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The Fat Burning Zone: Fact or Fiction?

We have all used a treadmill or exercise bike at the gym with a chart on it showing us how hard to exercise to achieve certain goals. The "fat-burning/aerobic" zone that many of us are so concerned with usually lies somewhere in the range of 60 - 70% of maximum heart rate (HRmax). But where does this number come from, and is it really the best intensity to exercise at if you are trying to lose weight?

To answer this question, first we must understand how the energy we consume (fat, carbohydrate, and protein) is actually used as fuel. In the general resting state, the average person burns approximately 60% fat, 35% carbohydrates, and 5% protein. Once activity (exercise) begins, relative fat metabolism decreases and carbohydrate usage increases. We use the fat/carbohydrate model because protein generally is not a substantial source of energy except in extreme conditions (starvation, etc.). The trend continues as intensity increases to where at "all-out" maximum exercise, the fuel used is almost 100% carbohydrates. Therefore, it is true that lower intensity exercise elicits a greater relative contribution of fats, and hence that "green fat-burning zone" we always see on the chart.

However, as exercise intensity decreases, so too does caloric expenditure. Remember, weight loss occurs when daily caloric expenditure exceeds caloric intake, so it makes sense to burn as many calories as possible through exercise. First of all, those additional carbohydrates you burned because you exercised harder now will not be stored as fat (the common storage form of most carbohydrates once glycogen reserves are filled in the liver and muscle). Second, even though relative fat metabolism decreases with increasing intensity, the absolute amount of fat burned increases. The following table illustrates this idea for a 30-year-old who weighs 200 pounds and exercises for 30 minutes on a treadmill at three different intensities:

Table 1. Calories burned and amount of fat used at different intensities.

Speed (mph) 3.5 5.0 6.5
Activity Walking Jogging Running
Work Level Low Moderate High
Avg. HR (bpm) 114 143 171
Intensity (% HRmax) 60 75 90
Calories Burned 175 412 522
Fat Used (%) 46 35 17
Fat Used (grams) 9 16 10

Although these values are approximations, this table should highlight two points. First, the total calories burned are tripled from walking to running (even though duration is unchanged). Second, although the relative contribution (%) of fat is markedly decreased from walking to running, the absolute amount of fat burned (grams) is still greater at the high intensity.

Even though most exercise aerobically to burn fat/lose weight, the primary goal should be cardiovascular improvement as underscored by the current epidemic of heart disease in the developed countries. In general, for the average healthy person who is normotensive (blood pressure < 120/80 mm/Hg), cardiovascular benefit increases as intensity increases to an upper limit of about 90% of maximal heart rate (Hrmax). Beyond 90% HRmax, little research exists to indicate further cardiovascular enhancement2.

Currently, for improvements in cardiorespiratory fitness, the American College of Sports Medicine recommends performing aerobic exercise 3 - 5 days per week for 20 - 60 minutes each day (continuous or intermittent) at an intensity of 55/65% to 90% HRmax3. Obviously, sedentary or older people will begin at the lower ranges and endurance athletes will be at the upper end. So if you plan on devoting 30 minutes to aerobic exercise, why not crank up the intensity? Sure, it will be harder, but you will burn more calories, more fat, and your heart will thank you in the long run.

Note: Anyone planning to begin an exercise program (aerobic or otherwise) should consult a physician for medical clearance prior to engaging in regular exercise.

References

  1. Brooks GA, Fahey TD, White TP, Baldwin KM. (2000). Exercise Physiology: Human Bioenergetics and Its Applications (3rd ed.). McGraw-Hill: New York, NY.
  2. McArdle WD, Katch FI, Katch VL. (1996). Exercise Physiology: Energy, Nutrition, and Human Performance (4th ed.). Williams & Wilkins: Baltimore, MD.
  3. Franklin BA (Ed.). (2000). ACSM’s Guidelines for Exercise Testing and Prescription (6th ed.). Lippincott Williams & Wilkins: Baltimore, MD.


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