Feel the burn: dispelling the myth of the "fat burning zone." - includes continuing education test

For several years the concept of the "fat burning zone" has rooted itself in the belief system of the general exercising public. Working within this "zone" refers to maintaining an exercise intensity of approximately 50 to 60% of aerobic capacity, also referred to as V[0.sub.2] max. It is believed that by restricting the intensity of an exercise program within this range, a person will burn more fat than carbohydrates. Determining the body's energy expenditure through methods of calorimetry lends support to this assertion. However, calorimetry identifies the ratio of the substrates (substance acted upon by an enzyme) utilized. Instead, weight reduction attempts should focus on the total number of calories expended. While the phrase "fat burning zone" is not accepted by the scientific community as bonafide physiological terminology, it is not too difficult to see how a fact taken out of context can lead to a complete misunderstanding.

The Respiratory Exchange Ratio

Substrate utilization, from rest to intense exercise, can be compared to a color continuum. At rest, the body's energy needs are derived from the breakdown of approximately 40% carbohydrates and 60% fats. As we move from a sitting to standing position, we are technically still at rest but are working harder. To proceed to a slow walking gait increases the workload, yet energy is still being supplied through aerobic metabolism. As the colors change from left to right of a continuum, energy requirements change as we increase the intensity of the exercise. When we move from walking to running, for example, there is an encroachment upon anaerobic metabolism. The shift from low to high intensity places 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% carbohydrate oxidation occurs at about 100% of maximal oxygen uptake.

To better understand this gradual shift in the utilization of fat vs. carbohydrate, we should look at the respiratory exchange ratio (RER). The RER is the ratio of carbon dioxide (C02) produced to the amount of oxygen (02) consumed and serves as a guide to the nutrient mixture being catabolized for energy. Table 1 shows the inverse relationship between the percentage of calories from fat and carbohydrate. As work intensity increases, the RER, also referred to as the respiratory quotient (RQ), approaches 1.0.

Table 1 Calorie Equivalence of the Respiratoyr Exchange Ratio
and Percent kcal From Choleterol (CHO) and Fats

Respiratory Exchange          Energy
        Ratio             kcal liters of           %kcal
                         oxygen([O.sub.2])   Carbohydrates    Fats
         0.71               4.69                 0           100
         0.75               4.74                15.6          84.4
         0.80               4.80                33.4          66.6
         0.85               4.86                50.7          49.3
         0.90               4.92                67.5          32.5
         0.95               4.99                84.0          16.0
         1.00               5.05               100.0           0

There are many issues to consider when designing an exercise program. Proper assessment of an individual is vital. Current health and physical status must be assessed. Even if high intensity exercise is a priority, prudent progression from low intensity work in addition to resistance training is crucial preparation. The desires and objectives of an individual must also be assessed. High intensity exercise is more difficult than low intensity work. If it causes a person to drop out of a program, 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. Therefore, if general conditioning is the objective, exercising at 50 to G0% V0, max might be an appropriate prescription. If weight loss is an additional priority and time is not an issue, a person may be successful working at a moderate intensity. However, the typical 60-minute aerobics class often does not allow enough time to expend the minimum 300 calories recommended by the American College of Sports Medicine for promoting weight loss. Much more time is required for low intensity work to reach the sufficient caloric expenditure. The time it takes to complete a warm-up, stretching, progress to desired work intensity, the cool-down and additional stretching does not leave much time for adequate caloric expenditure, especially if work intensity is suppressed. High intensity work done in the same period of time will yield a higher caloric expenditure. Sixty-minute aerobics classes can reach the 300 calorie minimum as long as the intensity is high enough.

There is an additional concern with low intensity work. Although low intensity exercise performed longer can expend calories similar to shorter term high intensity work, significant improvement in V[0.sub.2] max may not occur. The ramifications of this are important when performance objectives exceed general fitness and weight control. For example, for the firefighter, high intensity work is 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, training for specific outcomes, this cannot be achieved with moderate intensity exercise. Should the firefighter train exclusively at high intensity? No. Performing only high intensity work is taxing on the body and can lead to overtraining and injury.