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Can a Keto Diet Improve Your Running Performance 

By: James Williams

One of the questions I’m asked the most is ‘what do you eat and drink on long runs and in ultra-races?’

When I say that "I use a high-fat low-carb approach", that leads to a lot more questions.


So, I wanted to give you an overview of the what has worked for me as I’ve trained and raced for ultra-running on the ketogenic (keto) nutritional approach.


I also want to make it clear that I am not trying to convince you to take on this type of diet. My goal is to give you my insight and understanding of the approach.


You’ll find out:


  • What are the different types of high-fat, low-carb plans?

  • Why is the ketogenic approach believed to be good for ultra-running?

  • Is the ketogenic approach right for your running goals?

  • How to start a ketogenic diet as an ultra-runner.

  • What specific ketogenic-friendly foods do I use in training and races?

  • What are the potential side-effects of running on a Ketogenic Diet?

What are the Main Types of High-Fat, Low-Carb Plans?


Keto - The keto diet is a low-carbohydrate diet that leads to nutritional ketosis. This can be measured by the amount of ketones that your body is producing. This diet relies on variable amounts of fat and protein to work. The central feature of keto is its macronutrient ratio.


Paleo - The Paleolithic (paleo) diet is a diet based on what our ancestors supposedly ate some 12,000 years ago. Paleo excludes refined and highly processed foods and even all grains and legumes. Paleo does not cause nutritional ketosis.


Atkins - This plan is designed for weight loss, which does not rely on ketosis. The diet is carried out in roughly 4 phases, with the last phase reintroducing a moderate intake of low-GI carbohydrates.

Why is the Ketogenic Approach Believed to be Good for Ultra-Running?

Our bodies rely on two main energy sources to keep us going - carbohydrates and fat. Here’s my understanding for the use of both of them, in the context of running.

Carbohydrates Are Good For Shorter Runs


Carbohydrates are the body’s primary source of energy. Because our typical diets are very high in carbohydrates (most are more than 50%), this makes sense as we’ve evolved to rely on them.


Carbs are the best energy source for when your body needs a short, sharp burst of energy. Think about the 2pm slump in the middle of the day, when you automatically start reaching for the chocolate to help get you through, or if you were exercising and you need a good amount of speed.


This could range from 100m up to a marathon for some people.The problem with these short, sharp bursts of positive energy is that they’re often followed by a big crash. We also have a very limited amount of carbs stored as calories in our body. So we need to refuel very often if we are using them.

Fats are Good For Longer Runs


Fat is a better energy source for longer, sustained periods of movement. This is because it burns slower, so you don’t get the intense highs (and lows) you get with carbs. We have a huge amount of calories stored as fat in our bodies. Much more than carbs. So fat can keep you going longer. Seems like the perfect fit for ultra-running, right?

But it doesn’t just happen overnight when you decide to eat a low-carb diet. Our bodies have been used to using carbs as the primary source of fuel for centuries, and it takes some effort and time to teach your body to learn how to use fat more effectively as an energy source. 

Is the Ketogenic Approach Right for Your Running Goals?

As with almost any question that runners have, the first answer is always, ‘it depends’. First, it depends on your goals. If you run marathons at a pretty good pace, or you run shorter races, then carbohydrates are probably still a good option for you. If you run marathons at a very casual pace, or you run in ultra-marathons, the low-carb approach might be something to experiment with. Of course, there are people who succeed using various different approaches. Zach Bitter is one of the most successful examples of someone using a high-fat diet. His achievements include: The 100-mile trail World Record - with a quick time of 12:08:36.The American 100-mile record - in 11:40:55, which is an average of 7:01 minutes per mile, or doing almost four marathons at 3:03 pace.

But at the opposite end of the spectrum is Scott Jurek. Scott is one of the most well-known ultra-runners of all-time, with multiple victories in some of the most competitive races in the world. And what’s his diet? Vegan. Which is pretty much the opposite of a low-carb, high-fat diet! But despite the different approaches in diet from elite athletes, there is one common thought among them, summed up by Ben Greenfield: "Don't eat processed crap."

How Do You Start a Ketogenic Diet as an Ultra-Runner?


Starting a ketogenic diet as an ultra-runner can be difficult because you need to:


Do a lot of research - On what foods you can and can’t eat. This takes a long time to get used to, because carbs are absolutely everywhere.


Carefully plan and read labels - The ketogenic diet can be difficult in certain situations, like eating on-the-go or eating out. So you often need to think about what you’re going to eat in advance.


Find appropriate foods for very specific situations - As runners, we all know that deciding what to eat in long training runs or ultra-races can be very difficult. The restrictive nature of the ketogenic diet makes this even more difficult. Say goodbye to those race gels, Haribo, and peanut butter and jelly sandwiches.


Go ‘all-in’ - If you’re following a strict keto diet, you need to fully embrace the change. In the initial phases of the diet, you need to eat as low as 20g of carbs a day. That’s less than one banana!

It’s worth noting that my approach to diet has changed over the two years I’ve been using it. At first, I did follow a strict keto diet, but my thinking has evolved. I do increase my carb intake at certain periods - before key training runs and races - to improve performance - and after races - when I want to treat myself! Zach Bitter, the 100-mile trail world record holder, says you can expect a three to four-week “induction phase”. This is where your body begins to learn to use fat more effectively as a fuel source. During this time, he urges you to pay attention to keeping carb intake as low as possible so your metabolism can make the switch and get good at burning fat. After that, he suggests that some carbs can be introduced in small proportions.

The Keto Flu and Ultra-Running

If you’re trying to follow a strict ketogenic diet, you may experience the ‘keto flu’ in the first few weeks. This is where you may get side-effects including nausea, constipation and headaches. Not all people have these symptoms and for those that do get them, they usually last about a week. This can be particularly bad for ultra-runners as it can impact your training, but there are a few things you can do to reduce the symptoms:


Train at a very low intensity - Most of your running should be at a very low intensity anyway. This is to avoid injury and burnout. Another good reason is that it gets your body used to burning fat as the primary source of fuel.


Do ‘fasted’ runs - This is a slightly more advanced technique and should be used with care, especially by newer runners. This is where you do runs, often long runs, without any food before or during the run. Most people do this as their morning run, where their last food was the main meal early the previous evening. One safety precaution I would advise is to take some foods with you on the run, just in case you do need to use them.


Carb-replenishing - Some of your training will be done at a higher intensity. For example, speed sessions, tempo runs, fartlek, hill repeats, etc. To make the most of these sessions, eat some carbs before, and during, the workout. This is often known as carb-replenishing or carb-refeeds.

What Specific Ketogenic Foods do I Use in Training and Races?

This is the question that I’m asked most often and it’s taken me a long time to tweak my diet for something that works for me. You might notice that some of the foods to the right are on the ‘don’t eat’ for most keto diets but I have adapted my diet over time to what works for me as an ultra-runner. Here is a simple overview of the types of foods I typically eat in different situations.


Breakfast: My standard breakfast is to have 2 or 3 eggs, with tinned sardines or mackerel and some spinach. This is quite smelly and means that I often find myself sitting separately from work colleagues in the morning.


I used to boil eggs (which take ages and is fiddly). But my ‘hack’ is to whisk them in a bowl and microwave them for about 1 minute. Other breakfast options I eat include avocados and meats. I avoid toast, jam, honey, cereals and milk, which all have high amounts of carbs. Although I’m very partial to a bowl of coco-pops if I’m in a ‘down’ period!

Lunch and Dinner: I’ll usually have meat with vegetables or a salad. Although if you’re being strictly ketogenic, you need to avoid a lot of vegetables. I avoid pasta, bread, rice and sweets. I also try to stay away from most sauces, which are high in sugar and carbohydrates.


Eating at restaurants can also be particularly tough, as most meals come with a side of chips, rice or pasta. I’ve found that if you ask to swap these for something else, like broccoli, or a side salad, most places will do this. If they don’t, you can leave those on the side of your plate at the end of the meal. One of my favorite low-carb swaps is when we’re having fajitas or tacos at home. I simply use romaine lettuce leaves as the wraps and avoid the usual tortillas. Still delicious, although they can get a bit messy!

Snacks: Eating on-the-go is one of the most difficult aspects of the ketogenic diet. This is because most snack-type foods in shops are full of carbs - Sandwiches, wraps, chocolate and chips. My main snacks are nuts - Brazil nuts are a particularly good source of high fat and are low carb. I also tend to snack on cheese and, if I’ve been very prepared, boiled eggs. I will often also have a bowl of berries with mascarpone or full-fat cream at home and then for a real treat, I have 90% dark chocolate. Try to avoid products which are labeled as low-fat, as these often have a lot of hidden carbohydrates.

Drinks: My standard drink is water and I rarely have any sugary juices or energy drinks. I usually have black coffee. Although I sometimes add a tablespoon of coconut oil. This is often referred to as ‘Bulletproof coffee’. You can add butter to it, to make it really keto-friendly. I avoid beer and cocktails as they are very high in carbs.

Keto Foods for Long Runs and Races


Sticking to a keto plan while actually out training and racing for ultras is one of the most difficult parts of the diet. This is because many of the keto-friendly foods are not that easy to carry and eat in a race situation. There are, however, a number of things that I’ve found work for me. Remember, one of the most important aspects of a nutrition plan for racing in ultras is that it will almost certainly be unique to you. You shouldn’t be doing anything new on race day.


But here is what has worked for me. Maybe you can try these out in training and they may work for you.




In the 2 or 3 days leading up to a race, I do introduce more carbohydrates to some of my meals. For example, I may have pasta with chicken or fish and a very light tomato-based sauce. This is because I want to top off all my energy sources that my body will use. However, it’s important that you’ve practiced this in your training too. Use your less important races (‘B’ and ‘C’ races) and long training runs to practice what you do in the days before.


During A Race


I actually have a pretty specific nutrition plan for ultra-races. In the early stages of the event I will primarily use Pip & Nut nut butter sachets. The coconut almond flavor is my favorite! I also use unsalted mixed nuts and raisins. I tend to alternate between these two roughly every 20 - 30 minutes. The timing depends on the difficulty of the section that I’m doing, plus the length of the race. If it’s a difficult section or a longer race, I tend to eat more often, to keep my energy up. One of the biggest mistakes I see athletes making is not eating enough early in ultra-races. Especially longer ones. You don’t want to get into a big calorie deficit very early, and so I recommend to eat smaller portions, but more often. One trick is to set your watch so that it beeps every time that you need to eat. So every 20, 25 or 30 minutes, depending on your strategy.

I tend to try and eat when I would be walking anyway. So if there is an uphill section where I’ll be walking, I will eat. Even if it means it is slightly earlier than usual. I also like to try to eat solid foods early in ultra-races, which is why I use the mixed nuts and raisins. This is because I know that I probably won't be able to eat solid foods later in the race. I put these nuts and raisins in sandwich bags and make sure that I can get to them easily.


Staying hydrated is absolutely critical for an ultra-race. This is an area that athletes can get wrong. They often don’t think they need to be drinking because they are going at such a slow pace. But that is a trap that you shouldn’t fall into. Make sure you’re following the rule of ‘small, but often’ with drinking too. On particularly hot or humid days, you will need to increase the amount you take in. I’ll always try to avoid the coca-cola and juices that are offered at aid stations, because I want to avoid the sugar crash that comes after the high.


In the later stages of the race, when you may no longer want to eat solid foods, I bring out the big guns! I’ve used my almond butter smoothie for multiple races now, which tastes delicious, packs a lot of calories and goes down very easily. This is an adaptation of the low carb peanut butter milkshake. I replace the peanut butter with almond butter, because almond butter is even more keto-friendly. Baby food is a god-send at times too. Although I tend to go for the sweet, fruity ones. 

The trap that many ultra-runners fall into in races is eating anything they see at aid stations.You start picking up things that you haven’t tested in training, which can have big impacts on your stomach in a race. It’s always a good idea to check out what the aid stations will offer months in advance of the race. That way you can test some of it in training. Plus, you can prepare your drop bags very thoroughly, with all the nutrition that you’re likely to use. Organization is the key with races. Make sure that your bag is neatly packed, and use labels where possible. You don’t want to be messing about in aid stations trying to find stuff in your bags, which cannot only lead to losing time, but also to lots of stress.

Another tip is to take a few extra sandwich bags. Then you can stock up on food that takes your fancy and can quickly move through the aid stations, rather than stopping.

What are the Potential Side-Effects of Running on a Ketogenic Diet?

We’ve looked at the positive aspects of a keto plan for ultra-runners. There are, however, some possible negative side-effects too. I haven’t actually experienced many of these, but they are common.

Using It For The Wrong Reasons

The ketogenic diet works because you teach your body to burn fat more effectively, instead of using carbs. Fat burns much more slowly than carbs. Which is good if you’re racing at a slow pace and need to go for a long time, like ultra-racing. It’s not great if you need some intensity - like for shorter races. If your goal is to complete marathons or below at a good pace, the ketogenic diet probably won’t be the right one. Even if your goal is a very long distance race, you’ll still have harder sessions in training. For example, you should still be doing some speed work, hill repeats, and weight training. For these sessions your body will primarily use carbs as fuel. Therefore you will need to eat more carbohydrates before the sessions. Some studies have even shown that the ketogenic diet hurts efficiency, even in longer distance races.

It's Difficult

Like with any new nutrition plan that you try to take on, the ketogenic diet isn’t easy. The keto diet is particularly difficult to stick to in real life. First off, carbs are absolutely everywhere. Once you get into the details of what’s included in a piece of food or drink, it astonishes you at how many carbs are included. A lot of people say that to get into ‘ketosis’, which is the physical state that you are aiming for, you need to eat less than 20g a day for 2 - 3 weeks. That’s less than one banana a day. One banana has 23g of carbs. Once you are into ketosis - which you can measure by buying a ketone testing kit - you are then allowed up to about 50g of carbs a day. So… that’s two bananas. Imagine how difficult it is to stay under these limits, when: One slice of bread has 17g of carbs, two weetabix have 25g of carbs, one can of cola has 36g of carbs. 

That’s another negative thing about this diet. It is very restrictive. There are lots of foods on the ‘cannot eat’ list. Which makes it really difficult when you DO have the time to read labels for your own shopping and in the leisure of your own home. But it’s much more difficult when you DON’T have the time or you’re in restaurants, where you can’t count carbs very easily.

Almost all snacks in supermarkets are full of carbs - sandwiches, wraps, pastries, etc. I often find myself eating a lot of nuts and some fruit where there are little other options. Most restaurants offer dishes with bread, fries, pasta, or pizza, but I have found that most places are fine with swapping out the bread or fries for a vegetable or something similar. 

It’s also difficult to do it if the people closest to you aren’t following the diet. My family and I found it tough at first. We were trying to make meals that fit their standard diet AND my low-carb approach. Now we’ve got into the habit of having similar meals where I just remove the main carbs.

So we’ll have spaghetti bolognese and I just won’t have the spaghetti.

The diet can also take a long time to adapt to and reap the benefits. I’ve already explained the keto flu, where you start to feel worse before you get better. It can take even longer to see the benefits in your training and racing! In my experience, you have to stick with it for at least a month.

Most people give up way before they get to that point. It’s also very easy to lose the main benefits of this type of training. A few days or a week of messing up the diet can really set you back!

Physical Side Effects

There are physical symptoms that some people report, including: headaches, bad digestion when training or racing, general tiredness, losing too much weight, and/or gaining weight unexpectedly. From my experience the general tiredness problem has usually been because I haven’t eaten enough calories for my training intensity. Which is a problem you have to be wary of when you cut out an entire food group, like carbs. This can also lead to drastic weight loss, which isn’t good. On the opposite side, some people end up taking on far too many calories and gaining weight. This can happen because they haven’t restricted their carb intake enough, and are just piling on more calories, or they don’t recognize just how calorie-heavy most fatty foods are and end up over-eating. You stop eating certain, beneficial food groups, like fruit and vegetables, which can result in other health issues. I keep a lot of fruit and vegetables in my diet. Which strict keto-ers would laugh at me for!

As with any diet there are some benefits, but also some drawbacks. Talk to your physician and a registered dietitian to see which one would better suited to your needs. 

About the Author

James Williams is a father of two, husband, and runner with race victories at 100 miles, 100km, marathons, half-marathons, 10k's and 5k's. Most recently, he attempted to break a world-record by running more than 800 miles in 9 days from the bottom of the UK to the top. He writes informative articles to help other runners improve their own performance and achieve their dreams on

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