Should Runners Try Intermittent Fasting?


by Jen Rawson, RD

Intermittent fasting is one of the latest fad diet trends. With this diet, only the time of day you eat is restricted, not the type of food. With its promise of weight loss without muscle loss, many people are jumping on board. But are the claims true? And does intermittent fasting work well for runners?

Jonathan Pielmayer / Unsplash

When it comes to eating patterns, my usual disclaimer is that there is no “one size fits all” approach. While some people may swear by the regime and claim they have boundless energy, another person trying it may feel cranky and lethargic. Our bodies are different and respond differently to dietary changes.

What is intermittent fasting?
It is complicated to define this diet, as there are three different ways to achieve intermittent fasting:

1) 5:2: In this protocol, a person consumes very few calories 2 days of the week and maintains their normal dietary pattern on the other 5 days.

2) Alternate Day Fasting: In this protocol, a person alternates between days of regular eating patterns and days of very little dietary intake.

3) Time-Restricted Feeding: In this protocol, a person restricts the time period they eat. Typical patterns include fasting 16 to 20 hours per day and eating during a 4 to 8 hour time period.

All types of intermittent fasting ultimately result in calorie restriction. The draw to intermittent fasting versus continuous calorie restriction is it requires less focus on what to eat and instead relies on when to eat. Additionally, some research has pointed towards maintenance of lean body mass (muscle) while losing fat.

What does the research say?
The research on intermittent fasting is still limited. A lot of research has been conducted in animal models, which have limited applicability to the human population. Additionally, because intermittent fasting protocols vary widely, it can be difficult to make comparisons between studies.

A 2018 systematic review of intermittent fasting in overweight and obese adults found that intermittent fasting provided no statistically different weight loss than continuous energy restriction. This finding is consistent with other reviews of the intermittent fasting literature. However, when the data was pooled, the waist circumference and fat mass loss were significantly greater in the intermittent fasting group, indicating that they lost more fat while maintaining lean body mass. However, it’s important to note that none of the studies in this review
collected data beyond six months.

In a 2016 systematic review that included only studies that went beyond six months, it was found that all studies experienced weight loss at the final follow-up. However, the weight loss was “lower than experienced at earlier measurement points in the study.” In other words, participants were regaining the weight they lost when they continued with the plan for a longer period of time. Though intermittent fasting shows promising research for short-term weight loss and maintenance of muscle, sustainability is called into question when long-term studies show the lost weight being regained.

What are the potential drawbacks?
As with all diets, there is more to consider than just weight loss and you must consider the pros and cons. Dietary restriction can impact our physical, mental, psychological and social health. Some potential negative effects of intermittent fasting include:

Decreased running performance.
There have been limited studies performed on intermittent fasting protocols and running performance, but those conducted in people who practice fasting during Ramadan have shown a decline in running performance. Though our bodies are capable of burning fat as fuel during exercise, fuelling your body with carbohydrate-rich foods prior to running is still proven to be the most effective method for performance.

Overeating and digestive problems.
Because intermittent fasting restricts the time periods in which a person can eat, it inherently restricts the use of the body’s natural hunger and satiety cues. When a person is overly hungry or has been restricted from food, he/she is more likely to overeat during a subsequent meal or feeding period. A feeling of restriction can also impact the types of foods that are chosen during the feeding period, which can impact the overall quality of the diet. Lastly, eating larger volumes of food in a short period of time can cause digestion issues in some individuals.

Social interference.
Following a protocol that restricts your time period to eat will inevitably make it more challenging to engage in social engagements that include eating and drinking. Limiting social interaction can make the dietary pattern difficult to sustain and can impact mental health as it leads to social isolation.

What is the bottom line?
Although intermittent fasting is being touted as a more flexible eating plan, it is still another diet. Long-term research on diets of any type shows that the majority of people rebound in weight to their baseline or beyond. Weight cycling (the losing and regaining of weight) can be damaging to our physical and mental health. And because runners rely on fuel for their workouts, they might find intermittent fasting interferes both with their scheduling and their performance. Can an intermittent fasting protocol work for some people? Certainly. Is it good for all? Certainly not.


Jen Rawson is a Registered Dietitian from Calgary who works in a private practice specializing in intuitive eating, sports nutrition and gut health. She is passionate about running and travelling, often combining the two at destination races.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.