If you’re one of the many runners who lace up and head out the door first thing in the morning, you’ve probably faced the familiar question: Should you eat something before your run, or just go on an empty stomach?
Fasted running has become popular for two main reasons. For some runners, it’s simply a matter of logistics. There isn’t time to eat and digest before an early workout, or eating too close to a run leads to cramps and GI discomfort.
For others, running fasted is a deliberate choice, often driven by the belief that it can enhance fat burning or support weight-loss goals.
But is fasted running actually beneficial? More importantly, is it safe? And when it comes to performance and body composition, is it better to run after eating, or does heading out on an empty stomach offer real advantages?
What Is Fasted Running?
Fasted running refers to running on an empty stomach, meaning your workout takes place after an extended period without food. Most runners who try fasted running do so first thing in the morning, heading out before breakfast or skipping their usual pre-run snack.
Because of the overnight fast that occurs while you sleep, this can mean you’re running anywhere from 7 to 16 hours, or more, since your last meal.
There aren’t strict rules for how long you need to go without eating for a run to be considered “fasted.”
In general, if it’s been at least four hours since a small snack or six or more hours since a larger meal, your body is effectively in a fasted state. Fasted running can also occur later in the day for runners who follow intermittent fasting approaches, such as time-restricted eating or alternate-day fasting.
While the concept itself is simple, running without eating beforehand, it doesn’t always feel easy. Depending on your metabolism, training background, and how long it’s been since you last ate, fasted runs can leave you feeling hungry, low on energy, or sluggish, especially at first.
For most runners, the safest and most practical way to experiment with fasted running is to start small. Rather than attempting a long run or speed workout, begin with a short, easy effort.
For example, if you normally eat a banana or toast upon waking and run 45 minutes later, you might try skipping the snack and shortening the run to 20–30 minutes.
Paying attention to how your body responds can help you decide whether fasted running is something you tolerate well before introducing longer or more intense sessions.
What Are the Benefits of Fasted Running?
#1: May Increase Fat Burning
Fasted running has been shown to increase the relative percentage of fat oxidation, meaning that a greater percentage of the calories you burn while you run come from stored body fat rather than from stored muscle glycogen.
Your muscles need energy to do the work involved in moving your body for any type of physical activity or exercise. This energy (ATP) is created by burning fuel that your body has stored from the nutrition you’ve taken in through your diet.
Excess carbohydrates that you eat are converted into glycogen, which is then stored in the skeletal muscles and liver.
Dietary fat and excess sugars when your glycogen stores are maxed out are stored as triglycerides in adipose tissue (body fat), while protein forms the structural muscle fibers.
The body has limited glycogen stores in the liver and skeletal muscles, and these levels deplete overnight during your fast.
According to the Academy of Nutrition and Dietetics,1How to Fuel Your Workout. (n.d.). Www.eatright.org. https://www.eatright.org/fitness/physical-activity/exercise-nutrition/how-to-fuel-your-workout 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 or so.
Therefore, when you run without eating beforehand, your body tries to conserve the limited glycogen remaining by trying to burn more fat for energy instead.
Some studies2Vieira, 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 Nutrition, 116(7), 1153–1164. https://doi.org/10.1017/s0007114516003160 suggest that you may burn up to 20% more fat when exercising on an empty stomach.
However, it’s important to make the distinction that you are not burning more calories overall during fasted running; rather, the source of those calories shifts to include a greater reliance on fat rather than carbohydrates.
#2: Can Reduce Digestive Distress
Runners with sensitive stomachs often find that running on an empty stomach prevents cramping, side stitches, gas, runner’s trots, and bloating.
Studies have found3Kondo, T., Nakae, Y., Mitsui, T., Kagaya, M., Matsutani, Y., Horibe, H., & Read, N. W. (2001). Exercise-induced nausea is exaggerated by eating. Appetite, 36(2), 119–125. https://doi.org/10.1006/appe.2000.0391 that nausea during hard workouts is more likely to occur if you are running on a full stomach.
When you exercise, blood is diverted away from the digestive tract to meet the increased oxygen needs of the working muscles.
As a result, digestion essentially ceases, meaning that anything sitting around in your stomach or intestines will do just that—sit around.
This can irritate your gut and cause nausea, bloating, and gas, all of which can signal colonic contractions that lead to the sudden need to use the bathroom mid-run.
For runners with sensitive stomachs, even small snacks can cause stomach trouble. In these cases, fasted running may be a better option.
#3: May Lead to Greater Weight Loss
Although running on an empty stomach doesn’t burn more calories than running after eating, some studies4Bachman, J. L., Deitrick, R. W., & Hillman, A. R. (2016). Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults. Journal of Nutrition and Metabolism, 2016, 1–7. https://doi.org/10.1155/2016/1984198 have shown that people who exercise on an empty stomach end up consuming fewer calories throughout the day.
It’s possible that fasted running has a more significant effect on decreasing appetite by suppressing the hunger hormone ghrelin compared to the hormonal response to running after eating.
Therefore, while fasted running doesn’t directly lead to more weight loss because it doesn’t actually burn more calories than running in the fed state, it is conceivable that it could accelerate your weight loss results by influencing the number of calories you eat.
Weight loss is largely dependent on the relationship between the number of calories you are consuming and the number of calories you are expanding every day, so if fasted running causes you to hold back on some of the calories you would otherwise have eaten, it’s a potential means to losing more weight.
With that said, different runners have different appetite responses.
Some runners find that they have a tremendous amount of rebound hunger, meaning that later on in the day, their appetite spikes in response to the significant caloric deficit generated in the morning by running without eating first.
#4: May Improve Blood Sugar Regulation
One of the primary concerns that runners often have before trying fasted running is becoming hypoglycemic, which means your blood sugar is too low.
This can result in fatigue, dizziness, headaches, irritability, nausea, and performance impairments.
Surprisingly most studies5Scott, S. N., Cocks, M., Andrews, R. C., Narendran, P., Purewal, T. S., Cuthbertson, D. J., Wagenmakers, A. J. M., & Shepherd, S. O. (2018). Fasted High-Intensity Interval and Moderate-Intensity Exercise Do Not Lead to Detrimental 24-Hour Blood Glucose Profiles. The Journal of Clinical Endocrinology & Metabolism, 104(1), 111–117. https://doi.org/10.1210/jc.2018-01308show that fasted exercise does not cause detrimental decreases in blood sugar, even for athletes with diabetes.
Some studies6Rothschild, J. A., Kilding, A. E., & Plews, D. J. (2020). What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients, 12(11), 3473. https://doi.org/10.3390/nu12113473 have even shown that exercising in the fasted state can improve insulin sensitivity and blood sugar control.
Despite these findings, if you have diabetes, you should consult your healthcare provider before trying fasted running.
#5: It’s Convenient
From a logistical standpoint, fasted running is sometimes the easiest option, if not the only option, for early-morning runners who just have to get out the door quickly if they want to get their full workout in.
Instead of having to wake up significantly earlier and lose more precious sleep, many runners who otherwise don’t have time to eat something and then wait for it to digest before going running prefer the fasted running route.
As previously addressed, if you don’t wait long enough after eating to go running, you can end up with a bevy of digestive symptoms that will all negatively impact your workout.
The Drawbacks of Fasted Running
#1: Does Not Increase Fat Loss
Many people are motivated to engage in fasted running under the belief that doing so will lead to greater fat loss or favorable changes in their body composition.
According to research,7Schoenfeld, B. J., Aragon, A. A., Wilborn, C. D., Krieger, J. W., & Sonmez, G. T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1). https://doi.org/10.1186/s12970-014-0054-7 fat loss and body composition changes were the same whether cardio exercise is performed in the fed or fasted state.
Another review that combed through the findings of five studies found that performing fasted cardio in the morning after an overnight fast had no significant effect on weight loss or changes in body fat percentage in either direction.
Therefore, it’s important to have appropriate expectations and be informed that fasted running increasing fat loss seems to be more of a common misconception than a reality in practice.
#2: Can Cause Muscle Loss
As mentioned, exercise in the fasted state leaves your body in a position of glycogen depletion or limited glycogen availability.
To enhance survival, your muscles are metabolically flexible in that they can turn to the other two fuel sources—fats and proteins—to make up the difference in energy needs.
And they do.
While we discussed the increased fat oxygenation to be a benefit of fast running, it’s also imperative that we cover the other side of the coin: fasted running can increase the percentage of calories generated by burning muscle protein.
Although the study involved cycling rather than running specifically, there is evidence to suggest that fasted cardio exercise can increase the contribution of protein for energy, meaning that your body burns more protein for fuel when you do fasted running.8Schoenfeld, B. (2011). Does Cardio After an Overnight Fast Maximize Fat Loss? Strength & Conditioning Journal, 33(1), 23–25. https://doi.org/10.1519/SSC.0b013e31820396ec
This, in turn, can compromise gains in muscle strength and size, as it directly catabolizes your muscles.
Consistent fasted running can therefore lead to muscle loss.
Loss of muscle mass is detrimental to strength, athletic performance, health, and metabolic rate.
#3: Can Increase Cortisol Levels
Cortisol is one of the primary stress hormones in the body, and chronically elevated cortisol levels have been associated with triggering the body to store more fat, especially in the abdominal area.
Any type of exercise is a stressor for the body, as is hunger or prolonged fasting.
Therefore, the combination of the two by running without eating beforehand can significantly increase cortisol levels, according to studies.9Kim, T. W., Lee, S. H., Choi, K. H., Kim, D. H., & Han, T. K. (2015). Comparison of the effects of acute exercise after overnight fasting and breakfast on energy substrate and hormone levels in obese men. Journal of Physical Therapy Science, 27(6), 1929–1932. https://doi.org/10.1589/jpts.27.1929
#4: May Cause Hormonal Imbalances
In addition to upregulating cortisol, there is evidence10Rothschild, J. A., Kilding, A. E., & Plews, D. J. (2020). What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients, 12(11), 3473. https://doi.org/10.3390/nu12113473 that exercising in the fasted state can also cause additional hormonal imbalances.
Your hormonal milieu can influence your injury risk, recovery from workouts, and the gains or adaptations your body makes.
Therefore, the potential consequence of dysregulating your hormones by running on an empty stomach should not be taken lightly.
#5: Can Reduce Your Performance
For most runners, the most immediate concern with fasted running is that most research shows that athletic performance suffers when you exercise without consuming adequate fuel beforehand.11Levy, E., & Chu, T. (2019). Intermittent Fasting and Its Effects on Athletic Performance: A Review. Current Sports Medicine Reports, 18(7), 266–269. https://doi.org/10.1249/JSR.0000000000000614
Strength, speed, and intensity levels tend to be significantly better when exercise is performed in a fed state, particularly when adequate carbohydrates are available.
Your tolerance for running longer may also be impaired in the fasted state.
ALevy, E., & Chu, T. (2019). Intermittent Fasting and Its Effects on Athletic Performance: A Review. Current Sports Medicine Reports, 18(7), 266–269. https://doi.org/10.1249/JSR.0000000000000614
A large review12Aird, T. P., Davies, R. W., & Carson, B. P. (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian Journal of Medicine & Science in Sports, 28(5), 1476–1493. https://doi.org/10.1111/sms.13054 that tabulated the findings of 46 studies, found that when people ate before an aerobic workout, such as running, they were able to exercise for longer periods of time. In this way, fasted running can make it less comfortable or unworkable to run for longer distances or durations.
The rate of perceived exertion also tends to be higher with fasted running, meaning the workout feels harder than it actually is.
So, Should You Run On An Empty Stomach?
When you weigh the pros and cons of fast running, most runners find that it’s not worth the time savings to run in a depleted state.
This is particularly true if you are concerned about your performance, improving your times, and getting stronger and faster.
On the other hand, if you suffer from a lot of digestive issues or don’t find that you feel weaker or more tired running without eating, it can be a workable and safe option.
For some pre-run breakfast options, check out our guide to what to eat before running in the morning.
