As most questions worth asking in life, this is a complex one. In this post I will try to lay out the good, the bad and the ugly of doing cardio while fasting, so you, the reader, can make an informed decision on your training plan.
Let’s start by defining what is fasted cardio. As the name implies, fasted cardio is a cardiorespiratory exercise – such as running – that is performed in a fasted state. Before you call me captain obvious, most people actually have a somehow vague idea of what is a fasting state and more specifically, when your body is in that state.
The correct definition of a fasted state is when your body have completely digested and absorbed your last meal/snack and your insulin levels are at a low or baseline level. Physiologically speaking, this state can be attained as soon as four hours after a (light) meal. Most research studies on fasted exercise, however, are performed in the morning, before the first meal of the day, at least eight to twelve hours after the last meal. This longer fasting is a more adequate way to ensure that the insulin levels in the body attained their baseline.
As for cardio, it is simply any exercise that increases your heart rate, usually utilizing large muscle groups, in order to perform movement over a sustained period of time keeping your heart rate to at least 50% of its maximum level. Easy right? Now for the fun part.
Does fasted cardio burn fat?
Once the body has stopped processing and breaking down food, your insulin levels are low and there’s no fuel (glycogen) circulating in your blood. As a result, your body has to turn to another source of energy, usually fat, to power you through the workout.
In 2018 a study published on the National Library of Medicine(1) found that “fat metabolism fuels resistance training more than carbs when doing it fasted” meaning that doing cardio while fasting burns fat. Well, considering that carbs are metabolized much faster than fat, it is quite obvious that, after a twelve hour fast, there will be no carbs left to burn… so, the answer to the question becomes obvious: Yes, fasted cardio burns fat!
Does fasted cardio makes you lose weight faster?
Since working out fasted burns fat, it would be easy to jump to the conclusion that fasted cardio makes you lose weight faster. However, it is a little more complex than that.
According to a study published on the Journal of Functional Morphology and Kinesiology in 2017(2), fasted cardio hardly led to changes in body mass.
But how can science disproof empirical data? We know that bodybuilders have been using fasted cardio as a fat-loss technique for many years. We know it works for them – just ask four times Mr. Olympia Jay Cutler(3).
So what a professional bodybuilder does different than the subjects of that research? How to make fasted cardio work? Two things:
The first one is the simplest of all: do not stuff your face after running.
Working out while fasting is a sure way to open a monster appetite. It simply becomes too easy to eat more – much more – than what you should be eating if you wish to lose weight. It is as simple as that. A good tactic that I personally use is to count my macronutrients. It only requires a scale and a simple cell phone app that can be downloaded for free. Just search for a macro calculator, there are literally dozens of those.
The second factor is to focus on your objectives. Do you want to burn fat and show that six-pack you have been working hard on, or to run a marathon?
Have you ever saw a muscular marathon runner? That is because too much cardio will break down proteins in your muscles. It will also increase your cortisol levels, an hormone that decreases muscle mass, eating away your gains.
Full disclaimer, don’t take it the wrong way, if cortisol wasn’t important, your body wouldn’t make it. However, too much of it will curb your muscular grow, and we don’t want that. Also, I am not saying that a marathoner cannot build muscle, that would be a lie. Any professional marathoner can easily build muscle… if he stops resistance training and begins heavy weight training. We will talk about that again when we reach the dreaded proteolysis.
The benefits of fasted cardio (the good)
A 2013 study (4) found that, when people ran on a treadmill in a fasted state, they burned twenty percent more fat compared to those who had eaten breakfast, helping the body burn stubborn fat.
Fasted cardio also increases endurance, which in turn helps you improve your cardio performance making you burn even more fat. This was proved by a research published in the Journal of Applied Physiology (5) comparing fasted individuals versus fed individuals over the course of six weeks. The research found that, when training at the same intensity, those who consistently trained in a fasted state showed a better improvement in their endurance exercise performance compared to those who ate before training.
The cons of fasted cardio (the bad and ugly)
In 2011, a research (6) published on the National Library of Medicine proved that, while you will be burning fat for fuel in a fasting cardio, you will also burn twice the amount of protein in your muscles than with fed cardio. According to the study:
“Lemon and Mullin found that nitrogen losses were more than doubled when training while glycogen depleted compared with glycogen loaded. This resulted in a protein loss estimated at 10.4% of the total caloric cost of exercise after 1 hour of cycling at 61% Vo2max. This would suggest that performing cardiovascular exercise while fasting might not be advisable for those seeking to maximize muscle mass.”
So why do bodybuilders still do fasted cardio if they are going to lose muscle?
Incredibly, this one is simple: Proteolysis, or the breaking of a protein molecule, usually occurs as a result of adverse cellular conditions, such as extreme temperature, acidity, or salinity, which disrupts the molecules in the peptide bonds and results in the bonds breaking (7). In clear English, it is the excessive stress in the muscle fiber that causes proteolysis, stress induced by a long cardiovascular activity, an effect easily countered by limiting the amount of cardio.
Most professional bodybuilders will do at maximum 20 to 40 minutes of cardio, enough to burn fat while keeping muscle mass. It is easy to understand this if you compare the bodies of professional marathoners versus the bodies of professional sprinters. Both are lean, healthy and run as a profession. One can run from one city to another in one go, something that I wouldn’t even attempt. The other only runs for a few hundred meters at a time, with explosive strength and a very impressive caloric expenditure compared to the short time of the event.
Is Fasted Cardio for me?
It can be. Ultimately, every single human body is slightly different from another and there is no answer that fits everyone. Fasted cardio can be the ideal exercise for those who enjoy waking up early, and can help a great deal in burning fat. The only way to actually know if it is for you, is trying.
Just remember not to run a marathon while fasting (if you want to build muscle).
1 Frawley K, Greenwald G, Rogers RR, Petrella JK, Marshall MR. Effects of Prior Fasting on Fat Oxidation during Resistance Exercise.
2 Hackett, D.; Hagstrom, A.D. Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis.
4 Gonzalez, Javier & Veasey, Rachel & Rumbold, Penny & Stevenson, Emma Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males.
5 Van Proeyen K, Szlufcik K, Nielens H, Ramaekers M, Hespel P. Beneficial metabolic adaptations due to endurance exercise training in the fasted state.
6 Schoenfeld, Brad MS, CSCS Does Cardio After an Overnight Fast Maximize Fat Loss?