain weight! This is known as the yo-yo effect and is easily explained.
Stated perhaps more accurately, the less lean body mass you have, the lower will be your metabolic rate, all else being equal.
Just reviewed my lectures on this subject...and i was right first time, the less you weigh, the less your metabolic rate. A decrease in total mass (does not matter whether fat or fat free mass) leads to your basal metabolic rate decreasing. Also the amount of energy expended doing exercise also decreases. All basic physics.