The “Fat Burning” Zone?

I encounter many clients that try to stay in the “fat-burning” calorie zone, which typically means keeping their heart rate between 130-140 beats per minute (bpm) during exercise for a prolonged period.  I’ll explain where this theory came from, and why it’s a huge problem for someone who is trying to lose body fat to follow this recommendation.

Our bodies utilize fat, carbohydrate, and protein to fuel physical activities at varying proportions depending on the intensity of exercise.  With regard to fat, for low intensity exercise (130-140 bpm) about 75% of our calories burned come from fat; for moderate intensity exercise (150-170 bpm) about 50% of our calories burned come from fat; and for prolonged high intensity exercise (over 170 bpm) about 20% of calories burned come from fat.

When you initially see the numbers above, its easy to assume that low intensity exercise would burn the most calories from fat.  That’s where the “fat-burning zone” recommendation originated.  However, when you look more closely, the problem with this theory is that the numbers are percentages.  When you look at it this way, you really need to know the total number of calories burned to know which percentage of calories from fat is favorable.

So, lets look at a specific example.  A 60-minute session of exercise at 130-140 bpm that burns 250 calories, 75% being fat calories, burns 188 total calories of fat.  A 60-minute session of exercise at 150-170 bpm that burns 750 calories, 50% being fat calories, burns 375 total calories of fat.  A 60-minute session of exercise at 180 bpm that burns 1000 calories, 20% being fat calories, burns 200 total calories of fat.  In other words, we have the following:  188, 375, and 200 calories of fat burned respectively as exercise intensity increases.  You can clearly see that the “fat burning zone” burned the lowest actual number of fat calories.

Another issue with steady state cardio at a low intensity is that it does absolutely nothing to build muscle.  Building muscle, which increases metabolism, is hands down THE FASTEST AND BEST WAY to lose body fat.