Can you Run A Marathon On The Keto Diet?

runner trying to run up a mountain

Interested in keto and marathon training? You’re in the right place.

Marathon runners have been told for decades that carbohydrates are integral to their training routines. Carbs provide energy and give you long-burning sustenance while you’re logging the miles for hours.

What’s not to like, really!

But, over the past few years, the keto diet has exploded in popularity, and easy to see why.

Science has reported that going low-carb and high-fat can help change body composition, improve mental function, increase energy levels, etc.

However, adopting the keto lifestyle can be tricky if you’re training for a marathon.

In fact, the low-carb, high-fat nutrition plan is often looked down on by endurance runners, coaches, and sports nutritionists. For that reason, it’s not always easy to find trustworthy information about how to train for a marathon while doing keto.

Fret no more. Today I got you covered.

In this article, I’m going to share with you some advice on adopting the ketogenic diet as a long-distance runner. I’ll also share tips on making the most out of your keto diet while training for a marathon.

Can you run a marathon on the keto diet?

The short answer is yes. Over time your body will adapt. But the long answer is multifaceted. How long will the keto-adaptation take? Will you be able to run at the same level pre-keto? Etc.

To get at the bottom of this, let’s explain what’s the deal with the whole ketogenic lifestyle as well as present forward some arguments for going keto during endurance training.

What Is The Keto?

Assumably, you’re here because you’re interested in marathon training while following a ketogenic diet.

But, just in case you’re here by luck and don’t know much about the keto lifestyle, let me explain what it is all about.

In short, a keto diet consists of high fat and very low carb consumption.

The objective of the keto diet is to force your body to start using ketone bodies produced from stored fat in the liver to provide you with the fuel you need.

The main principle is simple – starve the body of readily-available carb fuel and force it to switch to fat as its primary fuel source.

If you start limiting your carb intake, your body will enter ketosis, a metabolic state in which the body relies on fat for fuel instead of carbs, which are often the main energy source.

To reach ketosis, you’ll need to limit your daily carb net intake to no more than 20 to 30 grams for four days or longer.

Sticking to Keto Diet

The main foods to consume on a ketogenic diet include

  • Eggs
  • Meats
  • Fish
  • Avocados
  • Olives
  • Non-starchy vegetables such as kale, broccoli, and cauliflower
  • Nuts and seeds.

Avoid the following items on the ketogenic diet

  • All grains, including oats, wheat, barley, rye, rice, and surrounding grains.
  • Sugar and in all of its forms
  • Processed foods
  • Quinoa
  • Alcohol
  • Fruit juice
  • Dried fruit
  • Some fruits
  • How To Get Started

Why People Go Keto

The high-fat, low-carb eating plan was initially developed for epileptic patients. Still, over the past few years, the keto diet has gained so much popularity in the fitness world.

People choose the keto lifestyle for a range of fitness and health reasons, but it’s popular among people looking to lose weight and improve body composition.

The Pros and Cons of The Keto Diet For Runners

Like any other nutrition plan, the keto diet has many pros and cons.

Let’s discuss some of them.

Improved Sleep

Once you reach ketosis, you’ll find yourself falling asleep fast every night and waking up feeling refreshed. You’ll also experience less drowsiness throughout the day, especially during the post-lunch slump.

Improved Body Composition

Research has found that people who get on the keto diet start burning any excess fat fairly quickly, improving their body composition.

For example, this research reported that working out while on keto can boost weight loss from stores without affecting lean muscle mass.

Other than weight loss, the keto diet can help:

  • Improving digestion
  • Improving mental function
  • Lowering the glycemic index
  • Lowering the risk of heart diseases, some cancers, and epilepsy
  • And so much more.

The Downsides

Of course, the keto diet isn’t without any cons. Some of these include:

Poor Performance

During the early weeks of the keto diet, your performance will crash and will crash hard.

This is especially the case if this is your first time on the keto diet.

So don’t expect to break any new records or run your longest distance even when you’re in ketosis for the first time.

Keto Adaptation Takes Time

I hate to sound like a broken record, but becoming fully fat-adapted doesn’t happen overnight.

Sure, reaching the state of ketosis may only take a few days, but ketosis and fat-adapted are not the same.

So expect to be moving around with low energy for weeks before you’re back to full power. Complete fat-adaptation may take up months, during which your body burns fat much more efficiently than it does carts.

No Guaranteed Benefits

I strongly believe in the keto diet, but I’m also well aware that nothing guarantees results.

The scientific community is yet far from reaching a consensus on the impact of a keto diet.

As far as I can tell, most of the experiments performed are relatively small scale and inconclusive.

Marathon Training Fueling Needs

Adopting the ketogenic lifestyle means no grains, sugar, starches, loaves of bread, and the sort. This must be triggering all sorts of alarms if you’ve been around the long-distance running block for a while.

After all, aren’t carbs the main energy source on the run?

Most experts recommend that regular marathon runners consume around 400 to 600 grams of carbs daily. That’s over 20 times more than the recommended carb intake on the ketogenic diet.

The truth is a little bit more complicated.

According to my experience, as well as plenty of anecdotal evidence, once you’ve fully fat-adapted, you’ll be running on fat almost as efficiently as on carbs, especially during low to mild intensity training.

Let me explain more.

Keto Adaptation

Although most marathon training relies on carbohydrates as a source of fueling, runners who have been on the keto diet for a while find themselves fat-adapted.

This is what happens when the human body switches over to fueling on fat rather than carbs.

According to research, fat adaptation may take place anywhere from 4 to 8 weeks on the keto diet, depending on metabolism and other variables.

But how come this happens? Let’s look at the science behind it for a deeper dive.

Fueling for Endurance

To power your working muscles during training, you need fuel, which takes the form of the molecule known as adenosine triphosphate (ATP).

To produce ATP, the human body mainly uses either carbohydrates or fats. What determines which energy gets employed is the availability of carbohydrates.

When carb levels are abundant,  your body would likely use them (in the form of glucose) to make ATP. Conversely, a lack of carbs forces your body to change over fat for energy.

It’s not rocket science.

The argument for fueling with fats

During endurance training—think long runs, weekend rides, marathons, etc.—your body will require continuous carb fueling during these efforts.

The reason? Because we only have a limited store of carbs—around 500 grams of glycogen in muscles and liver—which translates to roughly 2000 calories of energy. Not a lot.

As you can already tell, 2000 calories might not be enough to get through two-hour sessions. But, that’s the reason endurance athletes take fuel on the go. They need the extra energy, or else they’d bonk. And that’s not nice.

However, IN THEORY, if you’re keto-adapted, you might be able to access a more copious fuel source: body fat.

Research estimate that body fat contains around 20 times more calories than glycogen, even in a lean person.

For this reason, the Keto diet preaches limiting carb intake to scarce levels to trigger this shift to fat for fuel. This not only gets you to burn off that stubborn fat around your belly, but it also unlocks an enormous energy reserve previously out of reach.

This sounds all nice and dandy, but what does the research say?

Again, you shouldn’t take my word for this. Let’s look at some of the research into keto and endurance training.

Research On Keto And Endurance Performance Training

Around 40 years ago, Stephen Phinney, a world-famous keto researcher, conducted an interesting experiment. He analyzed the endurance of six obese, untrained subjects on a treadmill under two different conditions:

  • Group I – A normal diet that contained carbohydrase
  • Group II – A hypocaloric ketogenic diet (protein-supplemented fast, or PSF)

At the end of the experiment, Stephen found that the keto group could stay on the treadmill for around twice as long before becoming exhausted compared to the carb-fed Group.

You can also watch Dr. Stephen discuss his finding in this YouTube clip.

Another experiment had 20 elite ultra-marathoners and ironman distance athletes perform a maximal graded exercise test and a 3-hour submaximal run at around 60 percent of VO2 max on a treadmill to assess metabolic responses.

During the experiment, Group I was fed the classic high-carb diet, whereas Group II was given a low-carb diet for an average of 20 months.

The result? The researchers concluded that long-term keto-adaptation results in drastically high-fat oxidation rates.

Additionally, the use of oxygen and the depleting patterns during and after the 180-minute run was similar. This may mean that the subjects in Group II were able to burn more fat without any impairments in VO2 max.

The Case of Zach Bitter

One of the most famous ones is the legendary ultra-marathoner Zach Bitter, the 100-mile American record holder who eats keto.

But keep in mind that Zach has been surfing the keto wave since 2011. According to some of his interviews and writings, Zach also cycles between ketosis and low-carb, so he’s not strictly keto throughout the year.

What’s more?

The guy focuses on an ultramarathon, which are run at a slow and steady pace. Fueling with fat makes more sense during long distances. On the other hand, the more intense the race, the more you’ll need carbohydrates instead of fat for fuel.

You can hear him talk about his keto experience on the Lex Friedman podcast:

Note – as I mentioned earlier, full-fat adaption may take up to two to three months. This is when the body uses fat as its primary energy source. However, most of the research I found did not last this long, leaving many questions about whether the subjects were fully fat-adapted, whether their ketone levels were measured, or whether they were in ketosis.

Can you Train For A Marathon While On Keto

Of course, the answer is yes. You just have to do it the right way.

If you’re willing to invest enough time and effort to make fat your main macronutrient and fuel source, you can train and run a marathon on keto.

How long it will take you when you’re newly on keto depends on you, but according to most experts, it can take months.

What’s more?

Some people may never seem to become completely fat-adapted while eating keto. If that’s your case, consider trying carb-cycling or switching from keto to low-carb eating during heavy training days.

Before you toe the line of a marathon race on keto, there are a few things to consider.

These include:

  • Your ketosis length. The first thing to consider is how long you have been in this metabolic state. In most cases, when you’re new to the keto diet, you’ll find it hard to muster up the energy needed for distance running at your pre-keto pace and speed.
  • Your calories. You cannot stay in ketosis while eating low fat. That’s the rule. Your body is primarily fueled by fat on the keto diet, so not meeting your calorie needs means you don’t have enough fuel in the tank.
  • Your fat intake. Serious about making fat your main source of fuel? Then your diet must reflect that intention. Simply increasing your protein intake won’t do the trick. If you don’t fuel your body with enough healthy fat, you won’t be able to power through those long workouts.
  • Carbo cycling. Consider adding a few low-glycemic index carbs during heavy training days to ensure you have enough fuel in the tank. Remember that to stay in ketosis, you’ll need to stay under 40 to 60 net carbs per day, depending on your metabolism and training volume.

And that’s all!


If you’ve been keto-adapted for a while and it’s working well for you, then nothing should be stopping you from running a marathon on a keto diet.

I won’t recommend trying the keto diet in the last few weeks leading your marathon.

Think long-term.

Three to four months is a good time range.

Transitioning from eating more fat to fewer carbs takes time for your body to adjust.

That’s why the off-season is the perfect time to transition to a keto diet—or at least when you’re not training for a specific race when you don’t have any race on the schedule soon.

Once you find out what works the best for you, you can start to train for races on a keto diet.