MythBuster: Is Fasted Cardio the best for burning fat?

January 12, 2018
Lucy Mecklenburgh

Fasted Cardio has been a technique used for many years, popularised by stage figure competitors and bodybuilders who were often looking for an edge to get leaner and burn fat more efficiently.  "Fasted" cardio as the name implies means doing cardiovascular exercise on an empty stomach.  The method is almost universally applied in the morning, when someone has been asleep for 8 hours, and probably hasn't eaten for 8-12 hours.

When we are asleep this is usually the longest amount of time people go without eating food, hence where the name BREAKFAST originated from, BREAK-FAST, as in breaking the fast! The name makes sense now ey?!

When your body is in a fasted state it's glycogen stores are depleted.  Glycogen is basically glucose but in a form that your body can store for use when it's needed, like in exercise.  So when you eat a meal, your body stores some of the energy from the food in the form of glycogen, then when you really need this energy, your body has access to this glycogen which is broken down into glucose that your muscles can use for energy.


So if you put 2±2 together, you can perhaps understand the theory, that in a fasted state your body is depleted of its glycogen stores, but if you force your body to exercise when glycogen is depleted, where will it get this energy?  Well we can also get energy from our fat stores, so the thought process and hope is that when one trains in a fasted state that their body is using up fat as an energy source, leading to a leaner body overall.


Sounds great, right? Does it really work this way though?


Two fantastic nutrition researchers (Alan Aragon and Brad Schoenfield) actually carried out a fantastic experiment to check whether fasted cardio really did work this way and was it a superior method for fat loss.  For you science nerds you can see the full paper by clicking here. 

In a nutshell they compared two groups of women, one group was doing fasted cardio and the other group was fed before doing their cardio sessions. However, overall caloric intake (which was 500 caloric deficit for everyone regardless of which group they were in) over the period of the study which was 4 weeks long was equated, as was the exercise kept the same, so in this way you couldn't simply say one group did better because they ate less or exercised more.  They used a fancy bodypod device to accurately measure body fat results.


What did they find?

Well, both groups lost significant amounts of weight.  However, there was no significant difference between either group.  When exercise and calories were controlled and equalled in both groups as best they could, it seemed fasted cardio fared no better than simply exercising when fed.


So, is fasted cardio worthless?

 Well no study is perfect, and this study was only 4 weeks long.  Proponents of fasted cardio suggest that a longer time frame may be needed to see the clear superiority of fasted cardio.  Nutrition research has a real problem with getting the participants to adhere to their eating plan, so it's very difficult to do these studies for longer than 4 weeks, so perhaps there is a case for the defense there.


This study was also done on pre-menopausal women, so it's difficult to say that this would apply directly to other age groups or sexes.


*The bottom line*

A single study is simply a piece in the puzzle and can never be considered the final word on a topic. What the researchers suggest from their study is that if there are any benefits from fasted cardio (still highly equivocal), they would be minor at best. So, the best advice for those who are simply looking to get lean is to focus on total energy and macronutrient balance; whether you perform cardio fasted or fed should depend entirely on preference.

Personally, fasted cardio sounds like torture to me, when I awake I am ravenous and need to eat!  After eating, I feel I exercise with more intensity and focus.  Others simply don't want or have no appetite for breakfast, and can happily train hard in the morning on an empty stomach.

So is fasted cardio a superior method for fat loss, this remains to be seen and so far the burden of proof is not there yet to conclusively say this is so.


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Lucy Mecklenburgh
Found & Fitness Guru

Lucy founded Results with Lucy to help ladies transform their health & lives. 

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  • Paula September 19, 2019

    What an interesting analysis of the concept of fasted cardio!

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