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The Fitness Zone

Fasted Cardio – The What, The Why and The How!
May 11, 2017

As a personal trainer or fitness enthusiast you may be hearing a lot about ‘fasted cardio’ and its benefits or its pitfalls. The topic has been made popular in mainstream fitness due its use in bodybuilding and the impact it has in fat metabolism. So, what’s it all about?

Australian Institute of Fitness Coach, Chris Young, explains before we look at the pros and cons of this training method we firstly need to understand what ‘fasted cardio’ actually is. Fasted cardio is just that, training in a fasted state. A fasted state is induced over a period of limited nutritional intake, for example waking up in the morning you are in a fasting state (hence the name breakfast, aka break fast). Once you are in this state, it is believed that carbohydrate (glucose or glycogen) levels are low within the body and you are more readily able to use fat/fatty acids as a fuel source during exercise, therefore decreasing fat mass.

Sounds great, right!? So why don’t many do it (or know how to do it) and why may it not be sustainable?

To answer, let’s break it down into two parts:

Part 1) How to do it properly.
Fasted cardio has a few tricks to performing it correctly. The biggest factor is intensity. If you are performing it outside of the right intensity you may be doing more harm than good. They key to this modality of training is that you should be in an aerobic energy pathway. If we look at the primary fuels for the aerobic pathway its CHO (carbohydrate), FAT and PROTEIN. For us to stay within this system we must maintain a low intensity (RPE 6-7 or 60-70% HRmax). If we step it up and push ourselves harder than these ratings we can move into a more anaerobic state which can be disastrous for our muscles, as higher rates of protein will be metabolised due to the lack of carbohydrate present in the body. Another factor we have to account for is time. On average we have 30-mins of CHO stored in our bodies, as we are in a fasted state we already know that CHO is very limited. This leaves minimal room to move with what we can use for energy. Anything over the 30-min time frame can also lead to higher levels of protein breakdown (again not good for them juicy muscles).

Part 2) Why isn’t it working as well as it first did?
As with any training our body will adapt to it and the old fasted cardio is not different in any way. Let’s say I do fasted cardio 3 days a week, I’m getting great results for the first few weeks and all of a sudden it slows down. Here I have two options: back it off or step it up and do it more or at a higher intensity (as we now know may not be the best idea). If you look closely at how the majority of people do it, they use it sparingly and to ‘shock the body’. It might be performed once a week and increasing in frequency closer to a bodybuilding comp, for that extra kick in fat loss.

Overall, we can see it can work well. However, it must be performed the right way to get the most from it.



This content is not intended to be used as individual health or fitness advice divorced from that imparted by medical, health or fitness professionals. Medical clearance should always be sought before commencing an exercise regime. The Institute and the authors do no take any responsibility for accident or injury caused as a result of this information.

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