Every weekend thousands of yummy mummies crowd into rooms heated at a stifling 40C, paying around $25 a session, in the hope it will do them some good.
But according to the Daily Mail, a study has found that hot yoga, also known as bikram yoga, is no better for your health than doing it at room temperature.
Yoga practitioners claim the heat causes blood vessels to dilate, lowering blood pressure, which could protect against heart attacks and strokes.
However, an experiment examining the blood vessels of people doing hot yoga and normal yoga found no difference in function.
Researchers led by Texas State University conclude that it is the postures, like the "half-moon pose" and "cobra'" which are responsible for better health, and not the temperature in which they are done.
Lead author Dr Stacy Hunter, from Texas State University, said: "The new finding from this investigation was that the heated practice environment did not seem to play a role in eliciting improvements in vascular health with Bikram yoga.
"This is the first publication to date to show a beneficial effect of the practice in the absence of the heat."
Bikram yoga was popularised in the seventies in America and is popular with sports stars David Beckham and Andy Murray, as well as Hollywood stars Demi Moore and Anne Hathaway.
But it is now more often known as hot yoga, following a scandal involving pioneer Bikram Choudhury, who has faced accusations of rape and sexual assault by former students.
Previous research has shown yoga at 40C improves blood flow in middle-aged adults after just two months.
This followed evidence that saunas and hot baths are good for blood vessels and may help to slash the risk of heart attack and stroke.
However the US study is the first to determine if it is the heat or simply the yoga itself which provides the health boost.
Researchers split 52 people into a hot yoga group performing poses at 40C, a group doing yoga at a comfortable temperature of 23C and people who did not do yoga.
Measuring blood vessel function using ultrasound, they found no difference between the hot yoga and traditional yoga group – but both groups showed better results than those who did no yoga.
The study, published in the journal Experimental Physiology, states: "These results indicate that the set sequence of 26 postures and two breathing exercises may be the key ingredient in producing favourable changes in endothelial function with yoga."
Endothelial function refers to the lining of the blood vessels involved in heart disease, which are less able to dilate as people get older. Any form of exercise which forces them to dilate can increase blood flow, lowering blood pressure.
It may also delay hardening of the arteries, which leads to heart attack or stroke.
Instead of the heat causing this, the study authors believe stretches and muscular contractions during yoga may be responsible.
The difference in participants' blood vessels was measured after three months of yoga sessions, done three times a week for 90 minutes. People involved in the study were aged 40 to 60.
Hot yoga was found to slightly reduce people's cholesterol and body fat percentage – however the change was deemed statistically insignificant.