Wednesday, January 5, 2011

Debunked!: The "Fat Burning" zone

There's a popular idea among exercise literature that low-intensity exercise burns the most fat calories.

This is a myth. Complete and total bunk serving only to confuse.

Every time you take a breath, you inhale more molecules of oxygen than you exhale molecules of carbon dioxide. The ratio between these two numbers is the respiratory exchange ratio. By measuring this ratio we can determine the respiratory quotient. The respiratory quotient is a measure of which fuel source (fat or carbohydrate) is currently being used by your body to generate energy to sustain an activity.  Let's take a look at the chart below:



The average Joe or Jane at rest has an RER of approximately 0.80.  This means that predominant fuel source at rest is fat, with approximately 70% of the calories burned coming from it.  So, while sitting around on your butt burns a higher percentage of calories from fat, this myth does not take into account that sitting on your butt does not burn all that many calories.

What happens when we add exercise into the picture? Well, let's use the average American woman as an example.
The average American woman is 5'4" and 140lbs.

She will burn approximately 51 calories an hour sitting at her desk doing work.
She will burn approximately 178 calories an hour if she goes for a brisk walk for exercise.
She will burn approximately 445 calories an hour if she runs a 12 minute mile.

Now let's look at how this breaks down into calories burned from fat and carbohydrates:


As you can see from the chart above, more calories are coming from fat as the intensity level ramps up, disproving the whole low-intensity is better for fat loss myth.

If you've been struggling with weight loss, and all you've been doing is low-intensity exercise, try to ramp up your intensity level a bit. You might just be rewarded for your efforts.

Are there any other fitness myths you'd liked to see debunked? Leave a comment to let me know!

Sources:
Respiratory Exchange Ratio
Average American Woman Statistics
Calories Burned Calculator