Fasted Running: Benefits & Risks of Fasted Cardio

Fasted running is the act of not eating within 10 to 14 hours of your exercise training. This fasted state may offer some benefits to athletes like improved performance and running economy, but there are some tradeoffs to consider.

In this article, we’ll dive into those considerations so you’re aware of all the benefits and risks involved with fasted runs.

So grab a snack and let’s dive in.

Fasted Running Benefits

If you are prone to rolling right out of bed, putting on your shoes, and hitting the ground running before the brain registers you are even awake, you may already be experiencing fasted workouts.

But you may not be aware of the potential benefits of these fasted runs, such as;

1. Increased Fat Loss: Because you have not yet taken in any carbohydrates for your body to use as fuel, your body is forced to burn fat.
2. Improved Aerobic Endurance: There are studies that tie fasted running to an improved V02 max, as well as other performance benefits
3. Fewer Stomach Issues: If you are one of the many runners who experience digestive issues, running on an empty stomach, you may want to consider testing fasted cardio as it may help reduce these stomach issues.
4. Efficiency: For those of you who are looking to just quickly get out the door, fasted cardio helps you to achieve that. Taking the time to eat something can slow your roll.

Megan Robinson, Board Certified Specialist Sports Dietitian  & Level 1 Certified RRCA Running Coach, explains the benefits of running fasted.

How Long Does It Take To Be Considered Fasted Cardio?

Although you might feel hungry if you run at 6:00 p.m. on an empty stomach, you are not likely experiencing fasted cardio though. You need to have not taken in any calories for somewhere from 10 – 14 hours to consider it fasted.

Using that logic, morning runs are the most obvious (and common) way to experience fasted cardio.

Does Fasted Running Burn More Fat?

Studies show that fasted cardio burns roughly 20% more fat than cardio completed after eating. However, they also found that there does not seem to be a correlation between the increased fat burning and actual weight loss. In addition, it does not appear to change the body composition of most athletes.

If your goal is to help your body to get fat adapted so that you do not need to take in as much fuel while workout out, this is an excellent reason to try fasted cardio. However, if you are hoping to shave inches off of your problem areas, fasted running will likely not have much impact.

The Downsides of Running on An Empty Stomach

Of course, there are some potential downsides to running on an empty stomach.

First, you need to expect that longer runs and more high-intensity workouts could cause be a struggle if you have not taken in any fuel.

If you have a difficult speed workout on your slate, you might want to take in some food beforehand. You might be short-changing yourself if you don’t. You want to put yourself in the best possible position to get a quality run-in, don’t you?

Running without food could cause you to get dizzy or lightheaded. This is especially true if you have certain medical conditions such as diabetes.

You could perceive your exertion to be harder if you have not eaten before a run. So if you are doing fasted cardio, you may feel like you are working way harder than you are. A small snack could have you powering through those workouts with a lower perceived effort.

Even though fasted cardio pushes your body to burn fat instead of food for fuel, there is no evidence that fasted cardio changes body composition in any way. In other words, if you are doing it strictly for weight loss, it may not be worth it.

How Long Can You Run On an Empty Stomach?

I am a firm believer that you should not run longer than 60 minutes on an empty stomach. Why? Because conventional wisdom says that you should start to think about fueling after 45 minutes to an hour of exercise.

If you will be running much longer than 60 minutes, you probably need to think about taking in some type of calories during the workout. Given that, it seems unwise to start off the run without any fuel in your body.

Having said that, I am certain that right now… someone is reading this, shaking their head and saying, “I run for 90 minutes fasted without any problems, and without taking in fuel or hydration.” 

Hey, if that works for you, good for you! I am basing my statements on 25 years of coaching experience and running norms, not anomalies.

Signs You Should Stop Fasted Running

If you are running fasted and feel different than you do when running after eating, you should really take note. Some particular things to look for include:

1. Lightheaded – Feeling lightheaded is a sign that you may not have enough fuel in the tank.
2. Dizziness – It’s one thing to feel light-headed, and yet another to be downright dizzy. Any benefits of fasted running are negated if you end up thumping to the ground!
3. Lethargy – Do you feel like you don’t have the energy to perform the workouts like you used to? Maybe this is your body telling you that you need fuel.
4. Declining Performance – Are your splits slower than they used to be? Maybe you are struggling to nail the times your coach assigned for that speed workout. These are both a sign that your body needs something.

Hydrating Properly During Fasted Runs

Hydration is always an important thing to consider, even more so if you are running fasted. Many people are unaware that approximately 20% of their water comes from food. It stands to reason that if you are not eating before running, you may end up dehydrated.

Be certain to drink some water before heading out the door to run, especially for fasted runners. Take in at least 8 ounces of water prior to starting your run to help clear your head and get the body primed for activity. For most short, early morning workouts, that should suffice.

In general, drinking more water will help you stick to your fast if you are really committed to doing so. Hydration helps your body feel full, longer.

Can I Do a Long Run Fasted?

For athletes acclimated to running on empty, like endurance athletes and long-distance runners, your body is likely capable of longer fasted runs. However, a recent meta-analysis shows that pre-exercise feeding enhanced prolonged aerobic exercise performance.

For this reason, you should really assess why you are challenging yourself with fasted long runs.

Most runners find they need some fuel during longer runs. If that has been your norm, going on a long run while fasted seems like a silly choice.

Also, consider what constitutes your long run. Are you talking 6 miles or 16? If your long run is 6 miles and you have run 4 or 5 without taking in any food, you can likely run 6 fasted miles.

However, once you get into double digits, you’re just asking for trouble and should weigh the cost/benefit of your choice.

Can I Do Speedwork Fasted?

Speedwork involves intentionally increasing your speed during your normal running pace. It’s intended to train your body to run harder which will improve your VO2 max, muscle fiber recruitment, and lactic threshold.

Little evidence suggests that speedwork, while fasted, poses any greater benefits or risks when compared to normal running regimens.

You may want to consider your speedwork within the frame of the total distance, which we allude to in the long run while fasted section above.

Is Fasting Good or Bad for Runners?

I guess that depends on what type of fasting you are referring to. I am a firm believer that people have to listen to their own bodies as they figure out how to fuel properly in order to support their personal goals. I have tried fasting and it does not go well for me.

However, I have friends who use fasting in different ways and for different purposes.

Guess what? It works out great for them! You have to analyze your goals for a particular day when determining if you need fuel prior to completing the task. In other words, it’s a gray area.

If you are looking to delve into the world of Keto and/or intermittent fasting, I encourage you to do some research and to recognize that fasted running could leave you feeling tired and sluggish in the beginning.

The process of waiting to get your body fat adapted takes time and patience.

As always, seek the guidance of your medical professional, dietitian, or running coach before attempting risking running practices.