The Myth of the Fat Burning Zone

When many people set out to exercise, they do so with the primary goal of losing fat mass. There is much advice floating around about how to optimize or maximize fat loss during exercise; one of the most commonly touted is that of the fat burning zone. Essentially, it is suggested that when performing aerobic exercise (walking, jogging, swimming, etc.) one should work at a fairly low intensity – the fat burning zone – to augment fat oxidation and thus fat loss (pictured above). The fat burning zone target exercise intensity varies from source to source but is approximately 50-60% of your maximal heart rate (maximal heart rate calculated simply as 220 – your age [with plenty room for error]).

The main error with the fat burning zone premise emanates from a basic
misunderstanding of absolute (total) versus relative (proportional)
values. One of the best ways I have found to explain this concept when I
lecture is to use a car analogy. I ask the following question: When
comparing 2 vehicles, a Honda S2000 and a Mustang GT, which is more
‘powerful’? Some background info is needed here – the Honda S2000 has a
2.2 litre engine and produces a total of 237 horsepower (hp) while the
Mustang GT has a 4.6 litre engine and produces 300 hp. To the above
question, most students will simply (and correctly) answer that the
Mustang GT is the more ‘powerful’ car as it generates more hp (300 vs.
237) – thus the Mustang is in an absolute sense more powerful. On the
other hand, the Honda generates more hp PER litre (107hp/L vs 37.5hp/L) -
hence, it is more ‘powerful’ in a relative sense.

The same type of thinking can be applied to fat oxidation at different
intensities of aerobic exercise. It is absolutely true that in a
relative sense, the lower the exercise intensity the greater the
reliance on fat as a substrate for energy. As the exercise intensity
increases, the relative proportion of fat oxidation decreases while that
of carbohydrate increases. However, the value of interest to anyone
attempting to maximize fat loss is not what percentage of energy comes
from fat during the exercise (relative), but how much fat is oxidized
(absolute). This is where the fat burning zone breaks down.

The following graph from my lecture which is based on actual data
illustrates this point. While exercising at 60% of maximal heart rate
(fat burning zone) the proportional use of fat is highest (63% – white
numbers inside each red bar) while the absolute number of calories of
fat burned is actually lower than that achieved at higher intensities
(70, 75, and 80%) – much like comparing the Honda to the Mustang. In
fact, although while exercising at 80% of maximal heart rate the
relative use of fat is much lower (33% vs 63%) the absolute amount of
fat burned is still greater (by approximately 10 calories).

A 50 year old man exercising at different
intensities for 30 minutes. Values inside each red bar indicate the
relative proportion of fat to total energy expenditure. Circled in green
is the intensity most commonly referred to as the ‘fat burning zone’.

Another reason the fat burning zone is inaccurate is because during
more intense exercise TOTAL CALORIC expenditure is much higher. Since
the body can for the most part inter-convert one macronutrient to
another (only exception being fat – carbohydrate conversion), a calorie
is a calorie regardless of the source (fat, carbohydrate, protein).
Thus, total caloric expenditure must be the focus if fat loss is the end
goal.

Of course, there is an obvious tradeoff between exercise intensity and
duration – you can walk for hours but sprint for mere seconds before
collapsing. Hence, each individual must adjust both variables to
maximize caloric expenditure – if you can jog rather than walk, your
workout can be half the duration to expend the same amount of calories.

Peter Janiszewski