The Dangers of Celebrity Yo-Yo Diet Transformations

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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.

PICS: Shocking Celebrity Weight-Loss Transformations

Matthew McConaughey dropped to 135 pounds for 2013's Dallas Buyers Club, but when filming finished, he had to put it all back on.

"I think the weight gain is kind of the more dangerous territory -- more dangerous than the weight loss," McConaughey told ET in 2013.

Dr. Stork also told ET that one should be careful before bulking up.

WATCH: Matthew McConaughey Goes Bald and Pudgy for Gold

"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.

WATCH: 6 Male Stars Who Lost a Shocking Amount of Weight

"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.

PHOTOS: Before & After: Celebs' Most Dramatic Transformations

In contrast, Dr. Philip Goglia has helped stars get in immaculate shape for roles. His client list includes stars like Chris Hemsworth and Chris Pratt.

"There is so much riding on their performance and appearance," Dr. Goglia told ET. "Chris Pratt lost over 60 pounds and looked incredible for Guardians of the Galaxy."

Watch the video for more.