IS THERE REALLY A “FAT BURNING” ZONE?

The last time you hopped on a treadmill or some other cardio equipment, you probably noticed a chart demonstrating different intensity zones based on heart rate (HR).

And of those zones, one is often labeled the “fat burning” zone. If you looked closely, you likely saw that the chart indicates that this special fat burning zone occurs at a pretty low exercise intensity and HR.

So what is the deal with this? If your HR gets above this value, are you not burning fat anymore? And if that’s the case, then why in the world is your coach having you do workouts that get your HR up so high?

FIRST, A LITTLE BACKGROUND …

There are three macronutrients that can be utilized for energy – carbohydrate, fat, and protein. Our bodies will do all they can to avoid metabolizing protein for energy, so we will just put that one aside. That leaves us with carbohydrate and fat as our bodies’ main energy sources. And, in reality, our bodies utilize both carbohydrate and fat to sustain our activities at all times.

Carbohydrate is the most readily-available and fast-metabolizing nutrient. Whenever there is a high energy demand (when HR is high or breathing is fast), the body primarily uses carbohydrate for energy. Our bodies have a relatively limited supply of carbohydrate (glucose in the blood or stored glycogen in muscle) to use for intense energy demands.

Conversely, fat is a nutrient with a high energy yield, and even the leanest among us have a lot of it (much more than the amount of stored carbohydrate).

These conditions make fat the body’s preferred fuel source in order to preserve carbohydrate for more strenuous demands. For example, as you sit and read this or as you sleep at night, your body is primarily burning fat to sustain you. This is why, technically, low intensity activity/exercise falls into a “fat burning” zone. Even in this zone, however, your body is still utilizing carbohydrate too (roughly 15% or so).

SO THEN, WHAT HAPPENS WHEN YOU DO HIGH-INTENSITY EXERCISE?

First, there is a shift from fat as your body’s primary fuel source to carbohydrate (since carbs are more readily available and faster to metabolize than fat). Despite this increase in carbohydrate utilization, however, your body continues to utilize fat for energy as well.

Now here is the good part—when you do high-intensity activity, your body requires more total energy to meet the exercise demands. While much of this increased demand is supplied by carbohydrate, your body continues utilizing the SAME (or even slightly more) fat than was used in the “fat burning” zone.

As an added bonus, your body utilizes more energy to recover after high intensity exercise, so your body will burn more fat after exercise than it would following lower intensity exercise.