We've written before about how celebs gain and/or lose weight for certain roles. It makes their portrayals of characters more believable, but it can't be a good practice for their health. Look at how these celebs changed their bodies for a role:

• Jake Gyllenhaal bulked up for his role in the upcoming film, Prince of Persia, by working out two to three times a day, six days a week. He followed that routine for three to four months, in addition to stunt training for the film.
• Irish actor Michael Fassbender lost 33 pounds in 10 weeks by severely restricting calories -- 900-1,000 calories for the first six weeks, then dropping down to 600 calories a day.
  
• Christian Bale survived mostly on apples and coffee when shedding pounds for his role in The Machinist.
• Robert de Niro packed on 60 pounds for his 1980 role in Raging Bull; by the end of shooting, he was visibly not feeling well.
• Renee Zellweger knows all about putting on extra pounds -- she's done it twice for the Bridget Jones movies. While she admits eating whatever you want many sound fun, she says: "After a week your glucose levels are going crazy. It doesn't feel good."
  

Sure, celebrities may seem to lead glamorous lives ... but a job that requires you to risk your health? As for me, I'm happier just being an average Jane.