The Fat Burning Zone: Fact or Fiction?
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?
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.
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:
Calories burned and amount of fat used at different intensities.
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.
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.
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.
Anyone planning to begin an exercise program (aerobic or otherwise)
should consult a physician for medical clearance prior to engaging
in regular exercise.
GA, Fahey TD, White TP, Baldwin KM. (2000). Exercise Physiology:
Human Bioenergetics and Its Applications (3rd ed.). McGraw-Hill:
New York, NY.
WD, Katch FI, Katch VL. (1996). Exercise Physiology: Energy,
Nutrition, and Human Performance (4th ed.). Williams &
Wilkins: Baltimore, MD.
BA (Ed.). (2000). ACSM’s Guidelines for Exercise
Testing and Prescription (6th ed.). Lippincott Williams
& Wilkins: Baltimore, MD.