The role of the ketogenic diet in exercise performance

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Medicina Sportiva (2016), vol. XII, no 2, 2756-2761
Journal of the Romanian Sports Medicine Society
The role of the ketogenic diet in exercise performance
Evan E. Schick
Physiology of Exercise Sport (PEXS) Laboratory, California State University, Long Beach,
California, 90840
Abstract. Muscle glycogen storage and degradation are nearly universally accepted as crucial metabolic processes for
ensuring adequate intramuscular energy levels during prolonged, high-intensity activity. However, a growing body of
data illustrates that alternative substrates, such as ketone bodies, may be equally as effective in transducing energy
during exercise. Ketosis, wherein ketones serve as the primary oxidative fuel, can be achieved nutritionally through a
low-carbohydrate, high-fat ketogenic diet (KD). Though the KD is unequivocally successful in facilitating weight-loss
with minimal sacrifice to lean mass, current research indicates a complex role for the KD in both anaerobic and aerobic
exercise performance. This review discusses: 1) the mechanisms behind KD adaptation, and the effect of KD adaptation
on 2) glycogen metabolism, 3) aerobic exercise performance and 4) anaerobic exercise performance.
Key words: low-carbohydrate, high-fat, metabolism, athlete.
Introduction
Use of a low-carbohydrate, high-fat ketogenic diet (KD) is widely recognized as an efficacious therapy for a
range of metabolic and neurodegenerative diseases and cancers (1-7). Clinical use of the KD has gained
popularity as its effects often mirror those reached pharmacologically yet are attained with little off-target
risk). However, interest in the KD extends beyond the clinical landscape as mounting evidence suggests that
the KD may also influence exercise performance and adaptation). Though its precise role within exercise
training and performance remains elusive, the notion that the KD might enhance exercise performance
remains contentious, as it challenges traditional, carbohydrate (CHO)-centric guidelines for exercise and
sport nutrition. Though past reviews have cohesively and comprehensively summarized ketone biochemistry
and its clinical role, this paper aims at unifying current findings most directly implicated in exercise
performance including: 1) the cellular mechanisms of KD adaptation as well as (2) the effects of KD on
glycogen metabolism, 3) aerobic exercise and 4) anaerobic exercise (8-12).
Due to its storage abundance, intracellular location and rapid energy provision, muscle glycogen has
long been held as the most important energy substrate during prolonged, high intensity exercise. This belief
has resulted in the decades-long practice of CHO loading prior to competition among endurance athletes
looking to achieve supra-maximal glycogen levels (13). The high fat and low CHO nutrient apportionment of
the KD conflicts with conventional, CHO-centered sports nutrition guidelines, which recommend up to 12
grams/kilogram body weight for those engaged in high intensity endurance programs (14). Depending on the
goal of the individual, the KD may be hypocaloric, eucaloric or hypercaloric and is typically comprised of a
3:1 to 4:1 energy ratio of fat to protein and CHO; though CHO restrictive diets with lower ratios of fat to
protein can also be ketogenic (2, 15).
The biochemical underpinning of the KD is hepatic synthesis of ketone bodies, Acetoacetate (AcAc)
and H-β-hydroxybutyrate (βHB). Hepatic ketogenesis, upregulated as a consequence of limited carbohydrate
supply and abundant fatty acid availability, converts acetyl CoA’s, generated at rates prohibiting tri-
carboxylic acid cycle entry, to AcAc and βHB (16). Both AcAc and βHB are transported via systemic
circulation to extrahepatic tissues, where they are oxidized as needed (16, 17). In healthy fed adult humans,
ketone oxidation represents only a minor fraction of total body energy expenditure, however, its contribution
to energy metabolism in the heart, brain and muscle significantly increases in many physiological and
pathological states including the neonatal period, fasting, starvation, repressed insulin production, insulin
resistance prolonged exercise and low-carbohydrate diets (2, 11, 12, 17-19).
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The role of the ketogenic diet in exercise performance
Evan E. Schick
Medicina Sportiva
2757
In fact, ketones -mainly βHB- supply up to 70% of the energy used by the brain during starvation with the
remainder provided by endogenously derived glucose (17). Traditionally, in the before mentioned conditions,
ketosis is seen as a metabolic means of holding serve until an adequate supply of blood glucose is available.
Mechanisms of KD Adaptation
Understanding the mechanisms by which the body responds to the considerable shifts in macronutrient
consumption and subsequent fuel utilization is crucial, as adaptation to a KD must be attained in order to
normalize or improve performance. Dietary fat, ingested in substantial quantities to induce a state of ketosis,
becomes the primary oxidative fuel by mass action. Subsequently, active tissues undergo a two-fold response
to accommodate elevated fatty acid flux: increased mitochondrial β-oxidation and reduced glucose oxidation.
Ample support for this tissue-level response has been provided, however, it is imperative to understand the
cellular mechanisms responsible for this metabolic shift. Heightened β-oxidation is accomplished through an
adaptive cellular response observed in studies of both animals and trained cyclists wherein prolonged KD
(>1 week) amplified the activity of skeletal muscle carnitine acyltransferase (CAT) and hydroxyacyl-
coenzyme A dehydrogenase (3-HAD) relative to the activity of citrate synthase (CS) (20-24). Analogous
gene expression data has revealed a pointed prioritization of fat oxidation over its storage as upregulation of
CD36, butyrate dehydrogenase (HBDH) and mitochondrial uncoupling protein-2 (UCP-2) was concomitant
to reduced expression of fatty acid synthase (FAS) in the livers of KD-adapted mice (25). Similar studies of
high-fat diet adaptation provide evidence of depressed glucose oxidation associated with reductions in
pyruvate dehydrogenase activity and both insulin and exercise stimulated muscle glucose transport (26, 27).
Provided that endurance training boosts capacity for fat oxidation, these tissue and cell-level adaptations
suggest that the KD might catalyze physiological responsiveness to an endurance exercise program (28).
KD Adaptation and Glycogen Utilization
Significant evidence links adaptation to the KD to altered muscle glycogen metabolism. Short term (3-14
days) KD has been reported to decrease baseline liver and muscle glycogen levels in rats and trained cyclists
(20, 29, 30). However, in a study of moderately obese subjects following six weeks of a KD, resting muscle
glycogen content was 57% of baseline after week 1 but increased to 69% of baseline values after week 6 (31)
Newer findings demonstrate that ultra-endurance runners fully habituated to the KD (>20 months)
experience similar baseline and post-exercise muscle glycogen levels compared to controls on a mixed-diet
(32). These studies indicate that initial depletion of muscle glycogen induced by the KD may be at least
partially reversed through habituation to the diet. Moreover, normal post-exercise glycogen repletion in KD-
adapted individuals may be attained by increased lactate-mediated hepatic glucose production i.e.
gluconeogenesis, and muscle glycogen synthase activity, especially in type II fibers (26, 33-35).
Additionally, KD-adaptation has been shown to preserve liver and muscle glycogen during exercise; KD-
adapted rats and trained humans have been shown to exhibit reduced exercising muscle and liver glycogen
degradation rates without sacrifice to endurance performance (20-22, 29, 35, 36). These evidence, though
somewhat indirect, suggest that chronic reliance on fat as a primary fuel increases its inherent rate of
oxidation, an especially useful adaptation that reduces muscle glycogen degradation while maintaining cell
energy levels during exercise (36).
Ketogenic Diet in Practice
Ketogenic Diet and Weight Management.
Perhaps the most understood and implemented application of the KD is within bariatric medicine, as obese
individuals undertaking a hypocaloric KD experience a significant reduction in weight that is vastly
attributed to fat loss with minimal loss in muscle mass (18, 37-39). Less studied, yet perhaps equally
intriguing, is apparently even a eucaloric KD may promote fat loss and lean mass preservation in both obese
and healthy weight individuals (15, 40, 41). Most recently, investigators reported that obese and overweight
individuals following either an eight-week resistance training (RT) with a eucaloric, carbohydrate restrictive
diet (<30grams/day) or RT with a conventional, hypocaloric diet saw equal improvements in fat composition
and strength (42). Driving these KD-induced anthropometric adaptations is increased sensitivity to both
insulin and thyroid hormone, which helps to limit rates of skeletal muscle catabolism during periods of
weight-loss (15, 43). These findings lend intriguing sports nutrition research opportunities for weight-

The role of the ketogenic diet in exercise performance
Evan E. Schick
Medicina Sportiva
2758
category athletes who must reduce body mass while retaining muscle mass during competition preparation
(43). The KD could represent a healthier nutritional strategy for these athletes compared to commonplace
rapid weight loss practices such as severe caloric and hydration restriction and induction of hyperthermia.
Ketogenic Diet and Aerobic Exercise.
The manner in which KD affects aerobic performance appears to be determined by three factors: exercise
intensity, training status and length of diet habituation. Bergstrom et. al., 1967, (12) illustrated that three days
of a KD was enough to compromise submaximal endurance (75% VO2max) in healthy, untrained
individuals. Similarly, a six-week high fat diet (HFD) significantly reduced work output in untrained subjects
during a 45-minute bicycle test. The authors attributed this finding to an increased proportion of fat oxidation
observed through decreased exercising respiratory exchange ratio (RER) (40). However, the purported
sacrifice to high intensity exercise performance may represent a tradeoff for increased fatigue resistance.
Indeed, moderately obese subjects adhering to a six-week KD exhibited reduced exercise intensity but
greater stamina during a treadmill exercise test to subjective exhaustion (31). These studies suggest that
enhanced fat oxidation may not be energetically compatible with high intensity exercise but it may augment
long duration exercise.
Interestingly, aerobic performance in KD-adapted endurance athletes appears to be less susceptible
to shifts in energy substrate utilization. Initial studies in trained cyclists revealed that 4-weeks of a KD did
not affect moderate intensity (~65%VO2max) endurance exercise performance; apparently, enhanced
exercising fat oxidation rates, observed through lower RER, were able to compensate for reductions in steady
state glucose oxidation rates (36). More recently, elite ultra-endurance runners habituated (>20 months) to a
low-carbohydrate diet (LC) displayed significantly greater rates of fat oxidation and lower rates of
carbohydrate oxidation during a 180-minute run at 64% of VO2max. Peak fat oxidation rates were also
significantly greater in the LC group and were reached at a higher percentage of VO2max compared to
controls (32). Furthermore, following just two-weeks of a high fat diet (~70% fat), trained cyclists
experienced lower RER values and enhanced endurance during moderate intensity (60% VO2max) exercise
(29). More notably, HFD did not impair performance during a high-intensity (85% VO2max) time to
exhaustion test. Likewise, 6-15 days of HFD enhanced fat oxidation and decreased CHO oxidation rates in
trained cyclists while either not affecting or even improving performance during 20-100kilometer cycling
time trials (22, 35, 44). However, Zajac and colleagues (20) provide new insights revealing that competitive
off-road cyclists undergoing 4 weeks of a KD experienced reduced exercising RER concomitant to
diminished maximal workload and workload at lactate threshold (45). Collectively, current findings
surrounding the KD and aerobic exercise performance, though inconclusive, highlight the need for further
examination of how training status impacts adaptation to the KD and resultant performance. Moreover, if the
KD does not impair higher intensity aerobic intensity exercise performance (<80% VO2max), future
evidence needs to support both morphological and functional changes in mitochondria that would allow the
cell to meet the increased rate of ATP demand requisite during high intensity aerobic exercise.
Ketogenic Diet and Anaerobic Exercise.
The existing pool of knowledge vis-à-vis the KD and anaerobic performance is scarce. In fact, equivocal
results from a mere six studies and one abstract currently encompass the subject. Four studies report that the
KD negatively impacts anaerobic performance. Of these, one study utilized recreationally trained subjects
and found that KD significantly reduced isotonic strength as measured by a three-set squat repetition total at
80% 1RM (46). Two other studies utilized cycle ergometer to measure anaerobic power output in healthy
non-highly trained subjects but their results differed slightly. While one found that KD limited both mean
and peak power, the other noted only a decrement in mean power (30, 40). In the former study, the authors
noted that the reduction in mean power was mitigated when corrected for a loss in body mass in the KD
group. These findings corroborate additional findings in which well-trained cyclists subjected to 6-days of
HFD followed by 1-day of CHO loading exhibited reduced exercising fat oxidation rates and impaired high
intensity cycle sprint performance (44).
Still, three other studies provide evidence supporting the use of the KD in conjunction with strength
and power training; the most compelling of which showed that elite female gymnasts following a KD during
normal training did not exhibit impairments in muscular strength, endurance or power (41). Significantly,
this was the only investigation of athletes. Likewise, a pilot study found that KD did not deter gains in either
strength or anaerobic power in response to an 8-week periodized resistance-training regimen (47).

The role of the ketogenic diet in exercise performance
Evan E. Schick
Medicina Sportiva
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One other study highlighted the clinical utility of the KD in showing that overweight women lost body fat
while maintaining lean body mass when combining a KD with resistance training (48-50)..Worth noting, two
of the aforementioned studies that approximated glycolytic flux while evaluating the effect of KD on
anaerobic power and muscular strength generated similar results. Both studies reported depressed blood
lactate levels in subjects adhering to a KD immediately following 30-second supra-maximal intensity cycling
attempt and 3 sets of squats (30, 51). The authors of each study speculated that this might account for the
observed reductions in power and strength since lactate production is positively correlated with glycolytic
capacity during high intensity efforts.
The overarching limitation of the above studies is that none allowed for adequate adaptation to the
KD, allotting a dubious task of synthesizing interpretations. Future studies must allow for at least 8-weeks
for diet adaptation before testing commences. Furthermore, significant attention must be given to how
adaptation to the KD alters intramyoceullular energetics during anaerobic exercise. Studies must specify
creatine phosphate (CP) turnover rates during sets and rest periods of anaerobic exercise as well as whether
KD affects intramyocellular CP storage levels. Finally, modalities of anaerobic activities must be diversified,
i.e. resistance based training, and should aim to better mimic “real world” anaerobic exercise.
Practical Application of the KD
Though the precise impact of ketosis on exercise performance remains unclear, practitioners looking to
integrate this nutrition strategy into their training may consider several methods. Dietary induction of ketosis
is most successful when >60% caloric energy is derived from fat and <5% is from CHO (52). Protein should
constitute a significant proportion of dietary energy (>20%) as it retards muscle wasting and amino acids
such as leucine and lysine are ketogenic (53). Upon absorption, medium chain triglycerides (MCT’s) avoid
systemic circulation and instead enter portal circulation for immediate oxidation by the liver, making MCT
oil a possible adjuvant to a KD (54). However, tolerability of MCT oil, especially at higher doses, may be
individualized, thus care should be taken when considering this method (55). Beyond the scope of nutritional
and supplemental methods, is an exciting new patent for a synthetic ketone body and ketone ester, providing
an intriguing new possibility for inducing ketosis, preventing muscle glycogen breakdown, aiding muscle
recovery and preventing muscle wasting. Before undertaking a KD, the practitioner must consider
performance goals, health and nutritional access in an effort to maximize effectiveness, adherence and safety.
Acknowledgements. The author declares no conflict of interest. The study was not funded. The results of the
present study do not constitute endorsement of the product by the author.
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Corresponding author
Evan E. Schick
Department of Kinesiology, HHS2-210
California State University, Long Beach
1250 Bellflower Ave.
Long Beach, CA 90840
Email: evan.schick@csulb.edu
Tel: (562) 985-4590
Received: September 6, 2016
Accepted: November 25, 2016