He is known for sleeping late, getting drunk and being, well, on the large side. Jack Osbourne, son of Ozzy and a former drug user, weighed in at 16st 7lb a year ago, aged 18. Then he found a new way to diet: a mix of extreme detoxification, including coffee enemas, and mountaineering.
The new regime, which could be copied by thousands in search of a quick way to lose weight, has admitted Osbourne, now a svelte 11st, to a select band of celebrities who stay in shape through extreme physical exercise.
Tom Cruise, who is keen to climb Everest, Cameron Diaz, who surfs big waves, and comedy actor Rowan Atkinson, who is 'addicted' to speed and loves motor racing, all enjoy pushing their bodies to the limit. They are known as adrenaline junkies.
Jack Osbourne is currently camping in California's Yosemite National Park at the base of El Capitan, a 3,000ft granite cliff. Tomorrow, he plans to start climbing. For the next few days, 19-year-old Osbourne and two colleagues will be living in space, sleeping on narrow ledges hundreds of feet above the ground, tied on to metal pegs hammered into cracks in the rock.
Baking in 30C heat, Osbourne and his companions will have to carry all their food and, most critically, their water. Each climber needs four litres of water a day, and with the ascent of El Capitan expected to take four days, the climbers will need to haul around 100lb of fluid as they inch their way up.
Jack, who shot to fame as a character in the reality TV hit The Osbournes, plans to lose still more weight and achieve his final ambition - 'a six-pack'. He has lost more than 60lb over the summer.
For the past five months, the part-time drummer and former full-time layabout has been living the life of an extreme sports athlete for the new ITV2 series Jack Osbourne, Adrenaline Junkie. Osbourne followed a punishing regime of indoor wall climbing near his home in Beverly Hills, before moving in April to a Thai kick-boxing school. Then came the mother of all colonic irrigations.
'That was one of the most miserable weeks of my life,' he said in London recently. 'It was hot, sticky and boring. I had a tube up my arse twice a day and five litres of coffee were poured in. It wasn't painful, but the pressure was uncomfortable.'
Out have gone the boozy nights on the town and the junk food. Jack is now tucked up in bed by 11pm and feasts on fruit and pasta. The chunky poodle hairdo has been axed and the heavy specs have been replaced by contact lenses.
'I was camping in a little hut in Thailand,' Osbourne said. 'So I went for weeks without seeing my reflection. And when I finally looked in the mirror four weeks later, I spent ages looking at myself, thinking: "Wow!"'
Osbourne has been hanging out with adventure athletes for some time, but it was only when he joined a group of British climbers in Croatia that he became actively involved. The team was filming a new form of the sport called 'deep-water soloing', climbing 50ft above the sea with no ropes. Anchored to the expedition's yacht by his excess weight, Osbourne was nevertheless intrigued to try it for himself.
Two of the Croatia team, Mike Weeks and Bean Sopwith, will be helping Osbourne climb El Capitan, up a route called the Salathe Wall. Seasoned climbers regard it as one of the hardest climbs in the world. Ahead of them will be another British team, including climbing cameramen Ben Pritchard and Paul Twomey. Pritchard is the only one of the team to have climbed the Salathe Wall before, also for a film.
Pritchard's partner on the climb, Rich Heap, said: 'It is arduous up there anyway, but they'll be concentrating on two things, the climbing and the filming. It can get blisteringly hot, but fortunately air rises off the valley and blows up the wall.'
Heap says the team hopes Osbourne will lead the way on some of the easier parts of the climb, but will rely on his more experienced partners for more difficult sections.
'The biggest thing you need up there is an overwhelming and insane desire to get it done,' says Heap. 'I can guarantee you one thing. He'll lose quite a bit more weight up there.' Which is bad news for mum Sharon, who fears Jack may be getting too thin.
Osbourne finished his preparations for the El Capitan climb by taking part in an urban adventure race in Manchester last month. Thirteen teams of two were required to scale landmarks throughout the city centre. Jack plunged into the Manchester and Salford Junction Canal while swinging hand-over-hand across a bridge.
Competing alongside Osbourne was Nick Colton, deputy chief executive of the British Mountaineering Council, a vastly experienced old hand. Colton says he was amazed at how well Osbourne was looking. 'Jack seems to have taken more responsibility for himself, and that's what climbing is all about. Good luck to the lad.'
A-list thrill-seekers
Surfing: Cameron Diaz
Professes to like nothing more than catching a big wave, once even breaking her nose in the surf off Honolulu's Waikiki Beach.
Motor racing: Rowan Atkinson
Unlike his mute alter ego Mr Bean, Atkinson admits he is addicted to speed and owns a number of sports cars. He even raced vintage cars competitively in the Eighties.
Boxing: Hilary Swank
The Oscar-winning actress donned boxing gloves for the 2004 film Million Dollar Baby. Despite the sport's visceral nature, the delicate Swank announced she plans to carry on hitting people in the ring.
Rock climbing: Tom Cruise
Another celebrity whose exploits on film would inspire a lust for adventure, Cruise learnt climbing moves for the 2000 movie Mission: Impossible II and became a dedicated convert.
And if it all seems a bit much ...
Knitting: Kate Moss
Not much adrenaline, admittedly, but when did rock climbing ever produce a nice scarf?
