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Christopher Bradley  •  Training •  29.07.2022 •  10 min read

The high fat, low carb approach to endurance sports

What should a runner eat on a daily basis to fuel their training? What have you been told? Maybe pasta? Porridge in the morning? Is pizza okay? Brown bread?

In a word: carbohydrates. The conventional wisdom for years has said that if you want to use your body like a high performance vehicle, instead of petrol or diesel, you need to run on carbs. 

However, if you’re an athlete focused on longer distance events, be that the marathon, ultra trail, or Ironman distance, you might be better served investing some time in a high fat, low carb diet.

*Please note, most of the scientific literature on high fat, low carb diets in sports was conducted with male test participants. With females being more susceptible to relative energy deficiency in sport (RED-S), this approach may not be as suitable for females as males.

Why do we rely on carbohydrates for endurance training?

Glucose is formed from the breakdown of carbohydrates in our diets and is stored as glycogen in the muscles and liver. When we exercise at higher intensities, we use this glycogen as an energy source.

Carbohydrates are quick for the body to break down, and they yield a large amount of energy.

They are the body’s main and preferred fuel source. Carbohydrates are also heavily featured across the Western diet, so it’s far more convenient for athletes to utilise them in all aspects of their training.

Drawbacks to carbohydrates

Carbohydrates sound pretty neat. Easy, convenient, high octane fuel… very cool. But while that’s fine if you’re focusing on the 5K, 10K, maybe even the half marathon, there’s a duration of intensity beyond which being heavily dependent on a high-carbohydrate diet can be a disadvantage. 

Depending on an individual’s physiology, they will be able to store between 60 to 90 minutes’ worth of carbohydrates to use as energy. Beyond these, they will need to top up their carbohydrate stores with gels and drink mixes if they wish to keep using carbs as their primary fuel source.

In longer endurance events, one of the main limiting factors for athletes is running out of carbohydrates, and then being physically unable to take on enough exogenous carbohydrates to keep working at that same intensity.

Man running in the woods

Misconceptions about fat burning zones

The other potential source of energy we have to use is by metabolising our fat stores - breaking them down into ketones which the body can then convert into energy. Fat doesn’t yield as much energy as carbohydrates and it takes longer to break down. 

In theory we should be burning fat at lower intensities, so it would be fair to assume that as an athlete burns through their endogenous carbohydrate stores and struggles to take on enough extra to sustain their pace, they could lower their intensity and switch to using their fat stores. 

Unfortunately, one side-effect of a carbohydrate-rich diet through all of your training is that it has a negative impact on your body’s metabolic plasticity - its ability to switch from metabolising carbohydrates to metabolising fat. Much in the same way that if you neglect your easy running your overall endurance and long-term improvement will suffer, if all you do is shovel down high-octane fuel you’ll become far less efficient at pacing long-haul endurance events. 

The tanker analogy

A good way to think about it is to imagine that you’re a lorry carrying an enormous fuel tanker. 

In this lorry, you’ve got your 60 to 90 minutes’ worth of endogenous carbohydrate stores, but in the oil tanker, you’ve got 60-80,000 calories’ worth of fats.

In theory, we’ve all got the capacity to work at a relatively high percentage of our threshold off of those fat stores for a very long time, far longer than 60 to 90 minutes. The problem is, most athletes aren’t able to connect to that enormous fuel source because they don’t have the necessary adaptations to metabolise fat, even at lower exercise intensities. 

Eating to compliment your training

To be clear, adopting a high fat, low carb diet is not the same as a keto diet, which breaks down to less than 50 grams of carbohydrates a day compared to a standard Western diet that consists of around 600-800 grams. A ketogenic diet can be incredibly restricting. To cap your carbohydrate limit at 50 grams is quite extreme, and would be virtually impossible to adhere to if your partner (or roommate, or family) wasn’t also onboard, and that’s discounting the issues you’d encounter navigating social situations involving food.

A high fat, low carbs-approach to eating for performance ultimately breaks down to dieting in a way that compliments your training. Eat carbohydrates when you need it. You don’t do the same training everyday so why would you eat the same thing everyday. 

Benefits of high fat, low carb

A low carb-diet, in the context of this approach, would see an athlete cap their daily carbohydrate intake at between 130-150 grams. 

The aim is to have the adaptations to tap into your body’s huge energy reserves in fat stores, while still being able to utilise carbohydrates when your training requires that high octane fuel - be it for hard workouts or for race day. 

How to begin getting fat adapted

While it would be incredibly difficult to sustain a keto diet while working and training, incorporating it for two weeks to kickstart your body’s ketone-production would be an excellent way to begin.

A non-negotiable early step in getting fat-adapted would be to start doing all your running early in the morning, typically in a fasted state. Liver glycogen is depleted overnight (because sleep is an active process) so there’s not as much glycogen readily available for your body to use in the morning, so an early morning fasted run can create optimum conditions for the breakdown of fats into ketones.

Run easy

There is some truth in the fat burning zone myth. You can’t just wake up and rip out an all-or-nothing 10k PB effort in a fasted state and expect to develop fat-burning adaptations, the energy output from fat just isn’t high enough for that intensity, so your body won’t try to use it. (You’ll probably just feel like steamed garbage.)

Initially, you’ll need to do a bit of benchmarking, as you can’t just cookie-cutter heart rate ranges and assume everyone will respond in the same way. On your first fasted run, go out and run at an easy pace and see how it feels - monitor your HR if you’re able, and be conscious and honest about your energy levels. You should feel no different at the end of your run to how you feel at the start. 

If you went out and ran a bit below your 10k-pace, and halfway you feel like you can’t sustain it, that’s your first sign that you’re trying to break down glycogen and there’s not enough of it to fuel your effort. You’re literally running on fumes. Use whatever tools you’ve got to monitor your effort levels and keep it easy to encourage that fat adaptation.

In daily practice you eat for your training

What we eat before we train has a huge effect on what fuel we use. If you do all your training in the evening and don’t think too much about what you eat, you’re probably going to be quite carbohydrate dependent, even if you’re running below your maximum aerobic function (MAF) calculation. 

If you were able to run at lunchtime, you could potentially just eat something rich in healthy fats like eggs or avocado for breakfast. Much in the same way that you might eat porridge and fruit before a race to top up your endogenous carbohydrate stores, taking on a healthy high fat meal just before you go for your run will encourage your body to use that as its fuel source.

Woman running in the woods

Low carb doesn’t mean low calorie

Similarly, just because you’re going low carb doesn’t mean you shouldn’t eat to fuel your training. It wouldn’t be wise to go for longer than a one-hour run in a fasted state. 

Most people get low carb wrong because they end up eating too little and then under fuelling. This will always result in a decline in performance. 

Executing a high fat, low carb approach long term

The dietary approach this article is examining here might be better described as high fat, necessary carbs. Hard kilometre intervals and short, high intensity hill reps require hard, high intensity fuel… and as most of you already knew, that’s carbs. 

The idea behind this is to develop metabolic plasticity as an endurance athlete. To be able to utilise carbohydrates on race day and reap their undeniable benefits, but be able to tap into the body’s fat reserves when the pace slows and taking on additional calories becomes difficult. (And potentially messy.) 


A high fat, low carb approach to eating may be an incredibly useful tool if your focus is on long endurance events such as Ironman distance triathlons, ultra marathons, and even marathons. Its worthwhileness and efficacy only improves as the duration of the event increases and the risks of bonking and gastric distress increase.

If your focus is on the half marathon and shorter distances, a high fat, low carb diet probably isn’t worth your time, and could stunt your efficiency at metabolising carbohydrates. 

Ultimately, no one diet fits all disciplines and all athletes. This is just one possible approach that, if diligently implemented, could yield massive performance results for endurance athletes focused on events of very long durations. 

“I think what surprised me the most about low carb, high fat diets is just the ability for athletes to perform at an intensity on no carbohydrates that in the past you just would have thought was impossible. 

“Also, the quality of training massively improves because you’re not just emptying the tank on every session.”

~ Finlay McAndrew, Run4It’s resident sports scientist, who has incorporated the high fat, low carb approach into his training and that of some of his athletes.

To listen to the full conversation, head over to the Run4It Podcast.

If you found this article helpful, you may also enjoy our How to fuel your marathon article.

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