Fasted Running: When You Decide to Run On Empty


Though it may sound like the latest workout fad, fasted running is actually something that endurance runners have been utilizing as a specific training tool for quite some time. The question is, is fasted running right for you or is it just something that sounds cool when you tell your friends you went for a run without eating any breakfast?

Here we’ll talk about what fasted running is and the purpose behind running on an empty stomach. Ultimately, you’ll have a better understanding of how to fit these into a regular running routine and if doing them is a good fit for you.

What does “fasted” mean?

“Fasted” is just a clever term used by those in the fitness world to describe a workout that you go into without any food in your stomach. And by food I mean you’re not allowed to have had any calories at all.

If you look up the term “fast” in the dictionary (and no we’re not talking about how you look on the track, Usain Bolt) it gives the following definition:

fast: to abstain from all or some kinds of food or drink

You’ve likely heard the term in a religious sense or perhaps your doctor has asked you to fast before morning blood work.

But here we’re talking about fasting before a workout, particularly before going for a run.

If a run is “fasted,” then you haven’t eaten since the night before – perhaps for 10 – 12 hours or more.

Image by skeeze from Pixabay

Fasted Running Vs. Fasted Cardio

You’ve probably heard friends say they’re doing “fasted cardio” or a “fasted workout.” Perhaps you know people who only do “fasted-state training” in which they never complete any type of workout with food in their stomach.

Fasted Running is simply one type of fasted cardio, since cardiovascular exercise is almost anything that bumps up your heart rate enough to make you sweat.

What is a fasted run?

For a run to be deemed as one that’s fasted, your insulin levels need to be low and your body won’t be able to rely on burning glycogen stores for fuel.

You’re completing a run without having eaten anything for at least 8 hours…usually up to 10 or more.

So, what might that look like?

Most people perform a fasted run by following a schedule similar to the one below:

1.) Eat dinner at your normal time – around 6 – 7 p.m.

2.) Avoid a bedtime snack and head to bed on time – let’s say 10 p.m.

3.) Wake up and throw on your workout clothes.

4.) Drink some water and coffee if that’s something you enjoy and then head out for your run. Plan to start your run within 12 hours of finishing your dinner from the night before – so in our case, by 7 a.m.

You can have coffee and water before a fasted long run. Image by JayMantri from Pixabay

What you CAN have before a fasted run:

  • Coffee: You lucked out, caffeine addicts. Since coffee is practically calorie-free, you’re welcome to a cup before starting your fasted run.
  • Water: Hydration is key! Read more on that here.

Related: Caffeine Before Running and How It Affects Your Performance

What you CAN’T HAVE before (or during) a fasted run:

  • No food of any kind
  • No sports drinks or gels
  • No gum

Anything with calories is OFF LIMITS as are “sugar-free” foods made with artificial sweeteners.

Though fasted runs usually occur in the morning (since it’s easiest to fast overnight) you could technically complete a fasted run at any time as long as your food has been completely digested over the course of several hours.

Now if you’re thinking that running on an empty stomach sounds both crazy and the opposite of enjoyable, I get it.

But fasted running can be beneficial, especially for endurance runners training for marathons or ultra marathons.

In order to compare a fasted run to a “normal” run, let’s start by discussing how you get energy during a “normal” run.

How You Get Energy During a “Normal” Run

Generally when we run, we get our energy in the form of glycogen stores.

As much as we all just love eating, its main purpose is to fuel our bodies so we can function as normal human beings day to day. And when we feed ourselves the proper amount of calories each day (in combination with other factors such as adequate sleep, etc.) we’re also able to complete the different types of exercises that we enjoy and keep us active. We rely on energy for those workouts.

What are glycogen stores?

Glycogen stores come from the carbohydrates we eat. When we eat carbs (grains, fruit, starchy vegetables, just to name a few) our bodies break them down into simple sugars, or glucose. This glucose is then converted into glycogen and stored in the muscles and liver. Sometimes carbs get a bad rap, but they are a macronutrient that should be eaten as part of a healthy diet. Most athletes depend on them to fuel their workouts.

How do glycogen stores help us get energy when we run?

As we start to run, or do another form of exercise, our body can burn any recent glucose (from your last meal) for fuel and also taps into its glycogen stores. Most people store around 2,000 calories of glycogen (give or take) which means there is plenty to pull from for your workout since you burn about 100 calories for every mile run.

As we eat our meals throughout the day, we keep a steady flow of glycogen. This makes it an excellent energy source that our bodies are used to using since we usually keep a steady flow…and since most people don’t run when they haven’t eaten for hours.

But in a fasted state, the process of the body obtaining fuel looks very different. So now we’ll look at how the body obtains energy in a fasted state.

How You Get Energy During a Fasted Run

Fasted running, like any type of fasted cardio, changes the way your body utilizes energy during the duration of the workout.

Since during a fasted run you haven’t eaten for up to 12 hours or more, your glycogen stores have been depleted. If they’re not completely gone yet, they’re close. And you definitely don’t have any recent energy burst through glucose since you skipped breakfast.

But even without eating, you’re still able to make it through a run. How is that possible?

Glycogen isn’t the only way for our bodies to get energy. We can burn fat for fuel, too.

The key here is that almost everyone has plenty of fat to utilize as energy.

Even the fittest athletes with a low percentage of body fat have enough stored that they can use to fuel a workout. VeryWell Fit notes that “converting stored body fat into energy takes a great deal of oxygen, which requires decreased exercise intensity.”

Teaching our bodies to efficiently burn fat for fuel is essentially the purpose of fasted running.

Your run will need to be slower initially when running in a fasted state. This is because fat is broken down with the help of oxygen, and running at top speed won’t allow you the oxygen you need to make this happen. As your body adapts to using fat as an energy source, you’ll be able to gradually increase speed for your fasted runs (though running without fasting for speed work or races is not ideal).

Why You Would Want to Complete a Fasted Run

Watch my YouTube video to find out when to try fasted running and when to avoid it.

I know you’re wondering: “What is the purpose of a fasted run?”

Like, if there was no benefit from running on empty, you probably wouldn’t do it, right? It’s not like hunger is an enjoyable feeling.

So, let’s talk about some of the benefits of running while fasting.

Fasted Long Runs Can Help Endurance Runners

When we’re talking endurance running, that’s referring to anyone training for a race at the marathon distance or more.

So why are fasted long runs good training for endurance runners?

Fasted runs in training can help marathon and ultra marathon runners’ race performance by allowing them to more efficiently burn fat for fuel once their glycogen stores have been depleted.

To understand the benefit that fasted running provides, let’s look at what happens when you run a race 26.2 miles or longer.

Remember how we discussed that people, on average, store about 2,000 calories worth of glycogen in their liver and muscles? What that means is that you have plenty of glycogen fuel for shorter races – even a half marathon. But because we burn about 100 calories for every mile run, that means you’ll have used up all your glycogen stores by mile 20. Yes, even when you’re taking gels or Gatorade all the way til the end of your marathon.

And because of that, those last 6 miles can look pretty grim for many people. Once those stores are used up, your body feels sluggish and almost like you’re having to suddenly pull dead weight. Now, there are different ways to avoid what we call hitting the wall, which I discuss in depth here, and one of those is training with fasted long runs.

Fasted long runs allow you to train your body to efficiently burn fat for fuel. Everyone will start burning fat for fuel if that’s all that’s available, but for someone who’s had practice running on an empty stomach it will come as much less of a shock.

Your body will say, “Hey. I’ve been here before and I know what to do.” You’ll be able to finish your long run or marathon strong and avoid what we call “bonking.”

Fasted Running Can Help Those With Stomach Upset

I know quite a few runners who really struggle with what to eat before they go running. They’ve tried eating less, eating more as well as various combinations of different types of foods. Though this isn’t something I’ve experienced in a very long time, when I was quite a bit younger I did have this issue.

One easy way to avoid stomach upset during a run (that is food-related, because there are other causes such as taking Advil before a run) is to do a fasted run.

Though it may sound a bit daunting at first, your body will adapt. That and you won’t feel like you’re going to toss your cookies the entire time. That’s for sure a win.

Fasted Running May Jumpstart Weight Loss (But so can running without fasting)

Everyone knows that ultimately the way to lose weight follows the process of calories in, calories out. So easy to understand, but a lot more difficult to achieve when food temptation is all around us.

Which means that fasted running isn’t going to necessarily catapult you into weight loss. If you start doing fasted runs everyday but then continue to consume the same amount of calories you always have, then it’s unlikely you’ll lose any weight.

There seems to be a consensus that neither fasted aerobic exercise nor non-fasted aerobic exercise are better than another for losing weight. The Journal of the International Society of Sports Nutrition writes,

“These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.”

In this study, all subjects maintained a caloric deficit, so both groups lost a significant amount of weight either way.

However, by eating later in the day after you run, it’s likely you will consume less calories which can help jump start your weight loss. In the end, weight loss is much more about being consistent with proper diet and exercise. Just doing fasted runs isn’t going to get you there.

We want to caution you to always, ALWAYS fuel the demands of your training…even if you’re trying to lose weight. Once your fasted run is over, you need to eat!

Photo by Nathan Cowley from Pexels

Executing the Fasted Run

So you’ve decided you want to try fasted running. You know to not have eaten for at least 8 hours, preferably more, so now what?

Here we’ll tell you how long your run should be, the speed with which to perform it, and how often you should do them.

Length of Mileage/Time for a Fasted Run

How long can you run fasted?

Fasted runs can be 1-mile or they could be 20 or more. It honestly depends on how fit of a runner you are and how fat adapted you already might be. Most people will want to keep fasted runs under an hour.

But, if someone can already easily run 16 miles while eating breakfast beforehand and eating gels during, there shouldn’t be any reason why they couldn’t run one while fasting.

On the flip side, if you’re running a certain amount of mileage for the very first time, don’t do it fasted. It should already be a distance you’re comfortable doing with food in your stomach.

Speed for a Fasted Run

What pace should I run during a fasted run?

Fasted runs are best completed at your easy pace. Easy pace is “conversation pace” and you can read more about those paces here.

It will be more difficult for you to pick up the pace while fasting. While implementing some speed or quality work during a fasted running once in awhile won’t hurt you, doing so won’t maximize the benefit you’re looking to get from a speed workout because you just won’t fun as fast as you would if you had more energy (think glycogen) to burn.

Instead, fasted long runs are perfect for a recovery or medium long run.

Related: The Importance of Recovery Runs in Marathon Training

Frequency of Fasted Runs

How often should I do fasted runs?

Just as you should be careful to weave speed training or hill training into your weekly workouts, fasted runs are the same. Doing them once or twice per week when you have an easy paced run in your schedule is enough to get the benefit of becoming a fat-adapted athlete without overdoing it.

Experienced endurance runners can do long run fasted every 2-3 weeks (also at an easy pace). This will help you build endurance while simultaneously helping your body learn to burn fat for fuel. Not every long run should be fasted, however.

How You’ll Feel During a Fasted Run

Running on a completely empty stomach doesn’t sound fun at first – even when you know it’s going to be an easy paced run. And when you’re already feeling hungry before you’ve stepped foot out the door, it can be really hard to muster the energy (the little that you have) to get running.

So should you plan to feel when you’re running while fasting?

You may start with hunger pangs but they will likely subside.

In my experience, and what I’ve found in talking to other endurance runners, is that the hunger feeling goes away very quickly.

Here’s a story of my first fasted run:

My eyes about popped out of my head when I saw a “fasted long run” on my schedule. 14 miles with no calories to get me through made me nervous.

I was really skeptical that I could do it, but me experienced teammates assured me I’d be fine. I threw a gel in my VaporHowe, just in case I needed emergency fuel, and headed out.

(If you’re new to fasted running – always bring an emergency gel. And anyone doing a fasted long run, even experienced runners, should have one on hand, too…just in case. Always better to be safe than sorry.)

I felt hungry almost the entire first mile and didn’t think that was a good sign at all. And then it completely went away and I was fine for the next 13 miles.

Once you have some fasted runs under your belt, you probably won’t even notice any hunger.

I do many short easy runs without any food in my stomach. I’ve gotten used to doing them, so if I head out right when I wake up, I don’t bother with breakfast. Since my body is used to it, I don’t feel hungry at all.

Intermittent Fasting and Running

Though I don’t personally intermittent fast, perhaps you are interested in changing up the way you eat by utilizing intermittent fasting.

And many people wonder if they can run while fasting.

The answer is yes, you can definitely continue to incorporate running into your schedule if you are starting an intermittent fasting way of eating. Long distance running while fasting is also doable. However, if you will be completing all of your running in a fasted state, we always recommend talking to your doctor before making any drastic change like this.

Healthline gives a great overview of Intermittent Fasting and its benefits. Including fasted running is a great way to boost the physical benefits your body will receive as well as teaching your body to use fat for fuel. But that doesn’t mean it’s for everyone. Just plan to follow the same advice in this article for fasted runs if you are intermittent fasting.

If you are interested in learning more about intermittent fasting, I highly recommend reading The Obesity Code: Unlocking the Secrets of Weight Loss by Jason Fung. The foreword was also written by ultramarathoner and sports scientist Tim Noakes.

Risks of Fasted Running

You should never feel pressured to complete fasted runs if you don’t want to. You can still be an amazing runner without doing them and they aren’t for everyone!

Hunger is certainly a side effect that people don’t want to experience for any extended period of time and that’s completely valid…even if you know you’ll be fine.

But let’s talk about the risks associated with fasted running.

Possible Hormone Imbalances for Women: Training that includes many fasted runs pose a potential risk for women, according to Trail Runner. They write that fasted running can raise cortisol levels which in turn can cause hormone imbalances leading to: fatigue, bone stress injuries, and weight gain.

Feeling light-headed or dizzy during or after a run: Fluctuating blood sugar levels due to fasting may cause you to feel faint during a run. Taking a gel with you on a run just in case is a good idea. That way if you feel like you’re just not able to continue running on an empty stomach, you can pop that gel for some fast fuel.

Issues with Food Restriction: If you already have a difficult relationship with food, fasted running is likely not for you as it requires you to restrict food. It’s important to make sure that you are always fueling your body in a healthy manner and meeting its needs, while also continuing to treat yourself!

We always recommend speaking to your doctor first about beginning any fasted running especially if you’ll be doing fasted long runs or more often than once or twice per week.

What to Eat AFTER a Fasted Run

No doubt you will be ready to eat all the things when you return home from your fasted run. Does that mean you should stuff your face with that leftover birthday cake as a reward for completing a run on an empty stomach? Probably not.

But your body will be craving something well-rounded at this point, so plan to give it what it needs. You’ll want to put together a meal that balances protein and carbohydrates that you eat within 30 minutes of finishing your workout. This timing allows you to get necessary nutrients to your body – especially important when your run was completed while fasting.

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So why should you eat protein and carbs after a fasted long run?

  • Protein is to repair overworked muscles. Since muscles is broken down during a run, protein intake within 30-minutes builds it back up to start the recovery process.
  • Carbohydrates are generally eaten after a run to replenish the lost glycogen stores that were used up during exercise. But in the case of fasted running, you’ll be eating carbs to build up glycogen stores that were already depleted before you even starting your run.

There’s no need to overeat just because you haven’t eaten in many hours. Aim for a few hundred calories and then you can eat again when that’s digested and you’re hungry again.

Here are some examples of how to get in your protein and carbs:

Protein: Eggs, chicken, Greek yogurt, eggs, protein shake or bar

Carb: rice, sweet potato, whole-grain bagel or toast, fruit

One of the most popular post-run recovery drinks for runners is chocolate milk. Because of its ideal ratio of carbs to protein, as we discuss in “Chocolate Milk Does a Runner’s Body Good,” it’s a great choice to have after a fasted run. However, it has a lot more sugar than some may prefer, so keep that in mind.

My go-to breakfast after a fasted long run? Toasted Dave’s Killer Bagel with Avocado (don’t forget the Everything Bagel seasoning) and Scrambled Eggs (and water, of course). And if I’m in a hurry, it’s always an RX Bar.

Jane

Hi, I'm Jane! I'm an avid runner who races 5ks to marathons. After a 4:59 first marathon, I came back to the distance years later running a BQ time of 3:36. I did a lot wrong for a long time and finally started doing a lot right. Now I'm an RRCA certified running coach and love sharing what I've learned to help others run their best.

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