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Ok, so maybe I’m prone to exaggeration, but in writing this article I plan to expose the truth behind this myth and clear up any confusion surrounding this misleading training zone.

This myth has been propagated in gyms and fitness studios for decades, plastered onto cardiovascular equipment instruction panels and preached as gospel by fitness instructors. The sermon is a simple one, and whilst elements of the theory have scientific merit, its application in the wider fitness community has been terribly floored. From a scientific standpoint the fat burning zone is simply a heart rate zone in which the body derives the majority of its energy from the metabolism of fat. This heart rate zone is considered to be roughly 60% to 70% of an individual’s maximum heart rate (MHR). In layman’s terms participation in low to moderate exercise will result in the majority of an individuals calories being burned from fat stores. Sound like the answer to your prayers? Well sorry to burst your bubble but this ‘no pain, heaps of gain’ theory is a false prophet.

If we break down the details a little further we begin to discover that there are more elements to consider, elements that will drastically alter our perception.  The first and foremost is that at any given level of exercise intensity the body will metabolise (burn) substrates (carbohydrate, fat & protein) at varying percentages given the level of oxygen consumption (low to high exercise intensity) present. As we discussed previously the dominant substrate metabolised in the ‘fat burning zone’ is fat, however we must consider the total amount of energy consumed from all substrates (typically measured in calories) to arrive at an accurate realisation of an exercises fat burning potential. What I’m about to say is common sense but needs to be spelt out none the less. The higher the level of exercise intensity, the greater the amount of energy required to fuel the exercise. Therefore, high intensity exercise will burn a greater amount of total calories than lower intensity exercise, it is this total calorie burn that we should be most concerned about.

To help me illustrate the importance of total calorie expenditure lets take a brief look at some simple examples of exercising at different intensities whilst mapping the details of energy expenditure. For the average exerciser, achieving the heart rate level required for the fat burning zone requires mild exercise, such as walking. When we exercise at this level of intensity calorific expenditure is split, 50% from fats and 50% from carbohydrates. Using a one hour walk as an example, the average energy consumption would be 277 calories1. Doing some simple math we can see that 138.5 calories would come from fat and 138.5 calories from carbohydrates. Now lets increase the level of exercise intensity and change the exercise from a walk into a run. As a result of increasing the exercise intensity, our heart rate and level of oxygen consumption will increase. For most people a run (not a jog,) means their heart rate will increase to roughly 80-85% MHR. At this level the percentage of substrate contribution to total calorie burn changes to 25% fats & 75% carbohydrates. However, the biggest change comes when we look at the total amount of calories burnt during this type of exercise; one hour of running will burn 986 calories1. Doing the calculations then tell us that 246.5 calories come from the bodies fat stores and 690.2 calories from carbohydrates.
 
See the table below for a first glance look at the figures.

                                                   Low Intensity Exercise                 High Intensity Exercise
                                                   60-70% MHR                               80-85% MHR (Heart rate Max)
                                                   60min (Walking)                           60min (Running)
Total Calories Burned                         277                                                  986
% Calorie Burn from Fats                   50%                                                 25%
Total Fat Calories Burnt                    138.5                                                246.5

When you put the figures side by side reality begins to dawn, the fat burning zone is a myth. Whilst the lower intensity exercise burns a greater percentage of fat, the total amount of fat burnt in the higher intensity exercise is greater due to the substantial increase in total calories burnt.

Stepping down from my soap box for a moment, let’s explore the reality of instigating the training outlined above. Walking is a style of exercise that is obtainable by the average exerciser. Even if not for an hour walking is still a beneficial, relatively low impact style of exercise. Compare that with running however and things change dramatically (as we’ve seen!) There is no doubt that it is superior in relation to fat burning as well as a multitude of other benefits (eg. cardiovascular fitness.) However the question must be asked, can everyone run for the same length of time that they can walk? For an athlete the answer may be yes, but the Mr. & Mrs. Smith down the street I would say the answer is a resounding no. Therefore it is not realistic to slip on the runners and proceed to pound the pavement for an hour without first building up to a certain level of cardiovascular fitness.

In my humble opinion, the approach to burning fat from a cardiovascular stand point must be balanced. Understand that both low & high intensity exercise has its place in any fitness regime. Depending on current fitness level, the interposing of low and high intensity cardiovascular fitness will provide you with the obtainable and most sustainable results. For example, next time you take a walk don’t just go at the same pace for the entire session, challenge yourself and break into a jog for 30sec every 5min. Then for every subsequent walk, increase the time you dedicate to your jog (15sec increase) until you can jog the entire length of the walk. Combining both styles of training will allow you to simultaneously burn fat and build cardiovascular fitness. As your cardio fitness improves so will your ability to exercise at a higher intensity for longer periods of time, therefore continuously elevating the rate at which you burn fat.   

In ‘Exposing the Myth: Part 2’ I’ll explore another vital part of the fat burning equation, excess postexercise oxygen consumption (EPOC). Part 2 will explore the requirements of the body post exercise, and the effects that different levels of intensity have on those requirements.

Joshua Humphrey
Lecturer
Australian Fitness Academy

1Figures relating to a 73kg individual, walking at 5.6kph and running at 12.8kph
Ainsworth BE, Haskell WL, Whitt MC, Irwin ML, Swartz AM, Strath SJ, O'Brien, WL, Bassett DR Jr, Schmitz KH, Emplaincourt PO, Jacobs DR Jr, Leon AS. Compendium of physical activities: an update of activity codes and MET intensities. Med Sci Sports Exerc. 2000 Sep;32(9 Suppl):S498-504.

Note: All figures in this article are taken from scientifically published studies. However these figures will always differ for each individual.

Article contributed by:

Australian Fitness Academy
701 Glenhuntly Road
Caulfield South VIC 3162
03 9532 7800
www.fitnesseducation.com.au