The Myth of "Fat Burning Zones"

September 15, 2021

December 4, 2020

The year is 2020. The fitness industry has changed a lot over the past few years, with the boom of Instagram “fitness models” and “influencers” and fad diets like Keto and intermittent fasting. There is so much information out there, but also much misinformation. Although there have been multiple advances in the industry, there have been some long-holding fitness “myths” that have yet to be debunked. It’s about time to shed some light on these fallacies.

The Problem

It seems at every gym you go, folks just jump on a treadmill, stationary bike, or elliptical and push a button that says “fat loss”. The problem is we often don’t see the fat loss results we want, no matter how long we spend on the treadmill. But why is this? The treadmill is monitoring your heart rate to ensure you stay in the elusive “fat burn zone”, so there’s no guessing that you’re doing it properly. Now we even have fitness trackers and watches that tell us exactly how much time we spent in this zone. “Congratulations! 60% of your workout was in the fat burn zone!” But what exactly does this mean? What even is the “fat burn zone”?

Why do we automatically assume that the “fat burn zone” is the best way to shed pounds instead of exercising at higher intensities or weight training? It sounds simple but the reality requires a deeper dive down the rabbit hole.

Fat Loss Basics

Before we dive in, we need to understand how fat loss works. Here are a few things we need to know:

  1. In order to lose weight in general, calories expended (burned) must be higher than calories consumed (eaten).
  2. When we burn calories, those calories can come from the breakdown of carbohydrate, fat, or protein. In most cases, we synthesize energy from carbohydrate and fat, protein is only used in extreme scenarios (starvation). In order to lose fat specifically, we must break it down to be used as energy.
  3. Depending on how we exercise, the proportion (key word) of energy coming from fats vs. carbohydrates varies. We will explain this more below.
  4. Lean muscle mass is more metabolically active than fat mass (it burns more calories), which means that we can potentially burn more fat if we have more muscle.

What is the “Fat Burn Zone”?

Without going into all the dry details, the “fat burn zone” is a relative intensity where our body uses a higher proportion of fat for energy (fuel) compared to carbohydrate. This ratio of fat to carbohydrate use can be placed on a broader spectrum, from resting to maximum intensity exercise. These intensities are relative to one’s VO2max.

What studies have shown is that at approximately 65% max heart rate, fat and carbohydrate usage is relatively similar. Lower intensity exercise uses fat as a primary fuel source and, as intensity increases, the proportion of carbohydrate usage increases. You see a crossover effect on the graph where we transition from using fat as the primary fuel source to carbohydrate. So naturally, exercising at lower intensities will use more fat, which is “optimal” in the range of 60-70%. This is what sounds so attractive, many of us wouldn’t mind losing a few pounds. We jump on the treadmill, power walk for 30 mins and wait for the "fat burn zone" to do its thing. Although the fat burn zone is real, there’s a catch.

What They Don’t Tell You About the "Fat Burn Zone"

Now that we’ve clarified what the fat burn zone actually is, we can start to understand why this system is flawed. In order to lose fat, we want to burn as much fat as possible, right? Sounds obvious, but we need to look at the whole picture.Here’s what they don’t tell you:

1) The "Fat Burn Zone" doesn't account for total energy expenditure

Let's do some simple math. We'll use a person exercising for 30 minutes as an example."Fat Burn Zone" Example30 minutes @60% intensity= 200 calories burnedRatio: 60% fat/40% carbohydrate60% = 120 calories from FAT40% = 80 calories from CARBOHYDRATEHigh Intensity Example30 minutes @80% intensity = 500 calories burnedRatio: 30% fat/70% carbohydrate30% = 150 calories from FAT70% =  350 calories from CARBOHYDRATEUsing this broad example, you can see that although the ratio of fat burned is less when doing higher intensity exercise, the OVERALL AMOUNT fat burned is higher. In addition to this, you’ve burned 300 more total calories because the overall intensity was higher! We get so caught up in exercising at low intensity burning very few calories that we forget the benefits of high intensity exercise. Total calories burned is more important than the relative ratio that comprises those calories.

2) The "Fat Burn Zone" is not optimal for building lean muscle mass

If we want to burn more fat, having more lean muscle mass is a good idea. Muscle tissue burns more calories than fat, even at rest. This means that more muscle mass=more fat burning potential! The best way to build lean muscle mass is to lift weights, and heavy ones at that. Low intensity exercise will not help us build muscle.

3) The "Fat Burn Zone" lacks the "Afterburn"

The harder we work, the longer it takes to "pay back". This is the general concept of the "afterburn" effect (or what we call EPOC). EPOC is short for excess post-exercise oxygen consumption, a fancy description for paying back O2 debt. Essentially, our metabolism is elevated for hours, even days, post-exercise. Higher intensity exercise means more calories are used during AND after our workout. When we exercise at low intensity, our body recovers very quickly and does not create a significant EPOC effect, even if we exercise for long periods of time. The graph below gives us a better visual as to what this looks like.

In Closing

For years we have been encouraged to exercise at low intensities to maximize fat loss. We've been conditioned to think lower and slower is better when in reality we need to incorporate a variety of exercise intensities (high being one of them). We must run, sprint, and lift heavy if we want to see the fat melt and muscle grow. Hopefully we now realize that high intensity exercise is much more effective at burning fat than confining ourselves to the “fat burn zone”

Author: Jen Winger