Intermittent Fasting and Running: The Complete Guide

Intermittent fasting and running may seem like a strange combination, but when done right many runners have reported energy level gains and improved wellbeing.

IF (Intermittent fasting) is seen by some as the newest diet craze, but it’s not exactly dieting.

Instead of restricting people from WHAT to eat, it’s restricting them from WHEN to eat which tends to be easier to do. But do intermittent fasting and running go together?

After all, runners NEED fuel to do what they do… 

This is what we set out to answer. We talked with coaches and nutritionists from around the globe and the answer is: it depends.

Intermittent fasting and running can be beneficial and complimentary if you do it right. 

In this article you will learn:

  • What is intermittent fasting?
  • What are the general health benefits of intermittent fasting?
  • What are the health benefits of intermittent fasting for runners?
  • What are the dangers of intermittent fasting for runners?
  • Should runners do intermittent fasting?
  • And, if so, how should runners do intermittent fasting?
intermittent fasting and running

What is intermittent fasting?

Intermittent fasting is an eating pattern in which people fast for a specific time each day to promote eating less while burning more fat.  

According to nutritionist Lisa Richards, author of The Candida Diet, there are three primary types of intermittent fasting; the 5:2 method, the 16/8 method, and the eat-stop-eat method. 

  • The 5:2 method allows the runner to take in only 500 – 600 calories each day for two non-consecutive days, and then eat a regular diet the other five days. 
  • The 16/8 method requires the dieter to skip breakfast, restrict their calorie intake to 8 consecutive hours only, and fasting for 16 hours. 
  • The Eat-stop-eat method is fasting in the more traditional sense where the dieter goes without food for 24 hours.
intermittent fasting for runners

What are the general health benefits of intermittent fasting?

Intermittent fasting is so popular because it is easy to do and because it is shown to have many health benefits such as weight loss, improved endurance, improved insulin resistance, increased muscle, and initiated cellular repair, among others. 

Cellular repair is a significant benefit of intermittent fasting. The body is better at being able to restore hormonal balance and clear itself of toxins which may lead to cellular damage,” explains Richards.

“IF leads to lower insulin levels, increase in growth hormones, and also an influx of norepinephrine. All of these elements work together to encourage the breakdown of body fat into energy.” 

Let’s look at some studies that show the health benefits of intermittent fasting:

“Our gut microbes’ activity also oscillates throughout the day with this circadian rhythm. According to researchers, intermittent fasting restores microbial health and the uprightness of intestinal health,” explains Dr. Rashmi Byakodi, a writer at Best for Nutrition

Dr. Byakodi adds that “People by changing their mealtime, eating early in the day, and fasting overnight can help them with better metabolism. When combined with a healthy diet and lifestyle, it can be an effective weight loss approach.”

But, what about for runners? 

intermittent fasting for runners

What are the health benefits of intermittent fasting for runners?

There are several studies that indicate there may be health benefits of intermittent fasting for runners.

  • One 2020 meta-analysis in the journal Nutrients suggests that intermittent fasting might improve VO2max, help decrease body mass, and fat mass.
  • Another small study in the Journal of Applied Physiology found people (men) who consistently trained in a fasted state over the course of six weeks showed more endurance improvements than those who ate before working out.
  • Intermittent fasting may also improve post-workout recovery by aiding in inflammation reduction and nutrient absorption, studies find.

Related: Hitting the Wall: A Survivor’s Guide

While this is all convincing research, runners should proceed with caution when considering intermittent fasting. 

intermittent fasting for runners

What are the dangers of intermittent fasting for runners?

If a runner is going try intermittent fasting, they need to ensure they’re doing their runs EASY. If not, it could be detrimental to both their health and training. 

Related: Rate of Perceived Exertion: Why RPE is the Best Running Metric

Scheduling a tough workout or run during, or at the end of the fasting period could leave someone especially fatigued or dizzy.

This is because stored glucose is depleted during the typical intermittent fasting period, which the body will need for additional energy when running, notes Coxall. 

He adds that endurance athletes run the risk of performing at a lower level due to lack of energy. Indeed, a 2020 review in the Journal of Sports Medicine on fasting’s impact on performance finds the evidence is unclear and recommends endurance athletes avoid high-intensity training while in a fasted state

Exercise physiologist and nutrition scientist Dr. Stacy Sims writes in her bestselling book ROAR, runners using intermittent fasting to lose weight may sabotage themselves:

“It nearly always backfires. When you go out and exercise in a fasted state, you’re putting your body under added and possibly undue stress. This is particularly bad in the morning when cortisol levels are already elevated…

“Your body wants to pump out more cortisol but needs the right ingredients (sex hormones) to manufacture it. So, your body steals those hormones to make more cortisol. Now your cortisol is very elevated which stimulates fat store, not muscle fat storage, so you’re storing more of what you’re trying to lose!”

intermittent fasting and running

Should runners do intermittent fasting?

Many runners successfully intermittent fast. Famed ultrarunner Dean Karnazes said in an interview that: “It’s very rare that I eat anything before noon. I obey a 15- or 16-hour window between my last meal…Basically, when I run in the morning, I’m just burning stored body fat.”

If a runner would like to intermittent fast, there are ways to safely do so if the runner is careful.

Related: Ultramarathon Nutrition Guide: What to eat before, during, after an Ultra 

How should runners do intermittent fasting?

Intermittent Fasting for Runners: A Guide

Run at low-intensity. 

Runners doing an intermittent fast should run at an easy pace ( RPE of 2-3 out of 10, or heart rate zone 2) which burns fat as energy rather than glycogen which will be depleted in a fasted state.

“Otherwise you risk hitting the wall,” warns Jack Coxall, strength and fitness coach. “It’s never good to starve your body from energy and output at high levels at the same time.”

Limit duration. 

If running in a fasted state, you do not want your runs to exceed 1.5-2 hours to start.

As you get more comfortable with long runs in a fasted state, you can push the envelope.

If you’re feeling dizzy or weak, stop and refuel with protein and carbs in a 3:1 ratio as soon as possible (within 30 minutes). 

Run at the right time. 

Runners who are doing an intermittent fast or alternate day fast should aim to do their runs during the time or days they are fueling.

If they eat between the hours of 11am and 7pm, then they should aim to run then (especially if it is at a higher intensity than an easy effort).

If they run in the morning after fasting while sleep, refuel quickly. 

Refuel properly. 

If a runner runs during a fast, for example, in you run in the morning, it’s important they refuel properly to avoid muscle wasting and keep the intensity low. 

Studies indicate that high protein intake can be beneficial to a runner doing intermittent fasting, providing greater satiety and increased energy. 

intermittent fasting for runners

Listen to your body. 

Thomas Watson, running coach and founder of Marathon Handbook, has experimented a lot with low carb diets and intermittent fasting – especially going for fasted long runs:

“The main advice I can give to anyone hoping to balance intermittent fasting and running is to take it very easy – start slow and short and very gradually push your fasted run length. Be mindful of energy level changes and check in regularly to make sure you’re confident you can comfortably proceed.”

“It’s something that you shouldn’t test out when in training for a half marathon or marathon, but instead if you’re interested in IF I’d recommended trying it out during your off-season when your running commitments are minimal.

Undue stress on your body can wreak havoc on your hormones and lead to increased fat and decreased performance, notes Dr. Sims:

“When you dig deeper and look at longevity data in terms of both intermittent fasting and exercise, they’re both beneficial, but you do not garner any additional benefits from layering intermittent fasting on top of exercising…And if you’re a (female athlete), it can be harmful to both your performance and your health.”

intermittent fasting for runners

The bottom line:

In summary, intermittent fasting can have a lot of health benefits for the general population. But for athletes, including runners, it runs the risk of stressing out the body and causing unwanted and detrimental effects. 

But that’s not to say that intermittent fasters need to pause all exercise: they just need to be mindful of the potential consequences of combining the two.

A well-fueled athlete is more apt to perform better than one who is fasting, says nutritionist Jen Scott: “An athlete optimizing their nutritional intake and timing before, during, and after their workouts and races, as well as balanced dietary intake throughout the day, is ultimately going to perform the best and have the best chance at maintaining lean body mass.”

intermittent fasting for runners
Photo of author
Whitney Heins is the founder of The Mother Runners and a VDOT-O2 certified running coach. She lives in Knoxville, TN with her two crazy, beautiful kids, pups, and husband. She is currently training to qualify for the US Olympic Trials marathon.

4 thoughts on “Intermittent Fasting and Running: The Complete Guide”

  1. Hi there, thanks for putting this guide together. Would doing IF when you only eat in the evenings (4.30pm – midnight) and also training for a marathon in 12 weeks’ time be workable? Usually run in the morning or at lunch and the plan includes speed work. Thanks.

  2. I recently started IF (3 weeks ago) and have started to see some results, for which I am excited. I have been challenged with my health and weight for 8 years and this is the first time I have seen progress. Anyway, I have a half marathon in September in Cedar City, Utah and wanted to know how to proceed. This has given me some things to think about. Thank you.


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