The Fat Burning Zone Myth

            You jump on the treadmill, and see that if you work at different heart rates, you get different effects. Theres the peak performance zone, the cardio zone, and the fat burning zone.

            You might have also heard that theres a specific heart rate that you need to work at to burn fat, or you have to do a certain amount of cardio (whatever the number is 20 minutes, 25 minutes, etc.) to burn fat. Before that time, youre not burning fat.

In this article, we will discuss dispel the fat burning zone myth.

The Argument for the Fat Burning Zone

            Myths have to originate somewhere, and most of them have at least a grain of truth, so lets at least explore how the myth of the fat burning zone came to be.

            Many decades ago, physiologists saw that people burn 2 major fuel sources to stay alive: sugar (glucose/glycogen) and fat. They also saw that the higher the intensity of exercise, the more sugar was burned as a percentage. Whereas at rest, the sugar:fat ratio might have been 50/50, during high intensity exercise, it skews heavily in favour of sugar. The higher the intensity of exercise, the more sugar you burn.

The opposite of that is true as well the lower the intensity, the more fat you burn as a percentage. That as a percentage qualifier is key.

So because of this interesting tidbit of physiology, marketers started jumping on it, with claims like “the science behind the fat burning zone”, and they told you to have your heart rate always in the fat burning zone… as if being above that zone (and therefore burning more calories) would not lead to greater fat loss.

Cardio equipment even started getting printed with “fat burning zone charts”. Like this one:

Fat Burning Zone vs. Cardio Zone

To continue this myth of the fat burning zone even further, as you can see from the chart above, the fat burning zone was distinguished from the cardio zone.

So what’s the difference? Intensity. Supposedly, the fat burning zone was in the 55-65% range (that’s 55-65% of your maximal heart rate), and your cardio zone was 65-85% of your maximal heart rate.

Now, there is a grain of truth to this, but it’s not the entire truth. To improve cardiovascular fitness, you do need to be exercising above 65% of your maximal heart rate. That’s called your “aerobic threshold.” But it’s not like you stop burning fat once you cross that threshold. You still burn fat.

The Truth About the Fat Burning Zone Myth

Now you know the origins of the fat burning zone myth. So why isnt it true? For 2 reasons:

Reason #1: Ratios Arent Absolutes

            We know that the lower the intensity, the more fat you burn as a percentage. Whats the lowest intensity thing you could be doing? Sleep. During sleep, you burn about 1 calorie per minute. And at that intensity, you burn about 50:50 sugar:fat. So if you burn 60 calories per hour of sleep, and only 50% of it is fat, you burn 30 calories of fat. And 1 gram of fat contains 9 calories. So you burn just over 3 grams of fat per hour of sleep. In 8 hours, thats 24 grams. Congratulations on your fat loss, by the way. I noticed your cheeks starting to look sunken in after that good nights sleep.

            When you are exercising, you may burn anywhere from 400-800 calories per hour, depending on your weight and heart rate.

            Of course, because the intensity of exercise is lower than the intensity of sleep, youre burning less fat as a percentage, and more fat overall.

            Lets say that during your workout, you burned 600 calories, of which only 30% was fat. Well, 30% of 600 is 180 calories. Which is more than the 30 calories you burned during 1 hour of sleep.

Reason #2: Fat Burned During Exercise Isnt Necessarily Body Fat

            Tapping into body fat stores is a difficult process. Fortunately, your body has fat reserves in its blood, called free fatty acids, and muscles, called intramuscular triglycerides. These are the first stores of fat that your body taps into during exercise, because theyre readily available.

            But this is fat inside your blood and muscles, not your fat cells.

            Only once your body has run out of fat in the blood and muscles, the next source is body fat. And it does tap into body fat after that point. But that doesnt necessarily mean youll lose body fat.

The ultimate determinant of whether youll lose body fat or not is net calorie balance. If you lose more than you eat/drink, youll lose body fat. If you lose less than you eat/drink, youll gain body fat. Simple as that. I go into this in much greater detail in my articles on The (Un)surprising Truth About Why We Get Fat, low-carb myths, as well as my article on what’s better: low carb or low fat.

            So now, my friend, you know the truth behind the fat burning zone. Go forth, and educate the uneducated masses