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.)