Matthew Fox (Lost 40 Pounds)
Credit: Everett

Fox apparently never felt compelled to lose weight when he played island-dwelling plane crash survivor Jack Shephard on Lost, but he worked hard to transform into a ripped serial killer for Alex Cross. For five months, the actor sweat with a trainer and eventually ate only around 1,100 calories a day to shed about 40 pounds – for an active average-sized man, the USDA recommends a daily 2,800 calorie intake.

The Bottom Line:
It's a wonder Fox was able to get through his workouts on such a limited amount of fuel. "The major concern is you're losing muscle mass, and when you do that, you lose strength, you lose stamina, you lose endurance," says Kushner.