The benefits and risks of fasted running on performance

What effect does fasted running have on our bodies? Is it really beneficial to our performance? Or are we simply approaching it wrong? Taren Gesell provides the answers that will transform your run PB…

Adult male runner in park at autumn sunrise

Fasted running has been a popular training method in the fitness world for some time now. In the 1980s, it was the norm to run on an empty stomach, with the familiar phrase being ‘eating is cheating’.


However, recent studies, that we’ll get into later, have shown that there are many drawbacks to exercising in a fasted state. Saying that though, many people find it uncomfortable to exercise after eating due to the jostling of food in their stomachs. So who’s right when it comes to fasted running?

In this feature, we’ll explore the benefits and risks of fasted running and look into the recent science to understand the effects fasted running has on our bodies. We’ll also provide a possible solution for getting the benefits of fasted running without the drawbacks.

Benefits of fasted running

There are some benefits to running in a fasted state before breakfast. Here are some of the most common benefits runners and studies report:

Benefit 1 – Less stomach pain

Many runners have found that their stomach tends to feel better when they run on an empty stomach. This is because the body is not trying to digest food while also trying to fuel the muscles during exercise. Running before eating breakfast can lead to less discomfort, bloating, and cramping during a run.

Benefit 2 – Potential additional fat loss

When the body is in a fasted state, it is more likely to burn fat as fuel instead of glycogen (carbohydrates stored in the muscles). A study showed that you burn 20% more fat when you exercise before eating breakfast. Fasted running can potentially lead to more significant fat loss over time.

Benefit 3 – Race performance improvements

Burning more fat during exercise strongly correlates with successful race performances in races longer than 90 minutes. One study found that the ability to burn fat during exercise was just as crucial as VO2 max and body fat percentage in predicting Ironman race performance.

Risks of fasted running

However, in the past 10 years, a lot of research has shown the significant potential downsides to exercising in an energy-depleted state.

Risk 1 – Hypoglycemia

One of the main risks for individuals who suffer from low blood sugar is a risk of hypoglycemia (low blood sugar) during exercise. Exercise causes your blood sugars to plummet. If you’re already starting in a depleted state, you may be inviting severe health problems.

Risk 2 – Your stomach remains untrained

To perform your best in races and training, you need to have eaten and topped up your muscle glycogen levels beforehand. If you’ve never trained your stomach to run with food in it, you’re inviting gastrointestinal distress when you carb load before race day.

Risk 3 – Increased appetite

This study found that fasted running may lead to increased appetite later in the day, offsetting potential weight loss benefits.

Risk 4 – Poor performance

To perform well on race day, you must have trained hard in your interval runs during training. If you perform your interval runs in a fasted state, your body won’t have the muscle glycogen it needs to push hard in training. Your training effect will be less significant, and you’ll perform worse on race day.

Risk 5 – Chronic cortisol levels

Fasted running can also negatively affect stress hormones. When you wake up, your stress hormone cortisol is relatively high. Eating can help to settle down cortisol, while exercise raises it.

Exercising without eating in the morning can lead to extended periods with elevated cortisol levels, resulting in poor metabolism, a poor adaptation to training, storing more fat, and an increased risk of injury and illness.

Risk 6 – Poor bone health

Chronic low energy availability due to a high degree of fasted running can lead to poor bone health, according to studies.

Risk 7 – Immunity impairment

One study found that fasted running can lead to immunity impairment.

Risk 8 – Additional risks for female athletes

For female athletes, fasted running can be particularly disadvantageous, affecting their menstrual cycle, muscle mass, bone density, and bone health negatively.

Female athletes, in particular, must weigh the potential benefits and risks before trying fasted running.

The benefits of fasted running without the risks

As we’ve discussed, there are many potential drawbacks to fasted running. Still, it’s clear that to perform well in races lasting longer than 90 minutes we need to burn a high amount of fat during exercise.

Is there a way to get the benefits of fasted running without the drawbacks? I believe so.

One study found that exercising after a protein-oriented breakfast, as opposed to a traditional western-world carbohydrate-focused breakfast, athletes burned fat at a very similar rate to exercising in a fasted state.

Instead of running wholly fasted, you could eat a protein and fat-based meal and still train your body to burn a lot of fat. Meals like omelettes, nut butter, protein bars, meats, or salads with protein are all great options.

In addition to that, there are other ways to reach higher levels of fat oxidation beyond just fasted running. Another study found that fat burning is highly correlated with VO2 max.

A well-designed training plan will naturally raise VO2 max and your ability to burn fat as fuel. If you have a well-designed training plan, you might not even have to worry about training yourself to burn more fat.

Is fasted running worth it?

Before wrapping up, a personal observation I’ve had with athletes who use our MOTTIV training app provides anecdotal evidence against fasted training.

When we speak with athletes who have plateaued in their training, despite doing every single workout outlined in their training plan, we always ask them, “What are you eating before, during, and after your workouts?”

One of the most common answers from these athletes who have hit a brick wall in their progress is, ‘Nothing; eating before training doesn’t feel good.’

In one case, we worked with an athlete to get them to eat more before, during, and after their workouts. Within a few months, they shaved 30 minutes off their half-Ironman time, set personal bests, and qualified for their first world championship.

We all train our muscles to work at a higher and higher intensity; we also have to train our stomachs to handle food while exercising.

Doing so with the proper nutrients at the right time can fuel your body for your workouts while teaching your body how to burn fat as fuel, leaving fasted running unnecessary.


Top image credit: Getty Images