Reporter: Stunning hollywood transformations. Christian bale, batman to fighter. 50 cent's bulging muscles, now barely recognizable, for his role in "all things fall apart." And hugh jackman said he... See More
Reporter: Stunning hollywood transformations. Christian bale, batman to fighter. 50 cent's bulging muscles, now barely recognizable, for his role in "all things fall apart." And hugh jackman said he lost about 25 pounds for his role on "les mis." I had gone on a water dehydration diet. Reporter: Celebrities, losing weight and fast. You can look. But you cannot touch. Reporter: In "magic mike," matthew McConaughey strutted his stuff on screen. But only a few months later, he looked old and frail. Dropping 50 pounds to play the part of a man with hiv. It takes a while for your body to understand that it has to feed off of itself. And you're not going to give it something else from the outside. Reporter: Now, he's packing the weight back on. Only five pounds shy from his goal weight. Health experts warn in kind of yo-yo dieting can be dangerous. People end up with gallbladder issues. Reporter: Ashton kutcher says he wound up in the hospital, after following the fruit diet to look more like steve jobs. When stars aren't losing weight for roles, many are gaining it. They take drastic measures to get drastic results. Reporter: Perhaps the best part of losing all that weight is gaining it back again. For "good morning america," abbie boudreau, abc news, los angeles. Bring on the milkshakes. What dedication.
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