Don’t be fooled by the fat burning zone

The fat burning zone is one of the biggest myths in the fitness industry.  It is everywhere – in magazines, books, stickers and charts on gym walls and there is even a ‘fat burning zone’ fat-burner1button on most CV machines.

During exercise your body uses energy stored primarily in two places: your fat stores or your glycogen stores (your glycogen stores come from the carbohydrates you consume).

The fat burning zone gains its name because in this zone (approx. 60% – 70% of your maximum heart rate (MHR)) you are working at an intensity where the majority of your energy is coming from your fat stores rather than your glycogen stores. So in theory it sounds great however here is the full science bit!

Take a look below at the example calories being used in two 30 minute session on a treadmill.  In session 1 the person is exercising in the fat burning zone (60%-70% MHR).  In session 2 the same person is working at a higher intensity of 70%-80% MHR.

Session 1 – 60%-70% MHR Session 2 – 70% – 80% MHR
Time

30 minutes

30 minutes

Total calories

140

190

Glycogen calories

56 (40%)

114 (60%)

Fat calories

84 (60%)

76 (40%)

Due to the increase in intensity in session 2 there is a larger amount of calories being used overall and even with the differing ratios of fat between both sessions the fat calorie figures are very close.  However the best is still to come………

The Afterburn Effect

Basically when you use your glycogen stores during activity the body has to restock them once the exercise has finished.  In order to replenish these stores your body uses an energy system called the Aerobic energy system.  The Aerobic energy system favours your fat stores as its main source of energy.  Therefore the more glycogen stores you use (as in session 2 above) the more of your fat stores are used to fuel the replenishment of them which is great for reducing fat!

Next time you read about the fat burning zone or use a CV machine and see the Fat Burning Zone button you now know the truth, the whole truth and nothing but the truth!

Take a look here to find out more on interval training and the afterburn effect

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