Can Fasting Improve Your Performance

What is Fasting? And Why Is It Believed to Be Good For You?



Fasting is where you don’t consume any calories for a certain period of time. This means not eating any food and not having drinks which contain calories. You are typically only allowed to drink water, sparkling water, black coffee and plain tea. There are a few reasons that I think fasting could be good for ultra-runners.


James Clear, the author of Atomic Habits, has been intermittent fasting for some time. He claims that it is a simple and effective way of losing weight. says that you can get significant reductions in blood sugar and insulin levels, as well as a drastic increase in human growth hormone. it also cites studies that have shown that fasting can cause weight loss, a reduction in waist circumference and leads to less muscle loss than other standard diets.


“This eating pattern can cause 3–8% weight loss over 3–24 weeks



Intermittent fasting is also a lot easier than normal dieting. Rather than cutting out entire food groups, you simply don’t eat for a few hours. In my experience, that is a lot simpler and easier to do than standard diets.

Which brings me on to the second benefit… 


Cutting down a meal or two will also lower the number of decisions that you have to make during the day. Which can only be a good thing in our complicated, fast-paced lives. This means that you can save time because you have fewer meals to plan and prepare for. And it can potentially save you money too because you’ll probably buy less food.


“Fasting is an effective “life hack” that makes your life simpler, while improving your health at the same time. The fewer meals you need to plan for, the simpler your life will be




There are a lot of theories that advise you to train your body to get better at burning fat so that you can improve your ultra-running.


If you’ve ever ‘bonked’ or ‘hit-the-wall’ during a race, this is because your body has run out of carbohydrates as it’s fuel source. And it’s switched to using fat.


But the reason you bonk is because your body isn’t used to this sudden switch. Fasting can help with your ability to burn fat, rather than relying on carbohydrates.


This could potentially mean that you don’t ‘bonk’ or ‘hit-the-wall’ as easily as you might do if you’re not fat-adapted.

Our bodies only typically enter the fat-burning state 12 hours after your last meal. So, under a ‘normal’ way of eating, your body would rarely enter that state.


However, fasting means that your body is forced to enter that state.


What Are the Different Types of Fasting?


There are a number of different methods of fasting. The first is called intermittent fasting.


“Intermittent fasting is not a diet, it’s a pattern of eating. 

It’s a way of scheduling your meals so that you get the most out of them. 

Intermittent fasting doesn’t change what you eat, it changes when you eat.

— James Clear


James Clear, author of Atomic Habits, gives a detailed explanation of intermittent fasting on his website.

He explains a few different options.


This is where you split a 24 hour day into two blocks. One block where you can eat and one where you can’t.

The most common method is the 16/ 8 - Dont eat for 16 hours and then have an 8 hour window where you can eat.

It doesn’t matter when you do the fasting period. You can choose whatever time works for you.

But, as states, most people extend the fast that they naturally do whilst they sleep.


“Most people already “fast” every day while they sleep. 
Intermittent fasting can be as simple as extending that fast a little longer.



The graphic below shows how a daily intermittent fasting schedule might look over the course of a week.

James Clear outlines how a week might look if you had a 16 hour window of not eating and an 8 hour window of eating every day


I’ve been trying a version of this. But with a couple of adaptations, based on my ultra-running training.

First, I don’t do intermittent fasting every single day.


I do it between 2 - 4 times a week. I generally don’t do it when I have a longer run or a more intense training session.

Second, I typically do 15 hours of fasting due to my lifestyle.


I usually don’t get in from work until 18:30. Then I spend 30 - 45 minutes with my children before they go to bed.

My wife and I prepare dinner together and always eat together. By the time we’re sat down for dinner, it’s usually after 8pm.

So I don’t usually start fasting until 21:00. Which I know isn’t great, because that’s very close to my bed-time. But that’s another habit I need to change!




I usually start eating again at mid-day the following day.


Here’s what my typical week looks like:

  • Sunday:

    • Stop eating at 21:00

  • Monday:

    • Morning - Easy 7 mile run

    • Mid-day - Start eating. This means I have a 15 hour fasting period

    • Afternoon - 7 mile tempo run

    • 21:00 - Finish eating

  • Tuesday:

    • Morning - Long run

    • Morning - Eat breakfast shortly after long run, by 09:00. This means I don’t really fast on these days. Although I would do a fasted long run.

    • Afternoon - Easy 7 mile run

    • 21:00 - Finish eating

  • Wednesday:

    • Morning - Easy 7 mile run

    • Mid-day - Start eating. This means I have a 15 hour fasting period

    • Afternoon - 7 mile tempo run

    • 21:00 - Finish eating

  • Thursday

    • Morning - Long run

    • Morning - Eat breakfast shortly after long run, by 09:00. This means I don’t really fast on these days. Although I would do a fasted long run

    • Afternoon - Easy 7 mile run

    • 21:00 - Finish eating

  • Friday

    • Morning - Easy 7 mile run

    • Mid-day - Start eating. This means I have a 15 hour fasting period

    • Afternoon - Easy 7 mile run

    • 21:00 - Finish eating

During the time I’ve experimented with this technique, I have had a lot of social commitments. Which means that I haven’t properly implemented it on weekends consistently.




Weekly intermittent fasting is when you have a 24-hour period where you don’t eat at all.


This is more extreme than the daily fasting, because you do it for longer.


But you do it less often.


Below is a visual example of how this might look.




James Clear’s example of how you might do one day a week of a 24 hour window of fasting


I haven’t experimented with this variation yet, because I haven’t been convinced of the benefits. But I may try it in the near future.


Even James Clear is less certain on this type of fast. He thinks that the biggest benefit is probably psychological.

“Perhaps the biggest benefit of doing a 24–hour fast is getting over the mental barrier of fasting. 

If you’ve never fasted before, successfully completing your first one helps you realize that you won’t die if you don’t eat for a day.

— James Clear



Alternate day fasting is the most extreme of the three types of fasting that James Clear outlined. This is where you have multiple 24-hour windows of fasting throughout a week. James Clear outlines how a week might look if you did alternative days of 24 hour periods of fasting For the amount of running that I do, this is not a practical option.


I also think that most ultra-runners would do too much exercise and activity to be able to do this method of fasting safely.

You may end up consuming far too few calories to support your training. And I think you would see your performances suffer.




This is where you choose two days of the week where you restrict calories to about 500 - 600 a day. You eat normally in the other days. gives a comprehensive overview of the different types of fasting. And for the 5:2 approach, they recommend that “there should be at least one regular eating day in between” the two fasting days.


This website also gives you some recommended schedules to help you start intermittent fasting. This could be something I experiment with in future. Although, I’m less convinced of the benefits of this approach.


It also seems to be a lot more difficult than any of the other options.




The previous four options were lifestyle-based approaches to fasting.


I’ve also tried running whilst fasted.


This article from Trail Runner Magazine explains the theory, benefit and risks of fasted runs.


“The theory goes that running on an empty stomach can make you more efficient at burning fat, pushing back the dreaded “bonk” and possibly even improving aerobic performance

— David Roche - On Trail Runner Magazine


As well as the physical benefits, I also think that there are psychological benefits of fasted runs.

When I’ve fasted on some of my longest runs, I’ve forced myself to experience the ‘bonking’ effect that I may feel in a race.

This gives me the opportunity to practise other things - such as psychological techniques - to get me out of that situation.

It has also meant that I have the really positive experience of pushing through a tough workout.


And I can use that positive memory during a race when it gets particularly tough.


But, admittedly, there doesn’t seem to be a scientific consensus on the usefulness of fasting.

This article provides some reasons why fasting is NOT a useful training tool.

And there are several things that I would encourage people to consider before using fasted running in their training.

  • Don’t Do it if You’re New to Running - This is a pretty advanced technique, where you’re looking to make smaller improvements and tweaks to your performances. As a new runner, there are lots more things to focus on, which will have significantly better improvements to your performance. Like building up your mileage. Or doing more ‘quality’ training sessions like these ones.

  • Start Gradually - Start with one or two short or medium-length fasted runs in the morning, when you haven’t eaten breakfast. Then build up to longer runs.

  • Don’t Fast Before ‘Quality’ Sessions - Being in a fat-adapted state means that you won’t be able to perform key workouts as well as you normally would. So if you have a speed, tempo or hill work session planned, make sure to have some food before.

  • Take Precautions - During fasted runs, you should always carry some food and water with you. Just in case you get into trouble. You can also plan routes where you can be near shops, home or somewhere else if you’re in trouble. Don’t do too many fasted runs in the middle of nowhere!

  • Remember to Get Race Specific - In a real race you won’t be restricting the amount of food you eat on purpose like you do with fasted running. In fact, you’ll be doing the opposite. You’ll want to be refuelling regularly and with the foods and drink that you know work. And the best way to know which foods work for you is to practise with them in training. So make sure that you always practise thoroughly with what you are planning to use in a race. This knowledge will be much more useful than the benefits you get from fasted runs!

If you take into account those considerations, I believe that fasted runs are worth experimenting with in training to see if they can improve your performance.


But, as with any running and nutrition advice, it’s highly dependant on your individual situation and goals.

“If you thrive on carbohydrates now, this may not be right for you and will probably just leave you feeling sluggish and irritable. 

However, any metabolic adaptations may be helpful for longer, ultra-endurance events

— Women's


Conclusion - Can Fasting Improve Your Ultra-Running Performance?


  • I believe that fasting can help you improve your ultra-running performance. But it’s more beneficial for improving your general lifestyle and help you get into good habits

  • I also believe that fasting as a way to improve running is a technique that newcomers shouldn’t worry too much about. If you’re relatively new to running or endurance sports, there are lots of other things that you will get bigger gains from.

  • There are a number of different types of fasting. You should experiment with these carefully to see which ones suit your needs (if any of them do)

  • There are some considerations to think about when doing fasting. Ease into it gradually and time your fasting around key workouts. If in any doubt, consult a nutritionist or a dietitian


Twitter: @James_Runs_Far


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