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Intermittent Fasting: What Runners Need to Know

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While some swear by the healing and performance boosting powers of intermittent fasting, others feel that it’s purported impacts are overrated. Especially for athletes.

What is Intermittent Fasting?

So what exactly is intermittent fasting (or otherwise referred to as time-restricted eating)? In short, it basically boils down to taking periodic breaks from eating. Some individuals do it by restricting calories on one or more days per week while eating normally on the other days, and others prefer going without food for a bit longer than usual during the day.

And while this may sound extreme to some, keep in mind that training on an empty stomach after a night’s rest also falls under intermittent fasting. According to well-known running coach, Jenny Hadfield, “…by delaying breakfast, you’re extending the time between meals (fasting) and training in a fasted state“.

But why fuel in this way? Why not just eat normally throughout the day? Some runners, especially ultra runners, follow this practice in order to lose weight and (so they believe) increase the ability of muscles to burn fat. An ability that is, in turn, believed to increase endurance.

But is there any truth to these claims? And is it advisable, from a long-term health perspective, to eat in this way?

The Pros and Cons of Intermittent Fasting


A number of scientific studies have been conducted on intermittent fasting, many of them on animal subjects, not humans. And from these studies, a number of supposed pros or benefits to eating this way have been identified. These include:

  • Weight loss. A 2011 study published in the International Journal of Obesity found that intermittent energy restriction was as effective as continuous energy restriction in the improvement of weight loss in overweight women.

  • The improvement of certain biomarkers for health. The same study also found that intermittent energy restriction was as effective as continuous energy restriction in improving insulin sensitivity along with a number of other health biomarkers.

The following potential risks and disadvantages are, however, also associated with intermittent fasting:

  • Encouraging over-or binge-eating on/during non-fasting days/times, oftentimes on unhealthy foodstuffs. While intermittent fasting has been found to have a positive impact on various biomarkers for health (see “Pros” above), this impact may be canceled out by overindulging on/during non-fasting days/times. An animal study conducted in 2010 concluded that, since mice overate on non-fasting days, they did not lose any weight or experience any significant health gains.

  • Why not keep it simple instead? Isn’t it simpler to avoid overindulging on all days instead of (often over-) restricting calories on some days and then eat normally (or overeating) on others?
  • An increased ability of muscles to burn fat as fuel as a result of intermittent fasting has not been intensively studied and confirmed to date.
  • It tends to promote a magic-bullet mentality. Many athletes are on the lookout for a revolutionary quick fix to take their running to the next level. Permanent lifestyle changes, such as following a nutritious, wholesome diet all of the time, will most likely take you where you need to be. But it will also take a mind shift and some dedication.

The Potential Impact of Intermittent Fasting on Running Performance

And while these general cons or potential disadvantages are also applicable to athletes, a small 2011 study focused entirely on the impact of intermittent fasting during Ramadan on running performance. This (albeit small) study, which involved 18 well-trained runners, concluded that running performance over the 5000 m was found to decrease by an average of 5% during a month of intermittent Ramadan fasting. The authors attributed this decrease in running performance to changes in both muscular performance and oxygen kinetics.

Should You Try Intermittent Fasting?

So should you try intermittent fasting? While some sports nutritionists and running coaches advise their clients against it, others, like Hadfield, sees no harm in experimenting to see if it works for you. Hadfield stresses the fact that we’re all different and that what works for one athlete may be disastrous for another. She adds that any change in the regimen should always be approached with care.

Should you, therefore, wish to experiment with this kind of fueling regimen, Hadfield recommends keeping the following in mind:

  • Pregnant women or those with hypoglycemia, diabetes, a high-stress life, or hormonal imbalances should avoid intermittent fasting.
  • Something that works well for the general public might not necessarily work for the running fraternity. As runners, our fueling needs and the demands that we place on our bodies are much higher than that of sedentary folk.
  • Training while intermittently fasting requires proper nutrition and a healthy diet. Trying to fuel on junk food when not fasting may negatively impact both your health and performance.
  • Only experiment with intermittent fasting in your off-season or when your training mileage and intensity are at their lowest.
  • Experiment with different running intensities while fasting. Some athletes find lower intensity workouts doable but struggle with more intense ones.
  • Hadfield recommends not to run long in a fasted state, i.e. anything longer than 90 minutes. She feels that the possibility of benefit does not outweigh the risks beyond this point.
  • Listen to your body and be honest about what it tells you. If you feel weak and dizzy during your workouts, intermittent fasting might not be for you.
  • Always break the fast with a meal or snack that includes both protein and complex carbohydrates.

Bear in Mind the Risks

So perhaps one of the most important lessons to take from all of this is the fact that what works for one runner may be disastrous for another. While not completely against this way of fueling, even Hadfield admits that, in hindsight, it cost her dearly. Training early in the morning on an empty stomach while trying to qualify for Boston, all while living a hectic lifestyle with numerous other stressors, ultimately contributed to adrenal and thyroid imbalances for her. Something that she still has to carefully manage to this day.

So if you do decide to give intermittent fasting a try while training, do so with great care. And keep in mind that it doesn’t come without risk.

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