Is Fasted Training A Good Idea For Athletes?

Fasted Cardio…A simple way to burn more fat with LESS effort and in LESS time, right? Well, we’ll just have to see about that, won’t we?!

First of all, let’s examine the word “fast”. In case you didn’t know, to fast means to not eat for a period of time.  For most people, the only time they are fasting is when they are sleeping while others prefer methods like Intermittent Fasting which is simply a way to extend this fasting window by several hours. This means that they have a smaller window of time to consume all their calories for the day.

But whatever the case may be for you, the first meal after a fast is still called breakfast.  Get it?  “BREAK-FAST? So whenever we refer to breakfast, that doesn’t necessarily mean the meal you have early in the morning.  It’s just the first meal of the day that breaks your fast.

What Is Fasted Training?

Now, there’s this theory that seems to go in and out of the spotlight in the fitness community that training on an empty stomach, or doing fasted cardio, produces more fat burning results.  This leads to loads of people literally rolling out of bed and going straight for a cardio session first thing in the morning.

But these people aren’t cranking out 6 minute miles.  Typically, we’re talking about low-intensity cardio because that’s likely the only thing you’ll be able to do without eating anything.  For most people high intensity interval training, for example, is out of the question, because you just won’t have the energy to do it and you might even feel extremely dizzy during or after.

The Theory Behind Fasted Training

So, the main question here is…are these early bird runners wasting their time or boosting their fat burning potential?  Well, the actual theory behind fasted cardio is sound and it comes down to this:

  1. A prolonged absence of food brings about a reduction in circulating blood sugar, causing glycogen (or stored carbohydrates) levels to fall. That leaves your body no choice but to rely more on fat, rather than glucose, to fuel workouts, especially those that last 45 - 60 minutes.
  2. Also, the low insulin levels associated with fasting are conducive to fat breakdown, increasing the availability of fatty acids to be used as energy during the exercise session.

In simple terms, this means that when you perform your cardio on an empty stomach your body favors burning fat instead of carbs because your carb deposits are already low and your body prefers to hold onto them as much as possible in case a life-threatening situation arises and you absolutely require the immediate burst of energy. Makes sense, right?

In addition to this, if you perform fasted cardio often enough you’ll actually TRAIN your body to eventually favor burning fat rather than carbs which is a good thing if you want to get lean.

Will Fasted Cardio Burn Muscle?

But the next question you all should be asking is, what about burning muscle?  Research has indeed shown that while you fast, your body breaks down amino acids into glucose. So fasted morning cardio can potentially mobilize more amino acids (which are the building blocks of muscles) for fuel in addition to more fat, which isn’t ideal if building muscle is your primary goal.

To combat this a lot of people who fast drink BCAA’s, while fasting and/or performing fasted cardio or they consume a fast digesting whey protein immediately after their workout to help lessen or eliminate muscle wasting.

3 Reasons Why Fasted Training Is A Bad Idea

Unfortunately though, fasted cardio will not produce better fat burning results for you, and that’s for a variety of reasons.

Reason No.1

It is a fact that a lot of studies have indeed shown that you burn up to 20% more FAT calories while training fasted. Your body though, is more complicated than that and it continually adjusts its use of fat and carbohydrate for fuel depending on a variety of factors.

As a general rule, if you burn more carbohydrate while exercising, you'll ultimately burn more fat in the post-workout period and vice versa.

So who cares if you burn a few extra fat calories while exercising fasted if an hour later the ratio shifts to a greater carbohydrate utilization? At the end of the day, it doesn't make a bit of difference and you’ll have burned the same overall calories. You need to evaluate fat burning over the course of days, not an hour to hour basis, to gain a meaningful perspective on its impact on your body composition!

Reason No.2

It comes down to simple thermodynamics guys! You’ll eventually lose fat and become leaner if you shift your energy balance in a way that expenditure exceeds intake. That simply means, eat less food than your body actually needs and you’ll lose weight.  This is where the term caloric deficit comes from.

For example, let’s say you did 30 minutes of fasted cardio and you burned 400 calories and 300 of those calories were from fat (very optimistically). Now, let’s say you did 30 minutes of cardio but had a meal just before the cardio session. This time, you also burned 400 calories but only 100 of those calories were from fat and the rest were from carbs.

Well, when we look at this from a calories in vs calories out for the day comparison, there’s no difference.  In both cases, you still burned 400 calories which is all that matters. Calories in versus calories out. Period.


Reason No.3

The afterburn effect. Yes, it is a real thing. We all know that intense exercise doesn’t just burn a lot of calories while doing it, but your elevated heart rate and blood flow will continue to help boost your metabolism for several hours after and that’s because of EPOC which is another way to say “afterburn” and it stands for Excess Post-Exercise Oxygen Consumption. The afterburn represents the number of calories expended AFTER training and guess what? Eating BEFORE exercise promotes substantial increases in the afterburn effect! And want to guess where the vast-majority of calories expended in the post-exercise period come from? You got it, fat!

So at this point, killing yourself to do fasted cardio probably makes even less sense. At best, the effects on body composition won't be any better than if you trained in a fed state; at worst, you'll lose muscle and reduce total fat loss. Therefore, why even risk it?


So what is the perfect approach? Well, there is no PERFECT approach. Everyone is different and you should always stick to what works for YOU.

Having said that, here at Gronk Fitness we are huge proponents of High Intensity Interval Training which is an intense way to train and cannot possibly be done on an empty and fasted stomach. Here is our advice: wake up, have a nice, healthy and high-carb breakfast, have your training session a couple of hours later and as always…get your mind right.

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