Skip to Content

Fasted Running: The Benefits & Risks of Training on Empty

Fasted running seems to have gotten a lot of attention over the past few years, and yet there still seems to be no conclusive answer as to whether or not it’s a good training strategy.

With some coaches and athletes saying that fasted jogging or running can help increase fat burning and efficiency, others say that it can actually lead to a loss of muscle mass. Let’s take a look at what running in a fasted state actually is, and dive into any potential benefits or dangers.

What is fasted running?

When most athletes mention fasted running, they usually refer to running after not eating recently beforehand. Fasted running in the morning is more common, as a lot of runners wake up early and find that they don’t have time to eat before heading out the door.

Technically, the scientific literature defines “fasting” as not eating for at least 10 – 14 hours. However, most runners would classify their training as a “fasted run” if they hadn’t eaten for just an hour or two prior.

The idea behind fasted jogging and running is that once your body is in a fasted state, it will look for alternate sources of fuel during a run – i.e., fat. Many runners attempt fasted running in hopes that it will burn more fat than a typical, fueled run.

Is fasted running in the morning safe?

A great deal of runners also find themselves running in a fasted state unintentionally, or purely out of convenience. Fasted running in the morning is much more common than later in the day, as early morning runners typically don’t have time to eat and then wait for digestion before heading out the door.

There is a bit of a discrepancy over whether or not fasted running is actually beneficial – or even safe – for that matter. However, the general consensus seems to be that fasted running in the morning is okay as long as you are completing easy runs that are less than an hour in length.

If you have time, some coaches suggest eating a quick, small snack immediately upon waking to top off your blood sugars after fasting all night. This snack is easy to digest on the run, and will boost glucose just enough to keep your body fueled. Some simple snack ideas are:

  • Banana
  • Instant oatmeal
  • Sports bar
  • Figs
  • Applesauce
  • White bread or rice
  • Potatoes
  • Dates
Fasted running has some promising benefits, but also quite a few risks. Here is how to safely incorporate it into your training.

What are the benefits of fasted running?

There’s a reason fasted running has taken off in popularity over the past few years – it is rumored to have some major benefits. And while a few have been proven, most are more of a speculation than anything. However, here are some of the more promising potential benefits of fasted jogging and running.

  • Percentage of fat burned is slightly higher
  • Teaches body to burn fat as opposed to glycogen over time
  • Burning fat prolongs the immediate risk of bonking
  • Molecular changes from burning fat could potentially lower cholesterol

However, although the potential benefits might sound really promising for someone looking to lose weight, there are quite a few dangers to consider.

What are the dangers of running in a fasted state?

Running in a fasted state actually poses a few surprising risks. While the potential for weight loss or fat burning might seem promising, it might actually be worth considering whether or not it’s worth the risk. A few scientific studies have shown some worrisome outcomes from continued fasted running.

  • Protein breakdown in muscles doubles
  • Potential reduction in muscle mass
  • Poorer performance and lower intensities
  • Lengthened recovery periods
  • Lower immune function over time
  • Potential to disrupt the menstrual cycle

Since there is not much conclusive research demonstrating a strong positive or negative outcome, the decision of whether or not to partake in fasted running is up to the individual runner. Many runners wind up running in a fasted state in the morning, or later in the day due to unforeseen circumstances.

If you do find yourself needing to complete a fasted run, the current guidelines are showing that it’s okay, as long as you do so during an easier, shorter run. Hard workouts and long runs should be saved for days when proper pre-run fueling can be completed.

Will fasted running improve performance?

Unfortunately, there is no solid evidence one way or another. Studies have demonstrated that a slightly higher percentage of fat is burned during fasted running. The use of fat for fuel may prevent immediate bonking, but is likely to actually increase the risk of bonking later on during the run.

If the run is short and slow, fasted running might help prevent bonking. However, it seems that there might be no long term benefits for running performance – and it might actually harm your performance in the end.

How long should a fasted run be?

Many experts and coaches recommend that fasted jogging and running be kept to under an hour. The shorter the run when fasted – the better.

How much fasted running should you do?

The answer ultimately lies with each individual runner. If you find that fasted running in the morning is much easier and more convenient than waking up at the crack of dawn to down a breakfast – do what works for you.

However, it’s best to save long runs and harder workouts for times when you are able to fuel beforehand. Grabbing even a quick, simple snack immediately before a run could be enough to top off your blood sugars and improve the benefits from your run.

Nearly every situation is different – just as every runner is different. Ultimately, the best solution here is to listen to your body. If you are finding no issues with completing easy, fasted runs in the morning each week – then keep at it. However, if you have the choice, fueling beforehand seems to provide more benefits than the potential of any fasted run.

More tips for running fueling: