“Fat burning Zone” A Mythical Belief

“Fat burning Zone” A Mythical Belief


For several years the concept of the “fat burning zone” has rooted itself into the belief system of the general exercising public. Working within this “zone” refers to maintaining an exercise intensity of approximately 50 to 60 percent of aerobic capacity, also referred to as VO2max. It is believed that by restricting the intensity of an exercise program within this range, a person will burn more fat instead of carbohydrates. Determining the body`s energy expenditure through methods of calorimetry lends support to this idea.1 The obvious attraction is that if working at a lower intensity will enable the person to burn more fat, then losing weight will be a more likely outcome. However, calorimetry specifically identifies the ratio of the substrates utilized in terms of percentages. Weight reduction at-tempts should focus on the total number of calories expended.2 While the phrase “fat burning zone” is not accepted by the scientific community as bona fide physiological terminology, it is not too difficult to see how a fact taken out of context can lead to a complete misunderstanding.


Substrate utilization, from rest to intense exercise, can be compared with a color continuum. At rest you are sustaining life through the aerobic process where the energy needs of the body are derived from the break-down of approximately 40 percent carbohydrates and 60 percent fats.3 As you move, for instance, from a sitting to a standing position, you are technically still at rest; however, you are working harder. To proceed to a slow walking gait is, again, increasing the workload, yet the energy is still being supplied through aerobic metabolism. Much as the colors change as you move from left to right of the continuum, the energy requirements change as you increase the intensity of the exercise. When you move from walking to running, for example, an encroachment on anaerobic metabolism begins. The shift from low to high intensity places greater demands for energy that the breakdown of fat is too slow to supply. As exercise intensity increases, the oxidation of fat gradually decreases until 100 percent carbohydrate oxidation occurs at about 100 percent of maximal oxygen uptake.4

To better understand this gradual shift in the utilization of fat vs. carbohydrates, look at the respiratory exchange ratio (RER). The RER is the ratio of carbon dioxide (CO2) produced to the amount of oxygen (O2) consumed and serves as a guide to the nutrient mixture being catabolized for energy.5 From the table above, we can observe the inverse relationship between the percentage of calories from fat and carbohydrates. As work intensity increases, the RER, also referred to as the respiratory quotient (RQ), approaches 1.0.

If you were to stop here without further pursuing the research, you might be satisfied to think that the theory of the “fat burning zone” is valid. It is clear that as the RER increases, so does the percentage of calories derived from carbohydrates. Conversely, increased exercise intensity yields less energy from fat. The failure of the “fat burning zone” theory, however, is revealed by the total calories expended per liter of oxygen. Referring again to Table 1, you can see that the total energy expended increases as the RER increases. Therefore, higher-intensity exercise is by virtue of total caloric expenditure a more effective weight-loss protocol.


Since old beliefs die hard, it is important to scrutinize the research in various applications. By manipulating the variables of exercise intensity and duration, the following scenarios will serve to support the data provided by the RER.

In the first example, Wilmore and Costill (1994) compare two separate, 30-minute workouts by the same 23-year-old female with a maximal oxygen uptake of 3.0 L/min. The first session was performed at 50 percent of her VO2max and the second was at 75 percent of her VO2max. The average RER for the two sessions were .85 and .90, respectively. Interestingly, the total calories from fat in both workouts were identical at 110. However, the average VO2max for the low-intensity work was 1.50 L/min compared with 2.25 L/min for the high-intensity work. This difference in work intensities yielded approximately 50 percent more total calories, at 220 vs. 332, for the respective exercise sessions.

A hypothetical example offered by La Forge and Kosich (1995) compares a person with an approximate maximal oxygen uptake of 49 L/min, exercising for 60 minutes at 50 percent and 70 percent of aerobic capacity. At 50 percent VO2max, their computations determined approximately 480 total calories expended. With an RER = .86, approximately 50 percent of the total 480 calories burned come from fat. Therefore, 480 divided by 50 percent = 240 fat calories. Converting this to grams of fat (240 fat calories divided by nine calories per gram of fat) produces 26.6 g of fat. At 70 percent VO2max, the total caloric expenditure was 660 calories. The RER of .90 yields approximately 40 percent of the total 660 calories burned, equaling 264 calories from fat. This converts to 29.3 g fat.

The significance of this example is that, although the percentage of calories from fat is less than the calories from carbohydrates at 70 percent VO2max, it is still more than the number of fat calories burned at 50 percent VO2max. In other words, the higher the work intensity, the more total calories burned. Additionally, the more total calories burned, the more fat calories burned regardless of the amount of expended calories of carbohydrate. Figure 1 clearly illustrates this point.

To gain a better understanding of the difference between percentages of calories and total calories,6 consider the following analogy. Two different people are taking two different tests. One test has 100 questions; the other has 200 questions. The person taking the first test answers 50 questions correctly, equaling 50 percent. The person taking the second test with 200 questions answers 80 of them correctly. This, however, equals only 40 percent. Which one answered more questions correctly? Obviously the person who took the second test answered more questions correctly yet appeared to score lower by the overall percentage. This is a case where a smaller percentage is actually a larger number because it is a percentage of a larger number from the beginning. It is clear that a smaller percentage of a larger number can be greater than a larger percentage of a smaller number. This is the very problem of the “fat burning zone” theory. Simply because a greater percentage of fat is burned at lower intensities, it is assumed that more fat calories are expended at lower intensities. The fact of the matter is that since so many more total calories are expended at higher work intensities, there are actually more calories coming from fat. It doesn`t matter that fat contributes a smaller percentage of those calories. A review of Figures 1 and 2 on page 69 will support this point.


In addition to the interest in the burning of calories during exercise, an increasing amount of supportive research regarding caloric expenditure at the completion of higher-intensity work has been done. At the beginning of exercise, the sudden demand for energy cannot be completely provided through the aerobic process. Likewise, during exercise, anaerobic metabolism must assist with any level of intensity above steady rate. Following the exercise, the level of oxygen consumption remains elevated to “pay back” what has been supplied through anaerobic sources. Traditionally referred to as the “oxygen debt,” this replenishment of oxygen during recovery is also known as “excess postexercise oxygen consumption” (EPOC). (3) It is now believed that the EPOC is needed for several other events of metabolic recovery. Contemporary thinking suggests that excess postexercise oxygen consumption reflects both the anaerobic metabolism of the previous exercise as well as the respiratory, circulatory, hormonal, ionic, and thermal adjustments that occur during recovery. (1) Table 2 on page 70 lists specific reasons for the continued elevation of oxygen consumption after heavy exercise.

The EPOC has gained a great deal of attention with respect to total caloric expenditure from exercise. Until now, this discussion of burning calories has been focused on low- vs. high-intensity work during actual exercise. Several studies, however, have measured the EPOC of different work intensities at various times on the completion of exercise. In a study of moderately trained men and women, Smith and McNaughton (1993)7 found that, with regard to energy utilized, the highest EPOC value was measured up to three hours after exercise at 70 percent VO2max. Treuth demonstrated that total 24-hour energy expenditure was greater with high-intensity, interval exercise (100 percent VO2max) than with low intensity, continuous exercise (50 percent VO2max). (4) Remembering that total caloric expenditure should be the ultimate goal of any weight loss protocol, this research adds further credence to the issue of exercising at a higher rather than lower intensity.


The exercise specialist must be able to prescribe an appropriate exercise protocol based on sound physiological principle. All too often a personal trainer or group exercise leader will take a “sound bite” of information and attempt to spin it into a new concept of fitness training. The “fat burning zone” is the result of this very problem. Everyone in the fitness industry has the responsibility to become educated before attempting to educate. After all, where did this theory come from if not from our own professional community?

Taking this point a step further, there is an additional concern. Now that the facts have dispelled the myth of the “fat burning zone,” can we be sure that high-intensity exercise will be prescribed in a prudent manner? In other words, should the leaders of the fitness industry advocate only high-intensity exercise as the “new fat burning zone”? Although the information would certainly be correct, the application may be completely inappropriate. This is the reason for the need for a thorough understanding of “exercise prescription.”

Many issues must be considered when designing an exercise program. Proper assessment of the individual is vital from two primary standpoints. Obviously, the first is the current health and physical status of the individual. Even if high-intensity exercise is a priority, prudent progression of low-intensity work in addition to resistance training is certainly crucial preparation. The second, and maybe even the more important, consideration is the individual`s objective. High-intensity exercise is more difficult than low-intensity work. If it causes a person to drop out of an exercise program, then we have defeated our purpose of instilling a positive lifestyle behavior. In terms of total caloric expenditure, significantly increasing the duration of low-intensity work can compensate for the decrease in intensity.8 Therefore, if general conditioning is the objective, then exercising at 50 to 60 percent VO2max would be a more appropriate prescription. If weight loss is an additional priority and time is not an issue, a person could still be successful at a more moderate level of intensity.

However, Stanforth and Stanforth point out that the typical 60-minute aerobics class is often not enough time to expend the minimum 300 calories, the recommendation of the American College of Sports Medicine for promoting fitness and weight loss. (6) With low-intensity work, more time is required to reach the sufficient caloric expenditure. Taking into consideration the time deducted for warmup, stretching, progression to work intensity, cool down, and additional stretching does not leave much time for caloric expenditure if the work intensity is suppressed. Higher-intensity work done in the same period of time will yield a higher caloric expenditure. Therefore, 60-minute aerobic classes can reach the 300-calorie minimum as long as the intensity is high enough.

An additional concern with low-intensity work is that although low-intensity exercise performed longer can expend total calories similar to shorter term, high-intensity work, significant improvement in VO2max may not occur. (8) The ramifications of this are important when performance objectives exceed general fitness and weight control. Take, for example, firefighters. Here is a population that has high-intensity work in the job description. An exercise prescription of moderate intensity for these individuals would be not only imprudent but dangerous. They must be prepared to work at intensities of near maximal heart rate. According to the law of specificity, this simply cannot be achieved with moderate-intensity exercise. Then should the firefighter train exclusively at high intensity? No. High-intensity work is very taxing on the body and can lead to overtraining and injury.

A prescription of exercise for this population, however, would certainly include a significant component of high-intensity interval training specific to the activities performed during a fire. Some choices could include treadmill work with elevation, intervals on stairs or graduating inclines, and high-intensity circuit-style resistance training. To add a fun component to the high-intensity program, the firefighter could try racquetball, full-court basketball, spinning, or mountain biking.

These ideas, along with moderate-intensity, longer-duration activities, balance the exercise program. Balance is necessary in all programs, regardless of personal interests or professional endeavors.

The problem with the “fat burning zone” mentality is that the public, as well as the firefighter, is being told not to work at high intensity because it is counterproductive to fat burning. Obviously, the research disproves this belief, yet the real message here is, first and foremost, to exercise consistently as a lifestyle. If an individual`s objectives surpass that of general fitness, such as weight loss or specific performance training or both, then more work will be inevitable.


1. McArdle, W. D., F. I. Katch, V. L. Katch. Exercise Physiology: Energy, Nutrition, and Human Performance, 4th ed. Philadelphia, PA: Lea & Febiger, 1996.

2. LaForge, R. D. Kosich, “Fat burning: just the facts,” 1996. IDEA Today, 13:1.

3. Wilmore, J. H., D. L. Costill. Physiology of Sports and Exercise. Champaign, IL: Human Kinetics, 1994.

4. Treuth, M. S., G. R. Hunter, M. Williams. “Effects of exercise intensity on 24-hour energy expenditure and substrate oxidation,” 1996. Medicine and Science in Sports and Exercise, 28:9.

5. Jannson, E. “On the significance of the respiratory exchange ratio after different diets during exercise in man,” 1982, ACTA Physiol. Scand, 114-103.

6. Stanforth, P. and D. Stanforth. “Burning fat: the rest of the story.” 1992. American College of Sports Medicine Certified News, 2:2.

7. Smith, J. and L. McNaughton. “The effects of intensity of exercise on excess postexercise oxygen consumption and energy expenditure in moderately trained men and women.” 1993. Eur. J. Appl. Physiol. Occup. Physiol., 67, 420-425.

8. Ballor, D. L., J. P. McCarthy, E. J. Wilterdink, et al. “Exercise intensity does not affect the composition of diet and exercise-induced body mass loss.” 1989. Medicine and Science in Sports and Exercise, 21:2.

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GREGORY L. WELCH, M.S., is an exercise physiologist and president of SpeciFit?, An Agency of Wellness And Competitive Performance Enhancement, located in Seal Beach, California. Additionally, he is on the faculty of the fire academy at Rancho Santiago College in Santa Ana, California. He also lectures nationally and has published several articles on training Ospecial populations.O

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