What Is The Fat-Burning Zone?

Dr. Westcott

During the past several years, considerable attention has been given to the so-called fat burning zone of endurance exercise training. Because weight loss is a high-order objective for most exercisers, most people would prefer to train in a manner that best reduces body fat.

Before we address specific training techniques and exercise zones, let’s review a little information on energy systems and calorie utilization during physical activity. Basically, the higher the exercise intensity, the higher percentage of carbohydrates used for energy production. Conversely, the lower the exercise intensity, the higher percentage of fats used for energy production. At the extremes, sprinting as fast as you can maximizes the contribution of carbohydrate fuel sources, and sleeping soundly maximizes the contribution of fat fuel sources.

Generally speaking, exercise physiologists recommend performing aerobic activities (running, cycling, stepping rowing, etc.) at about 70 to 80 percent of maximum heart rate.
This is considered an ideal range of exercise intensity to develop cardiovascular fitness, and numerous research studies reinforce this heart rate training zone concept.

The so-called fat-burning zone is typically identified as about 60 to 70 percent of maximum heart rate or even lower (50 to 60 percent of maximum heart rate). The fat-burning zone theory holds that endurance exercise performed at lower intensities uses a higher percentage of fat calories than endurance exercise performed at higher intensities. This is actually true, but can be quite misleading unless you see the entire picture.

Consider the following example, taken from the exercise physiology lab at the University of Wisconsin, LaCrosse. A 160-pound male walks for 30 minutes at a pace of 3.5 miles per hour. At this pace his energy is supplied 40 percent from fat sources and 60 percent from carbohydrate sources. On another day the same subject runs for 30 minutes at a pace of 6.5 miles per hour. At this faster pace (higher intensity exercise), his energy is supplied only 25 percent from fat sources and 75 percent from carbohydrate sources. On the surface, this would seem to validate the theory of a lower intensity-training zone that preferentially uses fat calories.

However, you need to know the rest of the story. At the 3.5 miles per hour pace, the subject burned a total of 240 calories during his 30-minute walk. He therefore burned 96 total fat calories (40 percent of 240 calories = 96 calories). At the 6.5 miles per hour pace, the same subject burned a total of 450 calories during his 30-minute run. He therefore burned 112 total fat calories (25 percent of 450 calories = 112 calories), which exceeds the number of fat calories burned during the lower intensity exercise session. So when you multiply the percentage of fat used by the total number of calories burned, you always utilize more fat calories when training at higher exercise intensities (higher heart rate zones) than when training at lower exercise intensities (lower heart rate zones).

Now that you understand this concept, please disregard the first part of this article. The whole truth is that the amount of fat you lose from your body fat stores is not determined by the type of calories burned but by the total calories burned. Let’s say this another way. The only thing that matters in body fat reduction is the difference between the number of calories you eat (including carbohydrates, fats and proteins) and the number of calories you burn (including carbohydrates, fats and proteins). Essentially, when you burn 3500 more calories than you eat, you lose one pound of fat.

Obviously, you burn calories faster during higher intensity exercise than during lower intensity exercise. Going back to the example, the subject burned almost twice as many calories in 30 minutes of running at 6.5 mph than he burned in 30 minutes of walking at 3.5 mph. Therefore, the so-called fat-burning zone is not really a fat-reducing advantage at all. In fact, it is a distinct disadvantage to train at a lower exercise intensity if your objective is to reduce fat weight in the most efficient manner.

Generally speaking, the same training zone that promotes cardiovascular fitness (70 to 80 percent of maximum heart rate) is ideal for purposes of fat loss as well. Unless your clients intend to exercise for a very long period of time, training at lower intensities reduces both the conditioning benefits and the rate of fat loss.

If your clients are just beginning an exercise program you may want them to start slowly and train within the 60 to 70 percent range of your maximum heart rate. However, as their fitness level improves they should feel comfortable exercising at a higher intensity (70 to 80 percent of maximum heart rate), and they will experience more fitness benefits as well.

If your fitness center has cardiovascular equipment that features fat burning zones, feel free to ignore those guidelines, and train your clients at a more effective exercise intensity. And if they don’t really understand all of the heart rate information, that’s okay. Simply have them perform their endurance exercise at an effort level that enables them to converse in short sentences, and they should be training at about the right level for both cardiovascular conditioning and fat loss.

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Written by Wayne L. Westcott PhD

Explore Wellness in 2021