Training

The Fat Burning Zone Myth | Lose Weight With Intense Exercise

At this time of year, many of you will be starting out on a fitness journey for the first time with the aim of getting in shape for summer. To improve your body composition and get in shape, you should be aiming to burn body fat and build muscle. So you may be a little confused by the title of this post.

With the rise in popularity of activity tracking devices over the last few years (and for the record, I think they have a great impact on getting people moving more), I feel it’s important to clear this one up for those of you that use them. The graphs they produce to show you the intensity you move at will highlight the amount of time you spend in your ‘fat burning zone’.

There are also pieces of cardio equipment that give you this information. The time spent in the fat burning zone is calculated using your heart rate, whether you use an activity tracker or the cardio equipment. But here’s why being in your fat burning zone isn’t actually optimal if burning body fat is the goal.

The Basic Physiology

Our bodies have different types of muscle fibers and different energy systems that provide them with energy (by producing ATP), using either creatine phosphate, glycogen (from carbohydrates) or fat. Which fibers and energy systems are most active and which fuel sources are most utilized, depends on the duration and intensity at which the muscles are contracting.

When exercising at relatively low intensities e.g. walking around or steadily cycling on a bike, we are predominantly using our slow-twitch, type 1 muscle fibers. These slow twitch muscle fibers have small ‘energy boxes’ within the muscle cells called Mitochondria. Using fat to fuel muscle contractions is an aerobic process and so it requires oxygen (and time), and it is the Mitochondria that uses oxygen to produce ATP, whilst using fat as the fuel source.

Our more explosive muscle fibers and energy systems, favor creatine phosphate or glycogen as the fuel source (depending on the duration and intensity) as producing energy using these fuel sources can be done much quicker as it is an anaerobic process and does not require oxygen. Therefore fat is not used as a fuel source when performing more intense work.

Now, in order to burn stored body fat and get in better shape, the most crucial factor is being in a calorie (energy) deficit. When we provide our body with less energy than it needs on a daily basis, it will make up for this missing energy by utilizing its stores (body fat).

With all of that information in mind, I have a question. If you perform 60 minutes of low intensity walking on one day and then a 60-minute intense resistance training session on another day, which one do you think would burn more energy and help create an energy deficit?

The resistance training of course. But if you looked at the time spent in the ‘fat burning zone’ during an intense resistance training session, it would likely be very low because we use predominantly the more explosive muscle fibres that do not use fat as a fuel source.

Supplements To Increase The Work Capacity of Explosive Energy Systems

Creatine – A good quality creatine supplement will enhance stored creatine levels within skeletal muscle and therefore enhance the capacity of our most explosive energy system to work at it’s maximum. This results in more muscle growth and more fat loss as we can train at peak intensities for a longer period of time.

Maltodextrin/Highly Branched Cyclic Dextrin – If you remember the information above, once our most explosive fuel source is depleted, the next energy system that becomes most active utilises glycogen as its fuel source. Again we only have a certain amount stored and available within the body, so using a dextrin supplement to drink during training provides the body with additional fuel for intense training.

The Bottom Line

Do I think that exercising at a low intensity is pointless? Absolutely not, I ensure myself and my clients are completing plenty of walking throughout the day. This contributes to the calorie deficit without causing any disturbance within the body, and then the main focus is the high-intensity resistance training which will ultimately get us in great shape.

So if you find yourself exercising at an intensity that is above the fat burning zone during your main sessions, don’t reduce your intensity to get back there. You’re actually moving closer to your goal by being out of the fat burning zone.

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Faye Reid

Faye Reid

Writer and expert

Faye has an MSc in Sport Physiology and Nutrition, and puts her passion into practice as goal attack for her netball team, and in competitive event riding. She enjoys a pun, and in her spare time loves dog walking and eating out.


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