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Fitness Tip: The myth of the fat burning zone

Monday, January 12, 2009 Posted by Peter Janiszewski, PhD
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.

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2 Response to "Fitness Tip: The myth of the fat burning zone"

  1. megha Said,

    There are so many myths, thanks to the greedy people.


    Posted on February 19, 2009 at 6:51 AM

  2. leangenix Said,

    Greedy people are everywhere, you cannot get away with them.

    Posted on February 28, 2009 at 10:15 AM


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We are PhD students in the School of Kinesiology and Health Studies at Queen's University in Kingston, Ontario. Our research focuses on the relationships between obesity, physical activity, and health risk. This blog is our attempt to consider the many "cures" for obesity that we read about on a daily basis. Enjoy.


The opinions expressed here belong only to Peter and Travis and do not reflect the views of any organization. Any medical discussion on this page is intended to be of a general nature only. This page is not designed to give specific medical advice. If you have a medical problem you should consult your own physician for advice specific to your own situation.

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