Fasted Running: How To, Benefits, + Drawbacks

If you are one of the many runners who heads out the door first thing in the morning to get your miles in, you have faced the ever-popular question of whether you should try to eat something first or run on an empty stomach.

Fasted running is popular for two different reasons. In one camp, you have runners who logistically don’t feel they have time to eat before running in the morning or find that doing so causes cramps because there’s not enough time to adequately digest the food.

There are also runners who deliberately choose to run on an empty stomach as a means of reaping purported benefits of fasted running, such as enhanced fat loss.

But is fasted running actually beneficial? More importantly, is fasted running safe? Is it better to run after eating, or are there weight loss benefits to running on an empty stomach?

In this guide, we’re going to look at:

  • What Is Fasted Running?
  • How Do You Do Fasted Running?
  • The Benefits of Fasted Running
  • The Drawbacks of Fasted Running
  • Should You Run On An Empty Stomach?

Let’s get started!

An empty plate with silverwear.

What Is Fasted Running?

Fasted running simply invokes running on an empty stomach, meaning that your run occurs after an extended period of time without food. 

Most people do fasted running first thing in the morning by running before eating breakfast or without having any sort of pre-run snack

Because of the overnight fast while you sleep, this may mean that you’re running anywhere from 7-16 or more hours since the last meal or snack you consumed.

Although fasted running typically occurs in the morning, the other scenario that can lead to fasted running is when runners who practice various intermittent fasting diets go running later in the day on days with time-restricted eating or on alternate-day fasting diets.

There aren’t really definitive guidelines for the amount of time it has to have been since you ate for your workout to be considered “fasted running.” 

However, depending on the volume and caloric intake of your last meal, if it’s been at least 4 hours since a snack or 6 or more hours since a larger meal, your body is essentially doing fasted exercise

A person running in the morning.

How Do You Do Fasted Running?

Fasted running is as simple as waiting to go running for at least 4-6 hours or more since you last ate anything, but just because it’s a simple protocol doesn’t necessarily mean it’s easy to go for a fasted run.

You might feel tired, hungry, and sluggish, depending on your individual metabolism, how long it’s been since you’ve eaten, and what your body is accustomed to.

For most runners, the easiest way to try fasted running is to go running first thing in the morning after you wake up and before you eat anything. 

Start with a short, easy workout rather than a long run or speed workout.

For example, if you normally eat a banana or piece of toast when you wake up and then run for 45 minutes 30 minutes later, try skipping the snack and reducing your run to 20-30 minutes. 

See how your body feels and responds to running on an empty stomach before trying a more intense workout.

A person running in the morning in the forrest.

What Are the Benefits of Fasted Running?

There are several potential benefits of fasted running. The benefits of running on an empty stomach include the following:

#1: Fasted Running May Increase Fat Burning

Fasted running has been shown to increase the relative percentage of fat oxidation, which means that a greater percentage of the calories that you are burning while you run is coming from stored body fat rather than from stored muscle glycogen.

Your muscles need energy to do the work involved in moving your body for any type of physical activity or exercise.

This energy (ATP) is created by burning fuel that your body has stored from the nutrition you’ve taken in through your diet.

Excess carbohydrates that you eat are converted into glycogen, which is then stored in the skeletal muscles and liver.

Dietary fat and excess sugars when your glycogen stores are maxed out are stored as triglycerides in adipose tissue (body fat), while protein forms the structural muscle fibers.

The body has limited glycogen stores in the liver and skeletal muscles, and these levels deplete overnight during your fast. 

According to the Academy of Nutrition and Dietetics, an endurance-trained athlete can store up to 1,800 to 2,000 calories of fuel as glycogen in the muscles and liver, though smaller runners might store closer to 1,500 calories or so.

Therefore, when you run without eating beforehand, your body tries to conserve the limited glycogen remaining by trying to burn more fat for energy instead.

Some studies suggest that you may burn up to 20% more fat when exercising on an empty stomach.

However, it’s important to make the distinction that you are not burning more calories overall during fasted running; rather, the source of those calories shifts to include a greater reliance on fat rather than carbohydrates.

A person running down the road.

#2: Fasted Running Can Reduce Digestive Distress

Runners with sensitive stomachs often find that running on an empty stomach prevents cramping, side stitches, gas, runner’s trots, and bloating. 

Studies have found that nausea during hard workouts is more likely to occur if you are running or exercising on a full stomach.

When you exercise, blood is diverted away from the digestive tract to meet the increased oxygen needs of the working muscles.

As a result, digestion essentially ceases, meaning that anything sitting around in your stomach or intestines will do just that—sit around.

This can irritate your gut and cause nausea, bloating, and gas, all of which can signal colonic contractions that lead to the sudden need to defecate mid-run.

For runners with sensitive stomachs, even small snacks can cause stomach trouble. In these cases, fasted running may be a better option.

A close-up of running shoes of someone running.

#3: Fasted Running May Lead to Greater Weight Loss

Although running on an empty stomach doesn’t burn more calories than running after eating, some studies have shown that people who exercise on an empty stomach end up consuming fewer calories throughout the day. 

It’s possible that fasted running has a more significant effect on decreasing appetite by suppressing the hunger hormone ghrelin compared to the hormonal response to running after eating.

Therefore, while fasted running doesn’t directly lead to more weight loss because it doesn’t actually burn more calories than running in the fed state, it is conceivable that fasted running could accelerate your weight loss results by influencing the number of calories you eat.

Weight loss is largely dependent on the relationship between the number of calories you are consuming and the number of calories you are expanding every day, so if fasted running causes you to hold back on some of the calories you would otherwise have eaten, it’s a potential means to losing more weight.

With that said, different runners have different appetite responses to fasted running.

Some runners find that they have a tremendous amount of rebound hunger, meaning that later on in the day, their appetite spikes in response to the significant caloric deficit generated in the morning by running without eating first.

Someone running in the fall.

#4: Fasted Running May Improve Blood Sugar Regulation

One of the primary concerns that runners often have before trying fasted running is becoming hypoglycemic, which means your blood sugar is too low.

This can result in fatigue, dizziness, headaches, irritability, nausea, and performance impairments.

Surprisingly, most studies show that fasted exercise does not cause detrimental decreases in blood sugar, even for athletes with diabetes. Some studies have even shown that exercising in the fasted state can improve insulin sensitivity and blood sugar control. 

Despite these findings, if you have diabetes, you should consult your healthcare provider before trying fasted running.

#5: Fasted Running Is Faster

From a logistical standpoint, fasted running is sometimes the easiest option, if not the only option, for early-morning runners who just have to get out the door quickly if they want to get their full workout in.

Instead of having to wake up significantly earlier and lose more precious sleep, many runners who otherwise don’t have time to eat something and then wait for it to digest before going running prefer the fasted running route.

As previously addressed, if you don’t wait long enough after eating to go running, you can end up with a bevy of digestive symptoms that will all negatively impact your workout.

Someone running across a bridge in the winter.

The Drawbacks of Fasted Running

Although there are some possible pros of fasted running, there are also potential drawbacks of running on an empty stomach, including the following:

#1: Fasted Running Does Not Increase Fat Loss

Many people are motivated to engage in fasted running under the belief that doing so will lead to greater fat loss or favorable changes in their body composition

According to research, fat loss and body composition changes were the same whether cardio exercise is performed in the fed or fasted state. 

Another review that combed through the findings of five studies found that performing fasted cardio in the morning after an overnight fast had no significant effect on weight loss or changes in body fat percentage in either direction.

Therefore, it’s important to have appropriate expectations and be informed that fasted running increasing fat loss seems to be more of a common misconception than a reality in practice.

A person stretching their quad.

#2: Fasted Running Can Cause Muscle Loss

As mentioned, exercise in the fasted state leaves your body in a position of glycogen depletion or limited glycogen availability.

To enhance survival, your muscles are metabolically flexible in that they can turn to the other two fuel sources—fats and proteins—to make up the difference in energy needs.

And they do.

While we discussed the increased fat oxygenation to be a benefit of fast running, it’s also imperative that we cover the other side of the coin: fasted running can increase the percentage of calories generated by burning muscle protein.

Although the study involved cycling rather than running specifically, there is evidence to suggest that fasted cardio exercise can increase the contribution of protein for energy, meaning that your body burns more protein for fuel when you do fasted running.

This, in turn, can compromise gains in muscle strength and size, as it directly catabolizes your muscles.

Consistent fasted running can therefore lead to muscle loss.

Loss of muscle mass is detrimental to strength, athletic performance, health, and metabolic rate.

A person running across a bridge.

#3: Fasted Running Can Increase Cortisol Levels

Cortisol is one of the primary stress hormones in the body, and chronically elevated cortisol levels have been associated with triggering the body to store more fat, especially in the abdominal area.

Any type of exercise is a stressor for the body, as is hunger or prolonged fasting. 

Therefore, the combination of the two by running without eating beforehand can significantly increase cortisol levels, according to studies.

#4: Fasted Running May Cause Hormonal Imbalances

In addition to upregulating the hormone cortisol, there is also evidence to suggest that exercising in the fasted state can cause additional hormonal imbalances. 

Your hormonal milieu can influence your injury risk, recovery from workouts, and the gains or adaptations your body is able to make from your workout.

Therefore, the potential consequence of dysregulating your hormones by running on an empty stomach should not be taken lightly.

A person running on a track.

#5: Fasted Running Can Reduce Your Performance

For most runners, the most immediate concern with fasted running is that most research shows that athletic performance suffers when you exercise without consuming adequate fuel beforehand.

Strength, speed, and intensity levels tend to be significantly better when exercise is performed in a fed state, particularly when adequate carbohydrates are available.

Your tolerance for running longer may also be impaired in the fasted state.

A large review that tabulated the findings of 46 studies found that when people ate before an aerobic workout, such as running, they were able to exercise for longer periods of time. In this way, fasted running can make it less comfortable or unworkable to run for longer distances or durations.

The rate of perceived exertion also tends to be higher with fasted running, meaning the workout feels harder than it actually is. 

The Pros and Cons of Fasted Running: Should You Run On An Empty Stomach?

When you weigh the pros and cons of fast running, most runners find that it’s not worth the time savings to run in a depleted state.

This is particularly true if you are concerned about your performance, improving your times, and getting stronger and faster.

On the other hand, if you suffer from a lot of digestive issues or don’t find that you feel weaker or more tired running without eating, it can be a workable and safe option.

For some pre-run breakfast options, check out our guide to what to eat before running in the morning.

Fasted Running: How To, Benefits, + Drawbacks 1
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Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, and contributes to several fitness, health, and running websites and publications. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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