I’ll admit, I was skeptical about keto at first. Carbs had always been my go-to for long runs, so I wasn’t sure how switching things up would affect my performance.
How could I possibly run without loading up on pasta and bananas? But curiosity got the better of me, and I decided to try it.
The transition was rough, but over time, I found that running on keto is doable—and for me, it has been a game-changer.
Curious about keto for marathon training? I’ll share what worked for me, how to avoid energy crashes, and ways to keep up your endurance without the usual carbs.
Can You Run a Marathon on Keto?
The quick answer is yes, it’s possible—but don’t expect it to be as easy as just skipping carbs and hoping for the best.
I quickly realized that my body needed time to adapt. The first few weeks on keto, my energy levels plummeted. Every run felt like I had bricks strapped to my feet. I kept reminding myself that it was part of the process and trusted that my body would adapt and learn to use fat for fuel.
After around six weeks, I started noticing a difference. I wasn’t breaking records, but I could run longer without needing gels or sugary drinks to keep me going. My body started tapping into fat as a fuel source, and I felt steady and strong, even during my long runs.
What is the Keto Diet?
First things first: let’s quickly cover what keto is. The ketogenic diet is a high-fat, low-carbohydrate nutrition plan designed to push your body into ketosis.
In this metabolic state, your body switches from burning carbohydrates (glycogen) for energy to burning fat. This can appeal to athletes who want to tap into fat stores for longer-lasting energy.
For ketosis, you’ll want to keep carbs really low—under 20-30 grams per day—and get the rest of your energy from healthy fats and a bit of protein
But staying there isn’t always easy. It takes time for your body to adapt to this new energy source, and it can be tricky to fuel properly, especially for endurance events like marathons.
What to Eat on the Keto Diet
On keto, your meals should primarily consist of healthy fats, moderate protein, and few carbs.
Here’s a quick guide to what to eat on the keto diet:
- Fats and oils: butter, olive oil, coconut oil
- Dairy: cheese, sour cream, heavy cream
- Eggs (a staple!)
- Meat: chicken, beef, pork, lamb
- Fish: salmon, sardines, trout
- Nuts and seeds: almonds, sunflower seeds, chia seeds
- Low-carb veggies: spinach, kale, zucchini, cauliflower
- Some fruits: avocados, berries (in moderation)
Here’s what avoid:
- Grains and starches: bread, rice, pasta, oats
- Processed foods and sugary snacks
- High-carb fruits: bananas, apples, grapes
- Beans and legumes
- Alcohol and sugary drinks
Pros and Cons of the Keto Diet for Runners
Like any diet, keto has its upsides and downsides, especially when it comes to running.
Pros:
- Improved fat metabolism: Once you’re fully fat-adapted, your body becomes more efficient at using fat for fuel, which can benefit endurance athletes.
- Stable energy: Some runners report fewer energy crashes during long runs, as fat provides a more steady energy source than carbs.
- Body composition: Many runners see a reduction in body fat while maintaining lean muscle mass, which can improve performance.
Cons:
- Performance dip: Initially, your performance will take a hit. Your body needs time to adapt to burning fat, so don’t expect to hit your personal best right away.
- Long adaptation period: It can take weeks, or even months, for your body to fully adjust to keto. During this time, your energy levels might fluctuate, which can be frustrating if you’re in the middle of marathon training.
- No guarantees: Keto doesn’t work for everyone. Some people never fully adapt to using fat for fuel; others may find that they perform better on a more traditional high-carb diet.
Marathon Training Fueling Needs
Adopting the ketogenic lifestyle means no grains, sugar, starches, loaves of bread, and the sort. This must trigger 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 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 and 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.
How Long Does It Take to Become Fat-Adapted?
Becoming fully fat-adapted can take anywhere from 4 to 8 weeks, depending on your metabolism, how strictly you follow the diet, and how much exercise you’re doing.
During this time, your body is getting used to using fat as its primary energy source instead of carbs. This transition can be tough—expect some low-energy days and potentially slower runs as your body adjusts.
Once you’re through the adaptation period, you should notice that you have more consistent energy levels, especially during long, steady-state efforts like marathon training.
As best as I can remember, it took around 6 weeks to feel fully fat-adapted. The first few weeks were tough—my runs felt sluggish, and I wondered whether I was making a mistake.
I even contemplated throwing in the towel and going back to carb-loading. But I stuck it out, and sure enough, I felt my energy levels even out.
By week six, I could go on a 10-mile run without any fuel, and I still felt like I could keep going afterward. That was a huge turning point. It made me realize that my body could adapt to using fat as its primary fuel source with patience.
The argument for fueling with fats
During endurance training, think long runs, weekend rides, marathons, etc., and your body will require continuous carb fueling during these efforts.
Let me explain.
The human body has 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.
I hate to state the obvious but 2000 calories ain’t enough for a three-hour hard run. And that’s the reason endurance runners take fuel on the go.
They need the extra energy, or else they’d bonk. And that’s not nice.
Here’s the plot twist.
IN THEORY (at least), if you’re keto-adapted, you can tap into a more copious fuel source: body fat.
Research estimates that body fat contains around 20 times more calories than glycogen, even in a lean person.
The Keto diet preaches limiting carb intake to scarce levels to trigger this shift to fat for fuel. This gets you to burn off that stubborn fat around your belly and 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 findings 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 were similar. This may mean that the subjects in Group II could burn more fat without any impairments in VO2 max.
The Case of Zach Bitter
Zach Bitter is technically the poster child of the keto diet for endurance runners.
But there’s more to the story.
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 is 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 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.
Marathon Fueling on Keto
One of the biggest challenges of running a marathon on keto is figuring out how to fuel your long runs. Traditionally, marathon runners rely on gels, sports drinks, and carb-heavy snacks to keep their glycogen stores topped up. But on keto, your approach needs to be different.
Once you’re fully fat-adapted, your body can rely mostly on fat for energy, even during long-distance runs. The good news is that body fat stores can provide far more energy than glycogen—so, in theory, you won’t need to refuel as often as someone on a high-carb diet.
That doesn’t mean you can go fuel-free, though. You’ll still need some fat-based snacks to keep up your energy on long runs.
Here are some keto-friendly fuel options for marathon training:
- Nut butters (almond, peanut, or cashew)
- MCT oil (medium-chain triglycerides) for quick fat energy
- Coconut oil
- Fat bombs (homemade snacks made with coconut oil, butter, or cream cheese)
- Avocados (though maybe not mid-race!)
Test your fueling strategy during training to see what works best for you.
Keto Marathon Training: What I Ate
When I’m doing keto (usually for a couple of months at a time), I pretty much overhaul my whole diet.
It meant giving up pasta and bread, but I learned to rely on things like avocados and healthy fats
Here’s what a typical day of eating looked like for me:
- Breakfast: Scrambled eggs cooked in butter, with avocado and a sprinkle of cheese
- Lunch: Grilled chicken with a big salad (greens, nuts, olive oil, and a hard-boiled egg)
- Snack: A handful of almonds or a fat bomb
- Dinner: Salmon cooked in coconut oil with roasted veggies and some guacamole on the side
- Pre-run snack: Coffee with MCT oil and a fat bomb or a spoonful of almond butter
It took a while to get used to not relying on traditional carbs, but once I found meals that worked for me, I started to love how I felt. I wasn’t bloated or sluggish after meals and could run without feeling weighed down.
7-Day Keto Marathon Meal Plan
Each day includes balanced keto meals and snacks with specific recommendations for pre-run, during-run, and post-run options. This plan is high in healthy fats and low in carbs, designed to support marathon training while keeping you energized and fat-adapted.
Day 1
- Breakfast: Avocado and Egg Bowl – 3 scrambled eggs with half an avocado, spinach, and a sprinkle of chia seeds
- Pre-Run Snack: A handful of macadamia nuts and a protein shake with MCT oil
- During-Run Fuel: Coconut oil packet or low-carb electrolyte drink
- Lunch: Grilled Salmon Salad – Mixed greens, cucumber, olives, and grilled salmon with olive oil and lemon dressing
- Dinner: Zucchini Noodles with Meatballs – Zucchini noodles with grass-fed beef meatballs and low-carb marinara, topped with Parmesan
- Evening Snack: A few cheese slices with olives
Day 2
- Breakfast: Keto Smoothie – Unsweetened almond milk, 1 scoop of protein powder, 1 tablespoon almond butter, and a handful of spinach
- Pre-Run Snack: Hard-boiled egg and a few almonds
- During-Run Fuel: Low-carb electrolyte drink (like LMNT or Ultima)
- Lunch: Turkey Lettuce Wraps – Sliced turkey, avocado, and cucumber in large romaine leaves with a drizzle of olive oil
- Dinner: Grilled Chicken Thighs with Roasted Brussels Sprouts and Cauliflower, drizzled with olive oil
- Evening Snack: Small serving of Greek yogurt (unsweetened, low-carb) with a few raspberries
Day 3
- Breakfast: Coconut Chia Pudding – Chia seeds mixed with unsweetened coconut milk, topped with a few raspberries
- Pre-Run Snack: Keto-friendly energy bite (almond butter, coconut flakes, chia seeds)
- During-Run Fuel: A coconut oil packet or MCT oil for easy fat energy
- Lunch: Spinach and Avocado Salad with Grilled Shrimp – Spinach, avocado, cucumber, and grilled shrimp with olive oil and lemon dressing
- Dinner: Baked Salmon with Asparagus, drizzled with melted ghee
- Evening Snack: A handful of macadamia nuts or walnuts
Day 4
- Breakfast: Keto Omelet – Eggs, sautéed spinach, mushrooms, and feta cheese, cooked in butter
- Pre-Run Snack: A few olives and a small piece of cheese
- During-Run Fuel: Low-carb electrolyte drink
- Lunch: Cauliflower and Chicken “Rice” Bowl – Riced cauliflower, grilled chicken, spinach, and a dollop of pesto
- Dinner: Beef Stir-Fry with Bell Peppers and Broccoli, cooked in coconut oil
- Evening Snack: Celery sticks with almond butter
Day 5
- Breakfast: Keto Pancakes – Made with almond flour, eggs, and a dash of cinnamon, served with a dollop of whipped coconut cream
- Pre-Run Snack: Handful of pecans and a spoonful of coconut oil
- During-Run Fuel: Electrolyte drink and a coconut oil packet
- Lunch: Grilled Pork Chops with a side of leafy greens and avocado, dressed in olive oil
- Dinner: Stuffed Bell Peppers – Ground beef, diced zucchini, and cauliflower rice, topped with cheese and baked
- Evening Snack: Greek yogurt with a dash of cinnamon
Day 6
- Breakfast: Smoked Salmon and Avocado Plate – Smoked salmon slices, half an avocado, and a few cucumber slices
- Pre-Run Snack: Small handful of almonds and a keto-friendly protein shake
- During-Run Fuel: Low-carb electrolyte drink
- Lunch: Chicken Caesar Salad – Romaine lettuce, grilled chicken, avocado, and Parmesan cheese with a keto-friendly Caesar dressing
- Dinner: Grilled Lamb Chops with Roasted Zucchini and Eggplant, drizzled with olive oil
- Evening Snack: A few olives and a cheese slice
Day 7
- Breakfast: Keto Breakfast Bowl – Sautéed kale, mushrooms, bacon, and a poached egg
- Pre-Run Snack: Keto energy bite or a handful of nuts
- During-Run Fuel: MCT oil or coconut oil packet for sustained energy
- Lunch: Tuna Salad Lettuce Wraps – Tuna mixed with avocado, diced celery, and mayonnaise, served in large lettuce leaves
- Dinner: Baked Chicken Thighs with Roasted Brussels Sprouts and a side of cauliflower mash
- Evening Snack: Dark chocolate square (90% cocoa or higher) with a few macadamia nuts
You can also check out this keto plan.
Frequently Asked Questions About Keto and Marathon Training
I hate to sound like a broken record but switching to keto for marathon training comes with a few unique questions. Let me adress some of the most common concerns.
Can I do speed work on a keto diet?
Yes, but it can be challenging! High-intensity sessions like speed work rely heavily on glycogen, which is more limited on a keto diet. Some keto runners find it helpful to use “targeted keto,” where they consume a small amount of carbs (15–30 grams) about 30–45 minutes before a speed workout. This provides a quick energy boost without kicking you out of ketosis.
What’s the best way to fuel long runs on keto?
When it comes to long runs, the goal on keto is to rely on fat as your primary fuel source. Many keto runners use fat-based options, like MCT oil or nut butter packets, to stay energized over longer distances. Electrolyte drinks are also essential, as keto can increase the risk of dehydration.
Do I need more electrolytes on keto?
Yes, keto runners generally need to increase their intake of sodium, magnesium, and potassium. Without carbs, your body retains less water, which can lead to quicker dehydration and muscle cramps. Adding electrolytes through supplements, or simply adding salt to your meals, can make a huge difference in maintaining energy and preventing cramps during your runs.
Join the Keto Marathon Training Conversation
How has keto impacted your marathon training? Do you have any go-to tips, favorite keto meals, or fueling strategies that have worked for you?
I’d love to hear from you—whether you’re new to keto, a seasoned fat-adapted runner, or somewhere in between. Sharing your story can make a huge difference for others who are also navigating this path.