The fat burning zone is one of the most pervasive myths in the fitness industry that just won’t go away. Magazines constantly promote workouts in the fat burning zone as an effective way to burn fat and most cardio machines around the world have some type of sticker, or image clearly visible (see image to your right).
The idea is if you keep your heart rate in the “fat burning zone,” which is roughly 55% to 65% of your workout to optimally burn fat., then you will magically burn more fat than at higher levels of exercise intensity.
Why work harder, when you can take it easy and burn more fat, right?
Well this, my friends, is why the fat burning zone myth is so attractive. The truth is at best, the fat burning zone is very misleading, and at worst, it’s complete misinformation.
This article will teach you 2 specific reasons why the fat burning zone is a myth so you can workout to optimally burn fat if that is your goal.
The Fat Burning Zone Confuses Absolute vs. Relative Fat Burn
To understand the fat burning zone myth, you need to understand how your body uses energy during exercise. To keep things simple, during exercise your body draws energy from two places: fat or glycogen stores. Glycogen is stored carbohydrates in your muscles and liver.
The fat burning zone was conceived because at lower exercise intensities more fat is burned relative to glycogen. Isn’t this awesome? Now you can hang out on the couch and lose a bunch of fat. I hope you are starting to smell something fishy with this idea of a fat burning zone.
At 50% of your max heart rate, your body burns a ratio of 60% fat to 40% glycogen. At 75% of your max heart rate, the ratio is 35% to 65%, and at even higher intensities, the ratio is even lower.1
So why the heck would you want to workout so hard if you burn so little fat?
The reason why is because it’s all about calories. You burn a lot more calories when you workout intensely than you do when you are sitting on the couch.
So here’s what the breakdown looks like assuming 30 minutes of exercise for a low vs. high intensity group. The high intensity group will likely burn double the calories as the lower intensity group, or 200 vs. 400 calories:
|30 Minutes of Exercise
|Fat Calories Burned
|Glycogen Calories Burned
|Total Calories Burned
|Low Intensity Group (50%)
|High Intensity Group (75%)
So now you can see you burn more fat calories at a higher exercise intensity than a lower exercise intensity (140 vs. 120) despite a smaller percentage of fat being burned. But I know you need more convincing because the higher intensity exercise represents only a 20% difference in fat calories burned for a 50% increase in intensity. Not a good tradeoff.
There is something important we are missing in these calculations, which you will learn in the next section.
The Fat Burning Zone Has No Afterburn Effect
When you exercise at low exercise intensities, you burn very few calories after the exercise is completed. When you exercise intensely such as during a HIIT workout, there is a metabolic disturbance that burns calories after the workout is completed. This is 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. But 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!2 Keep in mind the cycling group exercised for almost 5x longer than the sprint group (3.5 minutes vs. 45 seconds).
If this isn’t enough convincing, one study showed a significant amount of fat was broken down from fat stores in the muscle following high intensity cycling sprints.3 During high intensity exercise, you are burning primarily glucose, 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. Contrary to popular belief, getting up early in the morning to do low intensity cardio on an empty stomach will not help you lose more body fat versus other more intense methods. 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.
With all that said, I highly recommend not relying on exercise to “burn fat” to get lean. 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.
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