Does Exercising in the Fat-Burning Zone Actually Burn More Fat?

A look at how your body uses energy during exercise, and why high-intensity activity is more effective for fat loss

Maybe you learned about the fat-burning zone from one of those little stickers on the cardio machines at your gym (like in the photo above) or from a friend who told you that slower, steady-state workouts burn more fat — either way, all you really need to know about it is that it’s a total myth.

Exercising in the fat-burning zone does not burn more fat or help you lost fat faster.

We know, this sounds entirely confusing. If you don’t burn more fat in the fat-burning zone, then why even call it that?

First, to really understand why the fat-burning zone isn’t the most effective way to burn fat, it’s helpful to understand exactly how the body uses energy during physical activity.

Marc Perry, founder and CEO of BuiltLean and a top personal trainer and fitness expert in New York City explains the process in simple terms.

“During exercise your body draws energy from primarily two places: fat stores, or glycogen stores. Glycogen is stored carbohydrates in your muscles and liver.”

He goes on to explain that when you exercise at a lower intensity (i.e. the fat-burning zone) more fat is burned, but only relative to the amount of glycogen that’s also burned.

For example, if you exercise at 50 percent of your maximum heart rate (a fairly low intensity for most) about 60 percent of the calories you burn will come from fat and about 40 percent will come from glycogen.

Compare that to exercising at about 75 percent of your max heart rate (a much higher intensity) when the ratio is more like 35 percent of calories burned from fat and 65 percent burned from glycogen.

The catch? Even though compared to glycogen you’re burning a smaller percentage of calories from fat by working out at a higher intensity, you’re still burning more fat calories and more calories in total compared to working out at a lower intensity.

This concept is nicely depicted in the following chart created by Perry.

Essentially, this explains why high-intensity exercise is more effective for fat loss. 

Plus, there are several studies to back this concept up. One study from 2002 found that cyclists burned fat at the highest rate when exercising at a level equivalent to 74 percent of their maximum heart rate.

Another from 2004 that compared 30 minutes of running and cycling at three different intensities (55, 65, and 75 percent of VO2 max) found that the highest rate of fat burn occurred at 75 percent VO2 max.

Additionally, as Perry points out, another fat-burning benefit of high-intensity exercise is what’s called the “afterburn effect,” or excess post-exercise oxygen consumption (EPOC).

“When you exercise at low exercise intensities, you burn very few calories after the exercise is completed,” Perry explained. “When you exercise intensely such as during a HIIT workout, there is a metabolic disturbance that burns calories after the workout is completed.”

In other words, by stepping up the intensity of your workout, your total calorie burn increases significantly both during and afterw the exercise session.

What does this mean for your workout routine?
Well first, don’t entirely dismiss low-intensity exercise. As Perry mentioned, there’s a place for it in every workout regimen. You can’t perform every workout at a high intensity — you’ll burn out and greatly increase your risk for injury. Some workouts need to happen at lower intensities in order to promote recovery.

Second, don’t rely solely on exercise for fat loss.

“I highly recommend not relying on exercise to ‘burn fat’ to get lean,” Perry explained. “In the context of a fat loss program, exercise helps you keep your muscle, stay fit, make modest increases to your metabolism, and burn some fat. Because it’s a scientific fact that you must eat less calories than you burn to lose fat, nutrition has a much more powerful impact on this equation and consequently, it should be your main focus.”

More Reading:
Why Eating Low-Fat Doesn't Lead to Fat Loss
5 Habits You Must Master to Burn Belly Fat
Myths About Weight Loss, And What Science Really Says

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