Understanding the Fat Burning Zone

Have you ever heard of the “Fat Burning Zone”?  Many people use  this as a target when doing cardio training for weight loss.  And it’s true; There really is an exercise ‘zone’ in which your body will burn a higher percentage of fat than carbohydrates for energy.  However, this common target that most people strive to hit when doing cardio is somewhat misleading.  With a little more understanding of how the body burns calories at different levels of intensity, you may rethink your method of cardio for weight loss.     

Without giving you a complete lesson in exercise physiology, I feel the need to give a brief (and very simple) overview of how we burn calories during exercise.  Our bodies produce and utilize a compound called adenosine triphosphate (ATP), which is used as energy for everything we do.  From breathing and blinking our eyes to lifting weights and running marathons, ATP is always being broken down and is required for all muscular contractions. Basically, if ATP is not available, muscles don’t move!  ATP is stored within our cells, but the amount immediately available for muscle contraction is limited.  Therefore, the human body utilizes three energy systems, all of which are always active, to replenished (or resynthesize) ATP so we can continue to perform tasks such as running for several miles or lifting weights for several reps.  The rate at which the ATP is replenished depends on the intensity and duration of the activity.     

The three energy systems are the oxidative system, the glycolytic system and the phosphagen system. In a future post, I will go into more detail about all three energy systems.  In this article, we are going to focus mainly on the Oxidative and Glycolytic Systems.  These two energy systems breakdown carbohydrates and fat to produce ATP and are key to understanding the fat burning zone. 

For simplicity, let’s define the two energy systems like this:

Oxidative: Uses oxygen to burn calories (fats and carbohydrates).  To make this super simple, let’s just call this energy system aerobic (meaning “with air”).

Glycolytic System: Uses stored carbohydrates to burn calories (carbs).  For the sake of simplicity, we will call this energy system anaerobic (meaning “without air”).

Burning Calories

It is important to understand that the energy systems work as a team to burn calories for energy.  While exercising at lower intensities, typically under 60% of your maximum heart rate, the aerobic system takes the lead.  This is because the body is getting enough oxygen to break down carbohydrates and fat to fuel the task at hand.  The lower the exercise intensity, the more fat is being utilized for energy as opposed to carbs.  The anaerobic system is still working, but is more like an assistant and not doing as much work.  

As the exercises intensity increases, more of the work load shifts towards the anaerobic system.  This is because carbohydrates alone are a much more efficient and readily available fuel source than stored fat.  So, when the work load increases, the body calls upon its power-house energy source to handle the task . . . carbs!  Again, the two systems are still working together, but the aerobic is now more of the assistant. ]

Putting it Together

Let’s try and bring this together with a real-world scenario.  Say you do 30 mins of low intensity cardio around 60% of max heart rate.  This is right in the sweet spot of the fat burning zone where your body burns a higher percentage of fat versus carbs. On an average, at this intensity a person might burn somewhere in the range of 150 total calories.  At this level of intensity, the aerobic system will be providing about 58% of the energy while the anaerobic system will only be contributing about 42%.  So, if 150 calories are being burned, 87 of those calories (58%) will come from fat and 63 (42%) will come from carbohydrates.  Not bad, huh?  In only 30 minutes you burned 87 calories from fat!

Now let's see what happens if we increased the exercise intensity outside of the fat burning zone, up to say 85%.  At this intensity, the same 30 minutes of exercise might burn somewhere around 450 total calories.  The aerobic system is still working, but oxygen alone isn’t enough to create the energy needed for the more strenuous work.  Your body needs more carbs to create ATP for energy, so the anaerobic system steps up and takes the lead.  Now only 27% of the energy will come from fat while the other 73% will come from those power-packed carbohydrates.  Looking at these numbers it is easy to see why low-intensity cardio done within the “fat burning zone” has been the go-to for most people when it comes to wanting to lose body fat . . .

*Note: these percentages come from the respiratory exchange ratio, which we will not get into . . . if you want more info on this, fire away with questions in the comments section.  

But wait, there’s more . . . Let's take a closer look! 

At the higher intensity we burned 450 calories with 27% coming from fat.  27% of 450 is 108 calories from fat.  So although the percentage of fat/carbs were higher at the lower exercise intensity, the overall number of actual fat calories burned were higher.  Not only did we burn 21 more calories from fat, but we also burned 300 more calories overall! 

This chart represents an example of the breakdown of calories from fat and calories from carbohydrates during 30 minutes of low-intensity exercise (60% Max Heart Rate) and high-intensity exercise (85% Max Heart Rate). 

Although there is a higher percentage of fat calories burned for energy at a lower intensity, the actual number of fat calories burned, as well as total overall number of calories burned, is greater when the intensity is higher.   

So, when it comes to losing body fat, here is how you can get the most bang for your buck.  When you lace up your cross trainers for exercise, WORK HARD!  Working at a high level of intensity (remember, the intensity must be appropriate to your fitness level), will yield greater results than lower intensities for the same duration of time spent at the gym.  Save your “slow, steady-state cardio” for life stuff like walking the dog, riding bikes with your kids, walking on the beach with your spouse or taking a hike nice hike with a good friend.