The other study reports the administration of a gluten-free casein-free modified KGD (1.5:1 lipid:non-lipid ratio; medium-chain and polyunsaturated FA) for 14-months to a 12-year-old child with ASD and seizures with substantial medical comorbidities associated with a family history of metabolic and immune disturbances. Due to the improvements in seizure activity, improved electroencephalogram, cognitive and social skills, language function, and complete resolution of stereotypies, anticonvulsant medication doses were reduced without worsening of seizures. Of note, the administration of the diet was accompanied by a wealth of medications, a significant weight loss, and transitioning to puberty, so it is difficult to assess the sole role of the diet with this clinical background.
In mouse models of ASD [i.e., Rett syndrome, BTBR model, and succinate semialdehyde dehydrogenase (SSADH) deficiency ], the use of the KGD has improved behavioral abnormalities (increased sociability and decreased self-directed repetitive behavior) and/or decreased the number of seizures, normalized ataxia, and increased lifespan of mutant mice. However, while the KGD was originally designed to be administered under controlled caloric intake, most of the mouse studies have been performed under ad libitum conditions and/or for a relatively short period [see Ref. ]. Moreover, a ketogenic low-carbohydrate diet does not have a significant metabolic advantage over a non-ketogenic low-carbohydrate diet as judged by equal effects in body weight reduction and decreased insulin resistance; however, the former one was associated with higher inflammatory risk and increased perception of fatigue.
Although the exact molecular mechanisms underlying the effect of the KGD are still under investigation, several scenarios are reported below to explore the potential therapeutic effects of the KGD in ASD.
Peripheral blood mononucleated cell (PBMC) from children with high severity scores for ASD has shown impaired PDH activity. The KGD is recommended as an alternative source of the acetylCoA in patients with pathogenic mutations in PDH- or GLUT1-encoding genes leading to amelioration of some symptoms especially in those with milder phenotypes. Thus, the use of the KGD in ASD with PDH deficiencies might prove to be beneficial.
Some patients with ASD have been reported to have defects in fatty acid beta-oxidation evidenced as long-chain acyl dehydrogenase deficiency and high concentrations of short or long acyl-carnitines in plasma. Carnitine biosynthesis has been recently identified as a risk factor for ASD. Thus in these cases, it is advisable to limit the use of a high-fat diet or improve its safety by switching to short or medium-chain FA, which do not utilize the carnitine system.
The KGD might improve mitochondrial function by enhancing mitochondrial biogenesis in murine models. The medium-chain triglyceride diet has been shown to produce significant increases in citrate synthase and Complex I activity in SH-SY5Y neurons. However, the increases in mitochondrial mass would need to result in an OXPHOS outcome of >=30% [30% as the limit for minor diagnostic criteria of mitochondrial RC disorder ] for that particular tissue, given that each tissue has a different ATP threshold. Otherwise the increases in mass might not be sufficient to rescue the already impaired ATP production in ASD individuals. Moreover, given the presence of mitochondrial DNA (mtDNA) deletions in PBMC from ASD, the KGD-driven mitochondrial biogenesis may result in an enrichment of defective mitochondria due to the proliferating advantage of damaged or deleted mtDNA over wild-type. Conversely, treatment of cells containing large-scale mtDNA deletions from a patient with Kearns-Sayre syndrome with KB shifted the heteroplasmy between and within cells. The observation that KB can distinguish between normal and respiration-compromised cells suggests that the KB may be useful in treating patients with heteroplasmic mtDNA disorders.
Children with ASD display an array of mitochondrial dysfunction (MD) of differing severity. Electron transport chain (ETC) deficiencies have been reported in ASD, primarily in Complex I and IV, but also affecting others such as Complex II, III, and IV. The prevalence of seizures (41%) has been observed to be significantly higher in individuals with ASD and MD than in the general ASD population (11%), raising the possibility that epileptic episodes observed in ASD might have a mitochondrial origin. Indeed, epilepsy is a recurrent feature of many inherited "classic" mitochondrial disorders, like myoclonic epilepsy with ragged red fibers, mitochondrial encephalopathy with lactic acidosis, and stroke-like episodes, and Leigh syndrome. In a small study on children with ETC defects (Table 1), the KGD has been proven to reduce epileptic attacks, with far better prognosis among children with Complex I deficits than Complex IV. These results are not surprising given that KGD generates more NADH/FADH2 than glucose (2 vs. 5).
Recently, KGD-fed rats showed increased brain expression of insulin-like growth factor receptor (ILGFR) and neuronal GLUT3. The KGD might have a beneficial effect in some ASD cases considering that IGFR is important for brain health throughout life, and that IGFR and GLUT3 have both been implicated in ASD.
Some energy-sensing molecules and metabolism regulators (including the mammalian target of rapamycin, mTOR) have been recently indicated as possible downstream targets of KGD and may be involved in neuroprotective effects associated to the diet. Defects in the mTOR pathway have been linked to ASD. Failure to inhibit mTOR pathway could lead to MD due to decreased mitophagy resulting in an accumulation of dysfunctional mitochondria as observed in a mouse model of ASD with phosphatase and tensin homolog on chromosome ten (Pten) gene haploinsuffciency. Indeed, inhibition of mTOR has been linked to a delay in the occurrence of the epileptic episodes and KGD-fed rats showed inhibition of the activation of the mTOR pathway in brain, thus representing an appropriate treatment to control seizures while enhancing the clearance of defective/damaged mitochondria.
Ketone bodies (without glucose and at concentrations 10-times higher than physiological ones) inhibit mitochondrial reactive oxygen species (ROS) production in rat neurocortical neurons by increasing NADH oxidation following glutamate (Glu) excitotoxicity. It has been suggested that the production of NADPH via oxidation of succinate semialdehyde (SSA) into succinate in the Glu decarboxylase (GAD)/gamma-aminobutyric acid (GABA) pathway may buffer the redox changes likely to occur in stressful conditions. However, other mitochondrial NADPH sources are quantitatively more important than SSADH and fatty acid oxidation produces more mitochondrial ROS than pyruvate oxidation.
Thus, the use of KGD could be beneficial in ASD given that higher rates of mitochondrial ROS production and compromised cellular antioxidant status have been reported in peripheral cells from children with ASD.
The GABA shunt bypasses two steps of the tricarboxylic acid cycle - the alpha-ketoglutarate (KG) dehydrogenase complex and the succinylCoA synthase - for the conversion of KG into succinate (Figure 1). It involves three enzymes: a GAD, catalyzing the Glu decarboxylation to GABA, a GABA transaminase, converting GABA to SSA, and an SSADH, catalyzing the oxidation of SSA to succinate. This metabolic route (the GAD/GABA pathway) is conserved from bacteria, through yeast and plants, to vertebrates. In higher eukaryotes, SSA can be reduced to gamma-hydroxybutyric acid (GHB) by an alternative reaction catalyzed by a GHB dehydrogenase. It has been proposed that KGD may limit the availability of oxaloacetate to aspartate aminotransferase, an enzyme involved in brain Glu metabolism, resulting in increased Glu or Gln availability to produce GABA. The increased conversion of Glu to GABA would be potentially beneficial in ASD (Figure 1).
Changes in GABA neurotransmission by KGD might explain the decrease in seizure frequencies and improved behavior observed in Rett syndrome. Studies in patients with ASD strongly suggest a dysfunction in the GABAergic system. However, changes in other components (including Gly, taurine, and GABA) cannot be excluded. In the case of SSADH deficiency (SSADH), the KGD may work through restitution of GABAergic neurotransmission, although the use of KGD in SSADHD has been strongly argued until more research is performed to test its potential detrimental effects in humans. Conversely, ketotic rodents fed on KGD showed no changes in whole brain (GABA) [between brackets = concentrations; ]; however, regional (GABA) changes cannot be ruled out, in addition to species-specific differences in the expression of GABA receptors subtypes. Considering that cerebrospinal fluid from children treated with KGD showed higher (GABA), it would be of interest to evaluate GABA and amino acid concentrations in different brain areas in animal models of ASD fed KGD.
Dysfunction in the cholinergic system has been observed when PDH deficits are present because a block in this enzyme decreases (citrate), the precursor of acetylcholine via citrate lyase. Studies in humans and animal models of ASD suggested that dysfunction of the cholinergic system underlies ASD-related behavioral symptoms. Trials conducted on ASD individuals have shown beneficial effects of galantamine (an acetylcholinesterase inhibitor) in the management of aberrant behaviors in children and adolescents with ASD. Treatment of BTBR mice with the acetylcholinesterase inhibitor donepezil hydrochloride improved social preference, social interaction and decreased cognitive rigidity. Thus, a KGD has the potential to exhibit beneficial effects in individuals with both ASD and PDH deficiency because the metabolism of KB overcomes the decrease in (citrate) and that of (acetylcholine).
Several side effects of KGD have been reported, among them: (a) limitation in protein, carbohydrate, and other nutrients intake can result in a lack of weight gain and growth inhibition, which could be detrimental in ASD because of a predisposition for being underweight and the presence of eating disorders. Thiamine, lipoic acid, and l-carnitine supplementation have been helpful in selected cases. (b) Dyslipidemia from KGD would need to be supervised in ASD patients with beta-oxidation deficits, including carnitine deficiency and, for older patients, the additional increased risk in heart disease and atherosclerosis. These patients should limit their fat intake or a modified KGD possibly with carnitine and/or coenzyme Q10 supplementation, should be used. (c) KGD has an increased risk of systemic ketosis, which may result in lower affinity of hemoglobin for oxygen, resulting in severe outcomes (e.g., coma and death) especially in anemic ASD patients. (d) Adverse events experienced by patients with RC complex deficits and epilepsy, which could be extrapolated to those with ASD, included symptomatic persistent hypoglycemia, persistent metabolic acidosis, aspiration pneumonia, and pneumonia followed by respiratory failure. (e) Initial fasting and prolonged caloric restriction can cause acute metabolic decompensation in ASD patients with metabolic disorders. To reduce the adverse effects of fasting, some studies have omitted the initial fasting period and substituted it with a gradual increase in calories. (g) Other side effects include constipation, slower growth, kidney stones, and gastroesophageal reflux, although most of them are treatable and/or preventable.