The concept behind “fasted cardio” first arrived in the 1970s.
It’s an idea we’ve been fascinated with since we first heard it, and one look in the cardio section of your local gym at 6am will show you that it remains as popular as ever in the 2020s!
Heck, it even made media headlines last year when J-Lo’s personal trainer claimed that it was “the secret behind her great body” (conveniently ignoring the fact that she reveals in the interview that she’s only been doing it for six months and has been in amazing shape for 25+ years…) so now it’s all the rage in Hollywood, too.
However, research shows us that this is all bullshit.
The Theory Behind Fasted Cardio And Fat Loss
The idea of why fasted cardio could help us burn more fat actually makes a lot of sense.
For many decades, we believed that training in a fasted state would force the body to burn through its existing energy stores (e.g. body fat) in order to provide working muscles with a fuel source.
This makes perfect sense, and it’s the very reason why so many of us “fitness freaks” get outta bed at Fuck O’ Clock in the morning to pound the treadmill, right?
Unfortunately, this theory is DEAD WRONG.
Putting Fasted Cardio To The Test
Yeah, it turns out the human body is a lot smarter than we gave it credit for!
A 2000 study published in the Journal of Applied Physiology was the first to really challenge the old theory that fasted cardio “burns more fat”. (1)
The Australian researchers who conducted this trial were able to clearly demonstrate that training in a fasted state did NOT lead to better fat loss results, showing that participants who trained in a fed state (a protein shake prior to training) achieved near identical results. Heck, if we want to nit-pick (and you know I do) the fed group actually recorded a slightly higher calorie burn overall, but the difference was very small.
This study sent shockwaves through the fitness world at the time, causing many people to wonder whether fasted cardio really was the “elite fat burning method” we’d always thought.
In the years which followed, we began seeing more and more studies re-confirmed the results of the 2000 trial.
The best of these, and the one I still consider to be the definitive research on this topic, arrived in 2011 via the Strength and Conditioning Journal. (2)
In this massive meta-analysis, researchers looked at the findings of 33 previous studies on fasted cardio and fat loss, reaching the conclusion that fasted cardio does NOT burn more fat.
“In conclusion, the literature does not support the efficacy of training early in the morning on an empty stomach as a tactic to reduce body fat.
At best, the net effect on fat loss associated with such an approach will be no better than training after meal consumption, and quite possibly, it would produce inferior results.
Moreover, given that training with depleted glycogen levels has been shown to increase proteolysis, the strategy has potential detrimental effects for those concerned with muscle strength and hypertrophy.”
– J Strength Cond (2011)
The data has now grown to such an extent that The American Council of Exercise no longer recognizes fasted cardio as an elite fat burning method. (3. 4)
So what gives?
Well, it appears that the body is able to balance our energy requirements by switching between fuel sources on a needs basis throughout the day.
This means if you train in a fasted state early in the morning you will definitely burn through fat in your workout, but your body will switch to using carbohydrates later in the day in order to redress the balance.
Likewise, if you train in a fed state (e.g. after a good breakfast) you’ll burn primarily carbohydrates for fuel during your workout, but the body will switch to burning fat later in the day to once again balance the books, so to speak.
We don’t get a say in it, because humans are “hard-wired” this way.
Russ’ Take-Home Advice
Listen up; I’m not saying fasted cardio is “bad”.
Heck, I’ve done it many times over the years and I actually enjoy it.
The research simply says it’s not superior for fat loss, which gives you the luxury of being able to choose whether you prefer doing your cardio in a fasted or fed state and you’ll achieve the exact same results. This will give you the consistency you need to build your best body.
Finally, I want to end this round-up by touching upon those closing words from the Strength and Conditioning Journal with regards to fasted cardio and muscle strength. You see, it’s important to remember that in this article we’ve been comparing the results of “regular cardio” (e.g. aerobic cardio) when done in a fasted or a fed state. There are several other forms of cardio training out there, like HIIT, where this topic wouldn’t even be a debate, because explosive cardio requires a readily available source of carbohydrates therefore training in a fasted state would lead to significantly worse in-gym performance. (5, 6)
Does that make sense?
Great!
References:
- Febbraio M. A., et al. Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance (2000).
- Schoenfeld B. Does Cardio After an Overnight Fast Maximize Fat Loss? Str Cond J (2011).
- Hackney K.J., et al. Timing Protein Intake Increases Energy Expenditure 24 H After Resistance Training. Med Sci Sports Exerc (2010).
- Paoli A., et al. Exercising Fasting Or Fed To Enhance Fat Loss? Influence Of Food Intake On Respiratory ratio And Excess Post Exercise Oxygen Consumption After A Bout Of Endurance Training. Int J Sport Nutr Exerc Metab (2011).
- Wright D. A., et al. Carbohydrate feedings before, during, or in combination improve cycling endurance performance. J Appl Physiol (1991).
- Schabort E. J., et al. The effect of a preexercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exerc (1999).
Great info Russ! I use If but make sure that it is not during my fast because I really go heavy.
Fantastic breakdown, learnt so much from this!