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Does Fasted Training Actually Work?

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As an athlete, you are constantly looking for new ways to enhance your training and improve your performance. And there is no end to the tips, tricks and hacks that you might encounter in the wild world of health and fitness advice. These different tweaks range widely from supplements to gear and novel nutrition strategies.

Over the past several years, one of these ideas that’s received a lot of traction is that of fasted training. But what is fasted training? What’s it supposed to do? Does it actually work?

What It Is and What It (Supposedly) Does

As its oh-so-descriptive name suggests, fasted training involves training while fasted. That’s really it.  Frustratingly, this fairly simple dietary strategy goes by many names. Depending on the source and situation, you might see it called fasted cardio, fasting cardio, fasted-state cardio or any related phrase.

Continuing this theme of confusion, there are also a lot of opinions regarding precisely what exercising on an empty stomach does to your body. These claims include arguments both for and against the practice.

According to fasted cardio’s supporters, you can ramp up your metabolism and force your body to burn more fat by heading out for your run before breaking your fast. In theory, this makes sense. By denying your body any outside sources of carbs – its preferred fuel source – you force the use of a back-up fuel. Fat generally fulfills this role. That’s how the thinking goes, at least. More on the science later.

Unfortunately for believers, the nay-sayers have many more complaints about fasted training. When you force your body to work without providing it with enough fuel, the detractors say, you make it more difficult for your body to perform at it’s best and thereby reduce the effectiveness of your workouts as a whole. There’s also a fear that, in response to the lack of other options, your body might start burning protein for fuel and even stripping that macronutrient from your hard-earned muscle.

On top off this, critics of fasted training have also expressed fears that running on an empty stomach could increase your appetite and make it more difficult for you to practice dietary self-control after your activity is over.

Macros 101

Before delving into the results of scientific inquiry, however, it’s important to be clear about what people are talking about when they discuss the different sources of energy used during exercise. While this might seem like a trivial thing, a proper understanding of this issue will help to explain a lot of the findings to be discussed later.


More clinically referred to as macronutrients, these primary fuel sources include carbs, fats and proteins. As mentioned, carbohydrates are your body’s favorite fuel – especially during short, intense workouts. Most of the time, though, your fuel mixture is pretty much evenly spilt between the fast-burning carbs and the sustained-release energy found in fat.

Proteins are not typically burned up for fuel, since it’s much more important that they be used for building and repairing various tissues throughout your body. When needed, however, proteins can be converted into carbohydrates.

Now, why does any of this matter? For a few reasons. First, remember that there is a clear priority in which macros get burned up when. It’s also important to realize that, when your body is deciding between using carbs and fat, it’s not all-or-nothing. These two fuel sources are typically used together, although the exact proportions will vary.

Fat vs. Fat

But, likely the most important thing to realize is this: dietary fat is different from body fat. In fact, body fat is much more accurately referred to as adipose tissue. While somewhat troublesome bodily tissue acts as a method for storing excess calories – regardless of the form they took before digestions. This means that adipose tissue can be formed from carbs, protein or fat. It also means that adipose tissue is sort of survival mechanism; your body is very hesitant to use it.

The fats that your bodily readily burns, though, are called free fatty acids. These are the fats that come from your food and are simply waiting in your blood to be used when needed.

When discussing a potential “fat-burning” strategy, then, a pretty major complications arises: just because a supplement or activity or diet burns more fat, that does not mean that it will reduce adipose tissue. In most cases, that fat is simply coming from free fatty acids.

What’s Actually Happening

Armed with this knowledge regarding the role of these various macronutrients, you can now jump into the research surrounding these various aspects of fasted training.

One particularly telling study conducted in 2012 was designed to examine several of these questions, rating changes in appetite, exercise performance and about of total calories burned. For the study, 12 healthy men participated in three different trials that lasted 10 hours each. While the subjects performed a 60-minute long treadmill run during each trial, the researchers adjusted other aspects of the routine, requiring the men to run fasted or after a meal and allowing them to eat both a standardized breakfast and all-they-could-eat from a buffet.


At the end of the trials, the researchers found no major difference in any of the measures between the fasted or non-fasted tests. Whether or not the men ate before their runs had no bearing on their performance, the amount of calories that they burned while exercise or the amount of food that they ate afterwards.

Interestingly, this study did find that fasted training actually does increase your body’s dependency on fat for fuel. Remember, though, that the total calorie burn was unchanged and that these fats were free fatty acids – adipose tissue was not a special target of fasted training.

This study, then, really settles most of the arguments surrounding this practice. What about concerns regarding muscle loss, though? While it is true that fasted training – and any exercise, in fact – can increase the breakdown of muscles, this doesn’t appear to be dramatic enough to cause any problems in the long-term. Of course, this is entirely dependent on your total recovery routine. If you damage your muscles with exercise, you must give them the time and resources to rebuild – regardless whether you were fasted or not at the time of your workout.


Really, then, it appears as though fasted training does not have any major advantages or disadvantages to regular ol’ fed training. That being said, if you prefer one over the other then go with that.