Introduction
Athletes, clinicians, and practitioners often seek novel nutrition strategies to optimize health and performance. Altering substrate availability through dietary manipulation has been the subject of increasing interest as a strategy to modulate training adaptations, and competitive performance (1,2). Dietary supplements also demonstrate potential for augmenting the adaptive response to exercise training (3), in addition to their traditional role as ergogenic aids (4). A topic of intense interest in sports medicine and beyond is the proposed performance and therapeutic benefits of nutritional ketosis (NK) (1,5–8). Because of the multifaceted effects of NK, its proposed utility has led to an expansion of research into these effects in athletic contexts, general health, and a diverse class of pathologies (8).
NK is best operationally defined as a nutritionally induced metabolic state resulting in blood Β-hydroxybutyrate (ΒHB) concentrations of ≤0.5 mM, regardless of endogenous or exogenous induction, or the systemic metabolic or signaling effects elicited (1,9). ΒHB along with acetoacetate (AcAc) and acetone are collectively termed ketone bodies (KBs). ΒHB is present in the body as R-ΒHB and S-ΒHB enantiomers of ΒHB (interchangeably termed D-ΒHB and L-ΒHB, respectively), with the R-enantiomer being the predominant circulating KB. NK can be achieved “endogenously” through dietary manipulation such as prolonged fasting or ketogenic diets (KDs), or “exogenously” with the oral administration of exogenous ketone supplements (EKs) (10). In athletic contexts, recent reviews have made the case for (1) and against (11) KDs for exercise performance. For EKs, however, despite initial positive findings (12), many studies since have failed to demonstrate benefits to physical performance (13).
While both approaches produce NK and share some common metabolic features and consequences, there are several divergent features of these approaches that should be recognized (Table) and have important implications for performance and therapeutics. Yet in our experience, it is increasingly common for clinicians, practitioners, and the general public to conflate these approaches. A recent review has attempted to disentangle the approaches to NK in the context of exercise metabolism (10), and the purpose of the present review is to provide an overview of methods to produce NK, the metabolic effects of KBs in key target organs, and features of convergence and divergence between endogenous or exogenous ketosis.
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Table: Features of convergence and divergence between NK achieved by KDs compared with EKs.
KB, Ketogenesis, and NK
Multiple physiological states increase the synthesis and circulation of the KBs. When glucose availability is low, and insulin is suppressed, an elevation in fatty acid oxidation in hepatic mitochondria results in an amplification of ketogenesis (Fig. 1). During prolonged fasting, starvation, severe calorie restriction, or consumption of a KD, the pancreas produces less insulin and more glucagon, resulting in increased rates of adipose tissue lipolysis and an elevation of circulating ketogenic precursor metabolites, namely, long-chain fatty acids (LCFAs).
Figure 1: Pathways of ketogenesis in liver and ketolysis in extrahepatic tissues, and methods to produce NK. Ketogenesis primarily occurs in the hepatocyte mitochondria wherein two acetyl-CoA, the primary metabolite generated during beta-oxidation of fatty acids, combine in a reaction catalyzed by ACAT to form AcAc-CoA. AcAc-CoA reacts with another acetyl-CoA to produce HMGCoA via catalysis by HMGCS. HMGCoA is then cleaved by HMGCL to produce AcAc and acetyl-CoA. AcAc can be reduced by to produce
R-βHB or can undergo spontaneous decarboxylation to produce acetone. Ketogenesis also occurs in small quantities in cells such as astrocytes, intestinal stem cells, and cardiomyocytes. NK can be achieved by dietary means incorporating CHO restriction, for example, fasting and KDs, or by exogenous ketone supplements that either directly increase blood
R-βHB concentrations (
e.g., ketone salts), or comprise ketogenic precursors (MCFAs, BD), or a combination of a KB and a ketogenic precursor in the form of a ketone ester. During ketolysis in extrahepatic tissues,
R-βHB is reoxidized to AcAc, before covalent activation of AcAc by CoA is catalyzed by OXCT resulting in generation of AcAc-CoA. Two molecules of Ac-CoA are liberated by thiolytic cleavage of AcAc-CoA by ACAT, after which Ac-CoA is incorporated into the TCA cycle. However, distinct from ketolysis,
R-βHB also can act as signaling molecule through interaction with cell surface receptors and regulators of gene expression (see
Fig. 2). ACAT, thiolase; ADH, alcohol dehydrogenase; BDH, 3-hydroxybutyrate dehydrogenase; HMGCoA, 3-hydroxy-3-methylglutaryl-CoA; HMGCS, 3-OH-3-methylglutaryl-CoA synthase; HMGCL HMGCoA lyases; OXCT, succinyl-CoA:3-oxoacid CoA transferase.
The liver synthesizes the majority of KBs in the body, but does not readily consume them, ensuring KBs can be utilized by extrahepatic tissues (Fig. 1). Once in circulation, KBs are rapidly taken up by the extrahepatic tissues, with the brain, skeletal muscle, and heart, being avid users. Rates of ketone utilization and synthesis are similar in normal weight adults consuming a standard diet, keeping blood KB concentrations typically ≤0.3 mM, even following an overnight fast (14). Within a few days of fasting or consumption of a KD, KB concentrations rise by approximately 10-fold due to increased ketogenesis, and can then serve as an alternative fuel source to glucose for most of the body (15).
Although NK is operationally defined as blood βHB concentration of ≥0.5 mM (1), there is no obvious metabolic “switch” that occurs at this threshold. Rather it is indicative of a state in which KBs serve as a prominent metabolite, and important substrate to overall energy supply on a whole-body level (1,14,16). For example, infusion of R-βHB in healthy young men to a concentration of just ~0.2mM to 0.5 mM results in reduction in estimates of hepatic glucose output and adipose tissue lipolysis, and an increase in cerebral R-βHB uptake (17). The magnitude of elevation of R-βHB in response to ketogenic stimuli varies depending on factors, including age, sex, height, body mass and composition, aerobic fitness and physical activity level, caloric intake, and even individual metabolic and genetic variability (1,5,7,18). In pathological conditions most typically associated with insufficient insulin, KBs can accumulate to produce a state of ketoacidosis wherein R-βHB is often ≥10 mM and blood pH declines and produces a life-threatening metabolic state known as diabetic ketoacidosis (18). In contrast, the normal physiological range of NK is commonly observed to be ≥0.5 to ~5 mM R-βHB (1,5,7).
Ketogenic Diets
NK has long been used to manage disease, regardless of the terminology used to describe it. Texts from antiquity reference anticonvulsant effects of fasting, and throughout the 19th and 20th centuries, ketogenic-type diets were described to benefit patients with metabolic disorders, such as diabetes mellitus. The “Classical Ketogenic Diet” developed at the Mayo Clinic in 1921 was restricted in protein (1 g·kg−1 body weight) and carbohydrate (CHO) (10 g to 15 g·d−1), with the remainder of calories from fat. The efficacy of this diet in epilepsy was established, and around the same time, KDs were being used to prolong the lives of type 1 diabetes mellitus patients. However, with the advent of antiepileptic drugs (AEDs) and the Nobel Prize-winning discovery of insulin, the classical KD declined in its therapeutic use despite its demonstrated efficacy.
Interest in KDs resurged in the 1990s as it became apparent that the KD was a useful tool for treating patients that failed to respond to AEDs (7), and evidence was accumulating that KDs may be efficacious for a host of metabolic disorders (19). Intrigued by the emerging and multifaceted mechanisms of therapeutic ketosis, other fields have expanded research efforts into NK. Encouraging data spurred public interest, especially in relation to widely applicable topics like weight loss and exercise performance, leading to mainstream popularity.
In the most general sense, a KD can be defined as any diet that amplifies ketogenesis, not just the strict therapeutic KDs used in early medical literature. There are now a number of defined alternative KDs including the medium chain triglyceride (MCT) KD, the Modified Atkins Diet, and the Low Glycemic Index Treatment Diet. Each dietary approach induces a state of NK, but are formulated with more liberal allowances of protein and CHO to improve palatability and compliance (20). Regardless of the specific alterations involved, it is now clear that KDs now expand far beyond the original strict 4:1 ratio of fat-to-protein and CHO and can be customized to meet individual preferences and tolerability without necessarily sacrificing benefits. Indeed, several studies suggest that these alternative KDs can elicit similar seizure control with improved compliance and palatability (20).
Exogenous Ketone Supplements and Ketogenic Precursors
Hopes of extending NK to a wider range of applications, and recognition of the pleiotropic effects of KBs, has led to the development of ingestible EKs (7,10). Until their development, the only reliable means of elevating circulating KBs outside of a research setting was through dietary restrictions that can present limitations regarding feasibility and practicality. EKs were developed to elicit a dose-dependent elevation of R-βHB and/or AcAc regardless of dietary macronutrient intake. EKs allow for rapid and controlled induction of ketosis and may offer an alternative tool to induce NK in individuals unable, uninterested, or unwilling to consume a KD.
Moreover, some benefits of NK can be attributed to mechanisms induced by the KBs themselves, rather than the other metabolic features of KDs. Thus, EKs could mimic beneficial effects of NK for specific applications, ranging from health optimization, to cognitive and physical performance, to medical therapeutics. Furthermore, there may be situations wherein KDs are contraindicated, with EKs providing the only feasible tool to induce NK (7,21).
A large number of natural and synthetic EKs are being developed and tested (22,23). The various formulations have distinct properties in terms of degree and duration of ketosis induced, and even in regard to metabolic and signaling effects (22). Therefore, research is needed and underway to elucidate the utility and application of individual EKs, and many are being evaluated for safety and efficacy for performance enhancement (10), as well as therapeutic potential (8).
Ketone salts
The most direct method of exogenously inducing NK would be to administer isolated KBs. However, R-βHB and AcAc in their free acid form can be unstable, expensive, and ineffective at producing sustained ketosis. Thus, the ketone acids can be buffered with sodium or other electrolytes to enhance efficacy and prevent overload of any single mineral. Balanced mineral formulations of ketone salts may even be useful in attenuating symptoms of mineral depletion that occur early in keto-adaptation, but at large doses, excessive mineral intake could lead to gastric hyperosmolarity or other adverse effects, such as inappropriate sodium load. Ketone salts have been the subject of several investigations for ergogenic potential in a variety of exercise challenges, but to date, none have produced performance benefits (13).
Medium chain triglycerides
MCTs contain fatty acids 6 to 12 carbons in length (medium-chain fatty acids [MCFAs]). Compared with LCFAs being absorbed via the lymphatic system, MCFAs can be absorbed via hepatic portal circulation and enter the hepatic mitochondria without requiring carnitine transport, where they are rapidly metabolized to acetyl CoA, and subsequently, to KBs. Therefore, MCTs are considered ketogenic fats as they result in ketogenesis without requiring dietary CHO restriction. A known side effect of high MCT consumption is gastrointestinal discomfort, which can be mitigated by a slow, progressive introduction over a 1- to 2-wk period (24).
Combination ketone salts and medium chain triglycerides
One promising combination of EKs is that of ketone salts and MCTs, which have been administered most typically in 1:1 or 1:2 ratios. Animal research suggests that this method results in a more sustained induction of NK because KBs are delivered directly in the form of ketone salts, while ketogenesis is stimulated by MCTs (22). This approach allows for lower dosing of individual components, with lesser potential for side effects from high intake of individual EKs or minerals. This formulation is available in popular commercialized products but has not been extensively evaluated in human trials.
1,3-Butanediol
1,3-butanediol (BD) is a Food and Drug Administration (FDA)-approved organic diol used as a food flavoring solvent and has been considered as a potential synthetic food for long-duration space missions (25). BD is metabolized by the liver to produce βHB, and in rodents, R,S-BD was demonstrated to produce a dose-dependent elevation of KBs in a ratio of 6:1 of R-βHB to AcAc (26). Extensive toxicology studies in a variety of preclinical models show that BD is safe and tolerable (27). BD is an EK itself and also is used as a backbone in the synthesis of some ketone esters. Like ketone salts, investigations, to date, of the ergogenic potential of R-BD in trained male athletes demonstrate a lack of benefit in endurance exercise settings (13).
Ketone esters
Several existing synthetic ketone esters (KEs) potently induce a dose-dependent elevation in blood KB concentration. KEs are held together by ester bonds that are cleaved by gastric esterases to liberate KBs in their free acid form from a backbone molecule. The choice of backbone molecule can vary, but often is a ketogenic precursor molecule, such as R-BD or R,S-BD. KEs are currently the most potent EKs available in terms of reliably producing NK, a promising feature but one that also requires a thorough investigation of their long-term safety. Encouragingly, some of the most well-characterized KEs have received Generally Recognized as Safe (GRAS) status by the FDA, such as the R,S-BD AcAc diester and R-BD R-βHB monoester, and the latter appears safe and well tolerated at known doses in healthy adults, whether acutely (12,23,28,29) or daily when consumed for up to 28 d (30).
βHB enantiomers
Currently, most commercially available ingestible EKs, with the exception of the R-BD R-βHB monoester and some specific ketone salt products, are a racemic mixture of R-βHB and S-βHB enantiomers of βHB. This is largely because the synthesis of racemic mixtures is more affordable than the pure enantiomers. R-βHB is considered the endogenous metabolite and an efficient fuel for ATP production. S-βHB is biologically present in small quantities, but does not contribute significantly to energy production and after ingestion of racemic EKs S-βHB remains elevated in circulation longer than R-βHB (23). Despite divergent metabolic roles, R-βHB and S-βHB have been shown to share similar molecular interactions and intracellular signal transduction cascades (31), but the specific effects of the two enantiomers remain a hotly debated topic.
Metabolic Effects of KBs on Target Organs
AcAc and R-βHB are pleiotropic signaling molecules and influence a diverse range of physiological processes (Fig. 2) (9). As such, KBs elicit multiorgan effects that can be similar or differ widely depending on the tissue of interest. Here we will briefly consider effects of KBs on brain, skeletal muscle, and adipose tissue.
Figure 2: Effects of ketone bodies on various tissues and organs. Ketone bodies βHB, AcAc, and/or acetone have been described to facilitate multiple systemic and/or tissue specific effects which extend beyond extra-hepatic energetic needs. These pluripotent effects include direct or indirect epigenetic regulation, reduced oxidative stress via antioxidant production, increased and/or neutral anabolic hormone secretion, increased skeletal muscle anabolic and regenerative signaling, decreased skeletal muscle catabolic signaling, altered tissue lipid metabolism and energetic efficiency, reduced methylglyoxal, multi-metabolite and systemic metabolic regulation, altered hunger hormones, altered intestinal stem cell fate, function, and regeneration, increased cardiac tissue hydraulic efficiency, neuroprotective mechanisms, and reduced cellular inflammatory signaling. Readers interested in further details on the effects described are directed to several other excellent reviews (
5–7,9,35–38). COX-2, cyclooxygenase-2; ERK, extracellular signal-regulated kinases; FOXO3a, Forkhead Box O3a; GABA, glutamate and gamma-aminobutyric acid; GPR, G-protein-coupled receptor; HCAR2, hydroxycarboxylic acid receptor 2; HDAC, histone deacetylase; IGF-1, insulin-like growth factor-1; MEK, mitogen-activated protein kinase/extracellular signal-regulated kinases; MnSOD, magnesium-dependent superoxide dismutase; mTORC, mechanistic target of rapamycin; NLRP3, nucleotide-binding protein, leucine-rich-containing family, pyrin domain-containing-3.
Brain
In the adult brain, glucose is typically the predominant and preferred fuel source, but as long as half a century ago, KBs were demonstrated to replace glucose as the predominant fuel during starvation (i.e., low glucose availability), supplying up to two thirds of the brain's energy requirements under such conditions (32). More recently, experimental elevation of R-βHB (infusion) or KBs (MCT ingestion) increases KB uptake and utilized by the brain in proportion to the circulating concentration (14,17). In turn, elevated uptake and utilization of KBs elicits a reduction in brain glucose utilization (14). KBs are, therefore, an alternative and prominent substrate in the brain when circulating concentrations are elevated to the level of NK. These developments have ignited interest in NK as a unique strategy to provide additional substrate during times of limited glucose availability or impaired brain bioenergetics, such as those that are featured in conditions as varied as epilepsy, Alzheimer's disease, and traumatic brain injury (TBI) (33,34).
Interestingly, NK enhances, or attenuates the decline in, cognition in a variety of preclinical and human settings, ranging from young to old, and including athletic settings, neurological disorders, and psychiatric disorders (34). Of particular interest to the sports medicine community is the potential of NK to mitigate adverse sequelae following traumatic brain injury (TBI). Preclinical studies of moderate and severe TBI via blunt force trauma strongly support a therapeutic role, showing reduced tissue death and cerebral edema, and improved cognition (33). Clinical studies have established safety of the KD in human TBI patients with diverse injury type with nonspecified origin (33). However, due to the diverse nature of TBI across athletic contexts and nascent interest in NK for TBI, larger studies across multiple settings are needed to evaluate the efficacy of NK in sport-related TBI.
Skeletal muscle
AcAc and R-βHB exert metabolic actions, particularly in relation to substrate utilization and anticatabolic processes, in many organs (16) (Fig. 2). In skeletal muscle, for instance, KB infusion results in inhibition of glycolysis and stimulation of protein synthesis in skeletal muscle (39,40) and attenuation of muscle protein breakdown (41). In vitro studies demonstrate attenuation of pathways of muscle atrophy with KB treatment of skeletal muscle (42), and an increased mitochondrial function and reduction of selected ceramide species (43). In rodent skeletal muscle, βHB impairs insulin-stimulated glucose uptake, but this effect is produced by R-, but not S-βHB, and occurs in oxidative, but not glycolytic, skeletal muscle (44). Thus, a better understanding the effects of these βHB enantiomers on skeletal muscle metabolism will be important given the aforementioned prevalence of racemic EKs. During exercise, elevated demands for ATP resynthesis in skeletal muscle are the primary driver of increases in rates of whole body substrate utilization (45). Ketosis achieved by ingestion of R-BD R-βHB monoester prior to exercise may attenuate CHO utilization, and increase reliance on intramuscular lipids during exercise (12), likely via the inhibition of adipose tissue lipolysis and consequent reduction in circulating free fatty acids (FFAs).
Adipose tissue
KBs also are known to directly and rapidly modulate lipid metabolism. R-βHB binds HCAR2/GPR109a and results in the inhibition of lipolysis by attenuating NF-κB activity in adipose tissue (46). To this end, EK administration, whether as ingestion of R-BD R-βHB monoester or infusion of R-βHB or R,S-βHB, reduces FFA and glycerol concentrations, with or without changes in insulin concentration (12,17,23,29,47), a phenomenon that appears to be dose-dependent (21). Conversely, AcAc binds GPR43 to upregulate lipolysis via increasing lipoprotein lipase activity in adipose tissue (48). These mechanisms and direct effects of KBs demonstrate differential regulation of adipose tissue lipolysis that may be dependent on factors such as the duration of fasting and KDs, or the type of EK administered. For example, prolonged fasting and KDs upregulate lipolysis and FFA mobilization in contrast to administration of EKs that primarily raise R-βHB concentrations given that R-βHB inhibits lipolysis via HCAR2/GPR109a (46) and acutely lowers circulating FFAs both at rest and during exercise (12). However, chronic administration of EKs that result in significant elevations in AcAc concentrations have resulted in adipose tissue atrophy that is not wholly explained by reduced calorie intake (49), suggesting that much remains to be learned on regulation of adipose tissue lipolysis during NK induced by different means.
Points of Convergence and Divergence in NK: Comparing and Contrasting NK Induced by Endogenous versus Exogenous Means
These divergent effects of KDs and EKs on adipose tissue lipolysis illustrate an important and often neglected aspect of NK—recognition of overlapping and unique features of endogenous versus exogenous induction of NK. In fact, there are many known points of convergence and divergence between the two approaches (Table), with many questions remaining about metabolic and molecular regulation under each.
Dietary requirements, safety, and compliance
Inherent to traditional KDs is the restriction of CHO intake with secondary considerations for protein quantity as both stimulate insulin secretion. However, EKs require no direct dietary CHO restriction. While data suggest that KDs and EKs can both be safe and tolerable in healthy individuals, only KDs have undergone extensive evaluation in pathological contexts. KDs have similar compliance issues as other dietary strategies (50), while compliance in short-term daily supplementation studies using EKs is reportedly high (30).
Circulating KB concentrations and βHB/AcAc ratio
KDs generally elevate circulating R-βHB to a range of ~0.5 mM to 3.0 mM at a ratio of approximately 4:1 R-βHB/AcAc (18). Typically, ketone salts, MCTs, and R-BD elevate R-βHB by approximately 0.3 mM to 1.0 mM above resting values (13), with their impact on circulating R-βHB being limited primarily by factors including the gastric tolerance of the dose provided of the different molecules, the capacity for hepatic conversion of BD to βHB, and proportions of the R- and S-enantiomers in racemic formulations. The elevation in circulating KBs and the respective contributions of AcAc and R-βHB after ingestion of KEs depends on the composition of the specific KE ingested. For example, the R-BD R-βHB monoester produces little change in AcAc concentrations but elevations of R-βHB in the range of ~3 mM to 6 mM at tolerable doses (23), and concentrations during exercise in the ~1.5 mM to 3 mM range (13). These lower concentrations likely reflect KBs being oxidized during exercise (12). Conversely, the R,S-BD AcAc diester elevates both R-βHB and AcAc in a dose-dependent manner in animal models (22), but little human data are available on this agent to date.
Onset, timing, and adaptation
KDs require a number of days before the amplification of rates of ketogenesis produces sustained NK, whereas ingestion of EKs elevate KB concentrations within minutes to hours. With prolonged adherence to KDs, the phenomenon termed “keto-adaptation” is proposed to occur after weeks to months. Although objective markers of keto-adaptation remain to be defined, the process refers to the symphony of physiologic and metabolic changes that accompany a shift from glucose-based to fat-based metabolism and requires variable amounts of time depending on the metabolic consequence of interest (1,10). Whether there are temporal adaptive responses to the metabolism of EKs with repeated consumption remains to be explored.
Direct effects of KBs
Importantly, features related to direct effects of KB metabolism and signaling properties tend to be a point of convergence between KD- and EK-induced NK. The superior metabolic efficiency of KBs has been known for decades, with studies showing that R-βHB and AcAc can uniquely increase work and ATP production while decreasing oxygen consumption (51) because of molecular changes resulting in an increase in the energy of ATP hydrolysis (52). Thus, KBs are often considered a more efficient fuel for ATP production compared with glucose and fatty acids. Once in circulation, KBs elicit a host of direct signaling effects that are not dependent on their metabolism and therefore likely to occur regardless of the method of NK induction, but in turn may be dependent on the circulating concentration achieved. This includes the ability to regulate intracellular metabolism by binding and activating HCAR2 receptor signaling, the ability to regulate epigenetic and redox function by acting as histone deacetylase inhibitors, and the ability to regulate cellular regeneration in response to insult (9).
Regulation of appetite
KDs produce subjective reports of reduced appetite and the desire to eat even in calorie deficit, and these effects have been attributed to the state of NK (53). Similarly, acute NK produced by the ingestion of the R-BD R-βHB monoester reduced subjective ratings of appetite and the desire to eat measured by visual analog scales, which coincided with the suppression of the appetite hormone ghrelin (28). While the exact mechanisms linking elevated KBs with the regulation of appetite hormones are not clear and likely involves multiple organ systems (54), this is an example of a physiological effect that appears common to KDs and EKs (15).
Glucoregulatory aspects
An effect on circulating glucose concentrations is a prominent feature of NK induced by both KDs and EKs, but the method of induction of NK results in divergent regulation of these effects. For example, as KDs require dietary CHO restriction, lower glucose concentrations, and an absence of postprandial glycemia are a consistent feature of their application (19). Conversely, several reports have demonstrated that EKs can acutely regulate glycemia by either lowering fasting glucose concentrations (12,17,28,47), and/or by attenuation the postprandial rise in glucose after a CHO-containing bolus (29). These effects occur with or without alterations in circulating insulin (12,28,47), and are most likely via attenuated hepatic glucose output, rather than an increase in skeletal muscle glucose uptake (21,29,55).
Anabolic and anticatabolic effects
KDs are reported to impact the adaptive response to exercise training measured by change in lean body mass (56), but currently, the understanding of this relationship is conflicting and confounded by the methods of assessment of lean body mass, relative protein consumption, anabolic hormone levels, and caloric intake (56). There also has been a suggestion that a KD may elicit an anticatabolic effect as evidenced by increasing circulating leucine and decreased circulating alanine concentrations (57). Similarly, anabolic and anticatabolic effects of infused EKs have been demonstrated in humans (40,41), in addition to anticatabolic effects of EKs in rodent models of atrophy-based pathologies (21).
Exogenous versus Endogenous NK: Lessons from Applied Contexts
Considering the above examples of convergence and divergence between KDs and EKs when inducing NK, it appears erroneous to assume that the respective methods will elicit identical effects or can be applied interchangeably in all contexts. Illustrative examples of this include applications in antiseizure and exercise performance contexts.
For example, in epilepsy, significant questions remain surrounding the mechanisms by which KDs exert antiseizure and neuroprotective effects (16), and debates on the therapeutic potential of EKs continue. The clinical data do not yet strongly support the notion that KBs elicit anticonvulsant effects independent of their role as a metabolic substrate as most human studies have failed to demonstrate a correlation between blood KB concentrations and seizure control (7). Despite this, all three KBs and various EK formulations elicit antiseizure effects in cell culture or animals (7). However, it also is clear that the individual KBs elicit unique effects as AcAc and acetone possess more potent anticonvulsant properties than R-βHB (7), highlighting an important need for EKs that can intentionally elevate specific KBs, rather than simply relying on the physiological equilibrium produced by KDs.
This relationship between endogenous and exogenous sources of KBs and their respective actions is further muddled by the experimental methodologies that are employed to understand NK. Much of what is speculated about the molecular mechanisms that underlie the pleiotropic effects of KDs (i.e., chronic effects) is extrapolated from studies that administer EKs or KBs directly (i.e., acute effects) in an attempt to isolate effects of these molecules. However, the central theme of this review (Table), and others (10), is to highlight that the effects of KDs and EKs are not synonymous and should not be conflated. Similarly, caution should be applied when mechanisms of effects of NK are extrapolated primarily from reductionist experiments performed in vitro with KBs exogenously applied to cells or tissues, in the absence of the sequalae produced by longer-term adherence to a KD. Moreover, recent studies have described EK-supplemented diets (54,58), that is, diets producing sustained NK in the absence of effects produced by CHO restriction of traditional KD approaches. Overall, these experimental approaches provide valuable insight into effects of NK per se, but conversely make teasing apart the effects of NK induced by endogenous versus exogenous means even more difficult, and further supports defining NK by R-βHB concentration, with the secondary consideration being the method of induction.
In exercise contexts, there is considerable debate about the merits of KDs for athletic performance (1,2), and the performance benefits of acute ingestion of EKs are equivocal (13). However, it is clear that NK alters the metabolic response to exercise at the level of substrate utilization, and in terms of contribution from circulating or intramuscular substrates (12,59–61). Common to KDs and EKs would be a reduction in CHO utilization during exercise, but this occurs under very different metabolic states. For example, low CHO availability is a de facto feature of KDs (2,60), whereas to date, most studies of EKs have been performed with EKs coingested with high rates of CHO supply (~1 g·min−1) (12,13). As described above, EKs have antilipolytic effects, which results in lower circulating FFAs concentrations and may augment the reliance on intramuscular triglycerides during exercise (12). Conversely, circulating FFAs and glycerol are elevated during exercise after a KD, and whole-body fat oxidation rates increase two-fold to three-fold over mixed/high CHO diets (1,60). These shifts in patterns of substrate utilization underscore the distinct metabolic states during exercise and have important implications for performance (10). Specifically, current speculation is that KDs may be beneficial for long duration (≥12 h), low-intensity ultraendurance-type event, where fats, being the primary source of ATP provision, can match lower rates of ATP demand, whereas the combination of NK and high CHO intake may confer performance benefits in endurance events of moderate intensity and durations of ≥8 h (62). Several years of research will be required to explore these contentions, but the salient point is that despite the common feature of inducing NK, KDs and EK are distinct metabolic states and should not be conflated as analogous fueling approaches for athletes.
Conclusions
Interest into the performance and therapeutic applications of NK has rapidly expanded. Regardless if induced by endogenous or exogenous means, NK can be defined as a metabolic state wherein R-βHB concentrations are elevated ≥0.5 mM, eliciting widespread physiological changes at both a systemic and cellular level. These effects can overlap or differ and can be universal or tissue-specific. Because NK is a potential candidate for performance-enhancing and therapeutic benefits, it will be critical to understand these points of convergence and divergence to fully evaluate its utility for sports performance and medicine in the future.
The authors thank Dr. Mark Evans for his constructive comments on the manuscript.
This submission was not supported by any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
AMP is a scientific advisor to Pruvit Ventures, LLC, and is an inventor on the following patent: Dominic P. D’Agostino; Angela M. Poff; Patrick Arnold; “Targeting Cancer with Metabolic Therapy and Hyperbaric Oxygen” (Patent Number: 9801903). AMP is an owner of Poff Medical Consulting and Communications, LLC and Metabolic Health Initiative, LLC. AMP and APK are inventors on pending patents “Compositions and Methods for Weight Loss Maintenance.” APK is an inventor on pending patent “Prevention of Muscle Wasting with Ketone Supplementation.” At the time of this publication, pending patents were still under review. Should patents become accepted, AMP and APK will receive a share under the patent terms prescribed by the University of South Florida. BE declares no conflicts of interest and does not have any financial disclosures.
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