Just what does it mean, anyway?

“Fat burning workout!”, or “A Workout That Blasts Away Belly Fat !”  Or any other variation on the theme.

The lure of the “fat burning workout!” via emails and newsletters comes across my desk often, as you can imagine. And I know you see it a lot, too.

Yet I’ve seen this phrase create so much confusion among those eager for fitness and fat loss.

Yes, exercise is important when it comes to helping you “burn fat”. It creates an extremely valuable fat burning effect. And I applaud my fitness pro colleagues for encouraging us to move as an important tool to reducing fat stores. But….

Which Brings Me To My Point

The truth of the matter is, though exercise is important for reducing body fat, you really “burn off” relatively little fat during exercise.

It is rather the overall consumption of calories and conditioning of the body during your exercise that are key.

A workout isn’t so much “fat burning” as it can create, more or less, a “fat burning effect”. And yes, some techniques and intensities do this better than others.

When you are exercising, though there are some fatty acids being utilized in the fuel “mix” for your muscle’s demands, much of the energy that you are using is coming from that which has been stored in your muscles and liver in the form of glycogen.

This fuel source is much more readily available than fat. It also allows your body to continue to protect its most prized fuel back-up resource, fat.  Wouldn’t you know it.

Your body does utilize fat and glucose in slightly different ratios depending on the intensity of your activity, whether you’re in couch potato mode or sprinting intervals. Fat requires a complex pathway to be accessed, released and utilized as energy for your exercise. So the more intense the exercise, generally speaking, the more you are utilizing glycogen stores vs. fat stores.

Sounds like an argument for lower intensity exercise, doesn’t it?

Not so fast.

Burning “Fat” Or “Sugar”?

Again, yes, it’s true that depending on the intensity of your exercise as well as the duration of activity, among other factors (your fitness condition, for example, being one of them), you are utilizing a greater or lesser amount of fatty acids for fuel; this is known as “lipid metabolism”.

Glycolysis is the term for the conversion of glycogen, as stored in the muscles and liver, for energy. (As noted above, there are other sources of energy for our activity as well, but for the purposes of our discussion, let’s stick with these.)

First: yes, lipids are accessed more deeply when it comes to sustained exercise of lower intensity, during the exercise itself.

And yes, if the anaerobic threshold is approached, then there isn’t enough time for the body to “aerobicize” much in the form of fatty acids for fuel.

However, these simple facts have led to misunderstanding about the overall effect of difference intensities of exercise in general, when it comes to “fat burning”. Let me explain.

When It Comes Right Down To It

Basically, here’s what it comes down to.

  1. You don’t use more calories at lower exercise intensities given an equivalent amount of time exercising at higher intensities..
  2. If you’re trying to lose fat weight and you have only 20 minutes to exercise, you would burn fewer calories walking at a moderate pace compared to walking at a fast pace, or climbing. Working out at higher intensities may cause you to burn a lower percentage of fat, but since you burn more total calories, you still use more fat calories overall.

At the same time, keep in mind that what is low intensity for one may be high intensity for another. Moderate-intensity exercise still burns calories over a longer duration, and it is important to be respectful of our individual fitness and condition when it comes to exercise selection.

What About 20 Minutes To“Fat Burning Zone”?

The practice of seeking the “fat burning zone” during exercise comes from the fact that, technically, after you have been exercising for approximately 15 minutes or more, your body has shifted into using a greater percentage of fat as fuel source.

Still, if you are trying to lose fat weight, then essentially you are more concerned about the overall number of calories burned, not necessarily the fuel source.

It is the overall energy demand creating a calorie deficit that has the effect of dipping into fat stores above and beyond that burned during the exercise itself. More about this in my articles on Excessive Post-Exercise Oxygen Consumption, or EPOC, .

The body relies on fat and sugar (glycogen) for energy all of the time; the ratio varies depending on exercise intensity. But the ratio does not vary so much that we need to be overly concerned about “staying in the fat burning zone” during exercise.

Let me give you an example.

As you are sitting here reading, you may be “burning” about 50+ percent fat and 50- percent carbohydrates. Overall, you’re not really utilizing much of either, because your energy demands are relatively low (even though your brain activity is the highest energy demand your body experiences, but that is another topic altogether).

Your calorie demand sitting still might be a couple of calories a minute. If you were to start vigorously walking, obviously you would need more energy that was quickly accessible, so you would shift into a higher percentage of glycogen utlilization, say 70 percent, depending on the intensity. After some time, given the intensity is not too high, your body would shift into a greater percentage of fat-burning to protect the glycogen stores for emergency.

Sounds Like a”Fat Burning” Workout, Doesn’t It?

Here’s the point; and here’s where we’ve gone awry in application of this principle.

These ratios of energy source don’t make a lick of sense to you when it comes to fat loss. What matters is how much of a calorie deficit you create overall. And higher intensity exercise creates a deeper calorie deficit than low intensity (again, see EPOC). There’s also the building of muscle tissue, but that again is another topic.


I remember years ago in aerobics classes being mindful of the aerobic training charts that were on all the studio walls. Remember them? They were color coded for intensity levels and “zones”. We religiously kept our heart rates within “fat burning zone” – with all of the best intentions, of course!

But intensity trumps long, slow distance when it comes to negative energy balance because of the excessive post-exercise oxygen consumption it creates. It is a deeper, longer term calorie demand that extends far beyond the exercise session itself.

And though I don’t like people to get obsessive or compulsive about how many calories they are burning during exercise itself, along with EPOC, there is a point to be made. Take a look at the following table:

Activity Approximate Calories Burned %Calories from fat
Walking 20 minutes 100 65
Jogging 20 Minutes 250 40

On the face of it, we see the greater percentage of energy draw from “fat”, right?

Yet looking at the overall calorie requirement, the higher demand exercise is going to be the one that creates the greater calorie deficit.

Does This Mean I Should Always Exercise At High Intensity?

Absolutely not. We need a variety of exercise modalities to be healthy, functional, fit human beings.

And walking – GOOD! I make a point to get in at least one leisurely walk a day, often two. Do not by any stretch of the imagination take this to mean that I think you should be jogging instead of walking. Oh contrere! Walking is excellent for overall health, and for many presents a fitness challenge, depending on individual condition.

The point is, we need to look beyond just the immediate fuel source at the overall picture to help us assess what is going to be most beneficial for our condition and overall fitness and fat loss or body composition goals.

Exercise Prescription?

1)  Be a good animal. I recommend daily walking, AND

2) 3 times a week do a challenging workout that is of enough intensity to push your energy limits to create a deeper calorie demand, EPOC, and create an overall conditioning response. As always, decisions about what type, intensity, and duration of exercise is a personal decision that must be made in concert with the wisdom of your health care provider. Disclaimer in place.


And as for those “fat burning workout!” commercials and emails? I’d like them to clarify that the workouts create a fat burning effect so that we don’t addle our brains with trying to figure out what kind of fuel we are using and instead put our energy into getting ourselves moving. Those with athletic endeavors involving endurance activities might try to dial this in to extend their aerobic capacity, but not so important for most of us who are looking for a little less avoir du pois.

About Lani
Lani Muelrath, M.A., CGFI, CPBN, is the creator of Fit Quickies™ & the Body Shaping Breakthrough Kit, a FREE collection you can download instantly to learn Fit Quickie #1: ‘7 Seconds to a Flat Belly‘ and your Burst Training Guide: How to fit 45 minutes of cardio into only 15 minutes. You can access your free kit here: Body Shaping Breakthrough Kit

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