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Does Yoga Count As Cardio?

We all expect that yoga will stretch your muscles, increase your flexibility and help you to relax and de-stress. But there have been a lot of Bikram yoga advocates who claim that it burns up to 1,000 calories per session. Can that be true?
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I'm no yogi master, but I've done a fair share of power, hatha, Bikram, and even accidental, freestyle yoga when out on slippery winter runs. We all expect that yoga will stretch your muscles, increase your flexibility and help you to relax and de-stress. But there have been a lot of Bikram yoga advocates who claim that it burns up to 1,000 calories per session. Can that be true?

Let's find out.

As you may know, calorie burn is highly variable depending on your body and the type of activity you do. For example, overweight or obese individuals burn more calories because their muscles work to carry more body weight. People with a genetically high metabolism also burn more calories, as do people with a higher percentage of muscle.

To put yoga into proper weight loss context, let's look at the average calorie burn of basic activities:

Rest: At rest, you'll burn 1 to 1.5 calories per minute (depending on your body weight) or 45 to 70 calories in 45 minutes.

Walking Slowly: Walking at a leisurely 2-miles-per hour pace, you'll burn 2 to 5 calories per minute, or 90 to 225 calories in 45 minutes.

Walking Briskly: Walking at a more brisk 4-miles-per-hour pace, you'll burn 4.5 to 10 calories per minute, or 200 to 450 calories in 45 minutes.

Running: Running at 6.7 miles per hour (a decent clip), you'll burn 9 to 20 calories per minute, or around 405 to 855 calories per hour.

So now that we understand the approximate calorie burn of familiar activities, let's compare these activities to yoga.

If you go to a gym, the average yoga class will last around 45-60 minutes. To burn at least 800 calories during that yoga class, it would need to be as strenuous for you as running at 6.7 miles per hour during the entire class.

And if you've been in a yoga class before, I doubt that you've felt the same cardiovascular fatigue as if you'd been charging down a treadmill the entire time -- especially considering that some of your time in that class is spent sitting still, or lying on your back and breathing. There is literally zero chance that you're burning the unbelievable 1,000 calories per session that some enthusiasts claim yoga actually burns.

But this is all hypothesis, right? So let's look at facts.

In 2005 and 2007, two separate studies measured the metabolic rate of people taking a beginner yoga class and found a calorie burn of 2.3-3.2 calories per minute, about the same calorie burn as strolling through the mall at a very leisurely pace. This would equate to 104-144 calories during a 45-minute yoga class. At this rate, to burn one pound (or 3,500 calories) of fat, you'd have to perform over 28 hours of yoga.

But hold on! Isn't it possible that people taking a beginner yoga class, such as in the studies above, may burn fewer calories because they aren't performing advanced movements or doing hot yoga at a high room temperature? In other words, could more difficult yoga routines burn more calories than a beginner class?

Another study completed in 2006 investigated that question and measured the heart rate of participants performing a more vigorous form of yoga called Ashtanga. Researchers compared Ashtanga to the same types of beginner yoga routines from the other studies.

In the study, the heart rate during Ashtanga yoga did indeed increase by over 30 beats from resting heart rate, whereas the heart rate during the easier yoga sessions increased by only half that much, or about 15 beats.

That may seem like a big difference, but when it comes to weight loss, a heart rate of just 30 beats over resting is not very significant.

To put this in context, your heart rate can go 30 beats over your resting rate when you're gardening, walking the dog, or vacuuming the house -- and could increase by 15 beats per minute through the mere act of standing, or even rolling over in bed.

So although it's true that an increase of 30 beats over resting heart rate will slightly improve cardiovascular fitness, there are much, much more efficient ways to lose weight than 15-30 hours of yoga per week.

But surely those extremely difficult hot yoga classes (which I've personally hours in) must burn more calories, right? In Bikram yoga, the temperature in the room is about 105 degrees, with usually at least 40 percent humidity. So you certainly experience more fatigue, a higher heart rate, greater amount of perspiration, and a significantly higher degree of perceived exertion by the end of the class.

But this seeming difficulty is not really due to the fact that you are burning more calories. As a matter of fact, you could simply walk into a hot room and stand there for 45 minutes, and your heart rate will significantly increase. That is because your body's primary mode of cooling is to sweat and to shunt blood to your extremities. As you sweat, you lose blood volume, and as you shunt blood, your heart has to work harder to deliver that blood. So your heart rate goes up,

But the increased heart rate is simply your body's environmental, temperature-regulating response to hot conditions, and not because you're moving more muscles or burning more calories -- and the only significant weight you're going to lose in a hot yoga class is water weight.

At the risk of having a yogi throw a brick (or perhaps a really hard yoga block) through my window, I also need to mention that yoga may actually slow your metabolism. In fact, a 2006 study measured the metabolic rate of yoga participants vs. non-yoga participants, corrected for body weight, and found a 15 percent lower metabolism in the yoga group. It concluded that the practice of yoga over time may in fact inhibit metabolism. To put this in context, it means that if you normally burn 2,000 calories at rest, you might lower that calorie burn to 1,700 calories at rest if you take up yoga.

That drop in metabolic rate is because yoga is a relaxing activity, and inhibits your body's "fight-and-flight" (aka "run-from-a-lion") response, also known as your sympathetic nervous system. This is highly beneficial for extending your life span, controlling stress and making you feel great, but it's certainly not going to shed any pounds.

So although I have nothing against yoga and I personally do some form of yoga every morning to improve my flexibility, balance and relaxation, I'm not fooling myself that it's going to contribute so any substantial weight loss. Actually, when it comes to calorie burning for weight loss, yoga is really one of the least effective activities.

But allow me to pacify any yoga zealots I've offended by ending with this: if yoga gives you less stress, greater mental discipline and enhanced body awareness, and you use those benefits to motivate yourself to exercise, eat healthy or stay active, then yoga may indirectly work toward your weight loss goals -- which is why a few studies have shown that people who self-report doing yoga on a regular basis are thinner than people who do not.

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Ben Greenfield is a fitness and triathlon expert and host of the Get-Fit Guy podcast on the Quick and Dirty Tips network. His book, "Get-Fit Guy's Guide to Achieving Your Ideal Body -- A Workout Plan for Your Unique Shape," will be published by St. Martin's Press in May 2012.