The Fat Burning Zone - Fact or Fiction?

On a message board that I moderate, someone recently asked a question about exercise intensity this morning. That, along with a book I’m reading by Jillian Michaels called “Making the Cut”, got me thinking about the often-discussed “fat-burning zone”. I wanted to clarify some misconceptions, highlight what’s true, and talk about what it all means.

What is true: At around 65% of your maximum heart rate, your body most efficiently converts fat into energy. This is because fat oxidation requires oxygen, and you have more oxygen available when you exercise at lower intensity. At this level, a higher percentage of calories burned will come from fat than when you exercise at higher intensity levels. Because of this, the idea that you must exercise at 60-70% max heart rate to burn fat has taken root.

But here’s the problem with the theory - even though a higher percentage of calories burned come from fat at this lower intensity, you burn a higher number of total calories when you exercise at higher intensity (for example, 85% max heart rate). Therefore, even though your body is converting fat into calories less efficiently, when you exercise over the same amount of time you’ll burn more total fat calories than when you exercise at a lower intensity. Workouts like Insanity rely on this concept in part – they push your heart rate well above the “fat burning” zone for significant periods of time, and you can drop significant amounts of body fat.

So, does this mean that everyone should be starting high intensity interval training and working out to exhaustion? No. A lot depends on your current level of fitness. If you are relatively new to exercise, you won’t be able to sustain that high level of intensity for enough time to burn any extra fat. Until your aerobic capacity improves, it probably makes sense to keep your heart rate at that lower, “fat burning” level that you can sustain over a longer period of time. As your fitness level improves, you can push yourself harder, and burn even more calories over the same period of time. But it’s important to build up to this level safely, and be patient with your body as your fitness improves.  




For more information, I found an article from Self Magazine, which explains all this in more detail (and probably more clarity). Happy reading!

Fat Burning Zone article

Laura