the point
My Lens on Ketone Esters Fueling Endurance Performance
This is my take on the Practical Application of ketone esters.
Athletes don’t train—or fuel—in laboratories. Much of what shapes fueling practices today comes from anecdotal experiences, influencer marketing, and commercial interests that often outpace the science.
Despite the current lack of evidence supporting ketogenic diets for endurance or high-intensity performance, I do believe there is potential in ketone ester supplementation when used strategically. Unlike glucose-based fuels that rely on insulin and cellular uptake to deliver energy—creating a natural limit to how much carbohydrate can be utilized—ketone esters provide an alternate substrate that enters the bloodstream directly. Once absorbed through the gut, ketones bypass many of the regulatory steps tied to carbohydrate metabolism, offering a more direct energy source that has minimal impact on blood sugar levels.
However, the effectiveness of ketone supplementation varies greatly by athlete.
For athletes interested in experimenting with ketone esters, context is everything. These supplements are not a replacement for carbohydrates, nor a shortcut to metabolic efficiency. Instead, they may serve as an adjunct fuel source under specific conditions—such as during long endurance events when glycogen stores are low, or between back-to-back training sessions when full glycogen restoration isn’t possible.
It’s also important to recognize that the response is highly individual.
- Well-trained endurance athletes, whose metabolic machinery is already primed for efficient fat oxidation, may experience subtle benefits in cognitive clarity or delayed fatigue.
- Less-trained or metabolically inflexible athletes, however, may feel little to no effect—or even mild gastrointestinal distress—if dosage or timing are off.
- Because ketone esters can alter substrate use and acid-base balance, it’s best to trial them first in training, not competition.
For athletes interested in experimenting with ketone esters, context is everything. These supplements are not a replacement for carbohydrates, nor a shortcut to metabolic efficiency. Instead, they may serve as an adjunct fuel source under specific conditions—such as during long endurance events when glycogen stores are low, or between back-to-back training sessions when full glycogen restoration isn’t possible.
It’s also important to recognize that the response is highly individual. Well-trained endurance athletes, whose metabolic machinery is already primed for efficient fat oxidation, may experience subtle benefits in cognitive clarity or delayed fatigue. Less-trained or metabolically inflexible athletes, however, may feel little to no effect—or even mild gastrointestinal distress—if dosage or timing are off. Because ketone esters can alter substrate use and acid-base balance, it’s best to trial them first in training, not competition.
From a practical standpoint, athletes should view ketone supplementation as part of a broader fueling strategy, not as a stand-alone solution.
Carbohydrates remain the most reliable source for sustaining power, while ketones can fuel performance in combination with physiology adapted to sport and metabolic efficiency.
What Does the Science Say?
Modern sports nutrition emphasizes periodization—adjusting your diet to match the demands of training and competition. This means strategically using carbohydrates when needed for high-intensity sessions and competitions, while lowering carbohydrate intake at times to promote certain adaptations.
Unfortunately, this nuanced approach is often misrepresented. Advocates of low-carbohydrate, high-fat (LCHF) or ketogenic diets sometimes claim that sports scientists recommend eating “lots of carbs all the time.” That is simply not true. Another distortion is the idea that scientists believe fat is not used at higher intensities. In reality, fat is always being used—even at 85% of VO₂max—but carbohydrate remains the dominant and essential fuel at those intensities.
Clinical examples illustrate this point:
- People who cannot use fat effectively (certain metabolic disorders) can exercise at high intensities but fatigue quickly.
- People who cannot use carbohydrate (such as those with McArdle’s disease) have very poor exercise capacity and can only sustain very low intensities.
Who Has Been Studied?
The body of research on ketogenic diets and performance is surprisingly limited, and the majority of studies have focused on small groups of cyclists, gymnasts, and taekwondo athletes. Studies on highly trained endurance athletes competing at elite levels are scarce.
Adaptations that allow trained endurance athletes to utilize more free fatty acids (FFA) for fuel include:
- increased mitochondrial density,
- greater storage and use of intramuscular triglyceride,
- enhanced muscular uptake of plasma FFA, and
- increased oxidative enzymes.
These adaptations help spare glycogen and may, in theory, extend endurance at low intensities (Erlenbusch, Haub et al. 2005). But how does this play out in performance studies?
What This Means for Athletes
Taken together, the findings suggest:
- At low to moderate intensities: Fat-adapted athletes can sustain exercise using fat oxidation, sometimes with little difference in performance compared to carbohydrate-fed athletes.
- At high intensities: Ketogenic diets appear to reduce performance capacity. This is consistent with physiology—depleted glycogen stores and reduced glycolytic enzyme activity impair high-intensity output, as evidenced by lower lactate levels and reduced maximal workloads in ketogenic conditions (Zajac et al. 2014).
- Strength and weight-class sports: Some preliminary data suggest ketogenic diets may help athletes in sports where weight control is key (gymnastics, taekwondo). However, data are limited and compliance is often poorly measured.
No studies to date have evaluated the long-term effects of ketogenic diets on athletes’ ability to train daily, recover, maintain immune function, or prevent injury—all of which are critical for real-world performance.
However, other studies have confirmed the presence of a
robust change in the muscle’s substrate use via observations of alterations in the concentrations or activity of
proteins or metabolites that regulate fatty acid availability,
as well as the persistence of increased fat oxidation in the
face of abundant carbohydrate supplies.
Louise M. Burke / SCIENTIST
What We Can—and Cannot—Conclude
At present, there is no compelling evidence that ketogenic diets improve exercise performance in well-trained athletes. If anything, the evidence suggests decrements in high-intensity work, with potential neutral outcomes at lower intensities.
What This Means for Athletes
- At low to moderate intensities: Fat-adapted athletes can sustain exercise using fat oxidation, sometimes with little difference in performance compared to carbohydrate-fed athletes.
- At high intensities: Ketogenic diets appear to reduce performance capacity. This is consistent with physiology—depleted glycogen stores and reduced glycolytic enzyme activity impair high-intensity output, as evidenced by lower lactate levels and reduced maximal workloads in ketogenic conditions (Zajac et al. 2014).
~Louise M. Burke
Considering that athletes might best benefit from a range of options in the dietary tool box is likely to be a better model for optimal sports nutrition than insisting on a single, one-size-fits-all solution.”
Moving Forward
- What’s needed now is more high-quality research, particularly in elite athletes, with outcomes measured on actual performance rather than just metabolic markers.
- Bottom line: For now, ketogenic diets remain a theory, not a proven strategy, for athletic performance. Carbohydrate periodization—sometimes high, sometimes low—remains the science-backed approach.
