Running a marathon on keto - Lots of Keto

Running a marathon on Keto: this is why you should try it!


After running 42.195 KM in Valencia on nothing but water & electrolytes, I am more than thrilled to share this amazing experience with you. Since I ran my first marathon last year on a high-carb vegan diet, I thought it would be an awesome idea to try running a marathon on keto as a sort of case study. I broke my last marathon’s time with 50 minutes, and totally experienced a runners high along the way. In this article, you can read about my personal experience and view on this journey. I also share my nutrition & training program before the marathon, nutrition during the marathon, and how I recovered after the marathon. Which differences did I notice according to performances and energy levels, compared to last year’s run?



Why I gave running a marathon on keto a shot!

The time of carbohydrate loading & eating bananas before a race, and taking sugary sport gels & drinks during runs might be over soon. Fat is no longer the bad guy (or girl). Currently, an increasing amount of endurance athletes covering extreme distances such as triathlons decides to run on fat. Jeff Volek, Stephen Phinney, and Tim Noakes – spent decades studying the effects of a low-carb, high-fat approach in endurance athletes, and more and more research indicates that a ketogenic diet can have significant positive effects on energy levels and endurance performances. 


To be able to provide the best advice to my clients who aim to improve their endurance performances, I decided to bring everything I learned into practice.  So what did I exactly do to be able to run in a fat-adapted state, and how did it end?



Running a marathon on keto - Lots of KetoThe basics of keto running, and carb running

Firstly, let’s talk about the differences between carb running and fat adapted running. In this previous article, you can read more about using carbs as fuel and using fats as fuel. How does this work in endurance athletes?

In general, a runner’s’ body first uses stored carbohydrates (glycogen) as fuel, since this is metabolized quicker than fats. Therefore, many runners still think carbohydrate loading is necessary to run a great race. The thing is that the body can only store a few thousands of calories from glucose, so you need to load up during the race. This is exactly what I did during my first marathon. I honestly didn’t know anything about running on keto, and I just did what most runners have been taught. Following a higher-carb diet, and preloading with LOTS of carbs right before the race. But, if you run on sugar, your body basically depends on this type of fuel, and cannot perform without it. If you don’t replenish on time, it results into a hard crash.

This year I completely did the opposite after I found out your body can store about 20 times as many fat calories compared to carbohydrates. Fats are the body’s ‘emergency’ fuels, and your body only turns to these if glycogen supply is lacking. If your body uses stored fats as fuel, you have a tank of about 40,000 calories to help you run faster and longer. Wow! There’s no need to replenish, you almost have an infinite energy supply.



It takes months of dedication to become fully fat-adapted

Of course, running a marathon on keto is not something you can decide the week before your race. It takes a lot of determination, dedication, and months of training to benefit from fat adapted running, and efficiently burn stored fat for energy. In July, I started to reduce my carbohydrate intake till a maximum of 30 grams per day. Falling off the wagon during the first weeks was not an option, since you basically have to start your ketosis journey all over if you do so. You have to realize that becoming a fat-adapted runner requires a complete physical and mental lifestyle change! Basically, your whole life starts to evolve around training, and eating keto.


Luckily, it’s all worth it, and a high-fat diet provides a steady long-lasting energy fuel throughout the day. I allowed myself at least one month to get used to this new lifestyle, and to slowly build up my mileage per week. Therefore, starting the ‘most intense part’ of my training program was actually not as hard as I thought it would be!



Practical implementation before the race, during the race, and after the race

Training & nutrition during the months before my marathon

For about 5 months I followed the Standard Keto Diet. I have to admit I didn’t calculate my macros, since I know it mostly by heart. Also, the amount of time I had didn’t really allow me to do so. On average, I consumed a daily amount of 2200 calories, whereof 80% fat, 15% protein, and 5% carbohydrates. This, divided into three meals. The main part of my diet consisted of whole food products such as leafy vegetables, avocados, nuts & seeds, fatty fish, grass fed butter, olive oil, coconut oil, eggs, and cheese & full-fat dairy in moderation.


During my training period, I ran 3 times a week on average, whereof one short tempo run, one slower long run, and one interval. After the first 2 months I increased my mileage, and I also started to experiment with intermittent fasting and fasted runs. I never thought I would be able to do a fasted run, but I felt amazing doing so! At least once a week I implemented a fasted run or a 16 hour fast. The reason for these fasted runs was as follow: to reduce my body’s available glycogen to be able to burn stored fat more efficiently during the final miles of my marathon.


I only experimented with this after 2 months, because if you’re not fat-adapted yet, you won’t experience a lot of benefits from it. I never ran more than 1,5 hour in a fasted state, and I always ran right after waking up. What I noticed, was that both my energy levels & average speed increased from doing these runs.


I performed my long runs mostly a few hours after a meal. On days I ran more than 1,5 hour, I took a running backpack with me. Filled with water and 2 teaspoons of a liquid electrolyte mix. After those runs, I felt like the recovery time was way shorter compared to running longer runs on a high-carb diet.


Nutrition right before & during my marathon

3 hours before my marathon I had my last meal: a fat-dense breakfast. This meal included 3 eggs, shredded cheese, a lot of butter, and ½ avocado. With this, I also drank one coffee with cream, and butter. Before my race, I prepared a 1-liter sack with 2 teaspoons of a liquid electrolyte mix, and ½ lemon juice. That’s all I had during my marathon, besides 1 liter of extra water. I have to admit that I was experiencing some stomach issues before and during the race. Eating raw gamba carpaccio the day before a race is probably not a very good idea! Keep this in mind 🙂


What did I eat after my run?

After my marathon, I had a huuuuge carb refeed. I ate bread, tapas, chocolate croissants, Burger King, some snacks, chips, and I drank beer & a cocktail. I have no idea how many calories I consumed, but after being so strict and dedicated for so many months, I just felt like eating & drinking everything I hadn’t been consuming during the months before my marathon. I could really enjoy the food at that moment, but afterwards I felt terrible! Eating those wrong types of food probably increased oxidative stress, and dampened down my immune system. The day after the marathon I only had one keto meal, and the day after that I went straight back to my normal Keto Diet. Although I went back to it pretty fast, I felt miserable for at least 3 days: no concentration, lethargy, skin problems, lack of purpose, feeling sad for no reason, and low energy levels. Is this what they call it the post marathon blues? I would say, blame it on the sugars… 


Keto running versus carb running: which differences did I experience?

Running on carbs

Running a marathon on keto - Lots of Keto

My first marathon was pretty much a roller coaster. During my training period (mostly the same training program as my second training), I developed a shin injury so I wasn’t my fittest self at the start. Mentally, I experienced so many thoughts during the run like: why am I doing this, what kind of crazy person am I? Physically, it was even more like a roller coaster. The first 10 kilometres went fine, but after these I started to get cramps & energy dips. After literally every hour I took a sports gel due to drained energy levels. I went from energy dips to energy spikes, which was definitely not an enjoyable feeling. You can see it pretty clear in the graphic on the left.


I was so glad when I finally arrived at the finish line, and after finishing within 4 hours, and 36 minutes, I was completely exhausted. I spent the rest of the day sitting on my bed, and the 2 days afterwards the soreness was killing me.


Running a marathon on keto - Lots of KetoRunning on keto

My second marathon, however, was pretty much the opposite of my first run. During the first 10 kilometres I felt okay, but not great. But, after those first kilometres (it may sound crazy), I could feel my body switching to using fat for fuel. My average average speed increased, but in a way I could maintain a steady pace. After 30 kilometres, instead of hitting the wall, my body and legs felt lighter, I experienced mental clarity, and I passed many people who looked like they were suffering. I was really able to enjoy the run, and was not worried whether I would hit the wall at a point: go keto! On the right, you can see the average speed during my second marathon. It’s incredible to see my average speed only increased! 


I finished strong with a sprint, in 3 hours and 47 minutes, and I almost broke my previous time with an hour. And, the best thing: I felt great, and was able to smile, and jump around. And this without eating anything along the way! The days after my marathon, my body was less sore and less swollen compared to last year. The only thing I experienced after having a carb refeed, was a mental challenge. This was probably caused by the inflammatory products I consumed. Also, the fourth day after the marathon, my hip felt quite stuck, and I could feel my injury from last year a bit. Not sure how to explain this. 


Conclusion: why you should give running a marathon on keto a shot!

Now, after the event, I realize it is a pity I did not measure any biomarkers such as fat percentage or blood ketones before starting keto, during my training period, and after my marathon. It should also be noted, that I did not experience any injuries during my second marathon. What if I got this injury because I was eating high-inflammatory food products at that time? It would be an interesting topic to research whether a Keto Diet could reduces the risk of getting injuries! 😉


Looking at these infographics, knowing the science behind running on keto, and just thinking about the energy levels and mental clarity I felt along the way, maybe already says enough. Keto has given me more confidence to set new goals (maybe a triathlon?!) and the experience of feeling the runners high. Of course, there are certain things I would do differently during upcoming races. But in the end, it’s all about the feeling you get from such an experience, and becoming a better person one run at a time. After all, the most important thing is to love what you do and be able to enjoy the journey. You can not have a happy ending without a happy journey!


I think it’s about time more runners should embrace fats, and give running a marathon on keto a shot! Not a sugary one, but an electrolyte shot 😉 


Are you a runner considering running a marathon on keto?

Are you considering running a marathon on keto yourself, and could you use help with your nutrition plan? Don’t hesitate to get in touch with me!


Important note to this article

This story is a personal experience! It depends on both the individual athlete and the event, if this approach is the right for you. Every athlete responds in a different way and should therefore always consult their doctor before implementing any extreme dietary change. Results all depend on your overall body composition, which food products you implement in your nutrition plan, your training program, how long you have been fat adapted, and the duration of your race.


More successful keto endurance stories

Ultra marathoner Alfie Pearce-Higgins completed his first ultra-marathon on keto, and also just shared his experiences in an article. Read it here!


Stay tuned, another personal experience story from one of my Instagram followers will be online soon! It’s the inspiring Sarah from, a passionate fat-fueled (trail) runner who enjoys her journey every step of the way.


Sources I used for this article

Jeff S. Volek & Dr. Stephen Phinney. The Art and Science of Low Carbohydrate Performance.

Ben Greenfield: Extreme Endurance Training & Ketosis

Timothy Noakes. The Lore of Running.

Running a marathon on Keto: this is why you should try it!

2 thoughts on “Running a marathon on Keto: this is why you should try it!

  • December 9, 2018 at 12:05 pm

    Thanks, good to see more of this 🙂

    (For context, I (have) run many marathons and ultras up to 50k on only water, and consumed <~120g Cho on typical 90km races)

    • December 10, 2018 at 10:24 am

      Wow, that is amazing!! And indeed, I am sharing this story so hopefully other runners will try it themselves as well, and experience how it can improve their performances! Have you written about your experiences as well? Would love to hear more about it! My next goal is probably going to be an ultramarathon.

      Also read this article, which was published on the same day:

      I tried to find out how many other runners were on keto during the Valencia marathon, but I couldn’t really find a way on how to do so! Was super curious if there would be more runners 🙂


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