Moments after I enter Joe Wicks’s London office, an almighty racket breaks out. It’s so loud and sudden that I assume something must have gone horribly wrong. Someone is screaming, literally screaming: “THE GUARDIAN IS HERE, DOING A DAY IN THE LIFE OF JOE WICKS!”

I look up and realise that the noise is coming from Wicks himself. He’s holding his phone at arm’s length, twirling around, yelling at the screen. This will be a Snapchat video for his followers, one of several he posts every day. I am the only person in the entire office (50 staff) looking up.

Wicks is having a moment. Two years ago, he was a down-on-his luck personal trainer who spent his days either running sparsely attended bootcamps or fruitlessly flyering outside train stations. Now the Body Coach, as he calls himself, is everywhere. He’s built a million-strong following on Instagram. His first cookbook, Lean In 15, was the bestselling non-fiction work of last year (despite being published on only 28 December). Every day, around 400 people sign up to his 90-day body transformation plan (paying £147 for the privilege). You may have seen him advertising Uncle Ben’s rice on YouTube, shrieking “WALLOP!” or “BOSH!” or “ARRIBA!”, like a cross between a 1990s Jamie Oliver and an animatronic chipmunk. With a book deal, upcoming TV show and numerous product endorsements, Wicks’s business is now earning £1m a month. It’s growing so fast that he’s hired a large team, led by his brother and best friend, to run it.

All this has come from shouting at his phone. The 30-year-old discovered Instagram as a tool for self-promotion in 2014, and his feed is a conveyor belt of 15-second recipe videos, before-and-after client photos and pictures of him in his pants. “I Snapchat in the bath,” Wicks tells me. “I Snapchat when I wake up. I’m giving people inspiration. It’s like a TV show.”

His popularity is staggering, but not all that surprising. He’s easy on the eye (almost every interview with him seems duty-bound to contain at least one Poldark reference) and his 90-day plan seems incredibly simple: do four full-blast workouts a week; drink lots of water; eat three protein-heavy meals every day. That’s it. It’s expensive (both the plan and the amount of food you need to consume), and it dictates your entire life for three months, but his testimonials are dizzying.

Wicks’s first job of the day is to put me through a high-intensity interval workout, the sort of 30-minute bodyweight circuit class he espouses in his plan. We do three circuits of battle ropes, bent-over rows, press-ups, sit-ups, sprints, burpees, and stints on the exercise bike. He is uplift personified, cheering me on at the start when I go off like the clappers (“You’ve got the strength of 10 bears!”), and at the end when I’m doubled up and want to vomit (“Come on, mate, 10 more seconds!”).

But after we finish, I realise that Wicks talks like this all the time, endlessly peppering his sentences with words like “positive” and “inspirational”. His monomania can seem impossible to penetrate. “It’s the power of social media, the power of a good message,” he tells me at one point. “I’m so ambitious. I’m just at the start. A hundred thousand people have done my plan, but my goal is millions and millions. Instead of Weight Watchers and Slimming World and Juice Plus, they’re all going to come to me.”

Which is all well and good, but I’d actually asked him what his parents do. I eventually discover that his mum’s a social worker and his dad’s a roofer. Wicks grew up on a council estate in Surbiton, south-west London, and still lives in the area. His parents weren’t particularly fit, but Joe, a self-described “really skinny kid”, grew up wanting to be a PE teacher. After joining a gym that cost half his monthly salary, and then completing a sports science degree, he eventually found work as a teaching assistant. But before long, he says, he realised that he “wasn’t cut out for it. So then I was like, ‘Right, what do I love doing?’ and that was exercise, so I was a personal trainer for five years, and all the online stuff was a progression of that.”

His parents are now separated; both are hugely proud of what he’s achieved. “They’ve become fitter because of me,” he says. “My dad wasn’t really that fit. He used to run a bit, but now he does yoga. He’s 55 and he’s the leanest he’s ever been. He did the London marathon this year and he beat me. I got injured and I had to walk a lot of it. I did it in six and a half hours, and he did it in five and a half.” His father now plans to do a yoga training course in Costa Rica.

Wicks’s mother helps adults back into education, after herself returning to education late in life. “She left school at 15, didn’t get any A-levels, was on benefits. Then, one day, she woke up and said, “I want to do something more with my life”, so she got her diploma and now she helps people who were in her situation.”

As the fitness plan started to take off, Wicks called in his older brother, Nikki – a genial, laid-back former Press Gazette features editor who is a constant presence in the office – to take charge of his social media strategy. I ask Joe if they’re close. “It’s the most incredible thing ever, having him here. He’s so smart. He was in Singapore editing a PR magazine, and he just kind of absorbed all this knowledge from CEOs with big media spends. He’s the Holy Grail. He’s helped me grow my business so quickly.”

Surely, I think, some of Wicks’s puppyish enthusiasm is a put on. It has to be. For example, it was reported last month that he had split up with his girlfriend of 11 years due to “the pressures of fame”. I scrolled back through his Instagram feed, searching for traces of sadness or vulnerability, and came up short. Instead, I found a video of him clutching a newspaper and gleefully hooting: “Here it is! My first bit of US PR in media print!”

Is it exhausting to be “on” all the time? Does he never think, “Screw it”? “I don’t,” he says. “It’s not as if I’m trying to say, ‘Look at how great and happy I am.’ I’m just trying to use my happiness and motivation to rub off on other people around me. I’m always giving out such a positive message that I never really get attacked. There’s not much you can attack me for. I help people get in shape, I’ve got a great book that’s giving loads of people loads of advice. It’s such a positive, powerful thing.”

We’re talking over a plate of “winner’s pancakes” that Wicks has made for me in the office kitchen. For him, making food also means filming food: his phone is out every step of the way, capturing full-volume snippets (“One banana! One egg! Whizz it up!”) that he can share with his followers. “This’ll get 150,000 likes,” he says as he updates Snapchat and Facebook with painstakingly composed pancake photos.

The health and fitness industry notoriously, desperately churns out one revolutionary new weight-loss idea after another. Wicks’s methodology is far from revolutionary – it’s nothing that any personal trainer won’t tell you – so I wonder whether Wicks is in this for the long haul? Yes, he says, and explains the contents of his next four books to me.

“I’m 30 years old and I’ve got my purpose ,” he says. “I know what I’m doing, and I want to reach as many people as possible. You know how Jamie Oliver has had that impact on schools and government, even at his age? I really believe that I’ll be that guy, doing big things and influencing the whole country. The NHS doesn’t know what it’s doing, and I’ll use my power of social media to get my message across. I’ve got a million followers, but my goal in the next few years is to have tens of millions, because I’ll just keep doing it. Every day. Never stopping. Never slowing down. I’ll slow down when I’m dead.”

His ambition is so pronounced, so difficult to see around, that I try another tactic to make him open up. I tell him how pleased I am not to be sitting in front of the screeching catchphrase machine from his videos, since that man is legitimately annoying. This approach works, to some extent.

“This is me now,” he says, leaning across the table to make his point. “I’m much more chilled out in real life. The Body Coach on social media is a character. People always think I’ll be going, ‘Bosh!’ and bouncing around like a hyper, but that’d be too much. I’ve tried doing videos where I’m a bit more quiet, but then people go, ‘Er, what’s wrong? Are you having a bad day?’ So I’m…”

“Trapped?” I suggest.

“Well, I’m kind of expected to maintain that energy. Things like ‘Guilty!’ and ‘Bosh!’ and ‘Wallop!’ just make it a little bit fun. I think it’s getting boring, but people love it.”

Once I finish talking to Wicks, I wander over to some of his ‘“support heroes”, the customer service employees who spend their days replying to emails from clients. It’s a slick operation that covers every stage from sign-up to £20-a-month aftercare. I approach one man and ask if he ever gets any complaints.

“It happens,” he admits, “but we can usually find a paper trail of reasons for their failure. Most of the time we can look back and go, ‘Well, you did tell us you went on holiday for a fortnight’, or, ‘You told me you were still going out every single night.’”

Really? People admit that? “Yeah, they’re really open. They tell you all sorts.”

I find myself wondering whether this is a side-effect of Wicks’s personality-led approach. When the face of the company has made his name by sharing every aspect of his life online, people are bound to share things back. And now he’s had to hire a roomful of people to deal with that.

As I’m about to leave, Wicks pulls me to one side. “Come here, mate. Look at this.” He gets out his phone and opens Snapchat. “This is what people write to me.” At random, he starts hitting messages he’s received. Without fail, they’re all messages of thanks from people who have increased their fitness and confidence by taking his plan. Messages of thanks for his book. Messages of thanks for changing their lives. It’s endless and overwhelming. His phone is full of people telling him how brilliant he is.

“How are you not a monster?” I ask.

“Because I love it,” he replies. And once you’ve chopped through all the self-promotional stuff, Wicks really does seem motivated by a desire to help people.

Every personal trainer I’ve ever met has had a plan like Wicks’s on the back burner. What’s his secret? “I truly believe that it’s a personality thing,” he says. “If you’re a likable person and you’re passionate, then people are drawn to that wherever you are in the world. And also hard work. Nobody wants to do the tweets. Nobody wants to write 60,000 tweets and post 1,000 videos when they’ve got no followers. People think you’re mad. But it pays off in the end.”

• Lean In 15: The Shape Plan, by Joe Wicks, is published this week by Bluebird at £16.99