The Fat Burning Zone: Fact or Myth?

It's that time of year again, for many the festive bulge is protruding over their favourite pair of jeans, New Year gym deals have been snapped up and already several fad diets have been tried and tested.

A common misconception for exercisers aiming to decrease body fat is to jump onto an aerobic gym machine such as a treadmill or cross trainer and begin exercising within the 'Fat Burning Zone' of 50 - 60% VO₂ max that is so frequently advertised. However, could training at such an intensity actually be doing exercise participants more harm than good? This article will discuss key factors to consider when training to reduce body fat and banish the notion of the elusive 'Fat Burning Zone'.

One statement that is true is that as exercise intensity increases we metabolise greater amounts of our carbohydrate (CHO) reserves as oppose to fat. This is due to fat requiring approximately 15% more oxygen in order for its utilization, as we train more intensely this oxygen cannot be taken in, diffused into the blood and transported to the performing  muscles rapidly enough for its effective use. We are able to determine the percentage values of carbohydrate and fat metabolism through our respiratory exchange ratio (RER), this is the ratio between the amount of carbon dioxide (CO₂) and oxygen (O₂) consumed in one breath. When this RER value is low (0.75), optimal fat utilisation is  achieved, as this value increases to 1.0 predominantly more carbohydrate is being used as an exercise fuel.  There is one problem with this, when at rest our RER is at its lowest and so predominantly fat is being burnt, implying our optimal fat burning zone is achieved when sat in front of the TV or asleep. Awesome!

However, when at rest our total calorific expenditure is very low and so in truth we are only burning a very tiny amount of fat. The data below is for two 30 minute exercise periods, one at a low aerobic intensity (50% VO₂ max) and the other at a moderate aerobic intensity (70% VO₂ max).

	50% VO₂ max	70% VO₂ max
RER	0.84	0.88
Fat : CHO	50 : 50	40 : 60
CHO Kcal	73 Kcal	124 Kcal
Fat Kcal	73 Kcal	82 Kcal
Total Kcal	146 Kcal	206 Kcal

The data shows that at a moderate exercise intensity the percentage of fat utilized is lower, but although less fat is metabolised as a percentage, the absolute value is greater as the total calorific expenditure is increased and so consequently more fat is broken down as the exercise fuel. Not only does this moderate intensity of aerobic training allow a greater number of total fat calories to be burnt but it also gains physiological adaptations to optimize the substrates utilization in following training sessions. Capillary and mitochondrial density within the muscles will be elevated, respiratory muscles strengthened and oxidative enzymes increased. All of which optimise aerobic endurance and fat oxidisation during physical activity by improving the transportation and subsequent utilization of oxygen.

Consequently,  in order to reduce body fat it is of greater benefit to achieve a larger total calorific expenditure through exercising at moderate intensities between 70 - 80% VO₂ max. This does not mean to say that training above this level does not gain these mentioned muscular adaptations, however the duration of such an intense workout is frequently too short to gain the optimal level of physiological change.  Not only will this equate to more fat calories being burnt as an absolute value but it also ensures appropriate adaptations are made to optimise fat utilisation in future training sessions.