In Nutrition

February 01, 2007

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In last month's CrossFit Journal, I said that we must always focus on performance when assessing fitness. Specifically, I argued that body fat measurement not only is futile (from an accuracy standpoint) but, more importantly, is irrelevant to athletes focused on what they can do rather than how they look. That said, fat loss is still a major and valid concern for many trainees out there, and the fitness industry is full of schemes and strategies for burning body fat.

One of these supposedly scientific nuggets of accepted "wisdom" is the belief, espoused by many personal trainers and reflected in the preset programs on most cardio machines, that the best way to lose fat is to work at a moderate intensity. A version of the graph above appears on the walls of innumerable gyms and training studios across the United States and Canada, graphically illustrating the supposedly "right" exercise intensity (as a percentage of an individual's mathematically estimated maximum heart rate) for maximizing fat loss. But it is not an accurate representation of the true picture. oxygen per min) will burn a greater percentage of fat than when working at the higher intensity.

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3 Comments on “The Myth of the Fat-Burning Zone”

1

wrote …

Not understanding the table on page 2. What I am seeing is that at 50% you burn a total of 4.86 kcal per minute and of those 4.86 kcal burned, 2.43 are fat kcal. So it makes sense that if you burn 2.43 kcal of fat per minute you will burn 73 kcal of fat in 30 min. (2.43 X 30 actually = 72.9 but close enough). However it states that at the higher intensity (70%)you burn 6.86 kcal per minute with 1.96 kcal being from fat. Yet it says the 30 min total of fat kcal is 82. This makes no sense since 1.96 X 30 = 58.8 not 83. Can anyone explain this?

2

wrote …

I think the problem is that you are assumming the same VO2 consumption over the course of the 30 minutes...

the 2.43 and 1.96 kcal / litre of VO2 rates given in the charts means that as you work at the 50%max and 70% max for every litre of VO2 consumed you can expect to consume that much energy.

the difference, and the reason for your confusion is the increased overall VO2 consumption in the 70%max versus the 50% max which makes the difference in the total amount of kcal burned.

Another way to look at it is a light rate of breathing versus heavy breathing. The light breathing will consume more fat per litre, while the heavy breathing will consume less per litre. but the heavy breathing will simple blow away the light breathing in total amount of VO2 over the course of that same 30 minutes.

hope thats clear, correct, and helpful

3

Oyden J Ortega C wrote …

Hi Jared, I perfectly understand the concept of burning more fat at 70% vs 50% over the same amount of time. My question is, if an average crossfit workout is say 15-20 minutes how can it burn more fat calories than a usual one hour work out at the gym.

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