The Doctors' Dr. Travis Stork gave ET the rundown on the potential risks that come with celebrity yo-yo diets.
While some use yo-yo dieting because of body image, these celebs went through drastic changes for movie roles. The tactic can be fruitful come awards season (Charlize Theron won an Oscar after gaining 30 pounds for Monster), but if done the wrong way, it could be detrimental to their health.
"When you're putting on all that quote unquote 'fat weight' around your gut, that's biochemically active and it puts you at risk for a lot of things," Dr. Stork said. "The biggest is increased heart rate and increased blood pressure."
Before Matt Damon got ripped for the upcoming Bourne film, he packed on 30 pounds for 2009's The Informant!. His weight-gaining strategy was simple -- lots of junk food.
"It was all fun, but by the end it was getting to me," Damon told ET in 2009.
Christian Bale famously became emaciated for 2004's The Machinist, dropping 60 pounds for the role.
"I don't think it's something I would wish to do again," Bale told ET at the time. "It ain't great for your health."
"In general with that rapid weight loss, you can get malnutrition -- certainly if you're not getting enough protein -- and even put you at risk for gallstones, which certainly anyone who's had them knows that's no fun," Dr. Stork said.
When 2013's American Hustle came around, Bale took the opposite approach, gaining 40 pounds to play Irving Rosenfeld in David O. Russell's crime drama.
"For a couple of months I just ate donut after donut, and sat on my ass until I felt right for the film," Bale said.