Is intermittent fasting a good idea for runners?

Intermittent fasting is a hot topic, but is it a sensible approach for active runners?


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Intermittent fasting was popularised as a weight-loss strategy when BBC broadcast journalist Michael Mosley wrote about it in his 2012 book, Eat Fast, Live Longer. Since then, many others have championed intermittent fasting, including some high-profile athletes.

So, what exactly is intermittent fasting – and where does it fit in with running performance?

What is intermittent fasting?

There are two main versions of intermittent fasting. The first one involves a prolonged period of fasting and a specific ‘window’ in which to consume food. The best known of these is 16:8, where the individual consumes their meals within an eight-hour window but fasts for 16 hours. The timing is up to the individual, but it needs to be maintained. So, if they choose to eat between 12pm-8pm, then they would fast from 8pm until 12pm. While this is the most popular, new variations have been introduced such as 12;12 or 14:10.

The second form of intermittent fasting is often referred to as the ‘5:2 diet’, where you eat ‘normally’ five days of the week and then two days of the week your intake is limited to just 500 calories a day. Similarly, to the time restricted version of intermittent fasting, different versions of this form of intermittent fasting have also arisen, including alternate day fasting. This involves eating as normal on alternate days and limiting calorie intake to 500 calories on the other days.

What are the benefits of intermittent fasting?

Beyond weight loss, the purported benefits of following an intermittent fasting model include slowing the aging process, reduced oxidative stress, improved body composition and improvement in metabolic biomarkers.

In the grand scheme of nutritional approaches, when looking at the time-restricted model, it is a fairly easy plan to follow with no food groups being avoided. Indeed, the individual can eat however they choose, the only caveat being that it needs to be restricted to the eight-hour window. So far so good, then.

However, supporting literature to suggest that this model of intermittent fasting can be a superior method of weight loss, improve athletic performance and reduced oxidative stress is scant. To date, the research supporting these claims has been done in animal models and all conclude with a recommendation for human studies. The handful of studies that have been done in humans, looking specifically at weight loss and body composition showed no significant difference between intermittent fasting and the usual, calorie-controlled methods often employed for weight loss.

One very small study, looking specifically at obese men with pre-diabetes, did demonstrate that when individuals consumed most of their nutritional requirements earlier on in the day, they were less likely to over-consume energy at night. This also led to improved insulin sensitivity and reduced blood pressure. While this is promising, it is important to note that, this study only composed of 12 subjects, of which only eight completed the trial. In addition, this was specifically looking at a particular population group, making it an unreliable source of information when extrapolating to those of us who run regularly.

Is intermittent fasting a good idea for runners?

Most of the information that has been reported relating to athletes has been via observational studies examining the effects of fasting in Ramadan. Beyond this, research is limited. In those studies, runners reported feeling fatigued and not being able to push hard in sessions during this period of Ramadan.

A few studies have been conducted on cyclists looking at the effects of intermittent fasting on endurance, high intensity and resistance training. In all three situations, it was concluded that intermittent fasting had a negative impact on performance, while body composition changes were not significant.

While we can appreciate that training will be impacted if conducted at the end of a fasting window, prior to consuming food, what about if you train later in the day once you have broken the fast?

2015 study researched the impact of cycling performance later in the evening when breakfast was omitted, as would occur in intermittent fasting. Not only did they show that cycling performance was negatively impacted when breakfast was omitted, but also that riders tended to consume a lot more energy at lunchtime, which could be one reason why body composition and weight loss was not affected.

What are the pitfalls for runners?

From a scientific point of view, the biggest pitfall is the lack of credible data available for this to be encouraged as a suitable approach for runners. It’s not just the low number of studies available, but also the lack of consistency around what format of intermittent fasting is being tested. Some studies look at 12:12, others at 16:8 and 20:4.

Performance wise, it is clear from the research available that there are no benefits. Indeed, one could argue that training in a depleted/fasted state not only impacts performance but has also been shown to depress your immune system, putting you at higher risk of infections and illness.

Professionally, I have huge concerns about using this approach in runners or anyone who is physically active. Nutrition is integral to both health and performance. In order to get the most out of your running, it is more important to ensure you fuel this appropriately, and that means fuelling well before, potentially during (depending on how long your run is) and then also in the immediate after for recovery. Sufficient nutrition, particularly in-day energy intake is necessary for the hormonal cascade needed not just for progression in your performance, but also to ensure that you prevent metabolic injury that in the long run can lead to the body downregulating and preserving energy, making weight loss even more difficult.

It is also important to appreciate that the human body is biologically biased towards energy balance. So, there is a real concern that in some individuals who may be susceptible to developing dysfunctional eating behaviours, these large periods of time of no eating, may then lead to periods of time of eating to excess, resulting in difficult thoughts and emotions, which then set the scene for more restrictive behaviours and the start of a binge/purge/restrict cycle.


As a weight-loss strategy, while it is not superior to calorie-controlled approaches, it may be easier to follow, especially the time-restricted model where you have a specific window for when food can be consumed. It will be this window that restricts the amount of food that can be consumed and thus works similarly to calorie control.

With regards to other claims around reduced blood pressure, oxidative stress and body composition, more human studies are required, as presently these are based on animal models.

There is no evidence to support intermittent fasting and running performance. However, some individuals whose focus is weight loss rather than performance may find this approach useful and easier than a restrictive eating pattern, especially in the first instance to lose body weight. However, this is caveated by the potential risk to metabolism if continued for long periods of time. So, runners who do follow the intermittent fasting approach need to be mindful about the potential negative consequences to health, performance and appetite.

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