Fasted Cardio: The Secret to Fat Loss or a Waste of Time?

What is fasted cardio?

Fasted cardio is basically exactly what it sounds like - you do some type of cardiovascular activity in a fasted state. Usually this is done by fasting through the night (while you're sleeping) and doing cardio first thing in the morning before you consume any type of calories for the day. 

The thought process behind advocating for this is that because you are training in a fasted state your body will preferentially use fat as a fuel source rather than carbohydrates, resulting in more body fat loss.

Sounds pretty awesome right?

If only it were that easy...

If you're looking for the quick answer, then I will go ahead and break it to ya. Unfortunately, fasted cardio is not the secret to enhanced rates of fat loss.

Another strike against countless Instagram fitness "influencers"...

Research has shown that when calories and protein are equated, fasted cardio does not result in greater body fat loss (1,2). 

For those of you looking for a more in depth breakdown of the flaws in the argument, I've got you covered. 

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You burn more fat during exercise...so what?

It seems logical to make the assumption that if you are burning more fat during exercise then you would therefore end up losing more body fat. But unfortunately, it's not quite that simple. The first thing we have to understand is that our body is constantly utilizing and storing different fuel sources throughout the day. Your body is rarely in these binary "fat/carbohydrate burning" or "fat/carbohydrate storing" states. It's basically a combination of both, all the time.

With that being said, it is more appropriate to look at the overall net storage or utilization over an entire day, rather than one specific point in time - especially if you are concerned with body composition changes. And guess what this net storage or utilization is going to be determined by? Overall energy balance. 

Even if you do "burn" more fat during fasted cardio, the majority of the fat likely isn't coming from actual subcutaneous fat or visceral fat,  but rather a stored form of fat found within the muscles (3). Which means that this stored form of fat has basically no bearing on health or appearance, so "burning" more of this fat would not produce the results that we are all looking for.

Basically what I am getting at here, is that it does not matter if you do burn more fat at a given point in time. What matters is your overall energy balance (calories in vs. out) at the end of the day. It is impossible to end up with an overall net storage of body fat independent of being in an energy surplus (taking in more calories than you are burning).

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The case AGAINST fasted cardio...

We've went over why fasted cardio isn't going to give you superior results. Let's get into a couple reasons that fasted cardio may actually be worse than traditional, fed cardio.

The first reason that fasted cardio could potentially be a bad option for you is the decrease in performance you might experience. There have been numerous studies that have examined the differences in performance between fed and fasted states, with an overwhelming majority of them showing decreases in performance associated with training fasted (4, 5, 6, 7). So if performance in the gym is something you care about, then fasted cardio may not be a great choice. In fact, even if you don't necessarily care about your performance during your cardio, the decreased ability to perform can lead to lower levels of intensity which can ultimately mean fewer calories burned. Even though the actual amount of additional calories burned might be negligible, if you are someone who is looking to maximize results and energy expenditure from your cardio, training fasted  could be less than ideal. 

The second reason that fasted cardio may not be a great option for you is if you are concerned with building/preserving muscle mass. There has been some evidence suggesting that exercising in a fasted state can lead to higher rates of protein breakdown, which is not ideal if you are looking to build or preserve that hard-earned muscle muscle mass (8).

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My recommendation... 

DO WHATEVER YOU ENJOY.

If you hate doing cardio on a full stomach, then don't eat before you do your cardio.

If you hate doing cardio on an empty stomach, then eat some freaking food beforehand.

The actual differences in outcomes between fed vs. fasted cardio are tiny. Basically it all comes down to personal preference and what is going to enable you to be the most consistent in your training. 

If you are completely neutral, then I would maybe suggest eating something beforehand- especially if you plan on doing some type of intense or long duration (>20-30 min) cardio. 

Find what works for you, be consistent, and results will follow.

-Clayton

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References
1. Schoenfeld, Brad Jon et al. “Body composition changes associated with fasted versus non-fasted aerobic exercise.” Journal of the International Society of Sports Nutrition vol. 11,1 54. 18 Nov. 2014, doi:10.1186/s12970-014-0054-7
2. Escalante, Guillermo & Barakat, Christopher. (2020). Fasted Versus Nonfasted Aerobic Exercise on Body Composition: Considerations for Physique Athletes. Strength and conditioning journal. Publish Ahead of Print. 10.1519/SSC.0000000000000565. 
3. Schoenfeld, Brad, Does Cardio After an Overnight Fast Maximize Fat Loss?, Strength and Conditioning Journal: February 2011 - Volume 33 - Issue 1 - p 23-25 doi: 10.1519/SSC.0b013e31820396ec
4. Wright DA, Sherman WM, Dernbach AR. Carbohydrate feedings before, during, or in combination improve cycling endurance performance. J Appl Physiol (1985). 1991 Sep;71(3):1082-8. doi: 10.1152/jappl.1991.71.3.1082. PMID: 1757304.
5. Sherman WM, Brodowicz G, Wright DA, Allen WK, Simonsen J, Dernbach A. Effects of 4 h preexercise carbohydrate feedings on cycling performance. Med Sci Sports Exerc. 1989 Oct;21(5):598-604. PMID: 2691821.
6. Schabort EJ, Bosch AN, Weltan SM, Noakes TD. The effect of a preexercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exerc. 1999 Mar;31(3):464-71. doi: 10.1097/00005768-199903000-00017. PMID: 10188753.
​7. Alghannam AF, Jedrzejewski D, Tweddle MG, Gribble H, Bilzon J, Thompson D, Tsintzas K, Betts JA. Impact of Muscle Glycogen Availability on the Capacity for Repeated Exercise in Man. Med Sci Sports Exerc. 2016 Jan;48(1):123-31. doi: 10.1249/MSS.0000000000000737. PMID: 26197030.
8. Lemon PW, Mullin JP. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol Respir Environ Exerc Physiol. 1980 Apr;48(4):624-9. doi: 10.1152/jappl.1980.48.4.624. PMID: 7380688.