Ketogenic Diet Tips for Endurance Athletes

Written by Toni Sicola
on March 23, 2020

Keto Diet for Endurance Athletes

Long-held wisdom is that endurance athletes need sustained energy availability for their activities. Whether you're running, swimming, cycling, power walking, or doing a through-hike of the Pacific Crest Trail, your energy and endurance are key to your success and the completion of your mission. The ketogenic diet is famous for its positive effects on energy and fat-burning, but is it advisable to combine a low-carb diet like keto and endurance training? 

Traditional Wisdom for Endurance Athletes

Most endurance athletes, nutritionists, and coaches agree that carbohydrates are the way to go for quick energy and an efficient fuel source. Your body is designed to convert glucose into fuel quickly, giving you the power you need to push through to the finish. 

But athletes on a high-carb diet can run into trouble. They need to refuel during their endurance events, usually with glucose blocks or some other form of quick energy. Sometimes this works great, but sometimes it can create gastrointestinal issues in the middle of a race. In fact, according to Chris Carmichael, GI distress is the number one cause of DNFs (did not finish) in ultramarathons. 

Fat Stores vs Glycogen Stores

If you've been working to become fat-adapted, to burn ketones instead of glucose, can you perform just as well, and without the GI distress to worry about? This question is a topic of hot debate among the keto community, nutrition experts, dietitians, coaches, and athletes alike. There are reasons for and against the ketogenic diet for endurance athletes. Let's explore the possible pros and cons and discuss the potential remedies you could employ to make the keto diet work for you.

Keto and Performance

The ketogenic diet was originally used as a measure to help reduce seizure occurrence in epileptic children. It was a happy side effect that adults experienced fat loss, improved mental clarity, and sustained energy when on the extremely low-carbohydrate, high-fat diet. We know, based on a substantial body of research that, when humans switch from burning glucose to burning fat, they can begin to burn body fat at a faster clip, using ketone bodies as fuel. But substantial fat loss or weight loss only takes place when you're running at a calorie deficit, whether you're working a keto plan or another sports nutrition plan. 

So what does that mean for performance? 

Indirectly, it could mean a lot. A lighter, leaner body is likely to perform better than a heavier one. Furthermore, fat is a more sustained energy source that we all carry around with us. It doesn't run out in the same way that glycogen stores will over the long duration of a distance race. Bonking is a lot less likely if you're relying on ketosis rather than glucogenesis to get energy. So a keto diet could, in fact, help keep a fat-adapted athlete in the race for longer. 

But will it help her go faster?

So far, there's no evidence that a low-carb, high-fat diet (LCHF) helps improve performance when measured by speed or strength on its own. In fact, there's research that seems to come to the opposite conclusion. The food we eat (regardless of its macro makeup) requires oxygen to be converted into a fuel source we can use. Fueling with ketones actually requires more oxygen than fueling with glucose, just to stay at the same pace. If you connect the dots on these findings, the keto diet could actually impair performance. 


That's because it takes longer for energy to become available to the athlete when it's sourced through fat rather than through carbohydrates. This is fine if you're doing a training session where you can keep the same low- to medium-intensity pace the whole time. But if you want to kick up to a higher level during an important race, you won't get to where you want to be by fueling on ketones alone. 

Most endurance sports aren't low-intensity the entire way through. Long-distance runners, cyclists, and triathletes all go through periods of low-, medium-, and high-intensity throughout the course of a race. It's one thing to consider endurance performance for a distance hiker who's not trying to beat a clock (or another person), but even then, trail variation could certainly mean an occasional ramp up in intensity. When it comes to racing, all of these sports alternate in intensity, and therefore, it's most ideal for the athlete to be adaptable to all possible fueling sources, not just fat. Nutrition science has demonstrated that carbs are more readily available in these times of higher intensity than fatty acids, simply because they require less oxygen for conversion to energy. 

So does all of this mean you can't improve performance on the keto diet? Maybe, but maybe not. 

How Endurance Athletes Can Use Keto

If you're an endurance athlete who's been curious about the health benefits of the keto diet, it's time to think strategically. We know that keto-adaptation increases your ability to burn fat and use it for fuel. We also know that fat oxidation takes longer than carbohydrate burn, requiring more oxygen for the same output. So if you want to try keto, there are two main things to consider: 

  • Timing
  • Alternate Forms of Keto (Cyclical Keto Diet and Targeted Keto Diet)


Just like with most things in life, timing is everything. In the case of your nutritional and exercise performance strategy, timing is critical for success. This is especially true for elite athletes, where seconds matter in a race. The best way to use the Standard Ketogenic Diet (SKD) is to drop weight in the off-season. Fat adaptation can take a couple of weeks, and during the adaptation period, you're neither getting enough glucose nor enough ketones to perform well. 

So if you know you have a race six months out, use keto to shed bodyweight and get lean. During the transition, and even after you've reached a state of ketosis, you might consider supplementing with exogenous ketone bodies. This might help ease the transition and get you into nutritional ketosis more quickly, providing you with extra energy as your body adjusts.

Alternate Forms of Keto

Both the Cyclical Keto Diet and the Targeted Keto Diet are designed to help improve athletic performance during a set period of time. If you're looking to gain muscle mass or do burst training on keto, either of these diets might work for you.

Cyclical Ketogenic Diet (CKD)

CKD is generally based on an alternating schedule. For five to six days, you'd follow the SKD, and for one to two days, you'd up your carbohydrate intake with healthy carbs like sweet potatoes, winter squash, legumes, or whole grains. The aim of doing this is to help temporarily replenish your glycogen stores for the purposes of building muscle mass or improving performance. In the days following your "refeedings" of carbohydrates, you'd go harder at the gym to get yourself back into ketosis more quickly and start the cycle again. Five days on SKD, 2 days for refeed, and so on. 

Targeted Ketogenic Diet (TKD)

TKD splits the difference between CKD and SKD. The word "target" refers both to the target activity you're attempting to fuel for and the target amount of carbohydrates you need to complete the task. 

For example, if you have a high-intensity work out planned for certain days of the week, add in additional carbs right before (or even during) that workout. As we already explained, high-intensity training requires glucose, so to keep your energy levels up for these particular workouts, choose an easily digested carbohydrate, and experiment with the amount until you feel you've found your target. 

Ketogenic Diet for Endurance Athletes

There's still a lot of work to be done to learn about the best way to improve physical performance on the keto diet. That being said, endurance athletes can experience performance benefit, not only by shedding pounds on keto, but by using it strategically throughout the year to keep blood sugar low. 

Experts recommend fueling with a high-carbohydrate diet just before a race to ensure that blood glucose is adequate for endurance events. For this reason, the CKD or the TKD could be the perfect balance between the various schools of thought on the matter.


    Author: Toni Sicola

    Published:  Mar 23, 2020 

    Toni is a wellness professional with a Master's in Integrative Health, is passionate about spreading health, happiness, and personal fulfillment to as many people as possible. She has a professional background in health and wellness, dietary supplements, and nutrition, and embarks every day to live a well, balanced, happy life.

    Written by Toni Sicola

    Published: March 23, 2020