Fitness gurus blow the lid on what The Rock used to get his insane new physique in just 12 weeks for new role

Dwayne Johnson earned the title 'The Rock' for his famous hulking muscular frame, bulging biceps and pecs.

But the 53-year-old stunned fans at the Venice Film Festival last night, debuting a much slimmer frame after a reported 60-pound weight loss for his new movie role.

Now, two celebrity personal trainers, including the coach behind She-Hulk, have weighed in on what might be behind his eye-catching transformation.

'The daily discipline and the constant grind that you have to put in when it comes to this transformation, the nutrition and working out are relentless,' Jill Charton, a celebrity personal trainer who works with Marvel and The Lakers, told the Daily Mail.

Katie Boyd, a fitness expert who previously trained members of the UK royal family, estimated it took him 'just 12 weeks to achieve' his new physique.

Both trainers guessed the transformation was almost entirely driven by diet. They said he would have also catered his exercise regime to work toward his new desired results.

The celebrity personal trainers the Daily Mail spoke to did not see Johnson in person, and evaluated him based solely on pictures.

While some commentators on social media have questioned if 'coming off' steroids could account for the weight loss, there is no confirmation that Johnson was taking steroids in recent years.

In 2009, he admitted in an interview to using the performance-enhancers in his late teenage years, but there is no mention of his taking steroids as an adult.

Johnson has slimmed down for his new role in The Smashing Machine, where he plays troubled UFC fighter Mark Kerr.

He took on the role because he was 'really hungry' for something new, telling the Hollywood Reporter, 'there was just a voice inside of me, a little voice, that said, 'Well, what if I could do more, I want to do more and what does that look like?''

The Daily Mail reached out to Johnson's representatives for comment, but did not receive a response.

Both trainers believe much of his transformation was down to tweaks he had made in the kitchen rather than changes to his exercise regime.

Charton said it was likely he had switched to a high-protein diet, which put him in a calorie deficit - meaning he ate fewer calories than he burned per day - in order to lose weight.

Boyd said his diet likely focused on lean proteins including chicken and white fish, and, if he did eat any carbohydrates, they probably came in the form of broccoli or other vegetables that would not spike his blood sugar.

They both guessed that he had been kept away from sugary snacks or pastries, with him previously sharing insane cheat days with plates full of pancakes.

'Food is the number one - abs and a slimmer frame are definitely made in the kitchen,' Boyd said.

The trainers also said his exercise routine was likely switched to focusing on cardio, such as running on a treadmill or using a stationary bike.

Cardio can help someone lose weight because it burns more calories compared to weightlifting and causes the body to burn its energy stores.

They added he was likely still weightlifting, but was now focused on lifting lighter weights for more repetitions rather than heavier weights for fewer.

Studies suggest lifting heavier weights boosts muscle size and strength by causing microtears in muscle fibers, causing them to grow larger, while lifting lighter weights boosts endurance but doesn't have the same growth effect.

More repetitions would also burn more calories, helping with weight loss.

'I bet you he is getting up in the morning and doing at least an hour of cardio first thing on an empty stomach,' Boyd said.

'And then working out five to six times a day doing high interval training and full body sessions.'

Neither expert believed he had used Ozempic or a similar weight loss drug. They said a GLP-1 inhibitor likely would not have helped his transformation.

Experts have said the drugs work primarily by suppressing appetite, which would not have benefited him because he would still need fuel for hours of training per day.

They said that side effects such as nausea, fatigue and digestive upset would also have made it very difficult to train at the level required for the role.

It was not clear how rapidly Johnson completed his transformation, but Boyd said it could have been done in around three months.

Still, she said the dramatic shift does not seem unnatural.

'What's really funny is that when I was like 18 years old, I was an extra for one of his movies, where they were filming in Boston,' she said.

'At that time, he was so thin, similar to the way he looks right now for this movie where he had to get really lean like that.

'So, you know, I think all big people like that have the capacity to get small again if they need or want to.'