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Keywords:

  • Ketogenic diet;
  • Epilepsy;
  • Metabolism;
  • Polyunsaturated fatty acids

Abstract

  1. Top of page
  2. Abstract
  3. MECHANISTIC INSIGHTS FROM STUDIES OF KD EFFICACY
  4. ANTICONVULSANT MECHANISMS OF THE KETOGENIC DIET
  5. HOW CAN THE KETOGENIC DIET BE OPTIMIZED?
  6. CONCLUSIONS
  7. REFERENCES

Summary:  The ketogenic diet (KD) is a broadly effective treatment for medically refractory epilepsy. Despite nearly a century of use, the mechanisms underlying its clinical efficacy remain unknown. In this review, we present one intersecting view of how the KD may exert its anticonvulsant activity against the backdrop of several seemingly disparate mechanistic theories. We summarize key insights gleaned from experimental and clinical studies of the KD, and focus particular attention on the role that ketone bodies, fatty acids, and limited glucose may play in seizure control. Chronic ketosis is anticipated to modify the tricarboxcylic acid cycle to increase GABA synthesis in brain, limit reactive oxygen species (ROS) generation, and boost energy production in brain tissue. Among several direct neuro-inhibitory actions, polyunsaturated fatty acids increased after KD induce the expression of neuronal uncoupling proteins (UCPs), a collective up-regulation of numerous energy metabolism genes, and mitochondrial biogenesis. These effects further limit ROS generation and increase energy production. As a result of limited glucose and enhanced oxidative phosphorylation, reduced glycolytic flux is hypothesized to activate metabolic KATP channels and hyperpolarize neurons and/or glia. Although it is unlikely that a single mechanism, however well substantiated, will explain all of the diet's clinical benefits, these diverse, coordinated changes seem poised to stabilize synaptic function and increase the resistance to seizures throughout the brain.

The ketogenic diet (KD) is a high-fat, low-protein, low-carbohydrate diet that has been employed as a treatment for medically refractory epilepsy for 86 years. The “classic” KD is based upon consumption of long-chain saturated triglycerides (LCTs) in a 3:1–4:1 ketogenic diet ratio (KD ratio) of fats to carbohydrates + protein (by weight). The vast majority of calories (>90%) are derived from fat. While clinical implementation of the KD has varied from center to center (Kossoff and McGrogan, 2005), diet treatment generally begins with a period of fasting followed by gradual increase in calories to a target KD ratio of 3:1–4:1. This is conducted in the inpatient setting over the course of several days, where blood glucose, urine ketones, and several other metabolic variables are closely monitored. The hallmark feature of KD treatment is the production of ketone bodies by the liver. Ketone bodies provide an alternative substrate to glucose for energy utilization, and in developing brain, also constitute essential building blocks for biosynthesis of cell membranes and lipids.

While the clinical effectiveness of the KD is widely accepted, surprisingly little is understood about its underlying mechanisms of action. Although some studies suggest that dietary constituents or metabolites have direct anticonvulsant effects, emerging evidence indicates that adaptations to chronic administration of the KD result in improved seizure control. These data suggest that the KD activates several endogenous metabolic and genetic “programs” to stabilize and/or enhance cellular metabolism, and that these fundamental changes help counter neuronal dysfunction associated with seizure activity.

MECHANISTIC INSIGHTS FROM STUDIES OF KD EFFICACY

  1. Top of page
  2. Abstract
  3. MECHANISTIC INSIGHTS FROM STUDIES OF KD EFFICACY
  4. ANTICONVULSANT MECHANISMS OF THE KETOGENIC DIET
  5. HOW CAN THE KETOGENIC DIET BE OPTIMIZED?
  6. CONCLUSIONS
  7. REFERENCES

The anticonvulsant efficacy of the KD has been examined in various acute and chronic animal models of epilepsy over the years (Stafstrom, 1999). Clinical and experimental studies have provided key insights into important treatment-related variables and, when considered together, these studies have helped direct mechanistic research. Commonalities between clinical and experimental studies of efficacy are summarized in Table 1 (adapted from Stafstrom, 2004).

Table 1. Translational correlations of ketogenic diet (KD) efficacy
VariableExperimental findingsClinical findingsReferences
  1. Abbreviations: P40 age, postnatal day 40; LCT, long-chain triglycerides; MCT, medium-chain triglycerides; PUFA, polyunsaturated fatty acid; Atkins diet, diet high in fats + protein, low in carbohydrates; CR, calorie-restriction; NAL, normal, ad libitum rodent diet; NCR, normal, calorie-restricted (by ∼15%) rodent diet; KAL, ketogenic, ad libitum diet; KCR, ketogenic, calorie-restricted diet; KD ratio, ratio of [fats / (carbohydrates + proteins)]; KAP, ketogenic/antiketogenic potential. Table adapted from Stafstrom (2004).

AgeYoung rats and mice <P40 at diet onset Adult rats and mice >P40 at diet onsetInfants, children and adolescents <19 years of ageMillichap (1964), Uhlemann and Neims (1972), Appleton and DeVivo (1974), Nakazawa et al. (1983), Kinsman et al. (1992), Hori et al. (1997), Bough et al. (1999b), Muller-Schwarze et al. (1999), Sirven et al. (1999), Nordli et al. (2001), Coppola et al. (2002), Kossoff et al. (2002), Mady et al. (2003)
Some evidence of efficacy in adults
Seizure typeEffective in a wide variety of acute and chronic seizure modelsEqually efficacious in a variety of seizure typesLivingston (1972), Appleton and DeVivo (1974), Mahoney et al. (1983), Otani et al. (1984), Schwartz et al. (1989a), Hori et al. (1997), Freeman et al. (1998), Vining et al. (1998), Rho et al. (1999), Su et al. (2000), Thavendiranathan et al. (2000), Greene et al. (2001), Bough et al. (2002)
However, protection however can be transient; maximal seizure activity can be prolonged
Dietary compositionQualitative differences do not affect outcomeQualitative differences do not affect outcomeHuttenlocher et al. (1971), Sills et al. (1986), Schwartz et al. (1989b), Freeman et al. (1998), Vining et al. (1998), Mak et al. (1999), Likhodii et al. (2000), Dell et al. (2001), Kossoff et al. (2003), Henderson et al. (2006), Kossoff et al. (2006)
Classic LCT = MCT = PUFA fat dietClassic LCT, MCT, Atkins can all diminish seizure frequency
KD ratioIncrease KD ratio = greater KD effectIncrease KD ratio = greater KD effectDekaban (1966), Livingston (1972), Bough et al. (1999a), Bough et al. (2000b), Freeman et al. (2000)
Calorie restrictionCR is anticonvulsantCR is an integral part of KD regimenBough et al.(2000a), Bough et al. (2000b), Freeman et al. (2000), Greene et al. (2001), Greene et al. (2003)
CR enhances KD anticonvulsant effect(CR associated with seizure control)
NAL < NCR < KAL < KCR 
Growth rateInitial transient decline in body weightDecline in weight/height over first few monthsRho et al. (1999), Freeman et al. (2000), Su et al. (2000), Vining et al. (2002), Peterson et al. (2005), Thio et al. (2006)
Followed by, resumption of normal or near-normal growth rateFollowed by resumption of normal or near-normal height/weight gain
Very young children do grow poorly
Latency to KD effect1–2 weeksSeizures can be reduced within 1–2 daysLivingston (1972), Appleton and DeVivo (1974), Freeman and Vining (1999), Rho et al. (1999), Freeman et al. (2000), Bough et al. (2006)
Fine-tuning is key to success of the diet; at least 1–2 weeks are required to see if changes are maximally effective
Reversal of KD effectSeizure are behaviorally more severe within hours, but threshold returns to baseline over the course of 1–2 weeksBreakthrough seizure activity can occur immediately with ingestion of carbohydratesAppleton and DeVivo (1974), Huttenlocher (1976), Freeman et al. (2000), Mady et al. (2003), Bough et al. (2006)
GenderMales = femalesMales = femalesMillichap (1964), Nakazawa et al. (1983), Schwartz et al. (1989b), Freeman et al. (1998), Bough et al. (2002), Mady et al. (2003)

Based on vast clinical experience, almost any diet that produces ketonemia and/or diminished blood glucose levels can induce an anticonvulsant effect. Ketogenic diets comprised of either LCTs (Freeman et al., 1998; Vining et al., 1998) or medium-chain triglycerides (MCTs; Huttenlocher et al., 1971; Sills et al., 1986; Schwartz et al., 1989b; Mak et al., 1999) can control seizures with similar efficacy. Even the high-fat, high-protein, and low-carbohydrate Atkins diet that produced a ketotic state, reduced seizures in epileptic patients (Kossoff et al., 2003, 2006). Similar effects have been observed experimentally. Ketogenic diets containing myriad types of fats (i.e., with low carbohydrate content) all produced similar levels of seizure control (Likhodii et al., 2000; Dell et al., 2001). Thus, available evidence indicates that dietary composition per se does not appear to affect the anticonvulsant efficacy of the diet, as long as there is a degree of sustained ketosis.

By comparison, KD ratios and calorie-restriction (CR) appear to be important variables in enabling seizure protection. Seizure control is reportedly optimized when KDs are administered in ratios of ≥3:1 (Freeman et al., 2000), and higher KD ratios increased both clinical (Dekaban, 1966; Livingston, 1972) and experimental anticonvulsant efficacy (Bough et al., 2000b). Similar to the KD ratio, increasing the extent of CR resulted in improved seizure control in epileptic mice (Greene et al., 2001), irrespective of the type of diet that was restricted (Eagles et al., 2003). In general, extra calories in the form of carbohydrates or proteins translate to additional metabolic substrates for gluconeogenesis and diminished KD efficacy. Breakthrough seizures are believed to result from overestimation and administration of excess calories (Freeman et al., 2000). As such, CR may share common anticonvulsant mechanisms and adjunctively optimize KD efficacy.

In rodents, maximal seizure control develops 1–2 weeks after initiation of a KD (Appleton and DeVivo, 1974; Rho et al., 1999; Bough et al., 2006). Similarly in humans, clinical efficacy does not reach its zenith in many patients until after 2 weeks (Dekaban, 1966; Freeman et al., 2000). One notable feature of the KD is the rapid occurrence of breakthrough seizures and loss of ketosis when carbohydrates are introduced (e.g., after a child sneaks a cookie; Huttenlocher, 1976). As a result, the KD must be strictly enforced in order for efficacy to be maintained. However, a breakthrough seizure may not necessarily translate to a total loss of seizure control. Studies have shown that, despite an abrupt discontinuation of the KD, the increased resistance to seizures waned gradually when switched back to control (Bough et al., 2006) or even high-carbohydrate, antiketogenic chow (Appleton and DeVivo, 1974). This decline in seizure threshold generally occurred over 1–2 weeks, mirroring the onset of seizure protection (Appleton and DeVivo, 1974; Bough et al., 2006). This indicates that a critical, minimal level of sustained ketosis is necessary but not sufficient to maintain seizure control. Thus, it would seem that metabolic adaptations to KDs underlie their key anticonvulsant actions.

Many studies and anecdotal observations have suggested that the KD is most effective in immature animals or infants and children (Livingston, 1972; Uhlemann and Neims, 1972; Otani et al., 1984; Bough et al., 1999b; Rho et al., 1999). This is perhaps due to enhanced metabolic capacity, more efficient extraction of ketone bodies from the blood (Morris, 2005), and/or greater compliance of KDs in the pediatric population. However, a lack of efficacy in older children or adults may simply reflect noncompliance or dietary intolerance rather than an inadequate response physiological (Livingston, 1972). The KD has been demonstrated to be similarly effective in infants (Nordli et al., 2001; Kossoff et al., 2002), adolescents (Kinsman et al., 1992; Mady et al., 2003), and adults (Sirven et al., 1999; Coppola et al., 2002). Furthermore, experimental KDs are effective in both young (i.e., <P40 days; Uhlemann and Neims, 1972; Otani et al., 1984; Bough et al., 1999b) and adult rodents (Appleton and DeVivo, 1974; Muller-Schwarze et al., 1999). CR, too, is equally anticonvulsant in both juvenile and adult mice (Greene et al., 2001). Thus, increasing evidence suggests the anticonvulsant effects of KDs do not appear to be age-dependent.

Clinical reports indicate that outcome is unrelated to seizure type or frequency (Freeman et al., 1998; Schwartz et al., 1989a; Vining et al., 1998). At least 50% of patients treated with a classical KD will exhibit at least a 50% reduction in seizures (Livingston, 1972; Freeman et al., 1998). Comparatively, anticonvulsant effects of KD in animals are more modest. Rats and mice only demonstrate a 15–20% increase in seizure threshold (Appleton and DeVivo, 1974; Bough and Eagles, 1999; Rho et al., 1999; Bough et al., 2000b). Despite efficacy across a variety of acute and chronic seizure models (Hori et al., 1997; Muller-Schwarze et al., 1999; Su et al., 2000), KD-induced anticonvulsant effects have been incomplete (Bough et al., 2002) or of limited duration (Hori et al., 1997). Further, there is little evidence to indicate that KDs diminish severity once a seizure begins. Indeed, many studies—including our own—have shown that CR and/or KDs can even exacerbate (maximal) seizures (Mahoney et al., 1983; Otani et al., 1984; Bough et al., 2000a; Thavendiranathan et al., 2000; Bough et al., 2003). If one considers that seizure activity requires large amounts of energy, seizure exacerbation may be a reflection of enhanced energy reserves after KD treatment, a situation that would allow for prolonged ictal activity once it begins (discussed below).

In summary, the following generalizations can be made about the KD: (1) its anticonvulsant effects appear independent of dietary formulation, but appear to be strongly linked to the total quantity of calories consumed; (2) the KD must be strictly adhered to, if the anticonvulsant effect is to be maintained; (3) CR may work synergistically with KD to limit seizures and optimize treatment; (4) maximum efficacy is not achieved for several days or weeks after initiation, suggesting that adaptive metabolic and/or genetic “programs” underlie KD-induced seizure protection; (5) these adaptations are likely generalized throughout the (epileptic) brain, irrespective of underlying pathology or genetic predisposition to seizures since the KD is an effective treatment for diverse epileptic conditions; and (6) efficacy is independent of gender and age, suggesting that KD treatment produces seizure control via a common set of pathways in all clinical responders.

ANTICONVULSANT MECHANISMS OF THE KETOGENIC DIET

  1. Top of page
  2. Abstract
  3. MECHANISTIC INSIGHTS FROM STUDIES OF KD EFFICACY
  4. ANTICONVULSANT MECHANISMS OF THE KETOGENIC DIET
  5. HOW CAN THE KETOGENIC DIET BE OPTIMIZED?
  6. CONCLUSIONS
  7. REFERENCES

Since the KD was originated over 85 years ago, several major hypotheses have been advanced, but none have been widely accepted. Several key aspects of the KD might ultimately result in seizure protection. Ketone bodies, free fatty acids (in particular, polyunsaturated fatty acids), or glucose restriction might each lead directly or indirectly to seizure control. While it is possible that any one of these KD-induced changes is responsible for the anticonvulsant action of the KD, available evidence suggests that improved seizure control, at a minimum, likely requires all three.

Role of ketone bodies

Beta-hydroxybutyrate (BHB) is the predominant ketone body measured in the blood, and as such, has been used as a clinical measure of KD implementation (Fig. 1). Accordingly, nearly all KD studies have attempted to establish a causative link between ketonemia and anticonvulsant efficacy. Although robust elevations in plasma BHB levels have been observed during KD treatment (Bough et al., 1999b; Thavendiranathan et al., 2000), there is no significant correlation between plasma BHB levels and seizure protection. Optimal seizure protection generally lags days to weeks behind the development of ketonemia, which occurs within hours of KD onset.

image

Figure 1. Metabolic pathways highlighting the production of ketone bodies fatty acids during fasting or treatment with the ketogenic diet (KD). Estimated fasting- or KD-induced concentrations of beta-hydroxybutyrate, acetoacetate, and acetone in blood are listed (large boxes). Measures of beta-hydroxybutyrate levels in blood are most commonly used as the clinical indicator of successful KD treatment. From Likhodii and Burnham (2004).

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Nevertheless, there is some evidence that ketones other than BHB may possess anticonvulsant properties. When injected into animals, acetone and its parent acetoacetate (ACA), prevent acutely provoked seizures. Seminal work in the 1930s revealed that acute intraperitoneal administration of acetone or ethyl-acetoacetate protected rabbits from thujone-induced seizures (Helmholz and Keith, 1930; Keith, 1933). Thujone is the active constituent of wormword oil, and is an antagonist of GABAA receptors (Hold et al., 2000). More recent experimental studies have shown similar results in rodents. Acetone (Likhodii et al., 2003) and ACA (Rho et al., 2002)—but not BHB—were anticonvulsant in a variety of acute and chronic models of epilepsy, consistent with earlier observations (Helmholz and Keith, 1930; Yamashita et al., 1976; Vodickova et al., 1995). Clinically, acetone levels of up to 1 millimolar (mM) were detected in the brains of five of seven well-controlled epileptic patients following KD using magnetic resonance spectroscopic techniques (Seymour et al., 1999). Although acetone could not be detected in two other seizure-free patients, the authors concluded that acetone contributes to the anticonvulsant effect of the KD. Interestingly, the concept that a lipophilic solvent may potently block seizure activity is not new. The classic example of this is valproic acid, which was initially used as a solvent to dissolve investigational anticonvulsant compounds, but was serendipitously discovered to possess intrinsic anticonvulsant properties.

Whereas in vivo pharmacodynamic studies have suggested that both ACA and acetone may act as anticonvulsant agents, there is no evidence that ketone bodies can directly modulate synaptic transmission and/or neuronal excitability. In vitro cellular electrophysiological experiments have failed to demonstrate an effect on the principal ion channels that mediate neuronal excitability and inhibition. Specifically, neither L-BHB nor ACA were found to modulate GABAA receptors, AMPA receptors, or NMDA receptors in both hippocampus and neocortex of rats (Thio et al., 2000; Donevan et al., 2003). Despite these negative observations, it remained possible that ketone bodies might affect network activity or synchrony. However, in field potential recordings conducted in vitro, Thio et al. (2000) demonstrated clearly that neither ACA nor BHB modified evoked excitatory postsynaptic potentials (EPSPs) or population spikes in the CA1 subfield of the hippocampal tissue. In summary, there is no evidence for direct anticonvulsant effects for either ACA or BHB, and acetone has yet to be studied in neuronal (CNS) tissue. This may, in large part, reflect the technical difficulties in investigating a compound that is highly volatile and can react with perfusion systems ordinarily used in pharmacological in vitro experiments.

Recently, it has been suggested that ACA and/or its metabolic byproduct, acetone, may activate a novel class of potassium leak channels known as the two-pore domain or K2P channels (Vamecq et al., 2005). K2P channels represent a diverse superfamily of channels that generally hyperpolarize cell membranes, and regulate membrane excitability both pre- and postsynaptically (Lesage, 2003). These channels can be modulated by changes in pH, osmolality, temperature, mechanical pressure, and certain fatty acids (Franks and Honore, 2004). Links between KD-induced elevations in ketone bodies (and/or fatty acids, as discussed below) and K2P channels, however, have yet to be explored.

In conclusion, although ketone bodies have been shown to possess anticonvulsant properties in vivo, there is no evidence to date that they mediate directly these effects. It is clear that some degree of sustained ketosis is required for clinical efficacy and that efficacy is maximized over a period of weeks versus days, despite a rapid onset of ketosis within hours. Whereas it is plausible that some dietary, pharmacokinetic factor(s) results in some level of seizure protection, the approximate 2-week time course for optimal seizure protection suggests a pharmacodynamic effect of the KD (e.g., parallel time course for changes in gene expression, mitochondrial proliferation, up-regulation of UCPs/transporters, etc) likely underlies the anticonvulsant nature of the diet. Thus, available data suggest that adaptations to, rather than a direct effect of, ketosis underlie the anticonvulsant nature of the KD.

Role of glucose restriction

Whereas most studies have suggested that persistent ketosis is essential to KD-induced seizure protection, others have posited that glucose restriction is the key feature (Greene et al., 2003). In addition to ketosis, it is clear that as ketonemia develops, another immediate consequence of CR and/or KDs is a ‘moderate’ reduction in blood glucose. Does caloric restriction simply act to limit gluconeogenic substrates that would otherwise reduce KD ratio and counter efficacy? Or, might glucose restriction result in another metabolic adaptation that helps quell aberrant hyperexcitability? Calorie restriction alone was sufficient to retard seizure susceptibility in juvenile and adult epileptic EL mice; and, blood glucose levels were inversely correlated with a decreased risk of seizures (Greene et al., 2001). Greene et al. (2003) hypothesized that CR reduces energy production through glycolysis, which limits a neuron's ability to reach (and maintain) high levels of synaptic activity necessary for seizure genesis.

Others have hypothesized that glucose restriction during KD treatment activates ATP-sensitive potassium (KATP) channels (Schwartzkroin, 1999; Vamecq et al., 2005). Interestingly, KATP channels are ligand-gated receptors broadly expressed throughout the central nervous system, in both neurons and glia (Thomzig et al., 2005). These channels act as metabolic sensors, linking cellular membrane excitability to fluctuating levels of ADP and ATP. Activation of this receptor by reduced ATP/ADP ratios opens the channel and leads to membrane hyperpolarization. When glucose is limited (e.g., during administration of a classic KD, which is typically CR by 25%), KATP channels might open to hyperpolarize the cell as the intracellular ATP concentrations fall. Conversely, when glucose is present and ATP concentrations rise, KATP channels close. As such, KATP channels may provide a measure of protection against a variety of metabolic stressors such as hypoxia, ischemia, and hypoglycemia, and are believed to regulate seizure threshold (Seino and Miki, 2003).

KATP channels are particularly abundant in the substantia nigra (Hicks et al., 1994), a region of the brain thought to act centrally in the propagation of seizure activity (Iadarola and Gale, 1982). KATP channels would therefore be ideally positioned to metabolically regulate the onset of several different seizure types, as does the KD. There is growing evidence that KATP channels may critically regulate seizure activity. Genetically engineered mice that overexpress the sulfonylurea (SUR) subunit of KATP channels were significantly more resistant to seizures induced by kainate, and showed no marked cell loss in hippocampus (Hernandez-Sanchez et al., 2001). Studies of KATP channel (Kir6.2-/-) knockout mice suggested that these channels help determine seizure threshold (Yamada et al., 2001). Following hypoxic challenge (∼5% O2), knockout mice exhibited myoclonic–tonic seizure activity, and, ultimately, death compared to controls who all recovered without sequelae.

Despite these observations, there is one important caveat in implicating KATP channels as mediators of a KD-induced anticonvulsant effect. Other studies have demonstrated an increase in energy reserves (specifically, ATP) after KD treatment (DeVivo et al., 1978; Pan et al., 1999). These data predict that KATP channels would remain closed, not open, during diet treatment, and would thus contribute to neuronal/glial cell membrane depolarization. Nevertheless, several findings are consistent with the notion that KATP channels are selectively activated during administration of a low-glucose, high-fat KD. First, KATP channels are regulated preferentially via glycolytic energy sources (Dubinsky et al., 1998). It has recently been shown that the glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) serves as an accessory protein to KATP channels and regulates directly their activity (Dhar-Chowdhury et al., 2005; Jovanovic et al., 2005). The observed reduction in glycolytic processes after KD treatment (specifically, the concentration of fructose-1,6-bisphosphate, the key regulatory enzyme of glycolysis) is consistent with this notion (DeVivo et al., 1978; Puchowicz et al., 2005; Melo et al., 2006). Glycolytic flux may be further limited as a consequence of elevated ATP (DeVivo et al., 1978; Otani et al., 1984; Pan et al., 1999; Bough et al., 2006) and citrate (Yudkoff et al., 2001) levels on KD treatment; both ATP and citrate are feedback inhibitors of glycolysis.

Second, it is hypothesized that the accumulation of free fatty acids over the course of KD administration (Dekaban, 1966) may boost KATP channel activation (Vamecq et al., 2005). Whereas PUFAs freely cross the BBB, saturated free fatty acids are transported across the BBB via carrier-mediated processes (Avellini et al., 1994). Fatty acids that intercalate within neuronal cell membranes have been shown to interact potently with KATP channels, specifically reducing their affinity for (and inhibition by) ATP (Shyng and Nichols, 1998). Overall, these findings suggest that the unique nature of low-glucose, high-fat KDs promotes KATP channel activation, despite observed enhancements in oxidative energy production.

Recent experiments involving 2-deoxyglucose (2-DG) provide further support for a glucose-restriction hypothesis of KD action. Two-deoxyglucose is a glucose analogue, which inhibits phosphoglucose isomerase and, hence, glycolysis. Stafstrom et al. (2005) reported that the addition of 1 mM 2-DG decreased epileptiform burst frequency to 25–80% of baseline in rat hippocampal slices exposed to elevated extracellular potassium. More significantly, the same group also showed that 2-DG (250 mg/kg, i.p) elevated the after-discharge threshold in olfactory bulb of perforant-path kindled rats, markedly reduced the progression of kindling, and limited the expression of BDNF and its cognate receptor, trkB (Garriga-Canut et al., 2006).

Interestingly, there are a number of anticonvulsant parallels between 2-DG (Stafstrom et al., 2005; Garriga-Canut et al., 2006) and KD treatment (Bough et al., 2003). First, both 2-DG and KD elevated electrographic seizure threshold in vivo; second, both 2-DG and KD potently retarded the progression of epileptogenesis in kindling models of epilepsy in vivo; and, third, both 2-DG (in vitro) and KD (in vivo) diminished measures of hippocampal hyperexcitability. These results collectively suggest that the anticonvulsant actions of KD may work, in large part, via an inhibition of glycolysis. Importantly, because 2-DG is fairly well tolerated when administered orally (Pelicano et al., 2006), this compound may represent a novel treatment strategy for epilepsy.

Role of fatty acids

Polyunsaturated fatty acids (PUFAs) such as docosahexanoic acid (DHA, C22:6ω3), arachidonic acid (AA, C20:4ω6), or eicosapentanoic acid (EPA, C20:5ω3) are believed to affect profoundly cardiovascular function and health (Leaf and Kang, 1996; Nordoy, 1999; Leaf et al., 2003). In cardiac myocytes, PUFAs inhibited fast, voltage-gated sodium channels (Xiao et al., 1998) and L-type calcium channels (Xiao et al., 1997). Similar findings have been observed in neuronal tissue. For example, DHA and EPA diminished neuronal excitability and bursting in hippocampus (Xiao and Li, 1999).

It is not surprising then that PUFAs are becoming an increasingly popular focus of KD research. After KD treatment, specific PUFAs (i.e., AA and DHA) were found to be elevated in both serum (Cunnane et al., 2002; Fraser et al., 2003) and brain (Taha et al., 2005) of patients and animals. Importantly, one report documented that the rise (or drop) in total fatty acids during KD treatment closely paralleled clinical improvement (or loss) of seizure control (Dekaban, 1966). An additional study found that dietary supplementation with 5 g of (65%) n-3 PUFAs once per day produced a marked reduction in seizure frequency and intensity in a few epileptic patients (Schlanger et al., 2002). These findings suggest that KD-induced elevations in PUFAs such as DHA and/or AA might act directly to limit neuronal excitability and dampen seizure activity.

PUFAs could ultimately block seizure activity in a number of ways (Fig. 2). First, PUFAs may inhibit directly ion channel activity. Omega-3 (ω-3) PUFAs have been shown to: (1) inhibit both voltage-gated Na+ and Ca2+ channels, (2) increase the resistance to bursting induced by bicuculline, zero Mg2+, pentylenetetrazole or glutamate, and (3) prolong the recovery time from inactivation in hippocampal neurons (Vreugdenhil et al., 1996; Xiao and Li, 1999; Young et al., 2000). Second, in conjunction with ketone bodies, PUFAs may activate a lipid-sensitive class of K2P potassium channels (Vamecq et al., 2005). And, third, PUFAs may enhance the activity of the Na+/K+-ATPase (sodium pump). Elevated ω-3 and diminished ω-6 PUFAs levels in plasma membranes significantly increased sodium pump function (Wu et al., 2004). These findings indicate that elevations in brain levels of PUFAs after KD treatment (Taha et al., 2005) could help reduce neuronal hyperexcitability via a variety of direct mechanisms.

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Figure 2. Potential pathways through which polyunsaturated fatty acids (PUFAs) may limit hyperexcitability in the brain. Acting directly, PUFAs such as arachidonic acid (AA), docosahexanoic acid (DHA), and/or eicosapentanoic acid (EPA) might inhibit both voltage-gated Na+ and Ca2+ channels, activate a lipid-sensitive class of K2P potassium channels, and enhance the activity of the Na+/K+-ATPase to limit neuronal excitability and dampen seizure activity. Acting indirectly, PUFAs might induce the expression and activity of uncoupling proteins (UCPs) to diminish reactive oxygen species (ROS), reduce neuronal dysfunction and induce a neuroprotective effect. Finally, PUFAs are expected to activate PPARα and induce a coordinate up-regulation of energy transcripts leading to enhanced energy reserves, stabilized synaptic function and limited hyperexcitability.

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Uncoupling proteins

In addition to their direct actions on neuronal excitability, PUFAs may also act indirectly to limit excitotoxicity and neurodegeneration. PUFAs regulate the expression of numerous genes in brain via transcription factors such as PPARα (peroxisome proliferator-activated receptor-α; Sampath and Ntambi, 2004). Through induction of PPARα and its coactivator PGC-1, PUFAs induce the expression of mitochondrial uncoupling proteins (UCPs) and activate these proteins directly as well (Jaburek et al., 1999; Diano et al., 2003). Recent evidence suggests that PUFAs are required for mitochondrial UCP activity (Garlid et al., 2001).

Uncoupling proteins are homodimers that span the inner mitochondrial membrane and allow a proton leak from the intermembrane space to the mitochondrial matrix. There are three major isoforms that have been identified in the brain, UCP2, UCP4 and UCP5 (a.k.a., BMCP-1 or brain mitochondrial carrier protein-1). UCP proteins are increasingly implicated in the regulation of neuronal excitability and survival (Andrews et al., 2005). The uncoupling effect, albeit of small magnitude, reduces the proton-motive force, disassociates or ‘uncouples’ electron transport from ATP production, and indirectly decreases the production of reactive oxygen species (ROS). Although it would seem that increased levels of UCP proteins would diminish cellular energy production, Diano et al. (2003) showed that chronic overexpression of UCP2 in neuronal tissue increased cellular ATP and ADP levels by triggering mitochondrial biogenesis. KD appears to do the same; that is, studies show that the KD induces UCP expression, stimulates mitochondrial biogenesis, and enhances energy production (see also below). Seizures, by comparison, increase ROS generation and/or mitochondrial dysfunction, which can lead to neuronal dysfunction and excitotoxicity (Layton and Pazdernik, 1999; Kovacs et al., 2001; Kovacs et al., 2002; Sullivan et al., 2003). Interestingly, UCP2 is up-regulated after seizures (Diano et al., 2003). The protective role of UCPs was recently highlighted by Sullivan et al. (2003) who demonstrated that dietary enhancement of UCP expression and function in immature rats protected against kainate-induced excitotoxicity, most likely by decreasing ROS generation (Andrews et al., 2005). Further work demonstrated that mice maintained on a high-fat KD demonstrated an increase in the hippocampal expression and activity of all three mitochondrial UCPs and exhibited a significant reduction in ROS generation in mitochondria isolated from the same brain region (Sullivan et al., 2004). In conjunction with reports that ketone bodies potently decrease ROS generation (Veech et al., 2001; Veech, 2004), these reports suggest that the KD compensates for seizure-induced elevations in ROS generation and neuronal dysfunction to provide a neuroprotective effect.

Energy production

Polyunsaturated fatty acids additionally regulate the transcription of numerous genes linked to energy metabolism (Sampath and Ntambi, 2005) through activation of PPARα, a scenario in which the KD is thought to re-program cellular metabolism (Cullingford, 2004). Indeed, numerous studies have described changes consistent with an enhancement in energy production following KD treatment. First, microarray expression studies demonstrated that KD induces a coordinated up-regulation of several dozen metabolic genes associated with oxidative phosphorylation after KD (Noh et al., 2004; Bough et al., 2006). Second, KD treatment stimulated mitochondrial biogenesis, resulting in a striking 46% increase in the number of mitochondria in the hilar/dentate gyrus region of rat hippocampus (Bough et al., 2006). And, third, levels of energy metabolites were increased after KD. Brain glycogen and ATP concentrations were boosted throughout rodent brain (DeVivo et al., 1978; Otani et al., 1984) and there was an elevation in the phosphocreatine-to-creatine (PCr:Cr) energy-reserve ratio in both animals (Bough et al., 2006) and humans (Pan et al., 1999). These findings are consistent with results that show ketones (4 mM BHB + 1 mM ACA) increased hydraulic work by 14% and improved energy status in perfused heart tissue (Sato et al., 1995). Further, there is an overall increased metabolic efficiency (DeVivo et al., 1978; Bough et al., 2006), decreased respiratory quotient (Bough et al., 2000b), and maximal mitochondrial respiratory rate in rodents following the KD (Sullivan et al., 2004). Collectively, these data provide compelling evidence that the KD enhances oxidative energy production by activating a variety of transcriptional, translational, and biochemical mechanisms in a concerted fashion.

Metabolic dysfunction has been identified in regions of hyperexcitability within the brain and is associated with several epileptic conditions. Impairment of mitochondrial function has been observed in the seizure foci of both human and experimental epilepsies (Kunz et al., 2000). Severe metabolic dysfunction occurred in both human and rat hippocampal tissue during periods of heightened neuronal activity (Kann et al., 2005). Kudin et al. (2002) demonstrated that seizure activity down-regulated mitochondrial enzymes involved in oxidative phosphorylation. In an earlier study, the same group demonstrated a specific deficiency in complex I activity and mitochondrial ultrastructural abnormalities within the hippocampal CA3 region of epileptic tissue resected from 57 human patients (Kunz et al., 2000). In view of previous studies demonstrating impaired oxidative phosphorylation capacity in pilocarpine-treated rats (Kudin et al., 2002) and in patients with epilepsy (Antozzi et al., 1995; Kunz et al., 2000), a KD-induced augmentation in oxidative phosphorylation and energy reserves seems likely to counter energetic deficiencies in epileptic tissue, making neuronal tissue more resilient to aberrant neuronal activity and, in this way, contributing to the diet's anticonvulsant actions.

Stabilized synaptic function

Intriguing as this argument may be, how exactly would enhanced energy reserves lead to stabilized synaptic function and diminished seizures? One possibility is via the sodium pump. ATP is primarily used to maintain ionic gradients, especially through actions of the transmembrane sodium pump (Hulbert and Else, 2000). Schwartzkroin originally hypothesized that KD-induced elevations in ATP concentrations might enhance and/or prolong the activation of the Na+/K+-ATPase, perhaps via an increase in the delta-G′ of ATP hydrolysis (Veech et al., 2001; Veech, 2004). In neurons, increased sodium pump activity might hyperpolarize the cell and/or reduce the resting membrane potential to diminish firing probability. Enhanced Na+/K+-ATPase function in neurons might also preserve normal neuronal functioning and/or delay a pathological buildup of high external K+ (Xiong and Stringer, 2000). In glia, increased activation of the Na+/K+-ATPase might slow glial depolarization and allow for prolonged uptake of extracellular K+ during periods of intense neuronal activity (e.g., high-frequency bursting). Increases in neuronal and/or glial action of the sodium pump would be expected to limit hyperexcitability and increase the resistance to seizures, as is noted after treatment with KD.

Although no studies have tested this sodium-pump hypothesis directly, a recent report suggests that KD tissue is more resistant to metabolic stress. When challenged with mild hypoglycemia, synaptic transmission within the dentate gyrus was maintained for approximately 60% longer in tissue from KD-fed animals compared to controls (Bough et al., 2006). These data suggest that the KD stabilizes synaptic transmission (both excitatory and inhibitory) for prolonged periods of time during metabolic stress such as during seizure activity. Hence, it seems likely that the KD induces seizure protection in part by preventing neuronal dysfunction (diminution of ROS/enhancement of energy reserves) and stabilizing synaptic transmission (enhancement in energy reserves).

A role for neurotransmitter systems

The noradrenergic hypothesis

One of the more intriguing observations regarding KD action involves norepinephrine, its receptors and signaling cascades. In general, increases in noradrenergic tone result in an anticonvulsant effect. Several lines of evidence support this view. Norepinephrine (NE) re-uptake inhibitors can prevent seizure activity in genetically epilepsy prone rats (GEPRs; Yan et al., 1993) and pharmacological NE agonists are generally, though not always, anticonvulsant (Weinshenker and Szot, 2002); damage to the locus coeruleus—the principal region of the brain from which ascending and descending noradrenergic innervation originates—converts occasional seizures into self-sustaining status epilepticus (SSSE) in rats (Giorgi et al., 2004); animals are more prone to seizures when NE is chemically depleted with reserpine (Weinshenker and Szot, 2002); and, interestingly, there are several reports of diminished brain levels of NE in several animal models of epilepsy, including GEPRs, kindled animals, EL mice, seizure-sensitive Mongolian gerbils, and tottering mice (Weinshenker and Szot, 2002).

Of significant interest is the observation that mice lacking the ability to produce NE (Dbh-/- knockout mice) do not exhibit an increased resistance to flurothyl seizures when treated with a KD (Szot et al., 2001). These data indicate that NE is required for the anticonvulsant effect of KD, at least in the flurothyl seizure threshold model. Weinshenker and Szot (2002) additionally reported an approximate twofold increase in NE levels in hippocampus following a KD, suggesting that KD increases basal release of NE. These studies indicate the anticonvulsant action of KD may result in part from an enhancement in noradrenergic signaling in the brain.

If the KD enhances NE release as described above, it may also promote the corelease of anticonvulsant orexigenic peptides such as neuropeptide-Y (NPY) and galanin. NPY has been shown to inhibit glutamatergic synaptic transmission and epileptogenesis in vitro (Rhim et al., 1997; Richichi et al., 2004; Vezzani and Sperk, 2004); galanin has been shown to limit SSSE (Saar et al., 2002) and diminish both excitatory synaptic transmission and ictal activity in vitro (Schlifke et al., 2006). Both neuropeptides are elevated after calorie restriction. However, there was no evidence for enhanced transcription of either of these peptides in the brain after KD treatment, suggesting that neither NPY nor galanin contribute significantly to the anticonvulsant actions of KD (Tabb et al., 2004).

The GABAergic hypothesis

One of the more popular hypotheses for KD action involves γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the mammalian brain. In general, the KD is most effective against seizures evoked by GABAergic antagonists. The KD potently inhibits seizures induced by pentylenetetrazole, bicuculline, picrotoxin, and γ-butyrolactone. In contrast, the diet demonstrates little if any efficacy in acute seizure models involving activation of ionotropic glutamate receptors (e.g., kainic acid), voltage-dependent sodium channels (e.g., maximal electroshock [MES]), or glycine receptor inhibition (e.g., strychnine; Bough et al., 2002).

Yudkoff et al. (2005) have proposed that ketosis induces major shifts in brain amino acid handling favoring the production of GABA. This results from a reduction of aspartate relative to glutamate, the precursor to GABA synthesis, and a shift in the equilibrium of the aspartate aminotransferase reaction in the ketotic state. As a result, there is an increase in glutamic acid decarboxylase (GAD) activity and GABA production (Fig. 3). Elevated GABA levels would, in turn, be expected to dampen hyperexcitability throughout the brain. Several studies support this possibility. First, KD and CR diet treatments both increased GAD transcript and protein levels in inferior and superior colliculi, cerebellar and temporal cortex, and striatum (the latter, KD only; Cheng et al., 2004). Second, both BHB and ACA increased the rate and extent of GABA formation in synaptosomes (Erecinska et al., 1996; Yudkoff et al., 1997). And, finally, KD treatment in vivo modified amino acid levels in a manner consistent with enhanced GABA production (Yudkoff et al., 2001; Melo et al., 2006). Although brain levels of glutamate and GABA have not been consistently elevated in rodents (DeVivo et al., 1978; Al-Mudallal et al., 1996; Yudkoff et al., 2001; Bough et al., 2006), two recent clinical studies report significant increases in GABA levels following KD treatment (Wang et al., 2003; Dahlin et al., 2005), further substantiating this view.

image

Figure 3. Metabolic modifications of glutamate and GABA synthesis as a consequence of diminished glucose and ketosis. In ketosis, beta-hydroxybutyrate and acetoacetate contribute heavily to brain energy needs. A variable fraction of pyruvate (1) is ordinarily converted to acetyl-CoA via pyruvate dehydrogenase. In contrast, all ketone bodies generate acetyl-CoA, which enters the tricarboxcylic acid (TCA) cycle via the citrate synthetase pathway (2). This involves the consumption of oxaloacetate, which is necessary for the transamination of glutamate to aspartate. Oxaloacetate is then less available as a reactant of the aspartate aminotransferase pathway, which couples the glutamate-aspartate interchange via transamination to the metabolism of glucose through the TCA cycle. Less glutamate is converted to aspartate and thus, more glutamate is available for synthesis of GABA (3) through glutamic acid decarboxylase (GAD). Adapted from Yudkoff et al. (2004).

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In addition to biochemical measures of KD-enhanced GABAergic inhibition, there is functional evidence as well. Electrophysiological recordings conducted in vivo demonstrated that network excitability was diminished in both KD- and calorie-restricted rats (Bough et al., 2003); greater stimulus intensities were required to evoke population spikes in both CR- and KD-fed animals compared to ad libitum controls. Paired-pulse inhibition was increased. Both CR and KD dietary treatments resulted in greater paired-pulse inhibition compared to controls at the 30-ms interpulse interval (Bough et al., 2003), a result consistent with an enhancement in fast, GABAA-mediated inhibition. Additionally, KD-fed animals exhibited an elevated electrographic seizure threshold and an increased resistance to a modified, 1-day kindling protocol (maximal dentate activation). These data suggested that both KD and calorie-restricted diets limited network excitability and elevated seizure threshold via an enhancement of GABAergic inhibition.

GABAergic interneurons, which at baseline have more depolarized resting membrane potentials, endure non-accommodating bursts of neuronal firing and must metabolically persist (Attwell and Laughlin, 2001), lest network inhibition becomes compromised. Previous studies have shown that a KD increases total brain [ATP] (DeVivo et al., 1978) and PCr/Cr or PCr/ATP energy reserve (Pan et al., 1999; Bough et al., 2006). Accordingly, KD-induced elevations in PCr are likely to play a pivotal role in maintaining the activity of the Na+/K+-ATPase during periods of intense seizure activity, in both glutamatergic and GABAergic neurons. In a recent study of human temporal lobe epilepsy (Williamson et al., 2005), the PCr/ATP ratio correlated with the recovery of the membrane potential following a stimulus train, which was inversely correlated with granule cell bursting. Because creatine kinase is predominantly localized within GABAergic interneurons (Boero et al., 2003), Boero et al. concluded that PCr and energy levels are especially critical to the maintenance of GABAergic inhibitory output. In this manner, a KD-induced increase in energy reserves might enhance GABAergic function in particular and improve seizure control.

HOW CAN THE KETOGENIC DIET BE OPTIMIZED?

  1. Top of page
  2. Abstract
  3. MECHANISTIC INSIGHTS FROM STUDIES OF KD EFFICACY
  4. ANTICONVULSANT MECHANISMS OF THE KETOGENIC DIET
  5. HOW CAN THE KETOGENIC DIET BE OPTIMIZED?
  6. CONCLUSIONS
  7. REFERENCES

Historically, few guidelines have emerged regarding the clinical implementation of the KD and its variants – including the medium-chain triglyceride (MCT) formulation (Huttenlocher et al., 1971) and more recent options such as the Atkins diet (Kossoff et al., 2003; Kossoff et al., 2006). This is largely a result of the fact that, until recently, few KD centers existed throughout the world. Even with a resurgence of interest in dietary approaches toward epilepsy treatment in the past decade, there remains a notable absence of Class I and II clinical studies. Today, few question the clinical efficacy of the KD in both young and older patients (Vining, 1999; Coppola et al., 2002; Mady et al., 2003), and many successful international centers have evolved (Kossoff and McGrogan, 2005; Freeman et al., 2006). However, since we do not fundamentally know how the KD prevents seizures, there exists as yet no rational basis for optimizing the efficacy of the diet, other than through trial and error.

When examining the accumulated clinical data, it appears seizure control can be achieved in the majority of epileptic patients as long as there is a shift from glycolytic flux to intermediary metabolism (resulting in measurable ketosis), irrespective of the precise dietary formulation (Henderson et al., 2006). On the other hand, the experimental literature suggests that different treatment protocols may result in differential efficacy or even lack of efficacy, despite significant ketosis (Bough et al., 2000a; Thavendiranathan et al., 2000; Bough et al., 2002; Thavendiranathan et al., 2003). Most of the published studies have been based on acute seizure models, and not on developmental epilepsy models. Hence, of course, one must bear in mind that, despite dozens of animal models of the KD, none recapitulate all of the essential features in the human epileptic condition (Stafstrom, 1999).

So how can we reach the goal of developing a safer and more effective KD? The reductionist approach posits that were we to identify the critical mediator of the diet's anticonvulsant effect, administration of this substrate alone would likely yield a similar clinical effect as the traditional KD, and importantly, spare the patient significant side-effects that may preclude its use—even in the face of clear clinical efficacy. The closest we have come to this situation is the recent use of BHB as an oral neuroprotectant. Promising results have already been demonstrated in Phase I clinical trials (Smith et al., 2005). Nevertheless, despite increasing experimental evidence that BHB and ACA both possess neuroprotective properties (Kashiwaya et al., 2000; Noh et al., 2006), a direct anticonvulsant effect of ketone bodies has not yet been demonstrated in epileptic brain, either animal or human. Intriguing, however, are animal studies indicating that ACA and acetone are anticonvulsant in acute seizure models. Yet, there remains a perplexing lack of an acute anticonvulsant effect of the principal ketone body, BHB.

Conversely, if we believe that certain PUFAs, in lieu of ketone bodies, are direct mediators of an anticonvulsant effect (Cunnane et al., 2002; Cunnane, 2004), as suggested by clinical studies (Schlanger et al., 2002; Fraser et al., 2003; Fuehrlein et al., 2004; Yuen et al., 2005), we may be closer to distilling the essence of the KD. However, there is likely no single fatty acid that is necessary and sufficient for an anticonvulsant effect. And experimentally, while it has been straightforward to demonstrate the inhibitory effects of PUFAs on specific voltage-gated ion channels and the resultant diminution of cellular excitability in vitro, it is not an easy task to demonstrate that ingestion of a specific fatty acid or fatty acid cocktail, acts directly on relevant brain receptor targets without first undergoing beta-oxidation. The collective data, from both animals and humans, indicate that the critical condition necessary for achieving seizure control is a metabolic shift toward fatty acid oxidation from glycolysis, reflected in the variable rise in blood/brain ketone levels and a concomitant (moderate) reduction in blood/brain glucose. Fatty acid composition may not ultimately matter, as long as this important metabolic shift occurs. And, interestingly, calorie restriction (Greene et al., 2001; Bough et al., 2003; Eagles et al., 2003; Greene et al., 2003) or intake of 2-DG (Stafstrom et al., 2005), both of which result in mild hypoglycemia, may be the only requirement for seizure protection, regardless of whether fats are consumed or not.

As we continue to explore putative anticonvulsant mechanisms of KD action, we are left with many outstanding clinical questions regarding dietary treatments for epilepsy. Well-designed, multicenter prospective- and controlled clinical trials are essential toward developing the optimum KD. If woven together with pharmacokinetic and pharmacogenetic investigations, these clinical studies will not only provide further insights into mechanistic underpinnings, but will also help differentiate responders from non-responders and identify patients in whom the diet is definitively contraindicated. Clinicians would be given the tools to make evidence-based decisions rather than rely upon a few case–controlled studies, anecdotal reports of efficacy, or clinical folklore as has been the practice in the past. Toward this end, information regarding the impact of pharmacogenetics on epilepsy treatment is now beginning to emerge (Depondt and Shorvon, 2006; Spurr, 2006), although much less is known regarding the genetically determined variables influencing dietary impact on brain function, particularly as it relates to the epileptic brain.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. MECHANISTIC INSIGHTS FROM STUDIES OF KD EFFICACY
  4. ANTICONVULSANT MECHANISMS OF THE KETOGENIC DIET
  5. HOW CAN THE KETOGENIC DIET BE OPTIMIZED?
  6. CONCLUSIONS
  7. REFERENCES

After nearly a century of clinical use, we still do not know how the KD works. However, much progress in KD research has been made in the past decade. Among other factors, current evidence indicates KD optimizes cellular metabolism. Endogenous biochemical and genetic ‘programs’ are switched on in the brain in response to ketosis, glucose restriction, and elevated free fatty acids. This unique metabolic state, if maintained, induces a shift away from glycolytic energy production (glucose restriction) toward the production of energy via oxidative phosphorylation (beta-oxidation of fatty acids and production of ketone bodies). The reduction in glycolytic energy supply may activate selectively KATP channels to increase the resistance to onset of ictal activity. An increase in oxidative phosphorylation coupled with an induction of UCPs and mitochondrial biogenesis can diminish ROS generation and increase energy reserves, both of which would be expected to prevent neuronal dysfunction, seizures and even neurodegeneration.

It is improbable that one mechanistic target or mediator will produce entirely the seizure protection associated with the KD. Rather, several factors likely contribute mechanistically to this broadly efficacious treatment for epilepsy. The challenge of finding key variables is made ever more difficult by the intrinsic complexity of metabolic effects and their resultant actions on neurons, glia and on the epileptic condition itself. We have reviewed here a number of seemingly disparate variables that must be sustained for a meaningful anticonvulsant effect to be rendered. These interrelationships are summarized in Fig. 4. The fact that a fundamental modification in diet can have such profound, therapeutic effects on neurological disease underscores the importance of elucidating mechanisms of KD action. Future studies will no doubt provide unique insights into how diet can affect the brain, both in health and disease, and likely provide the scientific basis for the development of potent new treatment strategies for the epilepsies.

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Figure 4. Hypothetical pathways leading to the anticonvulsant effects of the ketogenic diet (KD). Elevated free fatty acids (FFA) lead to chronic ketosis and increased concentrations of polyunsaturated fatty acids (PUFAs) in the brain. Chronic ketosis is predicted to lead to increased levels of acetone; this might activate K2P channels to hyperpolarize neurons and limit neuronal excitability. Chronic ketosis is also anticipated to modify the tricarboxcylic acid (TCA) cycle. This would increase glutamate and, subsequently, GABA synthesis in brain. Among several direct inhibitory actions (see also Fig. 2), PUFAs boost the activity of brain-specific uncoupling proteins (UCPs). This is expected to limit ROS generation, neuronal dysfunction, and resultant neurodegeneration. Acting via the nuclear transcription factor peroxisome proliferator-activated receptor-α (PPARα), PUFAs would induce the expression of UCPs and coordinately up-regulate several dozen genes related to oxidative energy metabolism. PPARα expression is inversely correlated with IL-1β cytokine expression; given the role of IL-1β in hyperexcitability and seizure generation (Vezzani et al., 2000), diminished expression of IL-β cytokines during KD treatment could lead to improved seizure control. Ultimately, PUFAs would stimulate mitochondrial biogenesis. Mitochondrial biogenesis is predicted to increase ATP production capacity and enhance energy reserves, leading to stabilized synaptic function and improved seizure control. In particular, an elevated phosphocreatine:creatine (PCr:Cr) energy-reserve ratio is predicted to enhance GABAergic output, perhaps in conjunction with the ketosis-induced elevated GABA production, leading to diminished hyperexcitability. Reduced glucose coupled with elevated free fatty acids are proposed to reduce glycolytic flux during KD, which would further be feedback inhibited by high concentrations of citrate and ATP produced during KD treatment. This would activate metabolic KATP channels. Opening of KATP channels would hyperpolarize neurons and diminish neuronal excitability to contribute to the anticonvulsant (and perhaps neuroprotective) action of the KD. Reduced glucose is also expected to downregulate brain-derived neurotrophic factor (BDNF) and trkB signaling in brain. As activation of TrkB pathways by BDNF have been shown to promote hyperexcitability and kindling, these potential KD-induced effects would be expected to limit the symptom (seizures) as well as the progression of epilepsy. Boxed variables depict findings taken from KD studies; up ([UPWARDS ARROW]) or down ([DOWNWARDS ARROW]) arrows indicate the direction of the relationship between variables as a result of KD treatment.

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REFERENCES

  1. Top of page
  2. Abstract
  3. MECHANISTIC INSIGHTS FROM STUDIES OF KD EFFICACY
  4. ANTICONVULSANT MECHANISMS OF THE KETOGENIC DIET
  5. HOW CAN THE KETOGENIC DIET BE OPTIMIZED?
  6. CONCLUSIONS
  7. REFERENCES
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