Fat Burning Zone

Originally Posted On June 18, 2012
Have you ever heard of that fat burning zone? Have you seen the charts in your gym or the programs on the cardio machines that will tell you what program to pick if you want to burn fat or where your heart rate should be in order to burn fat? Well forget all of that BS. Fat burning cardio is simply aerobic work.

For those who are beginners or extremely over weight, moderate-intensity exercise may be appropriate and can even yield weight loss, but for everyone else, it has way more negative effects than positive (I will hit on this in a future post!)

No one truly knows the origin of the fat burning zone concept, but it was most likely created by the fitness industry to sell cardio machines by having people believe they needed to do moderately intense cardio in order to lose weight. The industry focused on the fact that low to moderate-intensity exercise is fueled predominately by fat and that an optimum fat burning rate has been measured at 65% of maximal oxygen consumption (VO2 max)—the body’s ability to take in and use oxygen during intense exercise.

Without the equipment to measure VO2 max directly by gas analyzers, the fitness industry estimated it to be 60-70% of age-predicted maximum heart rate (age minus 220 beats/min). This makes two huge assumptions:

That exercise heart rate can be used to predict oxygen consumption
That everyone of a given age has the same max heart rate
Exercise Intensity and Fuel Use

If the exercise is of low to moderate intensity, energy demands are met predominately by fat where as exercise of a high intensity is fueled mostly by carbohydrates. Basically the body is smart and won’t waste any energy that it doesn’t need to so it will save its carbohydrate stores for more intense bouts of exercise since muscles can extract more energy per liter of oxygen consumed from carbohydrates than from fat. The body is also smart by adapting to workouts which in turn lowers calories burned.

Myths about the Fat Burning Zone

Most of us understand that in order to lose weight, we must take in less energy than we expend, and that the more intense a workout is, the more energy we burn. Think of it this way: If you exercised for 20 minutes, you will burn maybe half the amount of energy at the low/moderate intensity than if you exercised at a high intensity. The low/moderate intensity is met predominately by fat and is unlikely to burn a meaningful amount of calories. In essence, when it comes to weight loss, it is not whether fat or carbohydrates are being used for energy, it is the total calorie expenditure that is important.


The Real Fat Burning Zone

There are two components that go into calculating total energy cost of an exercise:

Energy used during exercise
Energy used in recovery while metabolism remains elevated above the normal resting level.
The post exercise energy use is fueled by fat (remember, low heart rate = fat used for energy). This energy is referred to as excess post-exercise oxygen consumption (body uses more oxygen in recovery process). Unfortunately, not all forms of exercise produce a meaningful excess post-exercise oxygen consumption (EPOC). The exercise has to be above 70% of VO2 max in order to get the EPOC effect (harder the workout = the higher your heart rate gets = EPOC effect). Although it is not entirely understood, it seems that the greater the intensity or training, the more metabolic disturbance which determines the magnitude and duration of EPOC.

In order for the body to recover from exercise, the body goes through several processes for up to an hour post-workout, all requiring oxygen which explains EPOC. It is further delayed due to the lack of glycogen (energy) in the muscles and increased hormonal activity which occurs under high-intensity training. And guess what is used as fuel during EPOC period as glycogen is being replaced in the muscles? FAT! But remember, this post-exercise fat burn hardly exists after moderate-intensity exercise!

Main Points:

Total energy expenditure is what matters if you hope to lose weight.
Energy expenditure is greater during and after high intensity exercise.
Moderate-intensity exercise is unlikely to produce prolonged excess post-exercise oxygen consumption (EPOC).
EPOC is fueled by fat

Next Post: The Negatives of Aerobic Training

Referenced article: T-Nation (I highly recommend reading it!)

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