The Myth of the Fat Burning Zone

A question I often get is “When I’m doing cardio (steady-state aerobic training), where does the fuel come from?  Because I want to be in the Fat Burning Zone!”  Everybody wants to burn fat…and most of us realize that we don’t want to ‘compromise’ muscle at the extent of losing weight.  Well let me first start off with ‘ a little education’ (I know, BORING, but it will help to understand my stance).

First, the respiratory quotient (RQ) is the amount of carbon dioxide (CO2) expired divided by the amount of oxygen (O2) consumed, measured during rest or a steady state of exercise using a metabolic analyzer.  When measured the RQ calculated during steady-state exercise, the relative contribution of fats and carbohydrates as fuel sources can be determined.  During steady-state exercise, an RQ of 1.0 indicates that carbohydrate is supplying 100% of the fuel, whereas an RQ of 0.7 indicates that fat is supplying 100% of the fuel for metabolism.  Any RQ between 0.7 and 1.0 indicateds a mixture of carbohydrates and fats are fueling metabolism.

Now some marketing departments of exercise equipment have misinterpreted the use of RQ, and that created the concept of the so-called fat-burning zone.  The thought is that people burn more fat at lower-intensity exercise because such easy work does not require getting energy quickly from carbohydrates.  Although this might be a logical concept, it is an inaccurate science.

To illustrate the fallacy of the fat-burning zone, it is important to compare two different exercise protocols.  For example, an individual partaking in low-intensity fat-burning exercise such as 20 minutes of walking at 3.0 mph may result in an RQ of 0.80.  An RQ of 0.80 results in 67% of energy coming from fats and 33% of energy from carbohydrates, respectively.  Further, at this pace the individual expends 4.8 calories per minute; 3.2 of which (67%) comes from fat and 1.6 (33%) from carbohydrate.  thus, for the full 20 minutes the individual expends 64 calories from the metabolism of fat and only 32 calories from the metabolism of carbohydrates.

If the same individual doubled the intensity to 6 mph for the same 20 minutes, the added intensity would require more carbohydrate as a fuel source and a subsequent RQ of 0.86.  An RQ of 0.86 results in 54% of energy coming from carbohydrates and only 46% of energy from fat.  However, this pace resulted in 9.75 calories expended per minute or 5.2 and 4.48 calories per minute from carbohydrates and fats, respectively.  Thus, for the full 20 minutes the individual expended 104 calories from carbohydrates and 90 calories from fat.  This increase in intensity raised the total caloric expenditure from fats, for the same time investment, above that of the low-intensity walk, to the tune of about a 50% increase.  Thus, the marketing statement that decreasing intensity puts one into a fat-burning zone is not entirely accurate.  In this example, a slightly higher intensity resulted in a greater contribution from fat despite the increased reliance on carbohydrates as a fuel source.

In a nutshell, the faster you go, the more calories you burn (duh)…even if the % of calories burned from fat goes down, the total amount of calories you burn from fat goes up!  The moral of the story is ‘if you are going to use your precious time to exercise, use it wisely’…you will have more fun, get better results and look and feel great!

Please don’t hesitate to send me questions about this article and/or post your comments below.

Thanks :)

About Michael

Michael Bonetti has been a certified personal trainer since 1994. His true passion is helping people. Whether you need help losing weight for a wedding, playing golf better, or living without pain; Michael is dedicated to learning everything he can to help you achieve all your health and fitness goals.

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