The Fat-Burning Zone: Fact or Fiction

Determining what fuel the body is using for energy when we are exercising can be quite confusing.  A metabolic analyzer must be used to determine the respiratory quotient (RQ).  This RQ can determine what fuel is primarily being used for energy.  A respiratory quotient is a ratio used to measure metabolism.  RQ measures the ratio of the volume of carbon dioxide produced by an organism to the volume of oxygen consumed.2 RQ=Vc/Vo.  This quotient is helpful in figuring out what energy is being expended. The volumes of carbon dioxide and oxygen produced depends on the fuel source being metabolized.2

A RQ of 1.0 indicates that carbohydrates are supplying 100% of the fuel, whereas an RQ of 0.7 indicates that fat is supplying 100% of the fuel for metabolism.1 Any number in between the ranges of 1.0-0.7 would be a combination of both fats and carbohydrates being metabolized.  With a healthy diet, protein is excluded because of its minimal contribution to energy production.1      

The fat-burning zone is a thought that people burn more fat at a lower-intensity exercise, because such easy work does not require getting energy from carbohydrates. Now that we know what a RQ is, we can put the numbers to the test.  If a person were to do a low-intensity fat-burning exercise such as 20 minutes of walking at 3.0 mph it would give them a RQ of 0.8.  An RQ of 0.8 would be equal to 67% energy from fat and 33% energy from carbohydrates.  At this pace, an individual expends 4.8 calories per minute. Then 3.2 calories would be from fat and 1.6 would be from carbohydrates.  So, for 20 minutes of exercise an individual would expend 64 calories from fat and 32 calories from carbohydrates for a total of 96 calories. Now, if the same individual were to double their intensity to 6 mph for the same 20 minutes it would require more carbohydrates as a fuel source with an RQ of 0.86.  An RQ of 0.86 results in 54% energy from carbohydrates and 46% from fats.  However, at 6 mph the individual would burn 9.75 calories instead of 4.8 calories and 5.2 calories from carbohydrates and 4.48 calories from fat.  This would equal a total of 104 calories from carbohydrates and 90 calories from fat.  So, increasing the intensity of this workout raised the fat expenditure nearly 50%.1

So, there is no magical fat-burning zone in lower-intensity exercise, you can actually burn more fat with higher intensity workouts in the same amount of time.

Resources:

1Clark, M., Lucett, S., & Corn, R. J. (2008). NASM essentials of personal fitness training. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.

2 EFFICIENCY OF ATP PRODUCTION. (n.d.). Retrieved May 29, 2016, from http://www.tiem.utk.edu/~gross/bioed/webmodules/respiratoryquotient.html