Fasted Running – Will Running Fasted Make you a Faster Runner?

Have you ever got out of bed and went straight for a run before enjoying breakfast?  If so, then you have more than likely engaged in fasted running. 

We all know (or at least I hope we do) that food is fuel for your run. Are there occasions where the fueling can or should be skipped prior to heading out the door?

Fasting seems to be all the rage currently as intermittent fasting has become one of the newest dieting crazes.  There are promises of quick weight loss, and potential running performance enhancement.  Which leaves us to question are these claims true?

This article will explain what fasting is, what roles it can play in running performance, and the safety of running fasted.

Ready, set, let’s go!

Fasted running – What is it?

Let’s start at the basics, although there are many variations of what fasting means, at its core fasting means going a certain amount of time between eating.  

Experts define fasting in endurance athletes (i.e. runners) to be 6-8 hours after a meal.  This definition takes into account the time it takes for glucose homeostasis to be achieved, and the effects of the fasting to occur.

Why run fasted?

First let’s talk about the science stuff. For many years, experts have recommended running fasted for endurance athletes as a way to increase fat oxidation.   Eating carbs prior to your run (aka NOT fasting) will raise your insulin, which in turn will suppress fat oxidation.  Which leads to the idea of fasting having the opposite effect, or stimulating fat oxidation.

The idea behind this was that in a fasted state fat being would be used for energy instead of carbohydrates, which can train your body to use fat as fuel.

This lead to the glamorous thought that fat would be burned at a higher rate, leading to a more lean body.  Which sounds great, right? Although this may sound like it would work, at the end of the day during your run you are still burning the same amount of calories. The source of these calories does not really matter. Just because fat may be used as the main energy source as opposed to carbohydrates, that does not mean fat on your body will be burned or reduced.

It is also important to point out that the body does not automatically move to using fat as an alternative source of fuel. Evidence has shown it may use additional protein, which can lead to muscle breakdown.

Is it safe to run fasted?

Typically, running in a fasted state is safe.  If you are in a time crunch and have to head out first thing for a  quick run prior to breakfast, there is no real problem with this. Your body will adjust, and if the run is short you will more than likely feel just fine.

If you are going out for a long run of over 60 minutes, be sure to eat some carbs prior to heading out the door.  Your body needs additional energy when spending prolonged time running.

It is important to take into consideration how much energy your body needs to sustain your long runs or workouts.  If you run without eating enough, this can lead to you feeling tired and sluggish.

Feeling tired and sluggish during a run or workout has a good chance of decreasing your performance during that run.  Decreased performance during your training may lead to a suboptimal race day outcome.

Any other potential problems?

Running fasted may lead to training in an under fueled state.  It is easy to underestimate how much you need to eat as a runner, and if you skip the first meal of the day it may be hard to make up for it later. So, if you do run after a fast, be sure to still eat an adequate amount of calories.

Also, regularly skipping meals may be a sign or symptom of disordered eating habits.  Unfortunately, disordered eating is prevalent in the running world.  Disordered eating can display itself in many different ways, and it can lead to some adverse and career ending outcomes for runners.

Runners with disordered eating habits are at a much higher risk for injuries, especially bone injuries.  They also are at a higher risk for developing RED-S or relative energy deficiency in sport.

Even something that may seem minimal like skipping breakfast prior to a run, may be a red flag regarding disordered eating patterns in runners. If you are engaging in fasting, it is important to think about why you are doing this, and how it may be affecting your health.

(If you or someone you train with are experiencing symptoms of disordered eating, please reach out to a doctor or registered dietitian to get help.)

So does fasted running improve performance?

The research does not support the idea that running in a fasted state will improve performance.  There is evidence that running in a fasted state will increase fat oxidation, BUT there is limited evidence that fat oxidation will actually improve performance.

Essentially, the body may use fat as fuel if there is no readily available carbohydrate sources, but the evidence is not that to show that this improves running performance.

So for now, from the available research, running in a fasted state does not appear to improve performance.

Final thoughts

Fasted running is a concept that has been around for years.  It may be getting more attention now from the general public as intermittent fasting is becoming more and more popular.

Although there is very limited evidence that supports the use of fasted running, athletes are still engaging in this. 

Everyone is looking for a quick fix to get faster, or lose weight fast.  Unfortunately, it does not seem that fasted running is that quick fix.  

The best way to get faster is by consistent training, allowing your body to rest, and good nutrition.  The main pillar of good nutrition in running is adequate caloric intake to support your exercise.

Fasting may get in the way of good nutrition.  So it may be good to just stick to what you know, and continue to learn more valuable nutrition information from a trusted source such as a registered dietitian (like me!).

Break the fast and eat some fuel prior to your run! As always, happy running!

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