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Swinging topless from rings high in the air, Zac Efron strains as his biceps bulge, his shredded shoulders swell and his sculpted six-pack is shown off to the max.

The actor, on the set of Baywatch , is so proud of his buff body he posted his workout pictures to his 11.6 million Twitter followers and 12.1 million Instagram followers.

One caption said: “Blood, sweat and Respect. The first two you give, the last one you earn.”

To get ripped for his role as lifeguard Matt Brody, Zac has done months of training and followed a strict diet. He tweeted: “Nine days of zero carbs and sugar. Only organic grass fed/free range protein and organic leafy greens.”

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Zac, 28, is one of many stars who have transformed from fit-but-lean to beefcake – and shared the results on social media. Mark Wright , 29, was almost unrecognisable in an Instagram selfie last week.

(Image: Instagram)

It showed him bulked up as the result of gruelling gym sessions and a protein-heavy diet. He captioned the image: “Someone said summer is on its way, I say I’m getting ready!!”.

Magic Mike XXL star Joe Manganiello , 39, singer Craig David , 34, and Jurassic World star Chris Pratt, 36, are also known for their muscle-mad selfies.

(Image: Instagram)

Former X Factor contestant Ray Quinn, 27, also caused a stir with gym selfies showing the Liverpudlian has transformed from baby-faced singer to mean-looking muscle man.

After years of famous men sporting toned-but-slim physiques the big beefcake is back with a vengeance.

(Image: Instagram)

David Charvet, who played Matt Brody in the Baywatch TV series in the 1990s, was nowhere near as big as transformed High School Musical star Zac.

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Experts fear that with the rise of the gym selfie, there is a much darker side to the example these men are setting – warning it could lead to serious psychological issues for guys who feel they need to achieve near-impossible body goals.

Dr James Byron-Daniel, senior lecturer in sports and exercise psychology at the University of the West of England , says: “There is massive pressure on men now because they see so many body images on social media, which we didn’t see 10 years ago.

“The problem comes when young men are not sure how to attain the body ideals they see but just want to get bigger and bigger.

“Constantly comparing themselves to others and not achieving unattainable aims potentially leads to psychological issues such as depression and anxiety.

“It’s a big problem – bigger than we realise because men are less likely to share their worries and seek help. It really is a ticking timebomb.”

(Image: FilmMagic)

Celebs work hard on their looks with what trainers estimate to be up to three hours a day in the gym and the right advice to do it healthily. But experts say the man on the street, who doesn’t have the correct advice, is increasingly turning to dangerous quick fixes.

“We’re seeing younger and younger men joining gyms,” says Dr Ruth Lowry, exercise psychologist at the University of Chichester. “We also have a rise in concerns about the obsession with highly muscular celebrity body ideals.

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“It can start off being proactive with exercise and healthy eating but can be taken to an extreme with people choosing a calorie-restricted diet and supplementation. All these things in moderation can have a beneficial effect but in a quest for that perfect image, which takes time, dedication and money, they are looking to achieve goals too quickly.

“We’re seeing men just concentrating on pumping iron instead of cardio health. Then there are conversations about other things, such as steroids.

“Becoming exercise dependent also means prioritising time at the gym to the detriment of other aspects of their lives. They can experience disordered eating patterns. And they can develop muscle dysmorphia, a specific type of thinking which means perceiving yourself as much thinner and less muscled than you are.”

(Image: Getty Images)

There is no suggestion any celebs featured here have psychological disorders or have used drugs to achieve their physiques.

But the number of men diagnosed with eating disorders has risen by almost 30% in the past 16 years.

Experts say the problem reflects increased pressure on men to care more about their image. Latest figures also show there was a 26% rise in males aged under-19 with eating disorders treated as inpatients in English hospitals in the five years to 2014.

Sam Thomas, founder of charity Men Get Eating Disorders Too, says: “Men are under much more pressure now as it’s become the norm to post gym selfies online. There’s no denying social media is to blame for the start of some eating disorders .”

A problem with some gym goers is orthorexia, a fixation with eating foods considered to be healthy. People become obsessed with restrictive diets and punishing exercise regimes. It’s feared it can lead to self-loathing and malnourishment, with some sufferers also falling foul of anorexia or bulimia as they battle to gain a sense of control.

(Image: Instagram)

Image-obsessed young men are being blamed for anabolic steroid use soaring to epidemic levels in Britain.

In 2015 the Crime Survey for England and Wales estimated almost 60,000 people a year take the drugs, which mimic the effects of testosterone and boost muscle growth. But experts say the true amount could be 10 times greater.

Professor Julien Baker, of the Institute of Clinical Exercise and Health Science at the University of the West of Scotland, says: “The real figure is definitely in the hundreds of thousands.”

Made in Chelsea star Spencer Matthews, 29, pulled out of reality show I’m a Celebrity... Get Me Out of Here! last year after admitting he’d become addicted to steroids. He said they were “very worryingly, readily available”.

(Image: Rex/Shutterstock)

One man who knows how it should be done properly is trainer David Kingsbury, from KL Fitness in Pinewood Studios, who has helped transform the likes of actor Hugh Jackman .

David says: “Huge numbers of fitness enthusiasts upload pictures but there’s a fine line between feeling inspired and feeling pressured. Don’t worry about what other people are up to.”

Campaigns in recent years have tried to change the way girls think about their bodies. Dr Lowry says the Government must widen the net. She thinks too little is being done to see how body image is affecting men, adding: “It is a worry.”