To effectively burn fat you should be working out at a spectacularly low intensity where your heart rate is rooted to a zone between 30-40% above resting levels
The theory goes that at this exercise intensity your primary fuel source will be fats. This may be true, but it won't help you get leaner in a hurry! In this article I want to try and explain why this mythical fat burning zone is such a joke.
At any moment in time you derive a proportion of your energy from carbohydrate and a proportion from your fat. Very, very rarely (if ever) would you ever derive all your energy exclusively from one source.
Instead energy is always derived from a combination of fats and carbohydrates. A Fuel Substrate Sliding Scale is a good way to visualise what I am talking about.
100% FATS----------------------------------100% CARBS
When exercising, you could be anywhere on this scale from the far left hand side to the far right or anywhere between. What determines your place on the sliding scale is the INTENSITY of your exercise.
As a general rule the lower your exercise intensity the further you will be to the left of the sliding scale (i.e. burning a greater % of fat). The higher your exercise intensity the further you will be to the right hand side of the scale (burning a greater % of carbohydrates).
This happens because fat requires the presence of oxygen to be successfully metabolised. When you work at high intensity’s the cardio-vascular system is unable to provide sufficient oxygen to the working muscles to burn a high % of fat. Instead other energy stores that don’t require the presence of oxygen to be converted into energy are utilised. Enter carbohydrates.
Ok so you are probably thinking that this means you should be exercising at a lower intensity to burn more fat right? Actually no it doesn’t, what the above sliding scale fails to take into account is;
a) Total Number of Calories burnt per workout
b) Afterburn Effect of Exercise (EPOC)
Let me expand on those points;
Total Number of Calories burnt per workout
Here are two examples to explain my point.
Fred goes for a 30 minute run keeping his heart rate in the fat burning zone. This places him here on the sliding scale
100% FATS----FRED--------------------------100% CARBS
Because Fred is working at a low intensity he only burns 200 calories at a ratio of 70:30 (fats:carbs) This means that of his 200 calories, 140 were derived from fat stores.
Roy also goes for a 30 minute run. However he performs high intensity interval sprints with short rest periods. This places him here on the sliding scale;
100% FATS--------------------------ROY-----100% CARBS
Because Roy was working out at such a high intensity he burnt 600 calories BUT at a 30:70 ratio (fats:carbs) This means that of his 600 calories, 180 were derived from fat stores
So even though Roy was burning calories using fuel comprised primarily of carbohydrates he still burnt more fat than Fred (180 vs 140) This is because the high intensity of his workout meant that Roy burnt an additional 400 calories more than Fred during his training session.
Even though he burnt fat at a lower ratio than Fred, his total fat burnt was greater
This is only the tip of the iceberg……
The Afterburn Effect of High Intensity Exercise
Shortly after working out Fred’s metabolism has returned to normal resting levels (homeostasis). Roy’s metabolism on the other hand will remain elevated for an extended period of time after working out because of the high intensity of his workout. This 'afterburn' effect of intense exercise significantly increases the metabolic cost of Roy's workout.
(Remember, it is not the amount of energy you burn in a workout that is important, it is the amount of energy you burn as a result of a workout)
The overall net result is Roy is infinitely more successful in his fat burning efforts than Fred!
In fact the theory of a fat burning zone suggests that your best bet to burn fat is to lie down on your sofa and watch DVD’s. Here your intensity is so low you should be burning exclusively fat – however we all know that this is not a successful fat loss strategy. So why replicate this theory in your training by working out at a pathetically low intensity?
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