Renee Zellweger wowed fans by gaining weight to play Bridget Jones and then quickly losing it not once but twice. And it's not just the ladies that repeatedly lose weight for plum movie roles. Rapper 50 Cent and actor Christian Bale have done similar stunts. Experts agree that yo-yo dieting is both psychologically and physiologically damaging. Effects include an elevated risk of heart attack, gallstones, stroke, diabetes, depression and, sadly, weight gain.