Who is getting burned? - Ask AFAA - explanation of the "fat burning zone" theory - Letter to the Editor

American Fitness, Sept-Oct, 2002 by Gregory L. Welch

Question:

I am confused about the "fat burning zone" theory. I have learned from research articles that more fat is burned at a higher training intensity and not a lower intensity, as the theory suggests. However, some instructors tell their students they will burn carbohydrates, not fat, if they get their heart rates too high. Life Fitness also includes the low-intensity "fat burning zone" on their aerobic equipment (i.e., bikes, treadmills and cross-trainers). Who is right?

Sincerely,
Jasmine Pearson
Huntington Beach, CA

Answer:

It is frustrating to see fitness equipment manufacturers still in the dark. In addition to printing the "fat burning zone" diagram on equipment, some machines even offer the "fat burning zone" as a workout option. When using this option, the machine eases the intensity if the heart rate gets too high, furthering the misunderstanding that more fat is burned at a lower intensity. However, it is even more disappointing to hear fitness professionals are misinformed in their understanding of exercise physiology--this is part of the reason the "fat burning zone" myth continues to exist. The public does not stand a chance when they hear misleading information from professionals, only to have it supported by prominent equipment manufacturers. Everyone in the fitness industry should become educated before they attempt to educate. American Fitness has previously published the following information, but it may be time for a refresher.

Exercising within the "fat burning zone" refers to maintaining an exercise intensity of approximately 50 to 60 percent of aerobic capacity (i.e., V[O.sub.2]max). Burning fat, carbohydrates and proteins is referred to as substrate utilization and is determined by methods of calorimetry (McArdle, Katch & Katch, '96). This is where the "fat burning" myth originates and appears to have validity. As the exercise intensity increases, the substrate utilization shifts from a higher percentage of fat to a higher percentage of carbohydrates utilized, as evidenced by the respiratory exchange ratio (RER) (Wilmore & Costill, '99).

However, calorimetry identifies the ratio of substrates utilized in terms of percentages. The failure of the "fat burning zone" theory is revealed by the total calories expended per liter of oxygen. Referring again to the RER, as the shift from more carbohydrates than fat utilized occurs there is an increase in the total calories expended per liter of oxygen. In table one, Wilmore and Costill (1994) demonstrate the increase in calories expended per liter of oxygen, as the percentage of carbohydrate utilization increases, due to an elevated work intensity. When an individual is attempting to lose weight, the focus should be on the total number of calories expended (LaForge & Kosich '96).

Taking a Closer Look

Figure 1 is an example by LaForge and Kosich (1995) comparing an individual, with an approximate maximal oxygen uptake of 49 L/min, exercising for 60 minutes at 50 and 70 percent of aerobic capacity. At 50 percent V[O.sub.2]max, their computations determined that approximately 480 total calories were expended. With an RER of .86, approximately 50 percent of the total 480 calories burned come from fat. Therefore, 480 divided by 50 percent equals 240 calories. Converting this amount to grams of fat (240 fat calories divided by 9 calories per gram of fat) equals 26.6 grams of fat. At 70 percent V[O.sub.2]max, the total caloric expenditure equaled 660 calories. An RER of .90 yields approximately 40 percent of the total 660 calories burned, equaling 264 calories from fat. This converts to 29.3 grams of fat.

The significance of this example is that although the percentages of calories burned from fat are less than the calories burned from carbohydrates, at 70 percent V[O.sub.2]max, they are more than the number of fat calories burned at 50 percent V[O.sub.2]max. In other words, the higher the intensity, the more total calories burned--regardless of the amount of expended calories from carbohydrates.

To gain a better understanding of the difference between percentages of calories and total calories, consider the following analogy offered by Stanforth and Stanforth (1992). Two people are taking two different tests. One test has 100 questions and the other has 200 questions. The person taking the test with 100 questions answers 50 correctly, equaling 50 percent. The person taking the test with 200 questions answers 80 correctly, equaling 40 percent. Which person answered more questions correctly? Obviously, the person who took the second test answered more questions correctly, yet appeared to have a lower score according to the overall percentage.

This is a case in which a smaller percentage is actually a larger number because it is the percentage of a larger number. Clearly, a smaller percentage of a larger number is greater than a larger percentage of a smaller number. This is the problem with the "fat burning zone" theory. Because a greater percentage of fat is burned at lower intensities, people assume more fat calories are expended. However, since more total calories are expended at higher intensities, more calories come from fat--it doesn't matter that fat contributes a smaller percentage of those calories.