Racing on Ketones: Here’s What You Should Know About This Unconventional Approach

Are ketones the key to newfound speed? Not so fast, say experts. Here’s what to know about the fueling approach some athletes say has helped them level up.

Photo: Derrick Lytle

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Dialing in a proper fueling strategy is personal in trail running, and those choices  can sometimes be contentious. With that in mind, an unconventional and controversial approach that is gaining attention in the ultrarunning community is running on ketones. 

Ketones are molecules produced by the liver during the breakdown of fatty acids. “Ketones are simply energy substrates that are produced by our liver, and these substrates can be used by all different tissues or by the brain but also by skeletal muscle,” explained Chiel Poffé, Ph.D, a postdoctoral researcher at KU Leven, during a recent conversation with ultrarunning coach Jason Koop. Basically, they serve as an alternative fuel source for the body whenever glucose (sugar) is limited or unavailable. This involves following a high-fat, low-carbohydrate (HFLC) diet or consuming an exogenous ketone supplement. 

“It’s important to understand the difference between the ketogenic diet and exogenous ketone supplements,” said Kylee Van Horn, RDN. “The ketogenic diet requires extremely low carbohydrate intake (usually less than 50 grams per day) or extreme calorie restriction and/or fasting, while exogenous ketone supplements can be taken and raise blood ketone levels within 30-60 minutes of taking them. The research on exogenous ketones is still in its infancy, but it appears as though they could offer similar benefits to following the diet without the side effects or potential negative performance impacts.”

Typically, an HFLC diet involves a carbohydrate intake of less than 20-30 percent of total daily calories, while fat intake comprises approximately 50-70 percent of calories. Reduced carbohydrate intake triggers the liver to produce ketones, shifting the body’s energy source from glucose to fats. 

Supporters of running on ketones maintain this metabolic adaptation holds multiple benefits for endurance athletes. 

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Researchers suggest the concept of following a low-carb diet with the goal of promoting “fat oxidation” may seem appealing, but it is somewhat misconstrued.

“It’s an intuitive and attractive premise, but at least somewhat misguided,” Nicholas Tiller, Ph.D., senior researcher in the Institute of Exercise Physiology and Respiratory Medicine at Harbor-UCLA Medical Center. “Such adaptations occur anyway as a byproduct of endurance training, especially when preparing for marathon and ultramarathon, which upregulates fat metabolic pathways irrespective of dietary strategies.”

While the idea of a full-on low-carb diet may be appealing, evidence suggests potential drawbacks. 

“There’s also evidence that low-carbohydrate diets prevent athletes from maintaining the intensity required to improve or retain maximal capacities, and experts generally agree that training extensively in a glycogen depleted state can compromise immune function, increasing susceptibility to infection,” said Tiller. “Both of these can be partially avoided by cycling carbs around harder training.” 

Experts caution against assuming that low-carbohydrate diets automatically lead to improved ultra-endurance performance. 

“In terms of ultra-endurance performance, there are no studies that have consistently shown that a low carbohydrate diet actually enhances performance,” said Tiller. “The best we can say at the moment is that, for some individuals, it may not impact negatively if done carefully.” 

However, rather than pitting ketones against glucose, we’ve gotten the input of two experienced ultrarunners and coaches to delve into their personal insights on the role of fats in performance and recovery and how an HFLC diet has worked for them.

Bronco Billy Bucks the Trend

Jeff Browning, a veteran ultrarunner fresh off setting the course record at 2023 Sedona Canyons 125, shifted to Optimized Fat Metabolism (OFM) protocol in December 2015. Peter Defty at Vespa Power pioneered the unique lifestyle nutrition and endurance training approach. 

“It leverages a ketogenic diet, less than 50 grams of carbs, for the initial four weeks to open up the metabolic pathway to burn on-board fat and teach the body to produce blood ketones naturally (e.g., nutritional ketosis),” said Browning, 51, who lives in Flagstaff, Arizona. 

“After that initial reset, I kept a high-fat, low-carb approach but upped my carbs from fruit and starches—sweet potato, red/gold potato, and occasional white rice—strategically timed around effort and volume. No longer strict keto, but 75-200 grams of carbs per day. Still technically low carb, but not keto.” 

Although Browning no longer strictly adheres to a ketogenic diet, he consumes approximately 75-200 grams of carbohydrates daily, strategically timing them around workouts and training volume. 

Jeff Browning at the 2023 Sedona Canyons 125 Aid Station. (Photo: Derrick Lytle)

“The bigger carb days are strategically timed around workouts, especially post-workout when the body is ready to top muscle and bone glycogen off and help with bone density issues seen in heavy endurance training related to a strict keto approach,” he said.

By cycling in and out of low-level ketosis throughout the training week, Browning said he benefits from ketones while ensuring sufficient glycogen replenishment and aiding recovery. He noted he occasionally experiences a temporary exit from ketosis after consuming higher volumes of carbohydrates during long runs. However, he promptly re-enters low-level ketosis the following day to facilitate faster recovery and reduce inflammation.

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The shift to running on ketones yielded notable changes in Browning’s performance and endurance. 

“The primary benefits are low inflammation and recovery,” said Browning. “The secondary benefit is no bonking anymore. I had been a high-carb athlete for 15 years and had 22 100-milers under my belt. I was always really inflamed post-100 milers with brain fog, and visible inflammation in my legs, ankles, and face.” 

While the benefits of running on ketones are evident, Browning also highlights the importance of avoiding potential pitfalls. He cautions against staying strictly keto for extended periods, emphasizing the need to strategically time carbohydrate intake. 

“The only issue I see is runners staying strictly keto too long,” Browning explained. “In other words, they get somewhat carb-phobic in training or trying to avoid simple carbs on long runs and races. That’s a mistake. Once that initial reset happens, runners have to shift to 75-200 grams of carbs per day in training. Plus, it helps to move from keto’s recommended moderate protein intake to a higher intake to aid recovery.” 

“The key is strategically timing carbs from a nutrient-dense paleo/primal food list (veggies, fruits, starches, etc.) and still using simple carbs on long runs and races,” he added.

For runners considering adopting an HFLC diet and utilizing ketones as a fuel source, Browning offers practical advice. Understanding that low-carb is not the equivalent of strict keto when in a training phase is crucial.

“If training, low-carb is not keto,” Browning said. “This is probably where the biggest confusion and misinformation is out there.” 

Zach Bitter’s Takeaways

An endurance athlete with multiple world and American records, Zach Bitter realized that his existing approach might not be sustainable for longer distances after completing an entire season of ultramarathon racing.

“I recognized that major dietary shifts are typically not wise during structured training and race seasons, so I took advantage of the off-season to try something new,” he said. 

Bitter, 37, who lives in Austin, Texas, emphasizes the importance of an individualized approach tailored to the runner’s needs, training phases, and intensity levels. He highlights the significance of balancing carbohydrate intake, avoiding excessive restrictions, and considering practical race-day nutrition. 

“For me, the things that improved the most were sleep quality, energy—throughout the day and during longer training sessions—became more stable, fueling during longer training sessions, and races became easier—both logistically and digestively,” said Bitter, who won the Hells Hills 50K in Smithville, Texas, in April.

Zach Bitter on the run. (Photo: Courtesy of Zach Bitter)

Bitter acknowledges that many individuals who adopt a low-carbohydrate approach have reported improved recovery and reduced muscle soreness. However, he cautions that these observations are currently anecdotal. 

“It is important to note that at this point in time, it is just an anecdote,” Bitter explains. “It is very possible that it is more to do with limiting or eliminating post-race nutrition that could have a negative impact on recovery, rather than anything specific to low carb or keto.”

Bitter acknowledges that there can be challenges associated with running on ketones for himself and others he has consulted with. He distinguishes between a strict ketogenic diet and a low-carbohydrate approach, highlighting the need for flexibility. 

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“It is pretty rare that I see someone who follows a strict ketogenic diet of less than 50 grams of carbohydrate per day continue to find that to be the best option available in the long term,” Bitter explains. 

Bitter advises adjusting carbohydrate intake based on different training phases, suggesting maintaining a balanced perspective on carbs and using them as a tool in moderation rather than avoiding them altogether. He recommends experimenting with new nutritional strategies during the off-season to find what works best.

“Performance should be your guide, so be honest with yourself and how your training is progressing compared to previous training cycles,” said Bitter.

Balancing Fueling Strategies: A Personalized Approach to Nutrition

“In general, while there are periodic success stories of athletes following low-carb and/or ketogenic diets for endurance sports, these instances seem to be the exception rather than the rule,” Tiller explained.

While running on ketones and following the HFLC approach has worked for these athletes, it’s important to note that it may not yield beneficial results for everyone. 

“Runners are making this way of eating and training work for race day performance and the proposed benefits of decreasing inflammation, no doubt about it,” said Julie Shobe, MS RDN. “However, I have had clients who have come from a low-carb, high-fat approach, and it did not yield beneficial results. Therefore this isn’t the theory of nutrition I personally encourage with my clients.”

As a nutritionist, Shobe encourages nutrition that prioritizes a balanced intake of macronutrients. 

“My clients see steady energy, improved recovery, controlled appetites, better sleep, decreased GI or stomach problems on long runs and race day, and overall better ultra running performance,” she said. “This is because they fuel with a rich amount of carbohydrates, protein, and fat. They also prioritize eating carbs on most training runs over 60 minutes long.” 

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For Shobe, this approach ensures that runners provide their bodies with the necessary energy to sustain the demands of ultramarathons, reducing the risk of injuries, stress fractures, sluggishness, and other complications. 

Athletes must recognize that diet and fueling are personal journeys requiring individualized consideration and adjustments. Moreover, it is essential to acknowledge the variability in individual responses and implement flexibility during different training phases. Seeking professional guidance, conducting thorough research, and discovering what works best for oneself are recommended. 

“Even if the physical benefits are seen, the diet can take a toll on mental health, leading to particularly restrictive eating practices and unnecessary food rules,” said Van Horn. “We have worked with many ultra-endurance athletes that were trying a HFLC diet and came to us with full-blown disordered eating and eating disorder behaviors. A better approach typically tends to stem from being intentional and periodized with carbohydrate intake, but not overcomplicating the diet.”



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