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Does the Fat Burning Zone Really Burn Fat Faster?

Do you really burn more calories working in the fat burning zone?

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Updated September 09, 2012

High intensity exercise burns fat faster

High intensity exercise burns fat faster

Inti St. Clair / Getty Images

The Skinny On the "Fat Burning Zone"

Reader Question: I’ve read that if I want to burn more fat and lose more weight while exercising, I need to exercise in the fat burning zone. Is this true?

The short answer is, no, not really. Although it’s technically true that exercising in the so-called “fat burning zone” (at a lower intensity level of about 60% to 70% of maximum heart rate) does use a higher percentage of fat calories for fuel, the overall total calories burned is still fairly low. The reason is simple. Fat is a slow-burning fuel that requires oxygen to convert it to a usable energy, so it's great for long, steady, slow exercise, like backpacking, or cycling a long distance. Most people have enough stored body fat to fuel low level activity for days and days without running out of energy, but if you want to go fast, work all-out, or burn the most calories per minute, you need to rely on the faster-burning carbohydrate (glycogen) for energy. Converting fat to fuel takes longer, and requires lots of oxygen. In the strictest definition, this is called aerobic metabolism.

High intensity training (HIT), on the other hand, tends to use anaerobic metabolism, or glycolysis, to quickly convert stored glycogen to energy for exercise. This process can happen with little to no oxygen. The downside of anaerobic metabolism is that it has a limited supply, and when you run out of stored glycogen, typically around the two hour mark, you’ll have to slow down and start using aerobic metabolism or refill your glycogen stores with some easy to digest carbohydrates.

To further complicate the fat-burning zone theory, you need to recognize that when we exercise we use a combination of energy systems throughout the workout. Athletes are rarely exclusively in the aerobic or anaerobic zone.

Read more about Aerobic and Anaerobic Metabolism.

Now to the real question. If your goal is weight loss, and burning calories is the way you are trying to lose weight, you are better off exercising at a higher intensity 2-3 times per week, and burning more overall calories from both fat and stored glycogen.

While it is true that you burn a higher percentage of calories from fat in the low intensity “fat burning zone,” you still burn more total calories, and more calories from fat, at a higher intensity. Pushing your pace, alternating bursts of high and low intensity (interval training), and adding some all-out efforts forces you to activate the anaerobic system (70-90% of your maximum heart rate), which results in a greater number of total calories burned during your workout, which is what ultimately determines fat loss from exercise.

Calories Burned in Fat Burning Zone

Still not convinced that the fat burning zone doesn’t really burn more fat? Let’s do the math. The chart below details the fat calories expended by a 130-pound woman during a typical exercise session. In this example, the woman burns more total calories and more fat calories at a higher intensity.

Calories Burned at Low and High Intensity

Low Intensity
(60-65% MHR)
High Intensity
(80-85% MHR)
Total calories burned per min.4.866.86
Fat calories burned per min.2.432.7
Total calories burned in 30 min.146206
Total fat calories burned in 30 min.7382
Percentage of fat calories burned50%39.85%
Source: The 24/5 Complete Personal Training Manual, 24 Hour Fitness, 2000

So, Is Low Intensity Exercise Just a Waste of Time?
Before you start doing all your workouts in the high intensity zone, keep in mind that exercising at a high intensity all the time is just not smart. It can easily lead to overtraining, injury, and exercise burnout. Low-intensity cardiovascular exercise has many benefits, and for some people, it is the best way to exercise.

One reason some people are better off sticking with a lower intensity workout is that working at the high intensity is difficult. Yes, it’s hard work. You can’t go very long without running out of fuel, so it’s not going to be something you can do for hours on end. Unless you train at a high level, you probably have enough glycogen to last about two hours before you run out of glycogen and need to refuel, or slow down. Refueling with the right food is one way to maintain a high intensity over hours and hours, and why ultra-endurance athletes snack on energy bars, bananas and power drinks. This isn't necessary for most recreational exercisers who rarely work out for more than an hour. If your typical workout routine includes an hour at the gym, you don't need to worry about running out of fuel if you work hard. So going for a high-intensity effort, at least a couple times a week, is a simple way to make the most of your workout time.

High intensity workouts aren’t for the timid. And they aren't for a beginner. They require a lot of effort and you will need to gradually build up your body to handle those efforts. You can’t go from couch potato to high-intensity queen overnight. You need to get your muscles, joints, cardiovascular system, and even your mind prepared to handle the stress, or you could easily overdo it and wind up injured or ill.

So, while high intensity gets you fit faster, and burns more calories, low intensity exercise should still have a place in your workout schedule. In fact, longer, slow endurance exercise should be a regular part of a well-balanced, healthy lifestyle. Recovery is faster with low intensity movement. Going for a hike, taking a casual bike ride, or just stretching are excellent low intensity activities. Mix it up with shorter, higher intensity workouts and power training to add variety, burn calories and build endurance quickly.

Be smart, listen to your body, and mix up your workouts in the way that works best for your goals and comfort zone.

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