Exercise and Fitness Forum

The Fat Burning Zone?

by Jeremy Gentles, BS, CSCS

Question: Will I lose more weight performing my cardiovascular exercise at a low intensity versus exercising at a high intensity? I?ve heard that exercising at low intensity puts your body in the ?fat burning zone.?

You know the graph displayed on every treadmill, elliptical machine and recumbent bike illustrating the target heart rate you should exercise at to be in the ?fat burning zone?? The basic premise of the fat burning zone is that while performing cardiovascular exercise at a low intensity, your body burns or utilizes a higher percentage of fat to fuel exercise; this is true. This is often interpreted to mean exercising at a low intensity will cause greater weight or fat loss than performing cardiovascular exercise at a high or vigorous intensity which employs a high percentage of carbohydrate to fuel exercise. While this sounds legitimate and sensible,it just doesn?t add up. While low intensity cardiovascular exercise certainly utilizes a higher percentage of fat for energy production than high-intensity exercise, it is flawed to assume this equals more fat or weight loss. If I were to give you 60 percent of $100 ($60) and gave your friend 35 percent of $200 ($70), who would make out better? Your friend, of course! While 60 percent is obviously a higher percentage than 35 percent, in this scenario it does not equate to more money. If you are wondering where this is going, let?s take the same percentages and apply them to exercise. In order to truly understand why the fat burning zone really does not exist, it will require a fairly good level of detail; follow along carefully.

Exercise Intensity and Fat
Heart rate is the easiest and most frequently used method of gauging cardiovascular exercise intensity. Regardless of what intensity you are exercising at, your body relies on two primary substrates for energy:fat and carbohydrates. As your exercise intensity increases, your body will use a higher percentage of carbohydrates to meet energy demands. Exercising at 60 to 65 percent of your max heart rate (MHR) is considered low intensity and happens to be in the supposed fat burning zone. Exercising at 75 to 80 percent of your max heart rate is considered high intensity or vigorous exercise and is most often associated with cardiovascular fitness. If you exercise at low intensity, roughly 60 percent of calories burned during exercise are from fat. If you exercise at high intensity, approximately 35 percent of total calories burned during exercise are from fat. Again, you can clearly see that low-intensity exercise certainly depends on fat for a larger percentage of energy than high-intensity exercise does.

Total Calories
Now, let?s take a look at the total number of calories burned during exercise, not just fat calories. During a 30-minute aerobic exercise session, an individual weighing 160 pounds will burn approximately 215 total calories while exercising at a low intensity. The same individual will burn 375 calories exercising at a high intensity. Still unclear? Take a look at the following table which illustrates total calories burned and total fat calories burned during high- and low-intensity exercise. Looking at this table, it is apparent that high-intensity exercise is just as effective at burning fat as low-intensity exercise; both burned a total of 130 calories from fat. The most glaring and significant difference appears when you consider total calories burned. During a low intensity 30-minuteaerobic exercise session, a 160 pound individual burns about 215 total calories. If the same 160 pound individual performed a 30-minute high intensity aerobic exercise session, they would burn 375 total calories. That is a difference of 160 calories! In this scenario, the individual would have to exercise for 12 more minutes at a low intensity to burn the same number of total calories as the 30-minutes high-intensity exercise period. Would you rather be on the treadmill for 30 minutes or 42 minutes?

What Matters
In relation to weight loss, total caloric burn or expenditure is far more important than what percentage of calories come from fat or carbohydrates to fuel exercise. In fact, given the information above, we now know that as exercise or activity intensity decreases, the body?s dependency on fat for fuel increases. I can name many activities which are lower intensity than exercising at low intensity, including:

• Sleeping
• Watching TV
• Driving
• Talking on phone

Does this mean sleeping and watching TV are better for losing weight than even low-intensity exercise? Of course not! If weight loss depended only on the percentage of fat burned during a particular activity, I would be suggesting sleep as the way to go.

It Doesn?t Stop There
One aspect of the fat burning zone debate that is often overlooked is the effect exercise intensity has on post-exercise metabolic activity. In other words, how does exercise intensity impact the number of calories you burn after exercise is complete? Following low-intensity exercise, there is very little recovery that takes place; therefore, very few calories are expended after exercise. High-intensity exercise, on the other hand, often requires a good deal of recovery after exercise is finished. For instance,high-intensity exercise depletes glycogen (glucose or carbohydrates stored in muscles and the liver); replenishing or recovering this glycogen burns more calories. As duration and intensity increase, so does the length of time required for recovery and the number calories you will burn after exercise is complete.

The Final Word
The object of this article is not to totally discount low-intensity exercise. Low-intensity exercise can be done most anywhere, it is associated with a host of health benefits, it is appropriate for many clinical populations, and individuals initiating a cardiovascular exercise program are recommended to begin with low-intensity exercise. The issue at hand remains, what exercise intensity is optimal for weight loss? The bottom line is that high-intensity exercise is just as effective at burning fat as low-intensity exercise, but high-intensity exercise burns far more total calories during and after exercise, making it superior for weight loss.

Jeremy Gentles, BS, CSCS is ObesityHelp?s staff health and fitness expert. Jeremy studied Exercise Science at East Tennessee State University and is a Certified Strength and Conditioning Specialist (CSCS) through the National Strength and Conditioning Association(NSCA). He has experience addressing the specific exercise and fitness needs of those who will undergo or have undergone bariatric surgery. You can contact Jeremy at [email protected].

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