Natalie Portman and Gwyneth Paltrow Take Bodies to Extreme

Dec. 3, 2010— -- When Natalie Portman dropped 20 pounds to play a ballerina in her new movie, "Black Swan," even her director worried the petite actress had gone too far.

"At a certain point, I looked at [Natalie's] back, and she was so skinny and so cut," director Darren Aronofsky told "Access Hollywood." "I was like, 'Natalie, start eating.' I made sure she had a bunch of food in her trailer."

Meanwhile, famously slim Gwyneth Paltrow gained -- then quickly lost -- 20 pounds, and no one, besides Paltrow and her trainer, seemed to notice.

Actresses have often shifted their shapes for a role. But today's crop of waiflike actresses hardly have any weight to lose. And when they do gain weight for a part, the way Paltrow did, the additional weight barely registers.

At least when Renee Zellweger gained 20 pounds to play Bridget Jones, she looked like a normal woman or, as New York Times critic Stephen Holden put it, "utterly real."

But there's nothing real about the weight fluctuations of today's starlets. Keith Ayoob, a registered dietitian and director of the nutrition clinic at the Albert Einstein College of Medicine in New York, warns, "Do not try this at home!"

"They do this for a role," Ayoob told ABCNews.com, "and it's often not the best thing for their metabolisms and bodies. But if they are getting $10 million for the role, I'm not sure the producer cares if their long-term health is at stake."

Here's the skinny on the latest actresses taking their bodies to extremes. Natalie Portman, left, in the movie "Black Swan," and, right, three years earlier.