ARTICLE - THE FAT BURNING ZONE

SUMMARY

THE ARTICLE

10/07/2006 UPDATE: This article has been published in Australian IronMan, Issue 13-6 p94-96.

23/12/2006 UPDATE: This article has been published in Fighting Fat, Issue 2006 p36-38.

I remember the first time I joined a gym - I hopped on a treadmill and was told by a personal trainer that my 'fat burning zone' was a simple low-moderate intensity jog - any more effort and I would exercise the heart and not lose any fat.  Then, today, I went shopping for an exercise bike and ended up at Workout World.  There the salesperson walked me through the various models and advised me that by sticking to the "fat burning zone" - approximately 55% of my maximum heart rate - I will burn the most amount of fat.  Any more than this and I would be focusing more on fitness than my fat stores.

After conducting a quick Google search, I found that the internet is absolutely swarmed with this misinformation.  Joe Blow on every second website and forum advises uninformed people to stay within the 'fat burning zone' so you can tap into your fat stores.  If you dare put any more effort into your exercise, you will apparently undo any of this hard work.

I must admit, way back when I was told about the 'fat burning zone' I remember being pleasantly surprised.  This means that I can only put in a bit of effort, about 55-65% of my maximum heart rate, and get big results...right?  WRONG.  Months down the track my fitness had got nowhere.  I was still out of shape, unfit and definitely not happy with my results.  But how could this be?

Let's begin by defining exactly what this proposed "fat burning zone" is.  Well the theory goes, by keeping your heart rate at about 55-65% of your maximum heart rate (which is low intensity), your body will utilise a higher percentage of your fat stores as a source of energy during the exercise period.  You know what - this is quite correct too!  So now you may ask..."If this is correct, then why on earth would I want to exercise any harder?"  Let me attempt to change your mind.

During a cardio vascular workout, we will consider two sources of energy - adipose tissue (or body fat) and carbohydrates.  Low intensity cardio (LIT), or staying within the "fat burning zone", will produce a greater utilisation of fat compared to carbohydrate.  Higher intensity cardio (HIT) will produce a greater utilisation of carbohydrate compared to fat.  BUT, lets assume that we exercise for 15 minutes for both LIT and HIT.  Your total energy expenditure will be much greater using HIT cardio - simply because you are going to be putting in A LOT more effort.  Whilst, relatively speaking, the HIT will use a lower PERCENTAGE of fat versus carbohydrates, in ABSOLUTE terms, because total caloric expenditure has increased, so too has your NET fat expenditure.  As a result, your NET fat utilisation DURING exercise may actually be higher during HIT!

Let's take a look at a study by LaForge and Kosich (1995).  For a 60 minute cardio vascular training session (at different intensities), these are the results obtained:

As you can see - exercising at a higher intensity for a given time proved to burn more fat than at a lower intensity!  Don't get fooled by the percentages - it's the AMOUNT of fat that is important!

Now we've established that fat utilisation is more effective at HIT for a given time period, we need to realise that at the end of the day, the fat burned during your cardio session is negligible to the amount you can burn in the other 23 and a half hours of the day.  Wouldn't it make more sense to burn calories for 24 hours straight, rather than for the hour that you are on a treadmill?  This is where your "fat burning zone" absolutely crumbles compared to HIT.  The whole purpose of performing HIT is to convert your body into a fat burning machine!  By performing a 15 minute HIT workout, you effectively tell your body to burn fat ALL DAY (AND NIGHT) LONG.  Your metabolism is raised for a much longer period than with your LIT or "fat burning zone" workout.  So, if we were to compare 60 minutes LIT to 15 minutes HIT and took a snapshot of a 24 hour period, it would look something like this:

You can exercise for a quarter of the time doing HIT and get massive results compared to your LIT.  We would generally not go longer than about 20 minutes for our HIT workouts.  This is because intensity is quite simply a function of the duration of your workout.  The longer the workout, the less intense it will be because the body will fatigue.  In addition to this, longer periods of exercise encourage higher cortisol concentrations - promoting excessive muscle catabolism (therefore lowering your base metabolic rate) and also inhibiting nutrient transport to cells - something not desirable for both recovery and building muscle mass.

There have been several scientific studies to confirm the results tabulated above.  A study was conducted by Angelo Tremblay, Ph.D. where they compared endurance exercise (EE) to high intensity interval training (HIIT).  The EE group would exercise for longer periods compared to the HIIT group.  Here's a summary of what they found:

It is important to note that a "fat burning zone" workout will typically last for 30-60 minutes (or more), otherwise insufficient calories would be expended due to the low intensity nature.

The conclusion from the study: "[Metabolic adaptations resulting from HIIT] may lead to a better lipid utilization in the post-exercise state and thus contribute to a greater energy and lipid deficit."  This confirms exactly what was stated above - the high intensity exercise stimulates your body to burn large amounts of fat (lipids) AFTER exercise.  As you can see, even though more than twice as much energy was expended in the EE group (due to the extended periods of exercise - keep in mind that previously we compared identical exercise durations), nine times as much fat was lost over the course of this study as a result of the HIIT cardio!  So far the "fat burning zone" isn't looking too good...

But why stop there?  It's pretty obvious that fat is just going to melt off doing your higher intensity sessions - what about your fitness?  Tabata et al conducted a study comparing 60 minute sessions at 70% VO₂max versus 4 minute HIIT at 170% VO₂max.  Let's take a look at the results of the study:

Look at those results!  So not only will high intensity burn A LOT more fat - your fitness levels will soar, you will reduce muscle catabolism and save time!  Why in the world would any healthy person want to waste their time spending hours upon hours on the cardio machine?

I am particularly passionate about this topic because I used to be in the same boat as most people - exercising in the "fat burning zone" and I got absolutely nowhere - I was overweight and my fitness was poor.  During my BodyBlitz challenge I performed regular high intensity cardio and the results speak for themselves - my resting heart rate dropped from 68 to 42 beats/minute, my VO₂max increased significantly and I lost 11kg of fat.

Of particular importance, when there are health implications for specific populations (eg. elderly, post-surgery, high-risk etc.), low intensity exercise may be more appropriate.  Always consult your doctor prior to beginning an exercise routine.

So, there you have it folks.  Next time you are told about the "fat burning zone", whether it be from the person on the treadmill next to you or a personal trainer, tell them to do their research.

References:

Tremblay, et al, Metabolism (1994) Volume 43, pp.814-818

Tabata, et al, Medicine and Science in Sports and Exercise (1996) 28, 1327-1330

Batman, Richardson, Richmond, Wilhelmi, The complete guide to Fitness Instruction, 2002 pp. 8

LaForge and Kosich (1995)

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