What is Fasted Cardio?

The term fasted cardio is pretty self-explanatory: it’s an approach to cardiovascular exercise that emphasizes working out on an empty stomach — AKA in a fasted state. It’s become popular amongst everyone from bodybuilders and athletes to the average fitness enthusiast interested in weight loss.  

While your body can be in a fasted state at any point throughout the day, many people perform fasted cardio first thing in the morning after fasting overnight, with the thought being that doing so maximizes your fat-burning potential.  

What is the Basis For Doing Fasted Cardio?

The rationale behind fasted cardio was developed by Bill Phillips and published in his book titled “Body for Life”.   In it, he argues that after an overnight fast, as little as 20 minutes of intense cardiovascular exercise is even more effective than an hour’s worth of cardio performed in a fed state.  

Phillips’ theory is predicated on the idea that fasting reduces your glycogen levels, which in turn, forces your body to move away from carbohydrates — which is its primary source of glycogen — and towards stored fat, thus improving your fat-burning potential.   

What Does The Science Say About Fasted Cardio?

While the theory behind fasted cardio may sound pretty convincing, there’s actually no scientific evidence to support it.  In fact, there are several reasons why it may not be all that effective when it comes to maximizing your fat-burning potential.    

The Issue of Perspective

To start with, one of the biggest flaws with the fasted cardio theory is that it only looks at your ability to burn fat during your workout.  

The fact of the matter is that the amount of stored fat your body converts into fuel is affected by a number of different factors and is constantly fluctuating throughout the day, not just when you work out. (1) 

In addition to exercise, things like your hormone levels and enzyme activity can also impact how much fat oxidation occurs in your body at any given time — your hormone levels and enzyme activity are constantly changing.(2)     

Ultimately, that means that just looking at how much fat your body is burning during your cardio session isn’t going to give you a clear picture of how much fat is being oxidized over the entire course of the day. 

Instead, in order to fully understand how much fat you’re actually burning off, you need to look fat oxidation over a multi-day period, not just during a workout session.    

Measuring Fat Oxidation During Training

Even if we’re just looking at the amount of fat burned during a single exercise session, doing cardio on an empty stomach still may not be any more effective for burning fat than performing cardio after you’ve eaten.  

While there is some evidence that eating carbohydrates prior to moderate-intensity aerobic exercise can potentially inhibit fat oxidation during the workout, it only appears to be true for people who are untrained.  In those with previous training experience, studies show that whether or not you eat before your workout has no impact on your ability to burn fat. (3)(4)      

For example, one randomized, double-blind study published in the Journal of Applied Physiology examined the effects of carbohydrate consumption both before and during exercise on individuals with previous training experience.  (5)  

The study consisted of four training sessions in which participants cycled for 120 minutes at about 60% of their maximum capacity followed by a short all-out sprint.  During the first session, subjects were administered only a placebo both prior to and during training. In the second trial, they were given a placebo beroe training and carbs during training.  For session three, participants were given carbohydrates 30 minutes prior to their workout and then a placebo during training. In the final trial, they were given carbs both before and during their workout.  

At the conclusion of the study, the researchers ultimately found that the amount of fat burned by participants was no greater in sessions in which they were given a placebo compared to when they were administered carbs.  The researchers ultimately concluded that ingesting carbohydrates both before and during exercise had no impact on the participants’ level of fat oxidation.  

On top of that, research also shows that when you perform fasted cardio at a high intensity, it actually breaks down more fat into free fatty acid than your body is able to use for energy.  Ultimately the free fatty acids that don’t get oxidized just turn back into body fat, erasing any potential benefit that working out on an empty stomach offers. (6)  

Targeting Intramuscular Fat

Another issue with the theory behind fasted cardio, particularly in well-trained individuals, is that as your training experience goes up, your body stores more intramuscular fat to be burned during endurance exercise.(7)  In fact, as much as 80% of all that fat that’s burned during a long cardio workout is intramuscular fat. (8)

The main problem with this is that intramuscular fat is stored in your muscles and has no effect on your body composition.   Subcutaneous fat, on the other hand, is the visible fat that lies just underneath your skin and is the main culprit behind an overweight appearance. (9) 

While your body may burn some subcutaneous fat during low-to-moderate intensity exercise, it’s usually a minuscule amount, which means that whether you’re in a fasted state or not, simply doing steady state cardio may not be all that effective when it comes to converting your subcutaneous fat into fuel.(10)  

Should You Do Fasted Cardio?

When it comes to working out on an empty stomach, the research doesn’t seem to illustrate that its effects on fat loss are any greater than exercising in a fed state.  In fact, studies show that in some instances, it can actually be detrimental, especially when it comes to maintaining the size and strength of your muscles.    

During a fast, your glycogen levels become depleted and consequently, the amount of protein that’s broken down within your body increases.  In fact, over 10% of all the energy your body uses during a low-to-moderate cardio session can come from protein if you’re working out in a fasted state.   (11)      

Protein plays a vital role in all aspects of your muscles. If you’re working out on an empty stomach, you may be breaking down more protein than your body is able to replace, which can actually lead to muscle loss when combined with a hypocaloric diet.  

On top of impinging on your ability to maintain muscle mass, fasted cardio can also negatively impact your performance when you’re exercising.  Ultimately, if you’re in a fasted state, your energy levels are going to be lower, which means that exercising at even a moderate intensity will be more difficult.  

And when it comes to high-intensity aerobic exercise like HIIT, it’ll be just about impossible to perform at your best, which ultimately reduced the total amount of calories you’re able to burn both during and after your workout.(12)

Another downside to fasted cardio is that you don’t get the same thermogenic effects as you do when you eat before your workout.    When you consume a meal before you exercise it actually increases your body’s production of heat, which can be beneficial both during and after your exercise.  

In addition to burning up more calories while you’re a workout, increasing the thermic effect of exercise also leads to excess post-exercise oxygen consumption, which ultimately elevates your metabolism in the hours after you exercise.  (13) 

Wrap Up

While fasted cardio has become a popular approach to fat loss, research suggests that it may not be any more effective than working out in a fed state.  One of the main reasons for this is that the theory behind fasted-cardio only takes into account the amount of fat you’re able to burn during your workout, not over the course of an entire day.  

Even if we’re only looking at your ability to burn fat during a single exercise session, the science indicates that there’s likely no difference between skipping vs. eating a pre-exercise meal, especially for those with previous training experience.  

On top of that, studies show that cardiovascular exercise — whether fasted or not — primarily burns intramuscular fat, not subcutaneous fat, which is the visible fat that lies just underneath your skin.  Unlike subcutaneous fat, intramuscular fat has no impact on your physical appearance.      

Not only does it appear that fasted cardio isn’t any more effective for losing weight, but it actually may not offer some of the benefits that consuming a pre-workout meal does. 

In addition to improving your performance, which, in turn, increases your calorie expenditure, consuming a pre-workout meal also increases the thermic effect of your workout, which helps to keep your metabolism elevated in the following hours.