Myth: More intense exercise burns fat better

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Moderate and low-intensity workouts are more efficient at burning fat.  

Heart Rate Zones

Figure 1: Heart Rate Zones – Click for larger version.

This statement centers on an age-old question that will be debated well after I am long form this Earth.  With mountains of research to explain the physiology, the question remains is low or high-intensity exercise better for fat loss?  The answer is different and the truth depends on what you mean by better.  You can define “better” as burning more calories and being more efficient.  Your intent is very important in determining which is more important to you.  

Let me clarify: Efficient exercise can be seen by two methods.  Calories burned per unit time or higher percentage of fat burned per unit of exercise.  I think the latter is a more accurate statement but many would argue with me.  Since I am writing the article, I will define the term.  

In prior years, experts promoted workouts in the fat burning zone as a more effective way to burn fat.  In fact, most aerobic exercise machines around the world have an image of the “fat burning zone”.  This ideal heart rate zone is what they claim to be the ideal heart rate to burn fat more efficiently.  I was always taught that this is 55-65% of the max heart rate for your age (Figure 1 shows the zones)[1].  Per the myth: If you maintain this zone, you will burn more fat than at lower or higher levels of exercise intensity.  Seems simple, right?  This simplicity is exactly why this myth is so attractive.  Weight loss is actually much more complex than this simplified and misleading model.  For more on this model, go to CREATING YOUR OWN EXERCISE PROGRAM – PART 3: DESIGNING CARDIOVASCULAR SESSIONS.  This website is a good reference on heart rate zones.  They have a place in developing a training program, but they guarantee a more efficient weight loss.  

So what is a low or high-intensity exercise?  Low or moderate-intensity exercise should leave you feeling breathless but not too breathless to be able to engage in a conversation with your workout partner.  High-intensity exercises will leave you speaking in short sentences while sweating and breathing rapidly.  It will be more of a challenge to speak with moderate and high-intensity exercise.  Low-intensity exercise includes cycling, jogging, swimming, and walking, swimming.  Examples of high-intensity exercise include aerobic classes, running, fast cycling, and high-intensity interval training.  I am sure you can think of more examples, but these will give you an idea.  

Background to the myth: The basis of the myth is based on the fact that the more intense the exercise requires more oxygen.  In this process, burn sugar, protein, and fat.  Fat oxidation requires more oxygen to burn for energy.  Since carbohydrates require less oxygen to metabolize aerobically than fat, it is felt that you will burn less fat during that period of high-intensity aerobic exercise.  This belief is misleading and overly simplified.  Plain and simple, there is more to this story.  

Exercise Effieciency

Figure 2: Exercise Efficiency: from Built Lean

So what is the truth?  Built Lean does a great job presenting the research behind both sides of this argument[2].  See Figure 2 for a chart for data I am about to explain (numbers differ from source to source – this is from Built Lean).  The truth is both sides are correct.  Low-intensity exercise is more efficient. With low-intensity exercise, you will burn 60% of your calories from fat.  With high-intensity exercise, you will but 35% of your calories from fat.  So, low intensity is better for weight loss.  Not exactly, look again!  You burned 20 more calories from fat and thus would lose more fat loss in less time with a higher intensity exercise.  So, high intensity is better?  Not so clear there either.  It is easier to maintain low intensity.  The truth is actually somewhere in between.  I recommend a mixture of both exercise methods but you should shift to more low-intensity as you age.  I am 50 years of age and I exercise almost entirely with low to moderate intensity exercise.  

The bottom line: Lower intensity exercise is more a higher portion of the calories from fat than high intensity, but higher intensity exercises burn more calories per unit of time.  I would argue that a mixture of both is the ideal way to exercise.  

References

[1]
“CREATING YOUR OWN EXERCISE PROGRAM – PART 3: Designing Cardiovascular Sessions – Verve,” Verve, 11-Dec-2015. [Online]. Available: https://verve-health.com/creating-your-own-exercise-program-part-3-designing-cardiovascular-sessions/. [Accessed: 15-Feb-2018]
[2]
“The Fat Burning Zone Myth: Don’t Be Fooled – BuiltLean,” BuiltLean, 01-Apr-2013. [Online]. Available: https://www.builtlean.com/2013/04/01/fat-burning-zone-myth/. [Accessed: 15-Feb-2018]
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About the Author

ChuckH
I am a family physician who has served in the US Army. In 2016, I found myself overweight, out of shape, and unhealthy, so I made a change to improve my health. This blog is the chronology of my path to better health and what I have learned along the way.

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