The Myth of the "Fat Burning" Zone

Have you ever looked down at a treadmill and seen a little chart with a "fat burn zone" on it?

I can't believe these are actually still on a lot of treadmills!!

This massive myth has again become in vogue among some fitness trends today. This theory was popular 60+ years ago and it regularly pops up again from time to time and today I hope to de-bunk it for you once and for all! One of the things I am most proud of as a trainer is that I actually have a Kinesiology degree. This is my profession and I take it seriously! Selling people myths just isn't fair to them or to you in the long run. If someone comes at you with this theory please slowly back away! This fat burning zone stuff can be confusing to understand at first but once you understand the original theory and the accurate science it becomes quite clear why this makes no sense.

What is the fat burning zone?

According to this theory the "fat burning zone" occurs when you keep your heart rate at 55 - 65% of it's maximum beats per minute during a workout. Anything over that will cause your body to burn alternative sources of fuel other than fat.

Fuel Source

Basically your body has three fuel options when it is taxed (like it is when you exercise). You can burn fat, carbohydrates or protein. Protein is the last possible fuel burn option and really only happens in starvation situations or ultra endurance events so I'm going to focus here on fat and carbohydrates.

During typical exercise your body draws energy from two places: fat and glycogen stores. Glycogen sounds fancy but really it is just stored carbohydrates. The fat burning zone concept was first conceived because at lower exercise intensities more fat is burned than glycogen. The idea was born that in order to lose more fat you have to work less hard - isn't that awesome?! And unfortunately too good to be true. 

Absolute vs. Relative Fat Burn

At 50% of your max heart rate, your body burns fat:glycogen at a ratio of 60:40, at 75% of your max heart rate, the ratio is 35:65, and at even higher intensities the ratio is even lower for fat. So would you want to workout so hard if you burn so little fat? The reason why is because if you just look at fat you're only looking at half the story. Glycogen is stored carbohydrate too which can also make you fat! 

Remember the low fat diet fad? When we thought that eating fat was making us fat? So instead of eating healthy balanced meals just have a vat of pasta for dinner and you will be skinny!! That's the same idea here. 

Have a look at this chart:

Source

So yes, when you exercise at a low intensity you will burn a lot of calories from fat but the important numbers to look at are the total calories burned numbers. And the after effect of working at high intensities.

The "Fat Burning Zone" Has No Afterburn Effect

:: Chart explaining the afterburn effect from one of my physiology courses at Western University ::

When you perform cardiovascular exercise at a low intensity you burn very few calories after the exercise is completed. When you exercise at a high intensely such as during a interval workout, there is a metabolic disturbance called EPOC (exercise post oxygen consumption) that burns calories after the workout is completed. EPOC is also known as the afterburn effect. Estimates of the afterburn effect vary wildly depending on the exercise method, the intensity of the workout, and even how its measured.
In a study by Dr. Christopher Scott and the University of Southern Maine, the total calorie burn of low intensity exercise vs. high intensity exercise was examined. A low intensity exercise group cycled at a steady rate of 3.5 minutes. The higher intensity exercise group required three 15 second sprints as fast as the subjects could run. What was the difference in calorie burn? Quite substantial. The cycling group burned 29 calories vs. 4 calories for the sprinting group during the exercise. When you take into account the calories burned after exercise, or the afterburn effect, the numbers look much different – 39 calories burned for the cycling group vs. 65 calories burned for the sprinting group. A surprising 95% of the total calorie burn occurred after the sprinting exercise! Keep in mind the cycling group exercised for almost 5x longer than the sprint group (3.5 minutes vs. 45 seconds). During high intensity exercise, you are burning primarily glucose (stored carbohydrate) during the workout, but after is when you burn the fat. This is the crux of the fat burning zone myth and the afterburn effect.

While low intensity exercise certainly has its place within an exercise regimen, relying on exercise in the fat burning zone to burn fat is not an efficient approach. For busy people, interval training and circuit training workouts are substantially more efficient to help you burn far more calories in much less time, and burn more fat in the process.