Several studies have demonstrated that the KD confers neuroprotection by restoring/promoting beneficial microbes in the GM of patients with neurodegenerative diseases. In addition, this regime can potentially improve and regulate memory, learning, and disease progression, reducing the frequency of relapses and attacks. During the KD regime, GM compositions and GM-derived metabolites were replaced, these changes possibly result in clinical improvements [1,2,3,4, 23,24,25,26,27].

Effect of the ketogenic diet on neurological diseases through ketone bodies

Although the underlying pathology of numerous neurological diseases has not been entirely determined, the role of inflammation, oxidative stress, and mitochondrial dysfunction in some neurological diseases, including Seizure, MS, ASD, PD, and AD has been identified. KBs produced from KD implementation can provide neuroprotective effects, including reducing oxidative stress, sustaining energy levels for CNS cells, adjusting deacetylation activity, and modulating inflammatory responses [28].

Several neuronal injuries result from glutamate excitotoxicity, calcium overload, mitochondrial dysfunction, and oxidative stress. The ability of KBs to counteract oxidative stress has been observed in studies, particularly in protecting the nervous system. Mitochondria is known to be the main source of reactive oxygen species (ROS) production, and glutathione peroxidase (GSH-Px) is an important enzyme involved in ROS formation process. In normal condition, superoxide anion production during oxidative phosphorylation is relatively low. However, when mitochondria are damaged, calcium ions become overloaded, and ROS level increases, leading to excitotoxic damage [29, 30]. KD helps lower blood glucose levels and promotes ketone production in the liver. The increase in KBs primarily occurs through the oxidation of fatty acids, particularly polyunsaturated fatty acids (PUFAs). PUFAs activate peroxidase by blocking voltage-gated sodium and calcium channels and regulating membrane receptors in neurons or inducing the expression of mitochondrial uncoupling protein (UCP). This uncoupling process reduces mitochondrial membrane potential, ultimately decreasing ROS production [29, 31]. The oxidative regulation has an impact on Complex I/III in the ROS/RNS respiratory chain. Research has shown that mitochondrial dysfunction and the inhibition of complexes I, II, and III can occur due to epileptic seizures. However, using KD can enhance the inhibition of complex II/III and significantly improve mitochondria function during oxidative stress [28, 32, 33]. Although the exact mechanisms by which KD reduces seizures are not fully understood, KBs and PUFAs, which can be increased through KD implementation and GM alterations, may play critical roles in its anti-seizure effects. As stated in the studies, KBs increase inhibitory neurotransmitters, activate potassium channels, and enhance energy production in the nervous system, thereby raising the seizure threshold in the brain, and PUFAs lead to increased energy transcripts, enhanced energy reserves, and stabilized synaptic function. This ultimately prevents neuronal hyperexcitability, which leads to anti-seizure function [34].

As mentioned above, KD has the potential to improve the functioning of mitochondria and alter glucose metabolism, leading to a decrease in the production of advanced glycation end products (AGE). The accumulation of AGE during the aging process can speed up the progression of AD. KBs, particularly β-Hydroxybutyric acid (βHB), have been found to mitigate the toxicity of 1-methyl-4-phenylpyridine (MPP +) on neurons cultured in vitro and reduce the toxicity of amyloid protein fragment (Aβ) on hippocampal neurons [35]. Additionally, according to the animal study of Beckett T, et al. 2013, KD can enhance the electrophysiological function of the brain in AD mice [36]. While animal studies have shown promising results, clinical research has not yet provided definitive conclusions.

Effect of the ketogenic diet on neurological diseases through gut microbiota

Recent studies suggest complex interactions between the GM and the central nervous system (CNS). The GM influences the development and balance of the CNS through immune, circulatory, and neural pathways, while the CNS affects the GM through stress and endocrine responses, called the "gut microbiota-brain axis." The vagus nerve is mainly responsible for the direct communication between GM and CNS. As stated in the studies, cutting this nerve reduced neurogenesis regulated by the GM and expression of brain-derived neurotrophic factor in the hippocampus [37]. Moreover, the GM also produces neurotransmitters and neuropeptides. Enterococcus spp., Streptococcus spp., and Escherichia spp. generate serotonin; Lactobacillus spp. and Bifidobacterium spp. produce gamma-aminobutyric acid (GABA); Escherichia spp., and Bacillus spp. produce noradrenaline and dopamine. Some species in Bacteroidetes and Firmicutes phylum produce short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate through the fermentation of insoluble dietary fibers. GM produces an enzyme called glutamate decarboxylase, which converts glutamate to GABA. These bacteria have also been shown to affect the expression of GABA and the N-methyl-D-aspartate (NMDA) receptors in the brain in animal models [37]. In animal studies, it has been shown that modulating gut microbiota composition could be effective on neurotransmitters’ concentration. For example, when mice were given Lactobacillus rhamnosus orally over a long period, it led to increased expression of GABAB1b mRNA in the cingulate and prelimbic region and accompanying reduced expression in the hippocampus, amygdala, and locus coeruleus. These neurotransmitters cannot cross the blood–brain barrier (BBB) and have limited direct effect on CNS function; however, they may indirectly influence the CNS system through the enteric nervous system, vagus nerve, and modulation of peripheral receptor expression. Moreover, imbalances in the GM can lead to increased intestinal barrier permeability and activation of an immune response in peripheral tissues. This can result in heightened signalling of cytokines/chemokines through neuronal or humoral pathways, potentially triggering an inflammatory response in the CNS where disruption of the BBB is considered an essential step [37, 38]. The study by Olson S., et al.2018, declared that KD alters GM and can protect against acute seizures in a mouse model. Mice treated with Akkermansia and Parabacteroides were protected against seizures compared to those in the control diet group [39].

Taxonomic changes in the GM and clinical outcome

It has been extensively studied that KD is effective in treating patients with severe and refractory seizures, and fortunately, the results of various studies have been admissible. The effects of GM changes on clinical improvement in epileptic patients treated with KD were examined in six studies. Since the KBs produced in the KD, as a source of energy for the brain, can pass through the blood–brain barrier by a special blood transporter, the issue of improvement of patients' symptoms using this diet was investigated. Various studies have explored its effects on the amelioration of patients' treatment [25,26,27, 40,41,42,43,44,45,46,47].

Although the difference in the amount of intervention period and follow-up time was different in several studies, overall, the changes in the composition of GM were in favor of an increase in the relative abundance of bacterial genera Escherichia (E. coli), Clostridia (Clostridiales, Clostridium, Lachnospiraceae, and Ruminococcaceae), Alistipes, Bacteroides, Desulfovibrio, Actinomycetaceae family and Bacteroidetes phylum and a decrease in the relative abundance of bacterial genera, Bifidobacteria (B. longum), Eubacterium (E. rectale), Dialister, Enterococci (E. faecium), Eggerthella (E. lenta), Cronobacter, and some genera of Firmicutes and Proteobacteria phylum were observed. As stated in these studies, more than a 50% reduction in patient seizure attacks was reported. This clinical improvement could be a result of these new GM combinations after KD treatment.

Unfortunately, most studies have had small sample sizes because KD is an expanding but uncommon treatment option in neurodegenerative diseases. The studies had varied designs, sample processing, analysis methods, and demographic characteristics. Therefore, a lack of consistent outcomes is expected. For example, two cohort studies and one clinical trial confirmed that Bifidobacteria abundance was reduced in drug-resistant epilepsy patients after ingestion of KD. This finding is not surprising since KD is usually fiber-free, and Bifidobacteria needs fiber to survive [5, 8, 16]. However, in a cohort study, the genus Bifidobacteria was increased in epileptic children compared with healthy age-matched controls after KD treatment [7]. Therefore, the results of studies regarding changes in bifidobacteria abundance were conflicting.

Tumor Necrosis Factor Alpha (TNF-α) is an inflammatory cytokine associated with epilepsy. In addition, Bifidobacteria species (B. longum and B. breve) were associated with TNF-α levels, and they were higher in patients who started KD and, they had experienced a reduction in seizure frequency. Bifidobacteria interact with the immune system through TNF-α, affecting the seizure threshold. Bacteroides has a role to digest and metabolize high-fat food and to regulate the secretion of IL-6 and IL-17 in dendritic cells (DCs), a process strongly associated with seizure severity of epileptic patients. Reduction in the Firmicutes, along with an increased level of the genus Bacteroides, is also related to the high production of SCFAs, which have antiepileptic effects [5, 8, 16].

Concerns about the increasing incidence of degenerative diseases such as AD and MCI, their irreversible complications, and the disproportionate response to standard treatments have led to examining another treatment method, including the KD, in these patients. One study evaluated this diet for GM changes and clinical improvement in patients. In the study by Nagpal R. et al. 2020, MCI patients had a higher percentage of fungal families such as Sclerotiniaceae, Phaffomyceteceae, Trichocomaceae, Cystofilobasidiaceae, Togniniaceae, and genera such as Botrytis, Kazachstania, Phaeoacremonium, and Cladosporium. In contrast, the control group had fewer Cladosporiaceae and Meyerozyma. After KD treatment, an increase in Agaricus, Mrakia, and a reduction in Saccharomyces and Claviceps were reported [6].

Fungi and bacteria coexist symbiotically in the human gut, and their interactions can be impaired in the disease states. According to the studies, a complex ecological co-regulatory network between them exists in a healthy person, which is disturbed in MCI. Different fungi play an essential role in the GM community stability and function, as seen in patients with MCI. Fungi like Meyerozyma, Wallemia, and Aspergillus correlate with several bacterial species in Firmicutes phylum and Bacteroides, Roseburia, and Eubacterium genera [3, 4, 6, 41,42,43, 48,49,50,51,52,53,54,55,56,57]. These data suggest that the KD modulates the fungal composition of the gut, which can influence the GM and the GM-derived metabolites.

In addition to the many treatment challenges associated with MS, immunomodulatory medications are the only treatments available to slow the disease's progress. Many studies have reported that MS patients have an underlying dysbiosis caused by reduced biodiversity and concentrations of essential bacterial groups, such as Faecalibacterirum prausnitzii. According to the study by Swidsinski et al. 2017, KD's effect on GM was biphasic. As mentioned in the study, first, bacterial diversity and concentration were reduced. After that, gut bacteria were restored at the end of the 12-week treatment period, and over time, they overpassed the baseline values [15, 27,28,29,30,31,32,33,34,35,36,37,38,39, 58,59,60,61,62,63].

An inherited disease known as GLUT1 DS disrupts glucose transport as a fuel supply for the brain, leading to seizures, impaired neurological development, and movement disorders. According to Tagliabue A. et al. [17], a survey was conducted on these patients evaluating KD's effects on GM. After three months of KD, Desulfovibrio spp. increased significantly. All patients experienced relief from paroxysmal dyskinesia and progressive global resistance to physical effort [17]. The results are preliminary in GLUT1 DS and MCI studies. Therefore, further studies must be conducted to prove and corroborate the results [6, 17].

Changes in GM-derived metabolites and clinical outcomes

Following KD, we observed reduced fecal SCFA concentrations, including acetate, propionate, butyrate, and branched-chain fatty acids, and increased BHB, trimethylamine N-oxide, and N-acetylserotonin [9, 12,13,14].

The KD's impact on the GM community alters the GM composition and metabolites. In several studies, metabolite changes have been examined, and their impact on neurological disease progression has been reviewed. Ferraris C. et al. 2021 read the effect of KD on GM-derived metabolites after a one-month KD diet treatment. Similar to this study, studies regarding GM in neurological disorders have also reported significant decreases in fecal SCFA concentrations, such as acetate, butyrate, propionate, and iso-butyrate [12, 39,40,41,42,43,44,45,46,47,48,49,50,51, 63,64,65,66,67].

ASD is a neurodevelopmental disorder characterized by multiple impairments in social interaction, repetitive behaviors, and interpersonal communication. ASD is associated with metabolism dysregulation and disruption of immune function, according to studies. Researchers have also investigated the effects of KD on ASD patients. In a three-month pilot study conducted by Mu C. et al. 2019, KD was examined for its impact on these patients. In their research, ketones and other metabolites, including 3-hydroxybutyrate, acetoacetate, acetone, and acetylcarnitine, increased their relative concentrations. Amino acid concentrations decreased, including glutamine, tyrosine, phenylalanine, histidine, and alanine. KD treatment resulted in a significant reduction in chromium levels and an increase in nickel and selenium levels. There was a significant negative correlation between acetoacetate and the comparison score. The ADOS-2 overall score was negatively correlated with the social effect score, whereas chromium and creatine were positively correlated with the comparison score. N-acetyl serotonin negatively correlates with behavioral index, and acetone negatively correlates with social affect scores. Lee R. et al. 2018, also assessed KD in ASD patients. Their findings showed that BHB serum levels significantly increased after three months of KD treatment. Based on the ADOS-2 score, they observed significant improvement in core autism characteristics. There were also substantial advancements in CARS-2 items related to imitation, body use, and fear or nervousness. There was no significant difference in restricted and repetitive behavior scores between patients [9, 14, 45, 47,