Dublin's WellFest weekend is Electric Picnic for gymgoers, and this year they have their own rock-star headliner. Joe Wicks – aka the Body Coach, the Bieber of burpees, the Jon Snow of jogging – appears thrice on the bill. A boot-camp class, a healthy-cookery demonstration and a Q&A session each draw nearly 1,000 people; afterwards, hundreds queue for hugs and selfies.

Before I meet Wicks I bump into a friend. When I tell her where I’m going she chokes down an excited, envious sob. “I love him!” she says. “Ask him to give you a body plan – and one for me, too.” His reputation as a weight-loss wizard, it would seem, is already solid.

By anyone's yardstick it has been a whirlwind three years for the 31-year-old social-media star. With five million online followers, including many in Ireland – 100,000 of his 1.8 million Instagram followers are Irish – Wicks has become the UK's foremost fitness guru.

Last year he signed an eight-book deal for a "ridiculous" advance; Irish booksellers have credited him, in part, for a 9 per cent jump in sales in 2016. While Wicks concentrates on creating content for his Instagram, Snapchat and Facebook platforms – a recipe here, a 15-minute workout there – a staff of 50 runs his online fitness programmes.

He spent years pushing his vehicle uphill, but now his experience is closer to being strapped to the front of a locomotive.

Wicks's team has issued a stern warning: no personal questions. He has had to get used to interest in his private life, and he is a fixture in celebrity magazines and tabloids. (Word has it that he is dating the glamour model Rosie Jones. ) And given that he looks quite like an approachable Poldark, it's easy to see why.

Burger and chips

Wicks admits that he doesn’t feel quite right this morning: he had a burger and chips and a chocolate brownie at his hotel last night – “they left a huge box of chocolates in the room for me!” – the combination of which is resting uneasily in his gut. Yet he still throbs with health. Clear skin, clear eyes – even his trademark curls gleam.

It would be remiss to credit Wicks’s rise on his boyband looks, though. Whether by accident or design, much of it has to do with timing: at a point when wellbeing and fitness were tipping into the pious and overly prescriptive, Wicks’s effortless dynamism was a palate cleanser.

Much like Jamie Oliver cut through the culinary establishment with his bish-bash-bosh approach, Wicks has his own thing going on, and his own catchphrases: weighing scales are "sad steps"; heads of broccoli are "midget trees". At a time when people are feeling stressed, overstretched and harried, his modus operandi resonates: spend quarter of an hour cooking a nutritious meal or working out and you'll reach your wellness goals. It's not a diet but an approach to life, he says. His passion is infectious.

Scratch the surface, though, and Wicks is no overnight success: there were plenty of hard yards early on. Initially, he trained as a PE teacher. “I wasn’t cut out for it. It was really difficult,” he says. “I love kids, but there was no way I could be a teacher.”

Taking a side step, he became a personal trainer. It led to handing out flyers to yummy mummies outside London train stations and eking out £1,000 a month from clients.

“I was living in a flat with my dad at the time, and I borrowed £1,000 to buy boxing stuff, and I couldn’t get any clients,” Wicks says. “I was really upset, and I told him, ‘I’m not going to be able to pay you back.’ He just said, ‘I’d give you my last penny to help you get some kind of head start.’ ”

The grafting continued until Wicks saw other trainers build online communities. In 2014 he turned his following into an income when he launched a three-month online fitness-and-nutrition plan, Lean in 15. More than 150,000 people have paid £147 for the plan, sold on a slight paradox: eat more, exercise less (but more effectively, using “high-intensity interval training”). The glowing testimonials grew, including one from a woman in Scotland who lost seven stone in a year. All the while Wicks turned his attention to Instagram and Snapchat, posting upbeat content.

“It may look like it happened overnight, but I was banging away on social media for two or three years,” he says. “My friends were all, like, ‘Why are you annoying me on Twitter?’ ”

Yet such platforms are ephemeral: many a MySpace superstar has had time to grieve former glories in the great recycle bin in the sky. Millennials – essentially Wicks’s core audience – get bored easily. Does he ever worry about his career or appeal disappearing as quickly as it came?

“It’s quite scary,” he says. “If they disappeared tomorrow business would struggle, but my guess is they’re not going anywhere. If a new platform comes along you just adapt. But I really enjoy the social-media side if it.

Food and wellness blogging took a knock recently when the motivations of the “clean eating” industry were discredited. Wicks is at pains to distance himself from the C-word. “Clean – what does it even mean?” he asks. “I had chocolate cake last night. Does that mean I’m dirty? I don’t even understand what the term means. It’s a silly thing.”

Wicks’s approach to wellbeing is much more relaxed. Does he ever get drunk? “I was pissed at Coachella for four days,” he says, referring to the music festival in California. “I don’t remember being there . . . No, I wasn’t that drunk: it was too hot. In the real world I drink about twice a month. Those things catch up on you.”

In tackling the obesity crisis, many wellness and fitness gurus extol the virtues of burning more calories than one eats. What’s rarely acknowledged – and this is perhaps where most regimes fail – is people’s emotional attachment to food: the fact that many find soporific calm in carbs or that others see themselves as “good” or “bad”, depending on what they eat.

“It’s a very sensitive subject,” Wicks says. “If someone asks on Twitter if they have an eating disorder, it’s not something I can comment on. If I have a bad day I’ll have a blowout, and I’ll connect my emotions with food, but people need to find their own balance. No one should ever feel guilty about what they eat. I eat the way I do because it gives me energy.”

Wellness crusade

When it comes to Wicks’s wellness crusade, it seems the child explains the man. Born in Surrey, Wicks grew up on a council estate, and his single mum – now a social worker – wasn’t overly fond of cooking.

“We used to have ‘picnics’ – sandwiches and crisps and chocolate,” he says. “There were a lot of Crispy Pancakes, waffles, pasta in tomato sauce. The cupboards were always full of crisps and chocolate, and I remember thinking, Mum, why isn’t there any real food in here? As soon as I could I got a job in Wilkinsons [supermarket], and once I could afford it I went to the gym.”

His mother is now a Lean in 15 convert, and has ditched her six-cans-of-Coke habit to boot. It’s not surprising to find that Wicks’s zeal has him already thinking of advocacy: “I could have a short career and sell lots of books and blitz it, but I want to get in on the schools.”

In the meantime he has in his crosshairs a podcast, a new TV show, a new book, YouTube content and onscreen collaborations with family and friends. All the while he is keenly aware that Lean in 15 has a shelf life, and he is keen to tweak his core values and offer his followers something different.

“I just feel like I’m starting out,” he says. “I’m so early on in my career. I’m just glad people are finding this useful.”

Cooking for Family and Friends, Joe Wicks's new book, is published on May 30th; thebodycoach.com