It is difficult for Miles to pinpoint the moment his muscle dysmorphia started. It was just always there, a background hum. “As far back as I can remember, I wanted a better-looking body,” says the 35-year-old US soldier, now stationed in Mons, Belgium. When he was 13, Miles spent a summer cutting grass to save up for a secondhand Soloflex exercise machine. The machine cost $1,000 (£800), but as Miles was too young to join a gym, it was worth the expense. With the help of the Soloflex, Miles started weight training and never looked back.

When he returned from a posting to Afghanistan at 24, things spiralled. He began obsessively working out and regimenting his meals. “I went all in ... it was full, hardcore dedication to the lifestyle.” Miles set his watch to beep every three hours, to remind him to eat. If it beeped when he was driving, he would pull over. Slowly, he whittled his body into shape. His muscles became striated, every fibre visible. Not big enough. At 95kg (210lbs) and 1.8 metres (6ft 2in), Miles wanted to be more muscular; leaner. He lost 22kg and started competing in amateur bodybuilding competitions. There was virtually no fat on him. “You pinch your skin and it just stays pinched.” His girlfriend left him. “She began to realise that my body dysmorphia was like dating another person.” The pursuit of muscularity took over his life. “I just thought, I am so lean, and shredded, and veiny, and masculine – I don’t ever want to go back to how I was before.”

Yet by 33, single again – the dysmorphia had claimed yet another relationship – it had all become too much and he was in a dark place. “I did not enjoy life in any way, shape or form.” All day long, he would starve himself, struggle through punishing workouts, go home and binge-eat before throwing it all up. One evening, waiting in line at the burger chain In-N-Out for more food to purge, Miles finally decided enough was enough. “I woke up so happy the next day, knowing it was over.”

A subset of body dysmorphic disorder, individuals with muscle dysmorphia feel they need to become bigger or more muscular, regardless of their size. Sometimes referred to as “bigorexia”, it typically affects men. About 30% of people with muscle dysmorphia will also have a medically diagnosable eating disorder, as those with the condition may follow extremely restrictive diets. Because men with muscle dysmorphia rarely seek treatment, estimating its prevalence in the general population is hard, but it is believed that about 10-12% of professional male weightlifters meet the criteria.

And muscle dysmorphia may be on the rise. A study published in June found that 22% of men aged 18-24 reported muscularity-oriented disordered eating. “The drive for a bigger, more muscular body is becoming very prevalent,” says the lead researcher Dr Jason Nagata of the University of California, San Francisco. Not everyone who benches 180kg has muscle dysmorphia. It is when working out takes over your life, occluding all else – work, family, friends – that you have a problem. “Their entire day is spent at the gym trying to bulk up,” says Nagata. “They may also be taking illicit supplements like steroids.”

What drives a generation of young men to slavishly pursue this physical ideal? “Over the past decades, the idealised male body image has got bigger and bulkier,” says Nagata.

This body type even pressed its way into our children’s bedrooms: studies show that action figures have become brawnier over the past 25 years.

This wasn’t always the case. “In the 70s, we saw very slim, almost androgynous men, like Mick Jagger and David Bowie ... to be muscular was to be defined as to be militaristic, at a time when, in the US, there were protests against the Vietnam war,” says Dr Roberto Olivardia of Harvard Medical School, an expert in male body image. “So that build was really frowned on and rejected by youth culture.

“But then the 80s came about, with figures like Ronald Reagan, who was pro-military, and men like Arnold Schwarzenegger and Sylvester Stallone.” This ideal male – hyper-masculine, militaristic, strong above all else – was exported globally through films such as First Blood (1982), Predator (1987) and The Terminator (1984). WWE wrestling was founded in 1980, and the likes of Hulk Hogan became celebrities. In the late 90s, a leaner but still very muscular aesthetic – popularised by Brad Pitt in the movie Fight Club – became fashionable.

Today’s muscle men stare down at us from the billboards of superhero films featuring stars such as Chris Hemsworth or Jason Momoa, the latter only last week body-shamed online after photographs emerged of him enjoying himself on holiday in fractionally less-than-superhero condition. On the small screen, the current crop of Love Island contestants mug for the cameras in tiny swimming trunks, all the better to display their perfect six-packs.

However, just as fashion magazines don’t cause anorexia, but contribute to a toxic environment in which extreme thinness is celebrated, Hulk Hogan, Dwayne ‘the Rock’ Johnson and Chris Hemsworth are not to be blamed for the disordered behaviours sweeping our gyms. According to the NHS, we do not yet know what causes body dysmorphia disorders but genetics, a chemical imbalance in the brain or a traumatic experience in your past may play a part. You are also more likely to develop it if you were bullied or abused as a child, something student Nathaniel Shaw knows well. The 28-year-old was bullied at secondary school – they called him a Borrower, on account of his slender frame.

“I was always the small kid in the corner that no one wanted to speak to.” Social acceptance came one squat press at a time. “From where I’m from in Nottingham, because it’s a very rough place, you’re training to protect yourself. The bigger guys are the serious people that no one wants to mess with. That was the main thing at the start – get big and be taken seriously.”

When Shaw went to college at age 17, he had a plan: work out all winter and reveal his buff body come summer. But when he took off his T-shirt playing football one afternoon, a girl said that Shaw “had no chest”. He instantly put his top back on. “I still wasn’t good enough.” The unkind comment laid waste to Shaw’s fragile self-esteem; he walked away from the wreckage determined to become even more shredded.

Shaw’s life became: the gym, home to eat enormous portions of tuna, pasta and cheese, move as little as possible to conserve energy and repeat. Shaw detached from the contours of normal life. He stayed in bed longer, later and missed exams. He was depressed. “The pursuit of this muscular ideal takes over people’s lives,” explains Nagata. “They become obsessed with it. They can’t function in their daily life outside of pursuing this ideal and it can lead to depression, missing school or work, and losing their ability to do basic living tasks.”

To an observer, Shaw – who weighed 80kg at 1.7 metres – was tank-like. But that wasn’t how he saw himself. “I would be in the gym and say: ‘I look like shit.’ Everyone would be like: ‘No you don’t, you’re huge, I wish I could look like that.’” Muscle dysmorphia is a disease of perception. Although its sufferers live in the material world – a place of grunting exertion and weighted bags and scoopable protein powders – they spend most of their time in an imagined reality, where they are incalculably huge. Their biceps are swollen watermelons; each muscle as finely striated as the delicate contours of a seashell.

But even if they eventually achieve that physique, it is not enough. As soon as one muscular ideal is achieved, a new goal appears. “There’s a saying: ‘Once you step into a gym, you’re forever small,’” says 27-year-old Rich Selby, an amateur bodybuilder from Cardiff. Miles agrees. “Every muscle could be bigger. I could be leaner. You look at yourself and feel like everything is small and weak. I have no chest muscles; I have no arm muscles ... you’re judging yourself against an impossible standard.”

Social media reflects this standard back at you. “You’re being sold a false reality,” says Miles. “I can get into really good shape, right before I’m doing a bodybuilding competition, and use lighting and angles and filters to make my physique look even crazier than it already is, and save a bunch of pictures and upload them to make it seem that I always look like that, all year round.”

Some turn to illegal substances to attain this ideal. Tony, 23, works for a pharmaceutical distributor in Dallas. He started taking illegal performance-enhancing drugs, including testosterone, equipoise and nandrolone, two years ago. The drugs created a dangerous feedback loop: the more he injected, the more his body changed and the more he took. “People are like: ‘Wow, that guy’s a tank.’ They have more respect for you ... I thought, heck yeah, I’m going to take more so I can get even bigger.”

It is a common experience: muscle dysmorphia can be fuelled by the positive reinforcement men receive from other men in the gym. “When you’re large, you get a lot of respect,” Selby confirms. Men come up to him and ask how much he can bench. Sometimes they try to start fights. “That’s why people become addicted. They’re insecure, so they need confirmation from other people.” Selby self-identifies as having muscle dysmorphia, but believes that he has it under control, because he has good self-esteem.

Up close, you can see the havoc muscle dysmorphia wreaks. “Your interpersonal relationships fall apart – but you are so caught up in the endorphin rush of affirmation from your gym buddies, you barely notice,” says Miles. “You’re kind of an asshole. You don’t realise it ... you just become this all-around grouch. It consumes not only all of your time and focus, but also the human part of you.” It’s also a lonely existence. All your time is spent preparing protein-rich food, but because you are over-exercising, you are often “hungry, and cranky, and don’t sleep well”.

Among the young men Nagata surveyed, 2.8% had used illegal steroids, and it is estimated that up to 1 million Brits take performance-enhancing drugs. “Steroids can lead to heart disease, kidney problems and liver damage,” says Nagata. There are also mental health risks. “People may have extreme irritability, aggression, paranoia and can be violent.”

Tony was one young man who used drugs to bulk up. He knew what he was doing was dangerous: he would even donate blood to get his blood pressure down. “I just genuinely didn’t care.” As he cycled on and off drugs, he experienced dramatic mood swings. He got fired from his job at a hardware store for screaming at a coworker in the break room. Eventually, his mental health deteriorated so much that he came off all the drugs in May last year.

What makes someone play Russian roulette with a steroid-filled syringe? Selby thinks people are driven to desperate measures because they cannot disassociate who they are from how they look.

It’s an obsession that can prove fatal. Freddie Dibben, 28, died in March 2017 after his heart became enlarged by the stimulant Clenbuterol. His father Clifford, 69, found him. “The hardest part was going back through to the kitchen and telling his mum what I found. It’ll never go away.” Like Tony, Freddie experienced mood swings. “He would snap at you,” Clifford says, recalling an incident where Freddie was “stroppy” with him as they tinkered with his car. He put the moodiness down to work stress – Freddie had been pulling a lot of night shifts at Wilton carpet factory, where colleagues referred to him as a “forklift”, on account of how much he could carry.

But Clifford felt blessed to have a health-conscious son; he didn’t see anything to be alarmed about. A keen gym-goer, Freddie even gave up smoking at his parents’ request. “He used to cook all his own food! He’d cook a meal before he went to work ... he had two sets of scales, and he weighed all his vegetables, everything. He even kept notes of what he was eating and what he was doing.” Clifford laughs, bitterly. “Except for the bloody drugs.”

And that is one of the issues of muscle dysmorphia – you can hide in plain sight. A pair of weighing scales in the kitchen; tupperware boxes of chicken and broccoli in your backpack. Most people view these as harmless, if idiosyncratic, behaviours. And when you look like you are hewn out of marble, it is hard to consider anything is amiss. It is only when you step past the facade that you realise these statue-like men are slowly destroying themselves.

It is a silent epidemic – Olivardia estimates as many as 10% of men working out in gyms may be suffering, but never seek help. Lately, Tony has started taking illegal substances again, insisting “it can be done safely”. Clifford still has Freddie’s weighing scales in the kitchen. Looking at them, he would probably disagree.

• Some of the names in the piece have been changed

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• This article was amended on 19 July 2019. An earlier version wrongly stated that you need to be in calorie deficit to be diagnosed with an eating disorder.