The Myth of the Fat Burning Zone

You may have heard that you burn more fat and can lose more weight through low-intensity exercise.  This is not true.  In fact, this is one of the biggest misunderstandings in the fitness industry!

The body uses fats and carbohydrates for energy, both at rest and during exercise.  At rest, the body burns about 85% fat and 15% carbohydrate.  As exercise intensity increases, those proportions change as the fuel source shifts from primarily fat metabolism to primarily carbohydrate metabolism.  During a high-intensity workout, the fuel may come from 25% fat and 75% carbohydrate.

The reason the fuel sources shift has to do with the physiology behind cellular energy production during exercise.  Basically, because carbohydrate metabolism requires less oxygen than fatty-acid metabolism, the body will use more carbohydrate when oxygen demands are higher.

This concept can be best understood by a simple example.  Consider two separate workout sessions of the same duration performed by the same person.

  • Session #1 is performed at low-intensity (let’s say walking/jogging) and 200 total calories are burned, 60% from fat and 40% from carbohydrate.  So, 120 calories were burned from fat and 80 calories from carbohydrate.
  • Session #2 is performed at high-intensity (running/sprinting) and 500 total calories are burned, 25% from fat and 75% from carbohydrate.  So, 125 calories were burned from fat and 375 from carbohydrate.

From the example you can see that during the lower-intensity exercise, there was a higher percentage of calories burned from fat, but the total number of fat calories burned was lower.  In addition, the high-intensity workout burned 300 more calories in total.

Bottom Line:  The body gets fuel to support exercise from fats and carbohydrates.  The mix of the two sources varies, depending on intensity (measured by the body’s increased need for oxygen).  Weight loss is determined by the total number of calories burned, regardless of the energy source.  Exercise at higher-intensities always burns more calories than low-intensity exercise.  So if you want to lose weight… work harder!

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