Should You Do Fasted Runs?

Fasting is one of the most highly debated topics in health science. The stances are polarized. People either believe it’s the secret to health and happiness, or they believe that fasting will cause a cascade of adverse effects. The same polarized debate occurs on a smaller scale in the running community. Some runners swear that fasted running led to breakthrough races; others question its appropriateness as it leaves them sluggish and hangry. This article will delve into the research around fasted running and whether it is an appropriate nutritional strategy for you. (Spoiler: it’s like not.)

Should You Do Fasted Runs?

What is Fasted Running?

Fasted running is when you run without eating yet for the day. In order to have truly fasted, you will be coming off an 8-14 hour fast (ie sleep) and run before you eat. If you run later in the day, after eating any meal, it is not fasted (even if your stomach is empty). 

Understanding the Science of Fasted Endurance Training

When approaching a topic such as fasted running, we want to first look at what the research says – and how the research is structured.

Studies are not simply about their conclusions. How a study is structured impacts the results. When you read any study, always look at their tested sample. Are the participants athletes or the general population? Male or female?

The energy demands and hormonal stress athletes experience are far different from that of the general population. So, if you encounter evidence that fasting improves energy, for example, look at the study. You cannot deduce that fasting will affect athletes with higher energy demands as it does more sedentary subjects. 

 Most studies on fasted running use 20-30 year old males, who are also a demographic that are seemingly invincible. Young men can also drink five beers the night before a race with little detriment, but that doesn’t mean such an approach is sound. Can those conclusions be applied to broader demographics of runners? 

Finally, when you also encounter a study, look at the methodology. Did the researchers have a control group? Or a third variable? How did those who did not fast respond? You may see that athletes who fasted improved their time to exhaustion by 3%; but those who performed after eating did so by 5% (this is just an example). The closer a study can get to real-world conclusions, the more useful it will be in this scenario. 

The Theory Behind Fasted Running

For the scope of this article, I will skip over theories of autophagy (cell turnover) and similar that people use to support intermittent fasting for the general population. Instead, I will focus solely on fasted running in relation to running performance.

The theory is that fasted running improves fat oxidation. When you run at a low intensity, your body uses both lipids (fat) and carbohydrates for aerobic metabolism. Fasting lowers your glycogen stores. As a result, your body resorts to using more lipids for energy production during a fasted run, according to research such as a 2011 study in the Journal of Applied Physiology

Proponents of fasted running argue that the increased fat oxidation yields performance benefits. However, the actual performance benefits are debatable. Whenever you are running at your aerobic threshold (roughly marathon pace) or faster, you burn a higher percentage of carbohydrates. 

Some individuals do opt for fasted running because of gastrointestinal upset. However, your gut is a muscle and it can be trained to tolerate food prior to running. It’s important that what you eat prior to runs is easily-digestible; high fiber, fats, and protein are not ideal pre-run foods. Skip the bran cereal or omelet and opt for a banana or slice of toast instead. 

The Possible Negative Effects of Fasted Running

Fasting does not improve performance. In a 2021 study published in Nutrients, researchers compared the 10K times of well-trained runners who practiced intermittent fasting with those who did not. Surprisingly, at the end of the intervention, the performances were the same – even though the intermittent fasting group lost body fat. Theoretically, the changes in body composition should have translated to faster 10K times. The researchers theorized that low carbohydrate availability negated any improvements to running economy that came through body composition changes.

Fasting can break down your muscles. According to a 2011 article in Strength and Conditioning Journal, fasted state endurance training actually leads to increased breakdown of the proteins in muscles. Increased muscle breakdown equates to a loss of strength and an increased risk of soft tissue injuries. You want neither of these risks as a runner!

Fasted running may compromise your endurance. A 2018 meta-analysis in the Scandanavian Journal of Medicine and Science in Sports found that pre-exercise eating actually improved aerobic endurance performance. Ultimately, that’s the goal of training – to improve performance – and your nutritional habits should support that goal. Thus, eating prior to runs supports your training goals. 

Your appetite may be poorly regulated. In one 2012 study, athletes who ran on the treadmill for 60 minutes had better appetite regulation when they trained after eating than fasted. Even outside the context of scientific research, this makes sense. You will be hungrier if you finish a run on an empty stomach; this hunger makes you more prone to make poorer eating choices. 

Your exercise ceiling will be lower on high-intensity runs. The higher the intensity of a run, the higher percentage of carbohydrates you burn for energy compared to fat. Any intensity above your lactate threshold relies on carbohydrates; fat oxidation cannot easily occur at these intensities. Thus, if you try to only rely on fat oxidation, your exercise ceiling will lower.

Gender is Also a Factor

Poor energy availability during exercise can have adverse effects on health markers for female runners. Poor energy availability can occur outside of the context of fasted running; however, fasted running can be a source of energy deficiency for some female runners, even if they are eating well otherwise. Fasted running creates larger windows of within-day energy deficiency. For women, within-day energy deficiency increases the risk of menstrual irregularities and poor bone health – risks that men do not have to worry about as much. (However, RED-S is a very real concern for male athletes and should not be ignored.)

So, Should You Do Fasted Running?

Ultimately, fasted running is a balance of risk versus reward. Yes, fasted running will improve fat burning. However, fat-burning does not have a significant impact upon performance, particularly when you are running at or above your aerobic threshold (50K or shorter). So, you may burn more fat while running – and that will be the only difference. In fact, your performance could suffer due to poor energy availability. 

Importantly, you need to consider whether or not the reward (possible increased fat oxidation) outweighs the risks. These risks include increased injury from soft tissue damage and poor energy availability, lower endurance, and possibly feeling like hot dog sh*t. If it does for you and you feel okay, proceed with caution. But if you have performance goals, have a history of disordered eating, have menstrual irregularities, or are injury-prone, it is best to skip fasted running and focus on fueling your body well. 

Most importantly, you must observe how you feel and how you are performing. If you are skipping pre-run calories and are experiencing a string of injuries and/or feel generally crummy on your runs, then you want to forgo fasted running. If your runs are suddenly slower, you may need calories before a run.

That said, some people who don’t have performance goals may run fasted out of convenience. If you opt for fasted running, only do so before easy runs. Long runs and hard workouts such as interval session have higher energy demands. Fat adaptation will not change how your energy pathways function at higher intensities. 

A pre-run snack does not have to be heavy or fancy. All you need is something easy to digest and rich in simple carbohydrates. A banana, a handful of dried fruit, a couple of graham crackers, or a stroopwafel are all light, quick snacks.

(Disclaimer: My master’s degree is in Applied Exercise Science & Sports Nutrition. However, I am not a registered dietitian. If you struggle with energy availability, GI upset on runs, or fueling in general, consult a registered dietitian.) 

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19 Responses

  1. Thanks for highlighting these studies and the risk/reward of fasted running, Laura!
    I think you make a good point that we should pay attention to how we feel. My husband does very well on fasted runs, me not so much.
    I usually have a few nuts and a chai before I got on my runs and that seems to do the trick.

  2. Great info! I used to run fasted a long time ago when I first started running early in the morning. But now I always have something to eat, and I think I would really notice the difference if I didn’t. I also tried a few fasted long runs back in 2014 or so. I don’t think they helped necessarily with my training, and now that we know so much more about the importance of fueling for runs they are not something I would do again. I like to wake up a few minutes early so I can eat something and drink a little coffee before I run.

  3. 738 / 5000
    Resultados de traducción
    Congratulations for the article.
    Each person is a world, and what can be beneficial to me, it is possible that another person does not.
    When talking about running fasting, it would have to be differentiated if it is a competition, or if you are training and you do one of your easy career days.

    In the first of the cases it does not seem right, especially in long distance races, but in the second I understand that it can have benefits of adapting your body to use more fats in the absence of glycogen deposits.

    Nor does it seem appropriate to go fasting to perform series training, for example.
    Everyone must experience and see what really works.
    Nothing is written in stone.

    Greetings, Juan.

  4. We talked about this during my nutrition certification course and the general consensus was it is not really recommended for athletes for many of the reasons you mentioned. There may be a place for it for more sedentary individuals who are not taxing their bodies in the same way. I tried it myself a few years back and did not have good results. I feel, learning to eat and train your body to use food as fuel for your runs is the way to go

  5. I only run fasted when I have to get in a run before work. I do drink coffee with half and half, so technically, it’s not truly fasted, but I’m pretty sure that the calories from the half and half don’t do a ton to fuel my run. I do much better when I’ve eaten and my other runs all take place about an hour after my smoothie. After all these years of running, my body just goes. It’s kind of funny how that works!

  6. Great article. I normally do my runs fasted, just out of convenience. I need to run early and get out the door asap. I’ve definitely found that something little, like an orange or half a banana, is better before long runs. I’ve been having some injuries so I think I’ll experiment with a pre-run snack on all runs. Thanks for the very clear information!

  7. I personally am not a fan of fasted runs and rarely do them. Occasionally, especially when I’m trying to beat the heat. But most times even then I’ll have a small snack because I do feel sluggish when I run and haven’t eaten anything.

  8. Interesting. I am not really into any form of fasting. I’ll grab something small before I head out for a run, but I can’t go out with nothing in my stomach. I’m actually about to have a snack right now, because I didn’t have a big dinner and I have to run in the morning 🙂

  9. I love how well you explain everything. I don’t even use the term “fasted running.” If I’m running long (or a long race), I’ll usually have something light to eat, like a banana. Otherwise, I’ll just run without eating, though I usually will have a Gu or something in case I need it.

  10. Thanks for all the background info.

    I have never tried it. I don’t think I want to.

    I enjoy my coffee and oatmeal. So far so good.

    I eat before everyone run even if it’s only a bar or banana. I feel crappy if I don’t.

  11. Thanks for bringing this up. It seems to me that sometimes it is worth trying new types of running and I do it periodically. But my experience of running without breakfast makes me disagree with those who admire this type of running so much. After my workout, I just unbearably want to eat and this can lead to the fact that I will not become too much in food, and because of this all the benefits of running can come to naught. It seems to me that this is very important and should always be taken into account. So it’s okay if you feel hungry after running, but it won’t get you great results.

  12. This article was really informative, thanks for sharing this amazing article, cuz I had never heard of something like fasted run before, it was great reading it.

  13. I love to run on an empty stomach, never any stomach problems or the need to go to the bathroom at the worst of times! Im trying to eat less sugar and do intermitted fasting and fasted running is very much part of that lifestyle, which is doing wonders for my health.
    It takes time to get fat adapted and to accept the feeling of working out on empty stomach so its no suprice if one feels bad when trying fasted running at first, so start small!
    It is no question that its healthy for the body to be in fasted state regularly but as with everything to much of a good thing can by unhealthy like not eating enough calories to support hard training sessions.
    I think at least that fasted running works well for some so it should not be discouraged outright.
    Also morning run on empty stomach is usually not fasted running – more than 14 hours from last snack – so there is usually plenty of energy in the liver for short hiit workouts and as a bonus you might get the benefits of fasting after the workout.
    Of course for a demanding workout or a race then fuel beforehand.

    1. Thank you for your comment!
      I do encourage thinking about fasted running in terms of science (peer-reviewed evidence from cohort studies, controlled trials, longitudinal studies, cross-sectional studies, etc.). HIIT workouts require anaerobic metabolism, which uses glycolysis to produce ATP (energy for muscle contraction). Glycolysis does require blood glucose as well as muscle glycogen, so fasting is not efficient energy for even a short HIIT workout. Fat-adapted running does not have a significant scientific basis for improving performance, particularly since fatty acid metabolism for ATP occurs only at 60% max or less, which is lower than performance levels for a majority of recreational runners. Endurance-trained athletes will undergo carbohydrate sparing effect adaptation from endurance training, even if they consume carbohydrates prior. That’s great it works for you, but it is important to look at the evidence rather than individuals responses when dealing with larger populations of athletes. Additionally, if an athlete truly desires to improve fat-adaptation, it can be achieved through the intake of a protein-rich meal without carbohydrates prior to exercise, as this encourages fatty acid metabolism without the negative effects of pre-exercise caloric restriction (Rothschild et al., 2020). Finally, all these studies have been done on well-trained males ages 20-30, and we should gather further evidence before generalizing these claims for most athletes.
      I also encourage providing peer-reviewed evidence for claims regarding health and to always look at the population addressed. Some research does support the benefits when treating obesity, insulin resistance, and hypertension, since most of its benefits are linked with caloric restriction (Anton et al., 2017) but that differs from dealing with an athletic population. Autophagy occurs through natural fasting (sleep). If an athlete is getting adequate sleep (7-9 hours), they are experiencing the health benefits of autophagy without needing to fast. According to Secor & Carey (2016), fasting can suppress the hormones essential for immune function and contribute to an energy deficiency that leads to the loss of lean muscle tissue.

  14. I’ve learned through personal experience that when my body is used to carbs, I will black out on an 18-hour fasted run. When my body is used to a zero carb diet, I can do a 24-hour fasted run without issue.

    1. Hello –
      Under no circumstances is a 24-hour fasted run recommended. Reaching the point of blacking out on a run is not safe. Nor are zero-carbohydrate diets recommended. Anecdotes do not outrank evidence compiled from statistical analysis of peer-reviewed data following the scientific method.
      You go ahead and do you if that really works, but please do not encourage others to do the same.

  15. In the early 1980s I lived in Carpinteria, CA, near Santa Barbara within about a half a mile of Highway 150 that goes from Rincon Point to Ojai. I was about one mile from Rincon Point. I used to get on my bike at 5:30 in the morning, ride that road to the ranger station near Casitas Lake on the other side of the double ridges on this road going toward Ojai, and back. I would shower, eat a big breakfast, and drive to work. I would be on top of the world all day from this pre-meal ride. I would feel terrific and loaded with both mental and physical energy until about 4:00 p.m. when I would drink a cup of coffee and continue feeling great until bed time. I really miss those days. This is a fasted workout as I had no food after dinner, which ended about 7:00 p.m. The wellness I felt and the energy I had from this routine was beyond description and has to be experienced to be believed. Perhaps a physician with a concentration on nutrition and sports physiology could explain what was happening here.

    1. Hi Bob,
      Thank you for commenting. While I am not a physician, my master’s degree focused on sport physiology and nutrition. In this field, we rely on data pools, not singular anecdotes. Some people experience feelings of euphoria when fasting, since it alters the release of certain neurotransmitters. However, that does not necessarily mean that fasting is favorable for long-term health. Athletes have different metabolic needs than the general population, which means that recommendations around fasting will differ. For example, research indicates that low-energy availability in athletes (a state that regular fasting would lead to) can negatively alter hormonal levels and lead to poor mental states.

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