Is the Fat Burning Zone Real?

Writers hate clichés and try to avoid them at all costs.

For example, the old cliché of “If I had a nickel for every time I heard X” gets used far too much.

Sadly, this is one circumstance where I have to employ a cliché in my writing, but with a twist and embellishment.

If I had a nickel for every time I heard about the “fat burning zone” or saw one of those silly graphs that are always plastered on pieces of cardio equipment at the gym, I would be living in Monaco, sipping martinis (I am classy like that), wearing a James Bond suit, driving an Aston Martin, and have a Bond Girl on my arm.

Except in my scenario she doesn’t get killed in some tragic way.

That was a roundabout way of saying the fat burning zone has been discussed ad nauseam for decades.

The real question, and what we are going to address here, is what is it and what does it actually mean for fat loss?

Buckle your seatbelts because we need to ride the bus to nerdville for a minute.

What Is the Fat Burning Zone?

History class in high school made me want to tear my eyes out; it was incredibly dull and usually about something so far removed from my life I couldn’t care less about it. However, history in science is a completely different animal.

All of science is really a story of how we come to understand the world and the history of the “fat burning zone” is actually quite interesting.

Related: The Best Cardio for Fat Loss According to Science

For all intents and purposes we can go back to 1993 when a paper was published by scientist by the last name of Romijn¹. In his experiments he had people exercise at varying levels of intensity and tracked what was being used to fuel the exercise: muscle glycogen, intramuscular triglyercides, and glucose from circulation or fatty acids from circulation.

Romijn and his colleagues were able to show that at different levels of intensity, specifically 25%, 65%, and 85% of their maximal work capacity that the body used substantially different sources of fuel. Let me reach into my bag of clichés again; a picture paints a thousand words.

Now it appears that at very low levels of intensity that the body uses mostly fatty acids from your blood to provide the energy needed to complete the work. As you increase your intensity you begin to tap into more of the muscle reserves.

At 65% of your maximal capacity you begin to utilize a substantial amount of muscle triglycerides (aka fat) and muscle glycogen. At 85% of your maximal capacity you begin to use less muscle fat and more muscle glycogen. Additionally, the usage of fat from your blood gets lower and lower as you increase intensity.

This experiment naturally leads us to the following conclusion: There are specific “zones” of training that utilize more fatty acids to provide energy to fuel your training. This, is where the idea of the fat burning zone originates.

What Does This Mean?

In one sense, the fat burning zone is real. There are specific “zones” of exercise where your body is oxidizing more fat than carbohydrates to produce ATP. There is not argument about that. The real question is does training in that zone and burning more fat for ATP during training result in more fat loss?

To adequately address this question we need to frame how we think about it. We can really view this on three different levels:

1. A cellular level 
2. A whole body level
3. An energy balance level

At a cellular level, the fat burning zone is correct. You do indeed have zones where you burn a higher percentage and total amount of fat to provide energy. This is fairly simple and straight forward. You can check of this box.

At a whole body level, it is also true that training in the fat burning zone your body will use more muscle triglycerides and release fatty acids from fat tissue to be used for energy. This is also fairly straight forward.

However, the energy balance level is where the idea falls apart. First, it is important to make the clear distinction between fat oxidation and fat loss. The oxidation of fat in muscle cells, regardless of the amount, does not override the concept of energy balance.

If more energy is coming into the system than leaving the system it does not matter what zone you are training in, there will be no net fat loss. The most important concept to think about is the total fat oxidation over a longer window of time than just training.

Think about it this way. If you train in the low intensity zone and burn about 400 calories in an hour and about 75% of that is from fat that means you have used about 33 grams of fat for that training session and about 25 grams of carbohydrates.

Related: Priming the Machine - The Importance of Pre-workout Nutrition

If you were to train at a high intensity for an hour and burn about 600 calories (of which maybe 50% comes from fat and 50% from carbohydrates), you use about the same 33 grams of fat and about 75 grams of carbohydrates. So even at that level of analysis the idea of the fat burning zone begins to break down a bit from a calorie balance standpoint.

The fat burning zone is equivalent to missing the forest for the trees.

We can also ask this question. Does an acute “optimization” of the ratio or amount of fat used to provide fuel for exercise increase the amount of fat loss over time? The answer given to us by controlled studies is a resounding no. There is clearly no benefit of the fat burning zone for fat loss^2,3.

The Wrap Up

The fat burning zone is one of those ideas that is true but misinterpreted. From a metabolism standpoint it is clear that there are “zones” of training that rely more heavily on burning fat for fuel. From a fat loss standpoint, the fat burning zone is a myth, there is no translation from cellular metabolism to fat loss.

The fat loss zone should be essentially viewed as a training tool to help modulate intensity and figure out where you are in terms of your substrate metabolism. It should not be used as a tool for maximizing fat loss.

Your fork is a far better tool for fat loss than the fat burning zone.