There's no refuting yoga's ability to boost flexibility, ease tension, and bestow practitioners with a general glow of health and calm wellbeing. But does it actually play a role in weight loss? In his new book, The Science of Yoga: The Risks and the Rewards, author William J. Broad claims that the fitness industry has long been overstating the ability of yoga to melt away the kilos.

"The science is extraordinarily clear - if yoga does one thing, it slows you down, it lowers your metabolism," Broad said.

A 2006 Bangalore study found that regular practice of gentle hatha yoga cut the metabolic rate of female participants by a whopping 18 per cent. "So that means, if all other things are equal and you take in the same amount of calories, you gain weight," Broad said.

What about the active forms of yoga where the emphasis is on building a sweat? Researchers at Long Island University in 2007 tracked the metabolism of yogis performing hour-long sessions of ashtanga, a brisk yoga. It found the oxygen demands placed on the body were similar to those of a leisurely stroll. In 2005, researchers at the University of Wisconsin studied "power yoga" and found: "It's a great muscular workout, and you certainly sweat, but it's not an aerobic workout."

Broad explained why: "You just don't get your heart beating fast enough the way you do when you run, or do other types of heavy aerobic activity. And the studies show that whatever you might gain from some minimal increase in your muscle mass, the overwhelming physiological response is that your metabolism slows down."

He's also dismissive of the weight-loss ability of Bikram yoga, which is performed in rooms heated up to 40C. "It's weight loss because of water loss through sweat; it comes back in a day or so."

Bikram Yoga Joondalup owner Jo Harvey disagreed with Broad's assessment: "Many Bikram students all over the world have achieved significant weight loss through their practice," she said. "The unique combination of yoga asanas (poses) and heat achieves a higher heart rate than other dynamic forms of yoga."

While Rob Schutze, co-owner of The Yoga Space in West Perth said yoga "won't get your cardio fitness up as much as running or doing a spinning class six times a week", he argued it had very positive effects on the cardiovascular system.

"Certainly in my experience, I gained a lot of fitness when I started practising ashtanga yoga." Mr Schutze added that in his 11 years of teaching, he's never seen yoga result in weight gain. Yoga Moves teacher Jane Underwood said yoga could assist with weight loss through its ability to calm the mind.

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