Intermittent Fasting And Running: Should You Try It?

Our CPT and nutritionist weigh up whether runners should try IF.

Intermittent fasting has become one of the most popular eating patterns for weight loss and health benefits.

While there are a long list of intermittent fast diet benefits such as managing insulin resistance, supporting fat burning, decreasing cortisol levels, and achieving a healthy body weight, the effects of intermittent fasting on energy levels, glycogen storage, muscle repair, and metabolic health for distance runners are not as well understood.

While runners can follow intermittent fasting diets, there are precautions and limitations to the efficacy of this dietary approach when you are following an intensive training plan.

In this intermittent fasting and running guide, we will discuss the pros and cons of intermittent fasting for runners and how to schedule running workouts around fasting periods.

A notebook with "intermittent fasting" written on it, an empty plate, utensils and an alarm clock.

Should You Do Intermittent Fasting And Running?

Before we discuss the potential benefits of intermittent fasting for runners and the effects of intermittent fasting diets on running performance and recovery, it is important to present the disclaimer that I am not a registered dietitian. This does not constitute medical advice.

I am a NASM-certified nutrition coach and a UESCA-certified endurance nutrition coach, and I have a master’s in Exercise Science and Nutrition.

Therefore, while I have a background in nutrition for runners and work with many marathon runners and endurance athletes on their nutrition strategy, the advice provided is not individualized and should not constitute medical advice.

Even though there can be health and weight loss benefits of intermittent fasting diets, time-restricted eating and alternate day fasting diets are not safe or appropriate for everyone.

Because restricting your eating window can limit your caloric intake, there are contraindications to some of the different types of intermittent fasting diets for certain individuals.

A plate made to look like a clock with food designated to only one section.

For example, intermittent fasting may not be appropriate for those who are pregnant or breastfeeding, have a history of an eating disorder, are underweight according to BMI, have diabetes or another metabolic condition, take certain medications, or have other chronic health conditions.

This is why it is important to speak with your doctor before trying running and intermittent fasting diets that involve prolonged periods of fasting. 

In general, most registered dietitians, sports nutritionists, and running coaches would not recommend that distance runners follow an intermittent fasting diet.

That said, there are many types of intermittent fasting and various goals that long distance runners have in terms of their running performance versus body weight, body composition, and health markers.

Therefore, it would be shortsighted to make universal claims that intermittent fasting diets aren’t good for runners or can’t be healthy for runners.

An alarm clock and a salad.

Can Intermittent Fasting Help Runners Lose Weight?

Numerous research and large-scale meta-analysis reviews1de Cabo, R., & Mattson, M. P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine381(26), 2541–2551. https://doi.org/10.1056/nejmra1905136 have demonstrated a variety of benefits of intermittent fasting on body composition.

These body composition benefits of time-restricted intermittent fasting dieting have piqued the interest of endurance athletes as well as strength athletes who are looking to support fat loss while maintaining muscle mass.

Particularly for endurance athletes and distance runners, carrying excess body fat decreases running economy and running performance.

A small study2Brady, A. J., Langton, H. M., Mulligan, M., & Egan, B. (2020). Effects of Eight Weeks of 16. Medicine & Science in Sports & ExercisePublish Ahead of Print. https://doi.org/10.1249/mss.0000000000002488 with middle- and long-distance runners found that eight weeks of 16/8 intermittent fasting helped runners lose fat without decreasing running performance (VO2 max, running economy, blood lactate concentrations, or heart rate during exercise).

However, there is a thin line because there can be a risk of disordered eating, clinical eating disorders, and extreme calorie restriction when long distance runners prioritize leanness over properly fueling the body.

In fact, having a very low body fat percentage can increase the risk of injury and lead to under-fueling for your running workouts.

Therefore, it’s important to reiterate that body composition is not the prime determinant of your athletic performance in any endurance sport.

Staying healthy so that you can train consistently and following an appropriate training program that is well-rounded and improves your cardiovascular fitness and endurance are paramount.

A tired runner, bent over.

How Does Intermittent Fasting Affect Running Performance?

Because there are different types of intermittent fasting diets regarding the time-restricted eating schedule (the length of the fasting window vs. eating window), not all types of intermittent fasting diet plans will work for endurance athletes following a training schedule.

In general, alternate-day intermittent fasting diets will not work if you follow a training plan that has you doing workouts most days of the week.

It is not advisable to perform any type of high-intensity exercise, including cross-training, when you are in a 24-hour fasting period.

If you are only performing low intensity exercise—such as walking or yoga—it might be possible to stick with your training plan with full-day fasting.

However, this won’t apply to most distance runners following a training schedule for a half marathon, marathon, or other long-distance race.

As can be surmised, the primary problem with running and intermittent fasting diets is that running, by nature, is a high-intensity type of exercise, so if you’re running on an empty stomach, your caloric intake and carb intake before your run won’t be ideal.

A peanut butter and jelly sandwich.

Carbs are the preferred fuel source for the muscles during vigorous exercise, and the body has limited storage for carbohydrates (in the form of muscle glycogen and liver glycogen).

According to the Academy of Nutrition and Dietetics, an endurance-trained athlete can store up to 1,800 to 2,000 calories of fuel as glycogen in the muscles and liver, though smaller runners might store closer to 1,500 calories.

Depending on your body size and running pace, you might store enough glycogen to support about 90-120 minutes of running at your marathon race pace effort.

This is why having a pre-workout snack high in simple carbohydrates is generally recommended for vigorous workouts for endurance athletes.

Plus, long endurance workouts can burn through your glycogen storage, even if you have a pre-workout meal or snack high in carbohydrates. 

This is why sports nutrition professionals recommend that half marathon and marathon runners and endurance athletes consume energy gels, sports drinks, or other forms of carbohydrates during long runs or workouts that exceed 90 minutes.

A person running on a track.

According to the American College of Sports Medicine3American College of Sports Medicine. (2009). Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise41(3), 709–731. https://doi.org/10.1249/mss.0b013e31890eb86 (ACSM), endurance athletes should ingest 30–60 grams of carbohydrates per hour during workouts lasting 1-3 hours or more. 

Fueling with simple carbs during your running workouts helps ensure that your blood glucose levels stay high enough and that there is a trickle of glucose coming in to supplement the glycogen stores.

The problem with combining intermittent fasting and running is that if you run first thing in the morning or during your fasting window, some of your muscle glycogen storage and most of your liver glycogen are already depleted.

This means that your body has to burn fat for fuel.4Vieira, A. F., Costa, R. R., Macedo, R. C. O., Coconcelli, L., & Kruel, L. F. M. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition116(7), 1153–1164. https://doi.org/10.1017/s0007114516003160

‌While even the leanest distance runners have plenty of body fat to serve as an energy source for marathon training and completing a full marathon on race day, it takes much longer to oxidize fat for ATP (energy) than carbohydrates.

This means that you cannot run at such a fast pace or high intensity because the rate of energy production cannot keep pace with the rate of energy consumption.

Many marathon runners and ultra-marathon runners, as well as Ironman triathletes, have likely experienced the dreaded sensation of “bonking“ or “hitting the wall“ on race day due to running out of glycogen or carbohydrates as a fuel source.

A runner who has bonked.

How Should I Schedule My Running Workouts Around Intermittent Fasting Periods?

A key tip with intermittent fasting for runners is to decide whether you are prioritizing your training plan and athletic performance or if you are prioritizing the weight loss or wellness effects of intermittent fasting diet plans.

This isn’t to say that you can’t find a happy medium between the two, but generally, you will need to make sacrifices on one or the other.

If your goal is to run faster or prioritize your running performance, you should fuel your body with well-balanced nutrition and space out your meals and snacks in accordance with your appetite, caloric needs, and training schedule.

You may be able to follow a fairly lenient time-restricted intermittent fasting diet, such as 12/12 intermittent fasting, 14/10 intermittent fasting, or 16/8 intermittent fasting diet.

However, once you start shrinking your eating window to much less than eight hours, it may be difficult to time your running workouts around your limited hours per day to eat.

As a certified endurance nutrition coach, I generally recommend that distance runners who want to combine intermittent fasting and running should run during the eating window.

A plate of breakfast food including pancakes, eggs and bacon.

You can have your pre-workout snack after your fasting period is over and then wait 30 to 90 minutes before you exercise.

Then, you should still have plenty of time to have a post-workout meal rich in complex carbohydrates and protein. 

Make sure to have this post-workout meal or snack within 30 minutes after finishing your running workouts, cross-training workouts, or strength training workout to help refuel and facilitate muscle repair and glycogen resynthesis.5Cintineo, H. P., Arent, M. A., Antonio, J., & Arent, S. M. (2018). Effects of Protein Supplementation on Performance and Recovery in Resistance and Endurance Training. Frontiers in Nutrition5(83). https://doi.org/10.3389/fnut.2018.00083

‌You can use the rest of your eating hours a day to take in energy-dense healthy fats, complex carbohydrates, and protein in nutritious meals and snacks.

This type of meal timing with running will not work well if you are following a restrictive intermittent fasting diet such as 20/4 intermittent fasting or OMAD (one meal a day) because there won’t be enough time to do your training and have proper pre-run and post-run nutrition within the restricted eating hours.

The exception to this suggested fasting timing and running would be in cases where your cardio workout is relatively short, such as with easy runs or low-intensity cross-training sessions.

Here, you could run in the morning after the overnight fast without having a pre-workout snack. 

Your fasted run would occur during the prolonged overnight fast, and then you could have a post-workout breakfast to start your eating hours.

I recommend running in the fasted state before you start your eating window when you are following a time-restricted intermittent fasting diet rather than running in the evening after your fasting hours have begun.

A nutritionist planning meals.

This is because eating after exercise is critical for supporting muscle repair and recovery, and if you have to go through the entire overnight fast and into the morning until you eat again, your muscles and tissues will not have more nutrients coming in for workout recovery.

For example, endurance athletes observing Ramadan should schedule their runs around the end of the fasting window before sunset or in the very early morning before they have to start fasting at sunrise. 

Then, they can have breakfast after running before their fasting hours begin.

Overall, just because there are benefits of intermittent fasting for everyday individuals looking to lose weight and improve markers of health does not mean that restricting your eating window will necessarily work well for distance runners who are following a training plan with high-intensity workouts, long runs, strength training, etc.

It is important to ensure you are fueling well before and after your workouts to support performance and recovery.

Consider working with a nutritionist to find the best running nutrition plan for your needs.

For detailed information on how to follow a 16/8 intermittent fasting diet, check out this next guide:

References

  • 1
    de Cabo, R., & Mattson, M. P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine381(26), 2541–2551. https://doi.org/10.1056/nejmra1905136
  • 2
    Brady, A. J., Langton, H. M., Mulligan, M., & Egan, B. (2020). Effects of Eight Weeks of 16. Medicine & Science in Sports & ExercisePublish Ahead of Print. https://doi.org/10.1249/mss.0000000000002488
  • 3
    American College of Sports Medicine. (2009). Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise41(3), 709–731. https://doi.org/10.1249/mss.0b013e31890eb86
  • 4
    Vieira, A. F., Costa, R. R., Macedo, R. C. O., Coconcelli, L., & Kruel, L. F. M. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition116(7), 1153–1164. https://doi.org/10.1017/s0007114516003160
  • 5
    Cintineo, H. P., Arent, M. A., Antonio, J., & Arent, S. M. (2018). Effects of Protein Supplementation on Performance and Recovery in Resistance and Endurance Training. Frontiers in Nutrition5(83). https://doi.org/10.3389/fnut.2018.00083
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

4 thoughts on “Intermittent Fasting And Running: Should You Try It?”

  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.

    Reply
  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.

    Reply

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