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Should Runners Focus on Fasted Runs


What is Fasted Running?


Fasted running is when you run without eating before the run. It could be an 12-16 hour intermittent fasting, 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 or feel empty).


The Theory Behind Fasted Running


The energy demands and hormonal stress athletes and runners experience are far different from the general population. So, if runners are planning of focusing on intermittent fasting and fasted runs, according to most studies, it will affect the energy level, and performance. According to most studies, only about 3% of runners following fasted runs could maintain or improve their performance levels. That is due to different body types.


The theory is that fasted running improves fat oxidation. When we run at a low intensity, our body uses both lipids (fat) and carbohydrates for aerobic metabolism. Fasting lowers our glycogen stores. As a result, our body resorts to using more lipids (fat) for energy production during a fasted run. When we run, especially at our aerobic threshold (fast pace, or speedwork, we burn a higher percentage of carbohydrates. That’s what our body needs for energy, glycogen (carbohydrates), If we do fasted runs, our body will only have access to lipids (fat), for energy, and lipids energy production is slower than glycogen, so it would take longer to get the energy. Body fat also doesn’t provide the same high level of energy as glycogen.


Some individuals do go for fasted running because of gastrointestinal distress. However, our gut is a muscle and it can be trained to tolerate food prior to running. It is important what we eat before our run is easily digestible; high fiber, fats, and protein are not ideal pre-run foods. Even to eat small pre-run snacks or drinks will help support our energy level. If we plan for a long distance run, we also need to have access to energy fueling and hydration.


Negative Effects of Fasted Running


1. Fasting can break down our muscles.

Fasted endurance training runs actually leads to increased breakdown of the proteins in our muscles. Increased muscle breakdown links to a loss of strength and an increased risk of soft muscle tissue injuries.


2. Fasted running may affect our endurance.

Pre-exercise eating actually improves aerobic endurance performance. Ultimately, that should be our goal of training, to improve our performance, and our nutritional habits should support that goal. Eating prior to runs provides good support for our training goals.


3. Our appetite may be poorly regulated.


According to a study, athletes who ran for 60 minutes had better appetite regulation when they trained after eating rather than fasted. We will be hungrier if we finish a run on an empty stomach, this type of hunger makes us more prone to make poorer eating choices, and also overeating for the rest of the day.


4. Our exercise level will be lower on high-intensity runs.


The higher the intensity of a run, the higher percentage of carbohydrates we burn for energy compared to fat. Any intensity above our lactate threshold.


Gender is Also a Factor


Poor energy availability during exercise, can have negative health effects on female runners. 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 daily energy deficiency. For women, within a day energy deficiency increases the risk of menstrual irregularities and poor bone health, men do not have to worry about these risks about as much. However, RED-S (Relative Energy Deficiency in Sport) is a very real concern for male athletes and should not be ignored.


Why Should We Avoid Fasted Running


Fasted running is a balance of risk versus reward. Fasted running will improve fat burning, however, fat-burning does not have a significant impact or benefits upon performance. So with fasted running, we may burn more fat while running, and that will be the only difference. In fact, our performance will suffer due to poor energy availability.


Importantly, we need to focus on these risks which include increased injury from soft tissue damage, poor energy availability, lower endurance, and possibly crash. It is best to skip fasted running and focus on fueling our body well to avoid injuries, menstrual irregularities, or eating disorders.


Most importantly, we must focus on how we feel and how we are performing. If we are skipping pre-run calories and are experiencing a string of injuries and/or feel generally sore on our runs, then we want to avoid fasted running. If our runs are suddenly slower, it might be a symptom that we need calories before a run.


Benefits of Light Pre-Run Snacks


  • Sustained energy for a better workout

  • Enhanced workout recovery

  • Increased performance

  • Hunger- Fasted running, too many hours since your last meal and you’re hungry

  • Increases food calories burned so less are stored as fat



A pre-run snack doesn’t have to be heavy. All we need is something easy to digest and rich in more simple carbohydrates.


  • Banana or 1/2 Banana

  • Dried Fruits

  • Dates

  • Figs

  • Dried Apricots

  • Bread with Honey

  • Rice Crackers with Nut Butter

  • Energy Drink

  • Light Smoothie

  • Energy Bar- Kind Bars, CLIF Bars

  • RX Nut Butter Bars

  • Honey Stinger Waffle

  • Gel



Good Pre-Run Fueling, Higher Energy Level and Performance, Great