Could A High Fat, Low Carb (Keto) Diet Improve Your Running Performance?

  • What are the benefits of a high-fat, low-carb (ketogenic) diet for ultra-runners?

  • Will a keto diet help you achieve your running goals?

  • And what are the specific foods that I use when training and racing on a keto diet?

One of the questions I’m asked the most is ‘what do you eat and drink on long runs of 25+ miles and in ultra-races?’

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

Taking a break during   my recent world record attempt   to run more than 800 miles in 9 days - and taking in my high-fat almond butter smoothie!

Taking a break during my recent world record attempt to run more than 800 miles in 9 days - and taking in my high-fat almond butter smoothie!

So, I wanted to give you an overview of the what’s 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. Just give you my insight and my understanding of the approach.

I’ll also provide you with some of the useful websites and resources that I use to guide my thinking.

You’ll find out:

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

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

  3. Is the ketogenic approach right for your running goals?

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

  5. What specific ketogenic-friendly foods do I use in training and races? - Including ideas for different situations like breakfast, lunch, dinner, snacks, eating out, training and ultra-races. And the foods I try to avoid

  6. What are the potential side-effects of running on a ketogenic Diet?

Disclaimer: I’m not a nutritionist and you should always consult a trained professional before changing your diet. The points in this article are based on my personal experiences. If you apply any of the advice in this article, it is under your own will. And I will not be responsible for any side effects.

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

Here’s a simple explanation of the three most common low-carb approaches from

  • 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 and that 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

Comparison of Keto, Atkins and Paleo Diets from (for

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 where 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 and so you don’t get the intense highs (and lows) you get with carbs.

We also have a huge amount of calories stored as fat in our bodies. Much more than carbs.

So fat can keep you going longer.

(And, no, for the men out there, I’m afraid that’s only referring to exercise… not other areas of life… at least it hasn’t proven for me!)

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.

This article also explains the broader lifestyle benefits of a low-carb diet.

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.

And, 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:

Whether or not I’m training, the piece that stays consistent in my diet is using fat as a primary micro-nutrient
— Zach Bitter - American World Record holder for 100 miles

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?


Which is pretty much the opposite of a low-carb, high-fat diet!

But despite the difference approaches in diet from elite athletes, there is one common thought among them, summed up by Ben Greenfield

Don’t eat processed crap
— Ben Greenfield

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:

  1. 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

  2. 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

  3. 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. And 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

  4. 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 and 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!

These macadamia fat bombs were a great high-fat treat

These macadamia fat bombs were a great high-fat treat

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 effective 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:

  • Reduce your training intensity - You could even plan to have a rest week or ‘down’ week when you are taking on the ketogenic diet at first

  • Stay hydrated - Make sure you drink plenty of water, even when you’re not exercising

  • Replace electrolytes - You could take a supplement to do this, or focus on eating foods like avocados and leafy green vegetables

  • Get lots of sleep

  • Make sure you eat enough - When removing almost an entire food group, it may lead to significant calorie losses

Training on a Ketogenic Plan

One of the main reasons for using a ketogenic approach in ultra-running is that it gets your body better at using fat as it’s primary energy source.

This is often called being ‘fat-adapted’.

Being ‘fat-adapted’ is good because we have lots more fat stored in our body than carbs.

And this is particularly important for ultra-runners, as we are competing for long periods of time, where we will rely on fat as an energy source.

Your training can play a big part in getting your body ‘fat-adapted’ too. There are a few things you can do:

  • 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. But 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 it can be a good idea to eat some carbs before, and during, the workout. This is often known as carb-replensishing 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.

Here is a simple overview of the types of foods I typically eat in different situations.

If you want a full list of what you can and can’t eat on a strict ketogenic diet, then read this article.

You’ll also notice that some of the foods below are on the ‘don’t eat’ list in that website.

But I have adapted my diet over time to what works for me as an ultra-runner.

You can see an example of Zach Bitter’s diet here.


My standard breakfast is to have 2 or 3 eggs, with tinned sardines or mackerel and some spinach.

Sardines, eggs and coffee are my standard breakfast. Very high fat. Very low carb. But also very smelly!

Sardines, eggs and coffee are my standard breakfast. Very high fat. Very low carb. But also very smelly!

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.

This article has a full list of which vegetables and salad to eat and avoid.

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.

And if they don’t, you can just leave those on the side of your plate at the end of the meal.

One of my favourite low-carb swaps is when we’re having fajitas or tacos at homes.

I simply use romaine lettuce leaves as the wraps and avoid the usual tortillas. Still delicious although they can get a bit messy!


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 crisps.

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% Lindt dark chocolate - You can buy it using this link

Try to avoid products which are labelled as low-fat, as these often have a lot of hidden carbohydrates.

Typical “low carbohydrate” meal replacement bars and shakes, ice creams or ice cream sandwiches, and other low carb or sugar-free snacks often contain potentially unhealthy ingredients
— Ben Greenfield -


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.

If I am drinking alcohol, I try to have vodka, fresh lime and soda. Which is quite low in calories and comparatively low in carbs compared to other drinks. Although I sometimes drink red wine too.

I avoid beer and cocktails as they are very high in carbs.

Keto Foods for Long Runs and Races

Sticking to a keto plan whilst 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…

But there are 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.

And you shouldn’t be doing anything new on race day.

You should have thoroughly practised what you’re going to eat before and during your race

But here is what has worked for me.

Maybe you can try these out in training and they may work for you.

  • Pre-Race

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 up all my energy sources that my body will use.

However, it’s important that you’ve practised this in your training too. Use your less important races (‘B’ and ‘C’ races) and long training runs to practise 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 flavour is my favourite!

Pip and Nut nut butter sachets   have worked incredibly well for me in my races. They’re easy to digest, high fat and taste good!

Pip and Nut nut butter sachets have worked incredibly well for me in my races. They’re easy to digest, high fat and taste good!

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’ll 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 usually 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.

Pip and Nut nut butter sachets are one of the key foods I rely on for my training and ultra-races. Including my wins at 100km and 100 mile events. You can buy them using the link below on Amazon. Plus, some of the other foods that I use, including electrolyte tablets and baby food!

I also 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.

And on particularly hot or humid days, you will need to increase the amount you take in.

I also use High-5 electrolytes mixed with my water. You can check them out here.

This is to keep my electrolytes up, but also because they help make the water taste better.

You can get very bored of plain water over the course of a race!

Lemon and lime flavour are my favourite!

And I’ll always try to avoid the coca-cola and juices that are on offer at aid stations. This is 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.

I use an adaptation of the low carb peanut butter milkshake on this page. I replace the peanut butter with almond butter, because almond butter is even more keto-friendly.

And baby food is a god-send at times too. Although I tend to go for the sweet, fruity ones.

I don’t think a mashed up lasagne or bolognese would taste that good at mile 80!

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 situation.

So it’s always a good idea to check out what the aid stations at a race 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. Organisation 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 can not only lead to losing time, but also to lots of stress.

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

Do you want help in organising yourself before a race?

Then grab your FREE copy of this pre-race checklist. Which can help you plan everything you need for before and during the race.

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.

But there are some possible negative side-effects too.

I haven’t actually experienced many of these, but lots of people have.

This article explains the possible side-effects of a low-carb diet in your broader life.

  • 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.

But it’s not great if you need some intensity - like for shorter races.

So 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.

And for these sessions your body will primarily use carbs as fuel.

So you will need to eat more carbohydrates before the sessions.

If you’re on a strict low carbohydrate diet, you can say goodbye to intense weight training, track intervals, or just about any activity that would be consider “tempo”, “threshold”, or “intervals”.
— Ben Greenfield -

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.

But the keto diet is particularly difficult to stick to in real life.

Firstly, 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.

So 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

Check out this carbohydrate reference list to find out the carbohydrate counts for a wide variety of food.

And that’s another negative thing about this diet. It is very restrictive. There are lots of foods on the ‘can not 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.

And most restaurants offer dishes with bread, chips or pasta. Or it’s a pizza.

But I have found that most restaurants are good if you ask them to swap the bread or chips 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.

But 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.

And it can take even longer to see the benefits in your training and racing.

So you have to stick with it for at least a month, in my experience.

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

I’ve already covered the side effect called keto flu. Which can be particularly bad for athletes who are doing a high volume of training.

And there are other physical symptoms that some people report, including headaches, bad digestion when training or racing, general tiredness, losing too much weight 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 so are just piling on more calories.

Or they don’t recognise just how calorie-heavy most fatty foods are. And they end up over-eating.

You also 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!

Low carbohydrate diets, if implemented improperly, result in low fiber intake from a sharp reduction in plant-based food consumption, which can increase risk of digestive cancers and cardiovascular disease, and also leads to constipation and bowel issues
— Ben Greenfield -

Ben Greenfield, a prominent expert in the triathlon industry and known as "The Brain" of triathlon, is “a pretty big fan of limiting your carbohydrate intake”.

But he also outlines some of the longer-term, non-sporting side-effects of a low-carb approach in this article.

A drop in fruit, vegetables, legume and grain consumption can result in inadequate phytonutrient, antioxidant, vitamin C and potassium intake
— Ben Greenfield -


  • A low-carb high-fat diet MIGHT improve your running performance - But it largely depends on your goals race distances

  • It will probably be better for those training at very long distance - ultra-races and above. Some slower marathon runners may also benefit

  • Starting a ketogenic diet is very difficult - It takes a lot of effort and some time

  • There are lots of specific keto-friendly foods and strategies for ultra-runners - But you’ll need to practise which works best for you

  • There are some potential side-effects of a ketogenic approach

What Do You Think About the High-Fat, Low-Carb Approach for Ultra-Runners?

Let me know in the comments at the bottom of this blog, on the Contact Page or on social media below. 

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