Fasted Running: Should You Run or Do Cardio While Fasting?

Is fasted cardio the best way to lose body fat? That’s the question that brought you here right?

I always want to be promoting the bigger picture in running…it will change your life.

BUT let’s be honest, the weight loss is what gets a lot of us in the door right? It’s why we lace up that first time and then the second time even though the first time was awful.

So if you’re looking for that, here’s my complete guide to running for weight loss.

We’ve already covered a few other big one’s like intermittent fasting, macros for runneres, keto for endurance athletes, high protein, high carb….and the list goes on on.

But fasted cardio is a topic that seems to pop up as a key training option every few years, so let’s dive in an explore what it means, how it works and whether you should use it.

What is Fasted Running?

Fasted cardio refers to doing your run or other aerobic workout (kickboxing, elliptical, OrangeTheory, etc) on an empty stomach. This means having not eaten for at least 8 hours, not just skipping your afternoon snack.

At this point your body is not utilizing energy for digestion.

Generally, this refers to exercising first thing in the morning before having breakfast. However, those who still utilize intermittent fasting and take long gaps during the day could also apply it mid-day or later.

So, fasted running entails running in a glycogen-depleted state. This means your body will have used the glycogen stored in your muscles and liver for repair, so you are currently low on carbs freely flowing for quick energy.

While this most often means you’re working out in the morning without eating breakfast, for evening runners that could mean not having a meal for 4-6 hours prior to the workout to return insulin levels to normal.

Is Fasted Cardio Safe?

Generally yes.

That still doesn’t mean it’s a great option for most people. And definitely not when we’re talking about runners heading out for 2-3 hours of running.

If you run out of energy far from home or on a trail, that can lead to some very scary situations. Or resulting in getting lightheaded, dizzy, or even passing out from low blood sugar.

What’s the Science Behind Fasted Running?

Fasted cardio and running is a topic that keeps popping up, but what really is the scientific theory behind it? And is it accurate?

But before we start figuring out whether or not science truly backs fasted running, let’s look at what the supposed effects are of running while on an empty stomach.

Those that support fasted running claim a number of different benefits, from improved performance to positive changes to body composition.

The main theory to support these benefits is that fasted running improves fat oxidation which in turn leads to performance benefits.

Fat oxidation is the process of breaking down fatty acids in our body, resulting in increased fat burning.

To oxidize fat, our body needs healthy mitochondria which are small structures in cells that serve as ‘power plants’ of our cells.

They generate energy for muscle contraction by burning fuel, using oxygen, and producing carbon dioxide.

And so, fasted running is said to enhance mitochondrial biogenesis, which is an increase in the number of mitochondria in the body.

This, in turn, theoretically will also improve our fat-burning potential, which means our body’s ability to convert fat for fuel.

But does it really work that way?

To explore this further, let’s first answer a question that’s probably on your mind.

Should You Eat or Fast Before Running?

The answer is like most things…complicated. Let’s try to break it down in the simplest way so you can decide what’s right for you.

Should you eat before running? Usually.

Sounds ambiguous, so let’s breakdown why you might choose to run without eating first and what you’re gaining or losing by dong so.

But let’s first note that a study looking at those who did fasted and non-fasted cardio found NO DIFFERENCE IN WEIGHT LOSS.

So don’t limit your performance or go hungry thinking this is a quick way to weight loss, the data doesn’t hold up.

You’ll probably quickly see that this is not a training approach we recommend. Though we certainly understand it can save time or you worry about your stomach handling food, our focus is on how to help be be healthy and hit your PR goals.

Benefits of Fasted Running

Since fasted cardio is one of the favorite tools that body builders throw out how to shred fat, let’s find out if that’s true. Here are the benefits that often encourage runners to try it out.

1. Increases the Body’s Utilization of Fat for Fuel

This is the main reason that people often site for how it helps with weight loss. What they’re referring to is that during EVERY WORKOUT, your body uses both fat and carbs for energy.

During lower intensity workouts, your body will rely more heavily on fat.

This is TRUE. But does not mean that it increases actual fat loss.

And in workouts where you have not provided any quick energy sources from a pre-workout snack, the body will work to convert fat in to fuel. However, the body will also start to dip in to muscle for fuel! That’s right, you’ll actually be losing muscle by not feeding your body when you do this consistently.

This is not an efficient process, which is why it should not be done for high intensity workouts or long workouts.

Below is a screenshot from a metabolic efficiency test, showing fat burning in red and carb burning in black. All of us have an intensity cross over point, where the body starts to rely more on one than the other.

Fasted workouts will NOT change this point, but changing your diet and training style can!

Let’s talk about this a bit more as it relates specifically to running and hitting your goals.

2. Improves Insulin Sensitivity

Here is a recent study focusing on utilizing fasted workouts for reducing symptoms of Type 2 Diabetes.

It shows there are  benefits when doing low intensity workouts, but does not go further to investigate any drawbacks or long term issues.

What Dr Stacy Simms has found is that while the health benefits can be great for those who are pretty sedentary or doing a very low level of movement, but backfires on those who are consistently training. Which is you!

Please know if you’re using it specifically for a health issue, you should work with a doctor and trainer to avoid injury.

3. Reduces the Risk of Runners Trots or other Stomach Issues

The last issue is one key reason that many runners skip fueling, which could improve their performance. Runner’s Diarrhea is understandably something we don’t want to deal with so many choose to forgo food to avoid it.

We talk a lot about this in our Sports Nutrition for Runners Course, but you can train your stomach!

Try a few of these tips to increase your pre-run meals:

• Start out by drinking a protein shake – then you are getting both hydration and some nutrition
• Use easy to digest foods like a banana or toast
• Slowly increase the amount over weeks and months, you are training your stomach

Drawbacks of Fasted Cardio

I’ve probably already pointed out a number of potential issues, but let’s dive in to this a little further.

Muscle Wasting

Doing long cardio sessions fasted could result in muscle wasting. In non-fasted endurance training, such as running, protein provides around 5 percent of the energy.

However, when you fast, the quantity of protein broken down in your muscles doubles.

Muscle tissue breakdown causes a drop in resting metabolic rate, a loss of strength, poor performance, and increases the risk of injury. Overall, this loss of muscle will then inhibit any weight loss goal.

Does Not Improve Performance

Even though fasted running is touted as a way of improving performance, there is little to no evidence of that. In fact, a recent study from 2021 actually shows that fasting nullifies any improvements.

In the study, the 10K times of runners who followed fasting were compared to those of individuals who did not.

Interestingly, even though the intermittent fasting group decreased body fat, the results remained the same at the end of the study.

The changes in body composition should, in theory, have resulted in faster 10K times.

However, low glucose availability, according to the researchers, nullified any increases in running economy brought about by changes in body composition for participants.

In short, fasted running won’t actually bring about any of the performance improvements you might have heard of.

Can Lead to Increased Cortisol Levels

Research shows that fasting is a big physiological stressor for the body.

Athletes who run on an empty stomach have increased cortisol levels, high tiredness, poor recovery, increased fat storage around the belly, and systemic inflammation.

These raised cortisol levels can lead to weight gain and overtraining.

Low Calorie Intake

While I know you might be reading this for weight loss, the results of having calories that are too low might start out as weight loss, but quickly turn in to other issues.

For women it results in the Female Athlete Triad – which is a loss of menstruation, increase in injuries, and hormonal imbalances.

For all athletes it’s a major cause of stress fractures. It slows healing and degrades performance.

Difficulty Performing at a High Level

Lack of quick fuel makes it harder to hit speed work goals or complete running HIIT workouts. Any sort of high-intensity runs whether in training or race day, require the body to have topped off glycogen stores and quickly available fuel.

The higher your heart rate goes the more your body relies on carbohydrates for fuel. This provides you with that quick energy needed to really push through.

Additionally, after a hard workout you have compromised recovery by not fueling. The body is already in a state of breakdown and that leads to each subsequent workout feeling harder and harder.

Should You Exercise on An Empty Stomach?

We’ve gone through the pro’s and con’s, but here’s a little more anecdotal evidence for you.

I did most of my weekday runs for years in Florida in a semi-fasted state. Meaning upon waking I would have a 1/2 scoop of protein powder and 1/4 scoop of pre-workout then hit the road.

For me that was about handling the heat, getting out quickly and I thought it was working out just fine…

But in fact, I lost muscle.

My low energy state raised my coritsol, which made every run feel worse and lead to decrease in my overall health and performance.

I’m not the only runner who had to learn this the hard way. Many of our athletes are blown away at the difference they feel in energy, ability to run longer or faster just by slowly increasing that pre-run fuel instead of doing fasted cardio.

Are Glycogen Depletion Runs Beneficial?

Occasionally, you’ll hear marathon runners talk about doing glycogen depletion runs with the goal of teaching the body to use more fat for fuel.

The more fat we can teach our bodies to use, the less we have to rely on tons of gels during race day. Again this is a huge part of why I do (and enjoy) low heart rate training. Having worked directly with Dr Bob Seebohar, we found I could take in far fewer gels on the run which made my stomach so much happier.

This is an advanced running technique and requires that you know how to do them very infrequently and under the right conditions.

BUT as noted, this is not an all the time choice.

• First thing in the morning to help alleviate hunger (and before you know what’s happening)
• This is not a no hydration run! Drink. Carry an electrolyte drink in your bottle.
• You must be training under your Low Heart Rate so the body is relying more on fat than carbs.
• Immediately post run you need to eat a high quality meal of protein and carbs to prevent further muscle wasting.
• If you start to feel faint or hit the wall, you need to being taking in fuel.
• Matt Fitzgerald recommends these only early in marathon training.
• Doing these too often will impair all of your workouts

As a certified running coach, I do not recommend these for our athletes.

However, we will often focus on utilizing a slightly lower carb in take around some long runs or testing out fueling with things like Muir or energy bars that are higher in fat.

🔊 So far the research seems to conclude that extended training in fasted states impairs performance and doesn’t show any clear signals of improving performance.

Where it can readily be shown that eating carbohydrates does improve the run. (side note: even better is eating a little fat with your carbohydrate slow down the use of carbs!)

Benefits of Eating Before Running

All right we’ve seen some good and bad for fasted running, what about if we choose to eat?

• Muscles well fueled for performance
• Can improve post workout recovery, which means you’re ready to go for the next run
• Allows for harder or longer workouts
• Increases total calorie burn of the workout
• Helps to prevent over training by stabilizing hormones

In other words, most of the time you’re going to benefit from eating prior to a hard workout or a long run.

If you’re doing a quick morning run during the week and you’ve been at this as many years as I have (yikes 20) then you probably won’t bonk or notice any difference if you skip the pb&j. But do your best to start making that fuel part of your routine.

We’re all an experiment of one, which means it’s great to know that there are potential benefits on both sides, but you have to decide what feels best for you.

IF YOU ARE FORCING YOURSELF NOT TO EAT TO LOSE WEIGHT please know that it’s not a long term solution.

If you can’t workout as hard or as long, you burn less total calories. Thus even if you “burned more fat than carbs” in the fasted state, you still burned less overall fat.

What Should You Eat Before a Workout?

If you’re now feeling a massive rush of relief that it’s ok to eat before your workout or you just want to fuel up with the options that are going to help you perform your best, this cheat sheet is for you.

Click here to download my cheat sheet

Looking for more runner nutrition?

• Running for weight loss guide
• 30 Day Runner Nutrition Course
• Sports Nutrition for endurance athletes – Who To Work With
• How many calories do runners need?

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