Fasted Running: How To, Benefits, + Drawbacks

What is Fasted Running: How To, Benefits, and Drawbacks. If you are one of the many runners who heads out the door first thing in the morning to get your miles in, you have faced the ever-popular question of whether you should try to eat something first or run on an empty stomach.

Fasted running is popular for two different reasons. 

  • Avoid cramps because there’s not enough time to adequately digest the food
  • Enhanced fat loss.

But is fast running actually beneficial? More importantly, is fasted running safe? Is it better to run after eating, or are there weight loss benefits to running on an empty stomach?

What Is Fasted Running?

Fasted running simply invokes running on an empty stomach, meaning that your run occurs after an extended period without food. 

Most people do fast running first thing in the morning by running before eating breakfast or without having any sort of pre-run snack. 

Because of the overnight fast, while you sleep, this may mean that you’re running anywhere from 7-16 or more hours since the last meal or snack you consumed.

Although fasted running typically occurs in the morning, the other scenario that can lead to fasted running is when runners who practice various intermittent fasting diets go running later in the day on days with time-restricted eating or on alternate-day fasting diets.

There aren’t really definitive guidelines for the amount of time it has to have been since you ate for your workout to be considered “fasted running.” 

However, depending on the volume and caloric intake of your last meal, if it’s been at least 4 hours since a snack or 6 or more hours since a larger meal, your body is essentially doing fasted exercise. 

How Do You Do Fasted Running?

Fasted running is as simple as waiting to go running for at least 4-6 hours or more since you last ate anything, but just because it’s a simple protocol doesn’t necessarily mean it’s easy to go for a fasted run.

You might feel tired, hungry, and sluggish, depending on your individual metabolism, how long it’s been since you’ve eaten, and what your body is accustomed to.

For most runners, the easiest way to try fast running is to go running first thing in the morning after you wake up and before you eat anything. 

Start with a short, easy workout rather than a long run or speed workout.

What happens to your body when you fast?

Fed State (0-3 hours after your last meal)

In the fed state, your body digests and absorbs nutrients from food. As your body absorbs nutrients, blood sugar levels and insulin secretion increase. 

Insulin helps transport glucose (sugar) from the blood into cells to be used or stored for energy. Excess glucose that is not immediately used for energy is either stored as glycogen in the muscles and liver or converted into triglycerides and stored as fat.

During and immediately after a meal, there is a natural shift in your body’s hunger (ghrelin) and satiety (leptin) hormones. 

Just before a meal, ghrelin levels increase to stimulate hunger. After a meal, ghrelin levels decrease while leptin, your primary satiety hormone, increases, helping you feel full and satisfied.  

Early Fasting State (3-18 hours after your last meal)

The size and composition of your last meal affect how long you remain in the fed state, but typically your body will begin to enter the Early Fasting State 3-4 hours after your last meal. 

As your body transitions from fed to early fasting, insulin levels decrease in the absence of carbohydrates (sugar) from food. Meanwhile, ghrelin increases as leptin decreases.

In the absence of food, your body begins to tap into its energy stores. Muscle and liver glycogen are used first and converted back into glucose. 

As glycogen stores become depleted, the body ramps up lipolysis, converting fat into triglycerides as an alternate fuel source. As lipolysis increases, the body also breaks down protein (including muscle tissue) as a source of glucose for the brain and red blood cells which rely heavily, on or exclusively, on glucose for energy.  

Fasting State (18-48 hours after your last meal) 

Around the 18-hour mark, your body will go into full-on fasting mode. At this point, your body’s glycogen stores are fully depleted, leaving fat and protein their main sources of energy. 

With insulin levels low, some fatty acids in the blood are converted into ketones and, at this point, your body starts to transition into ketosis. 

How fast this happens depends on the composition of your usual diet and your last meal. If you typically lean lower-carb, ketosis will kick in on the earlier side, around 18-24 hours. 

Prolonged Fasting (48+ hours after your last meal) 

Fasting for more than 2 days (48 hours) is considered prolonged, or long-term, fasting. During this stage, the body relies increasingly on ketones and muscle protein for sustained energy. 

Going without food for this amount of time is not recommended for most people, and should only be done under medical supervision. 

Adaptive cellular responses to fasting

In addition to the hormone and metabolic changes, fasting also triggers some beneficial adaptive cellular responses that lead to reduced inflammation, oxidative stress, improved stress resistance, and the repair and/or removal of ageing or damaged cells.

These cellular changes can improve metabolic, cardiovascular, and cellular health, as well as counteract ageing and disease processes 

What Are The Benefits Of Fasted Running?


  •  Fasted Running May Increase Fat Burning

Fasted running has been shown to increase the relative percentage of fat oxidation, which means that a greater percentage of the calories that you are burning while you run is coming from stored body fat rather than from stored muscle glycogen.

Your muscles need the 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 fibres.

The body has limited glycogen stores in the liver and skeletal muscles, and these levels deplete overnight during your fast. 


  •  Fasted Running 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 found that nausea during hard workouts is more likely to occur if you are running or exercising 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 defecate mid-run.

For runners with sensitive stomachs, even small snacks can cause stomach trouble. In these cases, fast running may be a better option.


  •  Fasted Running May Improve Blood Sugar Regulation

One of the primary concerns that runners often have before trying fast 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 studies show that fasted exercise does not cause detrimental decreases in blood sugar, even for athletes with diabetes. Some studies 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 fast running.


  • Fasted Running Is Faster

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 bevvy of digestive symptoms that will all negatively impact your workout.

The Drawbacks Of Fasted Running

Although there are some possible pros of fast running, there are also potential drawbacks of running on an empty stomach, including the following:


  •  Fasted Running Can Cause Muscle Loss

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.

A study involving cycling rather than running specifically has 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 fast running.

This, in turn, can compromise gains in muscle strength and size, as it directly catabolized 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.


  •  Fasted Running 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.


  • Fasted Running May Cause Hormonal Imbalances

In addition to upregulating the hormone cortisol, there is also evidence to suggest that exercising in the fasted state can cause additional hormonal imbalances. 

Your hormonal milieu can influence your injury risk, recovery from workouts, and the gains or adaptations your body can take from your workout.

Therefore, the potential consequence of dysregulating your hormones by running on an empty stomach should not be taken lightly.


  • Fasted Running Can Reduce Your Performance

For most runners, the most immediate concern with fast running is that most research shows that athletic performance suffers when you exercise without consuming adequate fuel beforehand.

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.

Fasting tips for runners

  1. Ease into it. Exercise puts high energy demands on your body. By easing into intermittent fasting, you will allow your body to adjust and learn how to safely fast around exercise. Rather than going for a full 24 or 16-hour fast from the get-go, start with 10-12 hours a couple of times per week and figure out what works best for you over time. 
  2. Keep tabs on your fluid and nutrient intake. If you’re highly active, it can be challenging to eat and drink enough while intermittent fasting. Make sure you’re getting enough protein and calories during feeding windows to avoid unwanted weight and muscle loss and stay well hydrated, especially during fasting windows. Consider taking a daily multivitamin to help fill any fasting-related vitamin and mineral gaps in your diet. 
  3. Keep fasted workouts low-intensity. Fasting and training can be dangerous when done together. Schedule higher-intensity workouts for non-fasting days and keep fasted workouts low-intensity. Better yet, take a rest day when fasting. 
  4. Refuel after a fast. Fasted training likely won’t help improve your performance, so be sure to refuel after a fast, and at least 2-3 hours before a post-fast run. Not only will waiting a bit minimize the chance of tummy troubles, but it will also give your body time to rebuild depleted glycogen stores so you feel and perform your best.  
  5. Don’t forget to strength train. To minimize potential muscle loss associated with weight loss, incorporate regular strength and/or resistance training into your running regimen, and be sure to refuel muscles with adequate protein and carbohydrates for optimal muscle repair and recovery. 
  6. Don’t lose sleep. Sleep is crucial to athletic performance and recovery, as well as hormonal regulation. Because fasting can negatively affect sleep, it’s important to make sure you’re getting enough shuteye each night. If fasting is keeping you awake at night, consider shortening your fasting periods.
  7. Keep eating like an athlete. To perform like an athlete, you’ve got to eat like one. While your eating schedule may change a bit when fasting, the macronutrient composition of the foods you eat before, during, and after a run should continue to support exercise performance and recovery. 



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