Just when you thought yoga couldn't get any hotter - because it seems everyone is doing it - here comes a New York insider's guide to the practice that is causing devotees to think again. In his investigation into Bikram, the most fashionable and extreme form of yoga, that is conducted in rooms of scorching heat, writer Benjamin Lorr warns that potential followers must go into it with eyes open: this is a fitness regime that should come with a health warning. Yes, it can it help you get into the leanest and most flexible shape of your life: but if you do as he did and follow the regime to its limit you are entering a world of almost addictive pain and taking huge risks with your body.
Lorr's book Hell-Bent: Obsession, Pain, and the Search for Something like Transcendence in Competitive Yoga, is making waves in the US with its shocking descriptions of 12 year-olds able to bend their bodies into perfect O shapes, women who exist on diluted grapefruit juice (fasting is part of the culture) and fans habitually starving and dehydrating themselves to fulfil the demands of their monstrously strict-sounding yogis (teachers).
Lorr's book describes his two-year journey from burnt-out slob to yogi fanatic and later sceptic, exploring why anyone would take up extreme Bikram - and how it could possibly be good for you.
Lorr stumbled across Bikram yoga in 2008 ''as it was the class nearest my house'' in an attempt to lose his ''conical man boobs and growing belly''. One of the most intense forms of body shaping, Bikram is taught in 1,600 studios worldwide and beloved of celebrities such as Jennifer Aniston and Gwyneth Paltrow. Andy Murray has studied Bikram for five years and describes it as "tough and ugly". The British No2 women's tennis player, Laura Robson, does it - as do members of Fulham and Chelsea football clubs. It has a fan base like no other for its ability to produce fast weight loss and a lean, almost skeletal shape.
''Bikram was completely at odds with my upbringing,'' Lorr explains. ''We were sceptical of all things New Age, and I'd always thought of yoga as something like eating more vegetables - good but worthy.''
Lorr says he was hooked after just one class - it was, he adds, a bit like ''being hit by a truck carrying rose petals''. He found classes ''almost pharmacologically addictive; pretty quickly I was doing it six to seven days a week, I'd lost 45lb in three months, and reinvented my personality… I even started using 'juice' as a verb. It was like a falling-in-love state; I was entering into a romance with Bikram.''
Lorr found he was pushing his body to its limits and beyond - and he liked it. He got further drawn in, enrolling on a nine-week course costing pounds 11,000 and holding down three jobs to pay for it. He withdrew from friends and family, started to enter yoga competitions (participants compete to hold the most perfect poses) and attended camps in secret locations, where extreme Bikram practice consumed 14 hours a day.
Yet the more Lorr delved into Bikram, the more he became aware of the risks he and others were taking with their bodies. In his classes serious pain was common, and blackouts and hallucinations not unusual. Class members routinely went to chiropractors to have ribs, which had been forced out of alignment by holding extreme positions, ''popped back in'': he came across one thirty-something yogi who had even suffered a slight stroke.
Could it be that something which felt so right could also be doing a great deal of wrong? Built around traditional Hatha yoga (see panel), in Bikram beginners learn a series of 26 (copyrighted) positions and two breathing exercises. The unusual intensity of the exercise is due to environment: the 90-minute classes are carried out in a room heated to 40C (105F). Over time, people learn to push their bodies to the limit in the most extreme circumstances.
The aim is to work progressively and systematically every muscle, ligament, tendon and joint, thus moving fresh oxygenated blood to every part of the body, restoring all systems to a healthy balance. Bikram teachers claim that through regular practice students can gain muscle strength, improve flexibility and achieve weight loss. It is said to help develop concentration, patience, determination and self-control.
Bikram's founding yogi is 67-year-old Bikram Choudhury, who began practising yoga in Calcutta at the age of four. He went to the US in 1973 at the invitation of President Nixon (who was suffering from phlebitis) and the urging of Shirley MacLaine, and now lives in the Hollywood Hills in some style. Lorr met him but felt conflicted about his leader.
Bikram poses defy expectation of what the human body can do. The most dramatic exercises involve backbending and wall-walking - where fanatics use a wall behind them as support while they bend their spines over, reversing them completely, walking down the wall with their hands, until the palms are flat on the floor beside the ears.
In a top level class, a yogi might practise this once or twice; after two years, Lorr was attending a secret group (Motto: ''What goes on in Back-bending Club stays in Back-bending Club'') who were repeating and holding the pose 60 times a day.
By the time Lorr started his book in 2010, he was still deep into competitive yoga. But writing it forced him to examine why he was so addicted - especially in light of the day-to-day pain that he was beginning to consider normal.
He freely admits that classes were agony - and sometimes afterwards too, with yogis encouraging students to ''open up to pain'', asserting that if there wasn't pain, they weren't doing the positions correctly. He would go to bed hating the last class, and wake up raring to get to the next one.
Not until Lorr experienced a temporary paralysis of his left shoulder after doing wall walks did he actually question the sanity of the experience - although he still carried on, and gradually his shoulder healed.
Yet despite his doubts, he found many people who say Bikram helped them. There was Joseph, who says yoga taught him to cope with debilitating paediatric rheumatoid arthritis; Sol, a former overweight diabetic who became an enthusiastic runner; and a car crash survivor who used yoga to avoid back surgery.
The experts Lorr consults are, however, sceptical. Most worryingly, Professor Susan Yeargin of the Department of Applied Medicine and Rehabilitation at Indiana State University, who conducts research into extreme temperatures and heat-related deaths, says: ''Most of my work is telling people to avoid that exact situation - when I study heat stroke, I put people into a room that is 104F to purposely stress their bodies. That type of situation can be devastating if core temperature rises to a dangerous level, leading to rapid deterioration of organs, coma, death.''
London-based physiotherapist Matt Todman is dubious about the benefits of such extreme poses as back bending, in particular for anyone who is vulnerable to back problems. He says: ''Bikram and back pain are a lot like oil and water. The heat may help you to feel better but feeling better, in this case, is not getting better.
''Back pain is caused by parts of your back moving too much without adequate control. Bikram yoga encourages more movement but doesn't deliver on more control, making the pain feel worse later.''
Others have been warning about the risks of Bikram for years. Back in 2005, Dr Robert Gotlin, director of orthopaedic and sports rehabilitation at the Beth Israel Medical Centre in New York, said he saw up to five Bikram-related injuries a week. The problem, he explained, is that if you stretch any muscle too far beyond its resting length, it will cause damage.
Lorr seems to agree - up to a point: ''If yoga was an FDA-approved medicine what would be on the warning label?'' He suggests: ''Acne, weeping, aches and sores, sudden weight loss, occasional puking, seizure, hallucination, irrational bouts of euphoria and/or horniness, diarrhoea.''
Yet he remains conflicted about Bikram and this is reflected in the book. He loves what it has done for his body but also sees the dark side. Nowadays, he still practises - but more moderately and in the cool of his apartment. He laughs and says: ''I will go once in a while to a hot studio. But that is not where I am right now. I guess I am enjoying more beer than green juice these days.''
The Daily Telegraph contacted Bikram Choudury's public relations manager Judes Yang at the Los Angeles HQ of the Bikram Yoga College of India to ask for a response to the health concerns expressed by experts last week, but at the time of going to print there was no reply