Zac Efron, Will Smith, Christian Bale and more extreme body transformations in Hollywood: How far is too far?

For decades, Hollywood actors and actresses have been pushing their minds and bodies to the limit to transform for movie roles, but are they going too far?

Zac Efron’s physique was on display as he geared up for his upcoming film "The Iron Claw." The actor, 35, was nearly unrecognizable as he has transformed his body, putting on massive amounts of muscle to portray a wrestler in the movie.

On the other end of the spectrum, Natalie Portman and her "Black Swan" co-star Mila Kunis revealed that they were "barely eating" while filming in 2010.

Dr. Michael Feiz, a practitioner of bariatric medicine, shared with Fox News Digital that it is unhealthy for anyone, even Hollywood’s finest, to go to such extremes to achieve a specific physique or figure.

"It is not safe for anyone, including celebrities, to go to extremes to achieve a certain body. We know that many celebrities feel the need to do so in preparation for a role," Feiz said.

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Here is a look at some other stars who have undergone insane body transformations for a role.

Christian Bale

Christian Bale is well known for going to extremes to transform his body to get into character.

In "The Machinist," released in 2004, Bale was dieting for months before production began. His diet reportedly consisted of nothing but apples, water and coffee. He lost 62 lbs. and weighed just 120 lbs. during filming.

After losing a large amount of weight, Bale gained back 70 pounds plus an additional 30 pounds of muscle for his "Batman" role.

"I keep saying I’m done with it. I really think I’m done with it, yeah," Bale said, regarding losing and gaining weight for roles, in 2019.

"It was more fun gaining the weight than losing it," he added.

But in 2010, Bale dropped weight again for "The Fighter." And in 2013, he put on some pounds for his role in "American Hustle." After that, the actor realized that it was getting harder for him to fluctuate in weight.

"I thought I was going to lose the weight I gained for 'American Hustle.' I said, two months, flat, that’ll do it. I was 185, and I went up to 228 for it. And I’m still working that off! It’s almost six months later," he said in 2013. "Now, I know that when I was in my early 20s, it would have been two months, and that’s it."

Fiez shared with Fox News Digital the downsides of gaining and losing weight quickly.

"When celebrities are transforming their bodies for a role, it’s often in a very short period of time. This time stress also places their bodies into a severe stress state," he said. 

"Imagine the fight-or-flight response that a person feels when faced with danger. Severe weight changes put your body in an artificially heightened state that you then maintain for, let’s say, 2-3 months. As a result, the body’s homeostasis is imbalanced. This includes aberrations in electrolyte levels, nutrient levels and hormone levels, including the sex hormones estrogen, testosterone and the body‘s stress hormones ADH, cortisol and thyroid hormones," he added.

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Chris Hemsworth

Chris Hemsworth is known for his built role as "Thor" in the Marvel universe, but he has taken a break from the weight room in the past.

In 2015, Hemsworth starred in a film called "In the Heart of the Sea," as Owen Chase, a man stranded in the ocean in 1820, facing starvation and despair as he tried to stay alive. 

In November of that year, Hemsworth took to Twitter to share that he "wouldn’t recommend" the diet he was on to perform the role.

"Just tried a new diet/training program called ‘Lost At Sea.’ Wouldn't recommend it," he wrote alongside an image of himself in character, showing off the dramatic weight loss.

Charlize Theron

Charlize Theron gained 50 pounds for her role as Marlo Moreau in the 2018 film "Tully."

Her character was an overweight mother of three who was struggling during that chapter of her life.

Theron shared that she forced herself to eat macaroni and cheese in the middle of the night to keep on the weight.

"I wanted to feel what this woman felt, and I think that was a way for me to get closer to her and get into that mindset," she said in an interview with Entertainment Tonight. 

However, she noted that she was not prepared for the mental toll that gaining a large amount of weight would have on her.

"You know, it was a huge surprise to me," Theron said. "I got hit in the face pretty hard with depression. Yeah, for the first time in my life I was eating so much processed foods, and I drank way too much sugar. I was not that fun to be around on this film."

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She detailed to the outlet that some mornings she would have to eat In-N-Out cheeseburgers and milkshakes to get a large amount of calories in.

"The first three weeks are always fun, because you're just like a kid in a candy store," Theron said. "And then after three weeks, it's not fun anymore. Like, all of a sudden, you’re just done eating that amount, and then it becomes a job."

Theron admitted it took a year and a half to lose the weight. "I was worried. I was like, ‘This is taking a really long time,’" she said. 

"Because on 'Monster,' I just didn't snack for five days, and I was fine. You know your body at 27 is a little different than your body at 43, and my doctor made sure to make me very aware of that. Like, you are 42, calm down, you’re not dying, all good."

Theron gained 30 pounds for her role in "Monster" in 2003.

Jake Gyllenhaal

Jake Gyllenhaal shocked the world when he transformed for a role as a boxer in "Southpaw."

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Gyllenhaal has been very transparent in past interviews about how he prepared for the role. He has admitted that he did 2,000 situps a day, as well as an eight-mile run.

The actor worked out anywhere from four to six hours a day in preparation to portray Billy "The Great" Hope.

He reportedly gained a total of 28 pounds of muscle for the boxer role.

Gyllenhaal went on "The Howard Stern Show" in 2015 and said he trained for five months and worked out twice a day. 

Natalie Portman

Natalie Portman admitted in an interview with the Daily Mail that she had been "barely eating" while filming "Black Swan."

"I was barely eating, I was working 16 hours a day," she said during the 2011 interview. "I was almost method acting without intending to. I do wonder now how people can do this kind of role when they have a family."

In an interview with Entertainment Weekly, Portman revealed that she thought she might "die" during the year-long preparation.

"There were some nights that I thought I literally was going to die," Portman told the magazine. "It was the first time I understood how you could get so wrapped up in a role that it could sort of take you down."

Mila Kunis

Portman’s "Black Swan" co-star Mila Kunis dropped down to only 95 pounds for the movie.

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She detailed her drastic weight loss in an interview with E! News in 2011. 

"I could see why this industry is so f---ed up, because at 95 pounds, I would literally look at myself in the mirror, and I was like, ‘Oh my God!’" Kunis said. "I had no shape, no boobs, no ass... All you saw was bone. I was like, ‘This looks gross.’"

She revealed that her friends and family had become concerned and even "panicked" by her weight loss. Kunis shared that her character on-screen did not illustrate how dramatic her transformation had been.

"In real life, it looked disgusting," she told the outlet. "But in photographs and on film, it looked amazing."

Kunis lost 20 pounds in five months for the role.

Renee Zellweger

Renee Zellweger gained 30 pounds to portray Bridget Jones in the 2001 film "Bridget Jones’s Diary," and she repeated the process again for the movie’s sequel, "Bridget Jones: The Edge of Reason."

Zellweger, now 53, opted for a different route in her recent true-crime series, "The Thing About Pam," about convicted killer Pam Hupp.

She wore a padded suit and facial prosthetics for the role, opting not to gain the weight needed for the character.

"Oh, gosh, if you don’t recognize an actor or an actress in a performance, that’s a great compliment," Zellweger said in an interview with Vanity Fair.

She explained what her look in the series had consisted of. "It was pretty much head to toe. It was prosthetics, it was a [padded] suit, it was the choice of clothing, it was the briskness in her step-step-step, her gait."

Matthew McConaughey

Matthew McConaughey said he had lost 50 pounds for his role in "Dallas Buyers Club" in 2013.

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During an appearance on the "Joe Rogan Experience" podcast in 2020, McConaughey said he had only eaten fish, egg whites and tapioca pudding to prepare for the role.

"I did not torture myself. I was militant. The hardest part was making the damn choice," he said at the time. 

The actor explained to Rogan that he had wanted to accurately portray an individual who had advanced stages of HIV, like his character did.

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"It was my responsibility," he said. "If I looked how I look now and played Ron Woodroof from 'Dallas Buyers Club,' you are out of the movie the first frame." 

McConaughey went from 185 to 135 pounds over the span of five months.

Will Smith

Will Smith gained 35 pounds of muscle to accurately portray Muhammad Ali in his Oscar-nominated performance in "Ali" in 2001.

"When the ‘Ali’ movie came up they’d done an international search for coaches," Darrell Foster, who trained Sugar Ray Leonard for two decades, told Esquire of the film. 

"A lot of top-rate professional coaches can coach you if you already know how to do something, but a lot of them can’t teach you from scratch. They needed to train Will from scratch because he couldn’t fight at all. He was athletic in other sports – football, playing around in the neighborhood shooting hoops – but he couldn’t fight at all. I think he had decided not to do the picture. I met with Michael Mann, they had spoken to someone who knew Sugar Ray, and they recommended me."

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Foster shared what it was like to train Smith.

"Smith had no star power for me," Foster told the outlet. "[With Sugar Ray] I had already been in 20 world-class training camps for world titles against people. Roberto Durán, the best in the world. Will Smith to me was like a 5-year-old kid coming in to the gym and saying ‘Hey, coach, can you teach me how to fight?'"

Dr. Feiz noted that celebrities like Will Smith get a lot of help when they transform their bodies, and are often "under close medical supervision and have professional trainers monitoring them." 

However, it does send a dangerous message to fans that going to extremes with their weight is possible.

"One of my concerns is the negative effect on fans of these celebrities. They falsely believe they can transform themselves in a similarly short period of time," Feiz shared. "The average person does not have access to the constant monitoring and testing by medical experts celebrities have, making it even more unsafe for the average person to undergo these changes in such a short period of time."