title pic for fasted runs

Fasted Runs – Why You Shouldn’t Run on Empty

I have always been an early morning runner. When it’s still dark outside, and only the sounds of a lonely blackbird and the whistling of tree branches disrupt the eerie silence, I lace up my running shoes.

Coffee has been my fuel of choice for the longest time. What’s been missing? Calories.

I ran fasted all the time over 15 years ago – before it was trendy or such a hotly debated topic as it is now.

However, as I progressed through my running journey, I realized that constantly running without any calories at all would not be a sustainable approach if I wanted to race marathons and ultramarathons.

But what about fasted training runs? What about running on empty now and then?

This article will shed some light on whether fasted runs are an advisable strategy if you want to improve as a runner.

But before we dive into some science, let’s first define what a fasted run is.

What Is Fasted Running?

Fasted running means you haven’t eaten anything for several hours before starting your run. Usually, you had your last meal 8-14 hours before lacing up your shoes. You also don’t consume any calories during your run.

That could mean eating an early dinner and running without fuel in the morning or eating breakfast and then going without any calories until your evening run.

If you are trying to become fat-adapted and opt for a butter coffee, you might be training in a glycogen depleted state. However, you are not truly running fasted.

What Are the Supposed Benefits of Fasted Running?

Many runners who want to lose weight think that running without fuel will help them shed pounds. Research has shown that running fasted can make your body more efficient at utilizing fats for energy and tap into fat stores quicker.

Many runners that choose to eat low carb and ketogenic diets also seem to be fans of running without fuel. This is just my subjective feeling from being active in all sorts of running communities, though. I don’t have any hard data to back up this claim.

However, I don’t think being ketogenic is the defining factor in your ability to run in a fasted state.

When I started my running journey in 2006, I ate a vegan diet and still did all of my runs in the morning. Without breakfast and any fuel during my runs. I also raced half-marathons on water only, without having breakfast. 

Many runners also don’t like the feeling of running with food in their stomachs. Or worse, they experience severe GI distress when trying to eat shortly before or during their runs. For those people running fasted means being able to run without needing the porta-potty every 20 minutes or so😆.

However, an alternative to simply not fueling your runs could be to experiment more and learn how to train your gut for long-distance running.

Besides, just because you can run fasted doesn’t mean you should do it. Why? Well, let’s look at the potential adverse effects of running fasted.

What Are the Potential Negative Effects of Fasted Running?

The most apparent drawback of running fasted is that you might feel like crap. Dizzyness and feeling weak or shaky are side effects you could experience.

When you avoid carbohydrates before and during your run, you risk that your glycogen levels run critically low. You know this as “bonking” or “hitting the wall.”

You will also very likely not be able to run at higher intensities.


Because the higher the exercise intensity, the higher the percentage of carbohydrates your body needs to fuel the exercise. That’s why I could never run as fast when on a ketogenic diet compared to eating a higher-carb diet.

Another significant negative effect of fasted running is that it can lead to poor recovery. Again my own experience and science back up this claim.

Contrary to what others have reported, my recovery on very low carb diets is worse than on higher carb diets.

While I tend to experience less joint pain when I eat more fats, my muscles are sorer and need longer to recover. I also need more time to recover performance-wise.

What You Should Consider if You Still Want to Run Fasted

As you might have guessed by now, training fasted all the time is not the best strategy if you want to develop your potential as a runner.

If you are still debating if and how to include running without fueling into your training, ask yourself the following question.

Does the hypothetical minor benefit of being a better butter burner outweigh the negative effects of having less power during your runs, less endurance, and potentially increased risk for injuries?

If you still think fasted runs are a good idea, consider that a 2021 meta-analysis of 407 studies on the subject showed that “[…] periodized CHO restriction does not per se enhance performance in endurance-trained athletes.“

That is not to say you should never run without eating first.

If you keep your runs short and easy, are healthy, and not prone to injuries, there’s probably no harm done. If I plan on lightly jogging for 30 minutes in the morning, I typically don’t bother taking anything before or during the run. In all other instances, however, some carbohydrate source is on board.

However, if you run fasted, make sure to refuel as soon as possible after finishing your run. Ideally, opt for a carbohydrate-rich snack along with some protein.

What to Do if You Are an Early Morning Runner?

Whether you head out in the wee hours of the day because you love it or because your hectic schedule leaves only the mornings for a run, you will need to plan your fueling.

While it’s convenient to down a coffee while getting dressed and head out the door, it’s not optimal.

If, like me, you can not fathom eating anything that early in the day, then have a couple of teaspoons of honey before heading out and during the run. It’s a perfect running fuel.

Alternatively, stash some gels into your pack and use them until you are ready for more solid options 😃.

Let's Keep In Touch!

I never spam! Read my privacy policy for more info.

Scroll to Top