The Role of Antioxidants in the Connection Between Microbiota, Neuroinflammation and Epilepsy

Several studies reveal that gut bacteria affect brain activity, while brain structures influence microbiota, demonstrating a dynamic gut–brain axis. The research demonstrates that Bacteroides and Marvinbryantia, among other microbial groups, correlate with neural activity and white matter changes. Magnetic resonance imaging reveals a negative correlation between Bacteroides and brain activity, and Marvinbryantia is tied to lower fractional anisotropy in the cingulate gyrus. Further analysis using reverse magnetic resonance imaging (MRI) suggests that the brain impacts gut microbiota. The negative relationship between the right hippocampus’ volume and Intestinimonas suggests stress response region changes might alter the gut environment via the HPA axis or autonomic nervous system, thus changing microbial niches [21,22]. Likewise, the positive correlation between the left superior cerebellar peduncle volume and Ruminococcaceae UCG010 genus, a SCFA producer, suggests that cerebellar integrity, also through the regulation of gut motility, may promote conditions favorable to beneficial microbes [23,24]. Based on these findings, particular bacteria might limit brain activity, potentially by creating neurochemicals or affecting immune reactions. In line with the above, previous studies have indicated that bacteria of the genus Bacteroides are instrumental in the production of SCFAs and other substances that can cross the blood–brain barrier, affecting brain activity [25,26]. Acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, and caproate, with their 1–6 carbon atom aliphatic tail, are short-chain fatty acids (SCFAs). These saturated fatty acids are mainly produced by the fermentation of dietary fibers by bacteria in the colon, including Bifidobacterium, Lactobacillus, Bacteroides, Ruminococcus, and Firmicutes [27,28].

SCFAs use Monocarboxylate Transporter 1 (MCT1), a monocarboxylate transporter, to cross the blood–brain barrier (BBB) [29], and Sodium-coupled Monocarboxylate Transporter 1 (SMCT1) (Solute Carrier Family 5 Member 8 (SLC5A8)), highly expressed in neurons [30]. Fatty Acid Translocase Cluster of Differentiation 36 (FAT/CD36) can mediate butyrate transport, as shown by in vitro models [30] (Figure 1).

Tight junctions prevent paracellular diffusion; therefore, transporters are the main way SCFAs enter the brain. The decrease in SCFAs, particularly butyrate, is connected with several neurological diseases, such as multiple sclerosis [31], stroke [32], traumatic brain injury [33], vascular dementia [34], septic encephalopathy [35], Alzheimer’s disease (AD) [36], and Parkinson’s disease [37]. One microbial metabolite that can cross the blood–brain barrier is tryptophan, which creates key molecules involved in gut–brain communication [38]. Since tryptophan cannot be synthesized endogenously, it must be obtained from the diet [39]. In the intestine, metabolism occurs through the kynurenine and serotonin (5-HT) pathways (with indoleamine 2,3-dioxygenase 1 (IDO1), tryptophan 2,3-dioxygenase (TDO), and tryptophan hydroxylase 1 (TPH1) involved), and the microbiota converts some into indole and its related forms [40,41]. Bile acids are also capable of crossing the blood–brain barrier via the Large Neutral Amino Acid Transporter 1 (LAT1) and act through the Farnesoid X receptor (FXR) and Takeda G-protein-coupled receptor 5 (TGR5). Bile acids and the microbiota engage in a bidirectional relationship that mutually modulates their composition [42]. The blood–brain barrier’s permeability to microbial bile acids, with known brain receptors, suggests these metabolites could aid the microbiota–gut–brain axis communication [43].

In mouse models of autism, problems with bile acid breakdown are associated with intestinal problems and behavioral changes, alongside a decrease in bile-metabolizing bacteria like Bifidobacterium and Blautia. Additionally, according to preclinical findings, bile acids may have a neuroprotective function in neuroinflammatory diseases such as Alzheimer’s, Parkinson’s, and Huntington’s, all of which are distinguished by neuroinflammation and microgliosis [44]. Considering the significant influence of the microbiota and its metabolites on brain function, interventions aimed at microbial composition could potentially change the risk or trajectory of neurological and psychiatric conditions, including depression, anxiety, neurodegenerative diseases, and developmental pathologies. Current therapeutic strategies aim to modulate the microbiota through diet, probiotics, prebiotics, and fecal microbiota transplantation (FMT). Diet is a primary way to alter gut microbiota, and it also affects the host metabolism [45,46]. Dietary habits significantly affect microbial balance, although the most substantial changes appear to occur at lower phylogenetic levels than the major phyla (Bacteroidetes and Firmicutes) [47]. Low-carb diets reduce Bifidobacterium spp., Roseburia spp., and Eubacterium rectale, thus reducing SCFA creation, especially butyrate [48]. The type and digestibility of carbohydrates also influence microbial composition. Resistant starch intake increases Ruminococcus bromii, Roseburia, and E. rectale, whereas a low-carbohydrate, high-protein diet can increase Oscillibacter valericigenes and reduce Roseburia and E. rectale [49]. Simultaneously, a diet rich in fiber supports the growth of Bifidobacterium, Ruminococcus, and the Lactobacillus–Enterococcus group [50]. According to the International Scientific Association for Probiotics and Prebiotics (ISAPP), probiotics are defined as “live microorganisms that, when administered in adequate amounts, confer a health benefit to the host” [51]. Probiotics typically add certain microbes to improve physiological processes and increase beneficial metabolites in the gut [51]. Preclinical studies have shown that supplementation with probiotics and prebiotics improves cognitive function in animal models by enhancing the integrity of the intestinal barrier and BBB, as well as modulating intestinal inflammation. Furthermore, both probiotics and prebiotics seem to help fix neurotransmitter systems, and beneficial outcomes have been seen in multiple neurodegenerative disease models [52,53,54].

Studies have shown a connection between GM issues and neurodegenerative and neurological diseases. Oxidative stress and neuroinflammation, though distinct, are connected and mutually influence each other.

An imbalance in ROS and free radicals, resulting in oxidative stress, has negative in vivo effects [55]. Furthermore, recent studies indicate that aging and various diseases are accelerated by increased superoxide anion (O₂⁻), resulting from excessive ROS and free radical production. A surplus of ROS can be harmful, causing mitochondrial failure, protein damage, and lipid membrane destruction, potentially leading to cell death [55].

Particularly, the role of the microbiota in the management of brain alterations, including epilepsy, is caused by mitochondrial dysfunction, excess of ROS, and neuroinflammation [56]. O₂⁻ is a major byproduct of mitochondrial metabolism, especially in oxidative phosphorylation and the TCA cycle, mainly at the level of respiratory chain complexes I (CI) and III (CIII). Thus, in mitochondria, peroxiredoxin 3 (Prdx3) and Thioredoxin reductase 2 (Trx2) frequently act as antioxidants. Physiological mtROS equilibrium is tightly maintained by the balance between free radical production and scavenging. However, once pathological conditions such as epilepsy occur, a surplus of ROS is produced, mainly in brain mitochondria, resulting in oxidative damage [55].

Epilepsy, a chronic neurological condition, involves repeated seizures. Drug-resistant epilepsy (DRE) develops in about a third of patients, despite the availability of anti-seizure medications (ASMs), which is a major clinical problem.

Epilepsy can be categorized into three main types: genetic generalized epilepsy (GGE), focal epilepsy, and epileptic encephalopathy (EE) [57]. Depending on the cause, epilepsy is usually classified into four groups: idiopathic, symptomatic, provoked, and cryptogenic; idiopathic and symptomatic are pure epilepsies [58]. Differences between various epilepsy types depend on things like seizures, electroencephalogram (EEG) patterns, age, and how the disease progresses. Generalized seizures impacting both brain hemispheres are a feature of GGE syndromes. Conditions such as juvenile myoclonic epilepsy (JME) and childhood absence epilepsy (CAE), which frequently appear in childhood or adolescence, frequently occur alongside normal development and intelligence. Unlike generalized seizures, focal seizures begin in one brain hemisphere. Examples include temporal lobe epilepsy, autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), and autosomal dominant epilepsy with auditory features. Severe, early-onset disorders known as epileptic encephalopathies (EEs) are characterized by uncontrollable seizures, developmental problems, regression, and persistent epileptic activity, frequently resulting in a negative outcome [59,60]. Studies on epilepsy genetics have shown new findings about somatic mosaicism causing focal epileptic lesions, repeat expansions, and familial epilepsy with myoclonus, and the increasing role of the non-coding genome in neurodevelopmental disorders.

Genetic studies identified specific mutations associated with GGE, particularly in genes coding for Gamma-aminobutyric acid (GABA)-A receptor subunits, such as Gamma-Aminobutyric Acid Type A Receptor Gamma 2 Subunit (GABRG2) and Gamma-Aminobutyric Acid Type A Receptor Alpha 1 Subunit (GABRA1) [61,62,63]. These mutations affect receptor affinity for GABA, transitions between active and inactive states, and membrane expression, resulting in neuronal hyperexcitability and a lower threshold for epileptic seizures [64]. Variants in GABRA1 are associated with JME but also with developmental and epileptic encephalopathies, characterized by early onset, pharmacoresistance, and psychomotor impairment [65]. Mutations in the Solute Carrier Family 2 Member 1 (SLC2A1) gene, which encodes the glucose transporter type 1, are present in 1% of patients with CAE, particularly in cases with very early onset, and may cause severe developmental encephalopathy or exercise-induced paroxysmal dyskinesia without cognitive impairment or epileptic seizures [66]. Genetic epilepsies encompass a broad spectrum of GGE syndromes, each characterized by specific seizure types, age of onset, and distinctive electroencephalographic findings. Other neurological diseases can cause provoked epilepsy, which is a secondary epilepsy [67].

Aging and genetics can cause increased brain electrical discharge in idiopathic seizures. Secondary, or symptomatic, seizures can stem from ailments, metabolic issues, and brain injuries such as focal lesions, abscesses, tumors, vascular malformations, and cerebrovascular accidents [67].

Additionally, the primary epigenetic mechanisms, including DNA methylation, histone post-translational modifications, and non-coding RNA expression/activity, are altered in both experimental and human forms of epilepsy [68]. In a study on temporal lobe epilepsy, the expression of DNMT1 and DNMT3A in patients with the condition versus healthy subjects was investigated. In particular, DNMT1 maintains methylation patterns, while DNMT3 drives new methylation. It was suggested by the data that both DNMT types are more common in individuals with temporal lobe epilepsy, a finding that points to their involvement in the condition’s pathogenesis [69].

A recent study analyzed the hippocampus of rats with chronic epilepsy for overall DNA methylation, revealing increased methylation in the epilepsy group compared to controls. A ketogenic diet was administered to the group, resulting in a decrease in seizure frequency and a modification of DNA methylation patterns [70].

Another study on the rodent hippocampus revealed distinct methylation patterns in animals with status epilepticus compared to those with epileptic tolerance. Preconditioning with chemical or electrical stimuli was performed on the latter group of mice before inducing status epilepticus; therefore, they exhibited tolerance to status epilepticus [71].

Evidence from animal epilepsy models suggests that histone modifications alter chromatin after seizures. Particularly, an increase in HDAC2 expression has been observed in tissue samples from temporal lobe epilepsy patients and in animal subjects undergoing status epilepticus, differing from control groups. Neurodevelopment involves the activity of HDAC2, a CNS-expressed HDAC, which also plays a crucial role in cognitive function by influencing the repression of genes involved in synaptic plasticity and memory creation [72].

Electrically induced seizures in a different animal model of epilepsy revealed alterations in histone H3 and H4 acetylation at the CREB promoter in the rat hippocampus. CREB, a key transcriptional factor, influences GABAa receptor expression in the hippocampus and is significant in epilepsy development [73].

MicroRNAs are involved in protein translation and also play a role in immune responses that regulate epilepsy. During the early stages of brain development, miRNA controls gene translation, which is vital for the correct formation and maturation of dendrites and synapses. Neuronal translation of proteins like the NMDA receptor subunit (Mir-125b) and morphological proteins (p250GAP) is regulated by miRNAs [74]. Three recent studies in animal models indicate that epilepsy is linked to aberrant protein production, including substantial transcriptional inhibition. The studies observed that rats with induced status epilepticus showed higher miRNA-132 levels in their hippocampus; miRNA-132 is known to have anti-inflammatory properties, and inflammation is implicated in the development of epilepsy [75].

Additionally, the expression of TLR4 increased in mice experiencing experimentally induced epilepsy, and miRNA such as Let-7i plays a role in regulating TLR4 levels. It is believed that HMGB1, released from damaged neurons, acts as the ligand for TLR4 and promotes epileptic seizures [76].

Current knowledge suggests that, in vitro, Mir-146a inhibits proteins that promote inflammation, including IRAK1/2 and TRAF6, as well as IL-1B, in a negative feedback loop, although its in vivo target in epilepsy remains undetermined [77].

Regardless of age, unprovoked seizures are the leading cause of epilepsy. The most prevalent causes of secondary epilepsy are brain vascular malformations and head traumas [67].

Specifically, all types of epilepsy are a major public health concern because extended seizures or status epilepticus (SE) can cause cell death, which can lead to varying degrees of brain damage. Despite advancements in anti-epileptic drugs, challenges remain; specifically, side effects and the chance of seizures returning after suspended medication [78].

Epilepsy is not only an important neurological disease alone, but also has close interactions with the occurrence and progression of numerous human disorders. In addition to epilepsy, patients may also have osteoporosis, fractures, and cognitive problems, and are at increased risk of death from suicide, vascular diseases, sudden unexpected death, and pneumonia [79,80]. Recent findings show that the GM is actively involved in regulating the GBA through direct mechanisms via the vagus nerve, and indirectly, by modulating the ENS [81]. Evidence increasingly points to systemic modulation within this complex GI ecosystem, potentially impacting both immunity and neurological processes, which supports the MGBA concept. Variations in the GM can cause systemic immune activation, leading to CNS inflammation, and conversely, neurological problems can initiate systemic inflammation, directly influencing gut microbiota. Such modifications frequently lead to problems with neuronal excitability and epileptogenesis, which results in seizure risk and confirms the clinical significance of the MGBA in epilepsy. As previously stated, the central role of BBB permeability suggests that changes in BBB permeability allow metabolite and neuropeptide production by the GM and GABA, which affects brain function by reaching the CNS, which influences seizure thresholds during epilepsy [82].

Thus, the MGBA is a multi-pathway network that is not completely understood. Thorough preclinical and clinical studies targeting these intricate pathways could help develop new therapeutic methods [82]. Indeed, 5-HT, vital for gut–brain interaction, acts as a key mediator; it is present in both the ENS and CNS [15]. Vagus nerve stimulation (VNS), used therapeutically since the 1980s for epilepsy, is particularly interesting. The activation of vagal afferent fibers changes the amounts of brain neurotransmitters like 5-HT, GABA, and glutamate, which could clarify its clinical success [83,84]. Changes to the gut barrier, for example, can affect the microbiota, causing more pro-inflammatory cytokines (e.g., TNF-α) and increased release of 5-HT by enteroendocrine cells (EECs) [85,86]. These processes contribute to sensitizing visceral afferent pathways, enhancing the impact of stress on motility, secretion, and permeability, which intensifies sensory input to the ENS [87]. Both in vivo and clinical research have demonstrated that prolonged stress can cause the up-regulation of central stress-response circuits, negatively affecting visceral and affective functions. Chronic stress is indeed related to the onset or aggravation of symptoms of irritable bowel syndrome (IBS) [88]. Patients suffering from IBS frequently show stress-induced alterations in gastrointestinal motility, intestinal sensitivity, autonomic regulation, and HPA axis activity [89]. Under physiological conditions, intestinal peptides maintain a synergy with the brain via homeostatic regulation; however, if the brain–gut axis malfunctions, this balance could be disrupted, resulting in irregular physical and pain neurotransmission. Constant increases in sensory signals may impact mood, anxiety, fear, and emotions [90,91], and epileptic seizures can be triggered by stress, which the HPA axis regulates [92]. Hormones affect epileptic activity differently; glucocorticoids rise in epileptics, and anticonvulsant deoxycorticosterone offers protection. Glutamate, along with CRF and corticosterone, may contribute to seizures by enhancing excitatory neurotransmitters (Figure 2) [93,94,95].

Neuroprotective and anticonvulsant effects have been observed in some beneficial bacteria, such as Akkermansia muciniphila and Parabacteroides, which influence the levels of GABA and glutamate in the hippocampus. On the other hand, dysbiosis might affect the GABAergic balance, increasing the chance of seizures [96,97]. Bifidobacterium spp. has been shown to boost immune responses and also release GABA, which can cross the BBB and affect the CNS. In vivo, Akkermansia and paramycetes colonization led to reduced glutamate and increased GABA in the hippocampus, with anticonvulsant effects [98]. Enterocromaffin cells in the gut produce roughly 90% of the body’s 5-HT, but the microbiota could highly influence its production [99,100]. The 5-HT is synthesized from tryptophan by some bacterial strains (e.g., Lactococcus, Lactobacillus, Escherichia coli, and Klebsiella) using tryptophan synthetase. In addition, studies show that sporigenic bacteria in the gut of mice and humans promote 5-HT synthesis in enteric cells via changes to the enzyme TPH1 [99]. In an in vivo study, reserpine-induced 5-HT depletion increased seizure susceptibility due to a lower epileptogenic threshold [101]. This implies that gut bacteria might influence gut and brain electrical function by controlling 5-HT, also affecting immune–inflammatory reactions. Both epilepsy patients and animal models display lower brain N-acetyl aspartate (NAA), which implies that the neuronal metabolism might be altered [102,103]. Indigestible fiber is processed by certain gut bacteria, including Firmicutes and Bacteroidetes, which then create SCFAs, which are responsible for microglia maturation and function. Increased seizure risk is connected to alterations in microglial function and BBB permeability [104,105]. Patients with DRE show significant differences in their microbiota when compared to those with drug-sensitive epilepsy or healthy individuals, which is interesting [106,107,108]. A study found that GM changes and SCFA reductions occurred before seizures in post-traumatic epilepsy, suggesting a connection between gut issues and seizure likelihood [109]. In genetic models, like the Wistar Albino Glaxo from Rijswijk (WAG/Rij) rats, which spontaneously develop seizures, studies showed lower brain SCFAs; however, butyrate helped decrease seizures, improve mitochondria, and increase seizure threshold [110]. Additionally, propionate supplementation demonstrated neuroprotective actions in the hippocampus, lessening mitochondrial damage and increasing seizure latency [111].

While the interconnection between gut microbiota/epilepsy and epigenetic modifications is still developing, it is hypothesized that microbial metabolites, including SCFAs, act as signaling molecules capable of influencing host epigenetic modifications [112]. These modifications can subsequently affect the genes that control nerve cell activity and inflammation. It is recognized that epigenetic modifications are crucial in the development of epilepsy. A potential strategy for preventing or treating epilepsy involves targeting these mechanisms, possibly through diet-induced, microbiome-mediated alterations [113].

Indeed, epigenetic changes, driven by the interplay of environmental risks and genetic vulnerabilities, are the source of significant phenotypic variation in neuropsychiatric disorders, without altering the DNA sequence itself [114]. According to evidence, an imbalanced gut microbiome and heightened gut permeability could be harmful, enabling the entry of molecules like pro-inflammatory cytokines and chemokines into the bloodstream, which may be neurotoxic and improperly activate the immune system [115].

The objective of this relationship is to enhance the immunity barrier and thereby control the microorganisms [116]. Clinical studies suggest a potential connection between neuropsychiatric disorders like epilepsy and alterations in gut microbiota composition and the gut–brain axis. Nevertheless, whether this link is directly correlated or proportional to the disorders’ severity is still under investigation [117]. The purpose of the ongoing observational study EPiGUT (ClinicalTrials.gov ID NCT07253701↗) is to understand the link between the gut and oral microbiota and various epilepsy types, and to determine if they affect how well medications for seizures function.

Genetic factors, age, region, and diet can all affect GM composition [148]. The ketogenic diet (KD) has been widely used in the treatment of refractory epilepsy, with patients reporting positive results [149]. For example, 16S ribosomal ribonucleic acid (16S rRNA) sequencing studies have explored how the KD impacts the GM in mitochondrial epilepsy patients, suggesting it may enhance seizure control. This is thought to occur through changes in fatty acid metabolism, activation of the cyclic AMP (cAMP) signaling pathway via adenylate cyclase 3 (ADCY3), and increased neuronal inhibition. Akkermansia and Parabacteroides significantly increase in mouse models within four days of diet initiation. These microbes, when used for gnotobiotic colonization, protect against seizures in germ-free or antibiotic-treated mice [89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151]. Reduced γ-glutamyl transpeptidase (GGT) activity causes this effect, decreasing the production of γ-glutamyl amino acids like γ-glutamyl-leucine. Since these amino acids cross the blood–brain barrier through distinct transport mechanisms and participate in the synthesis of glutamate and GABA, their reduction increases the GABA/glutamate ratio in the brain and attenuates neuronal excitability [151,152,153,154]. Carbohydrate restriction may be linked to a decline in beneficial gut bacteria such as Bifidobacteria, Eubacterium rectale, and Dialister, while potentially increasing Actinobacteria and Escherichia coli, according to pediatric research. Functional analysis indicates depletion of the bacterial pathways involved in carbohydrate degradation [155,156]. A recent review highlighted a close relationship between the microbiota and GBA, clarifying the understanding that restoring the GM through ketoacidosis, probiotics, and FMT can improve DRE [157]. The ketogenic diet has demonstrated a beneficial impact on gut SCFA levels, especially with the inclusion of leafy greens, berries, and nuts [158]. After adhering to a KD, the study observed that 50% of the participating children had reduced seizures, and 10% achieved complete remission [159,160]. Traditional protocol rigidity and frequent adverse effects prompted the creation of alternative ketogenic formulations for better tolerability and adherence. Current primary dietary approaches consist of the classic ketogenic diet (cKD), KD, medium-chain triglyceride (MCT) diet, modified Atkins diet (MAD), and low glycemic index treatment (LGIT). Directly affecting neuronal excitability is supported by the experimental findings regarding ketone bodies. For example, acetone causes neuronal hyperpolarization and decreases excitability by activating K2P channels, which are essential for resting membrane potential. Neurotransmitters like glutamate, norepinephrine, and adenosine can be influenced by ketone bodies, which also enhance mitochondrial function; furthermore, they can exert epigenetic effects by regulating gene expression associated with seizure susceptibility, reducing DNA methylation through elevated levels of adenosine and histone hyperacetylation.

Medium-chain fatty acids have demonstrated a greater ability to influence phosphoinositide signaling in model organisms like Dictyostelium compared to valproate [161,162,163,164]. Decanoic acid might prevent seizures by blocking AMPA receptors in a noncompetitive manner, a proposed additional mechanism for its potential anticonvulsant effects. Decanoic acid reduces postsynaptic excitatory currents, acts as a non-competitive antagonist of amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and activates the nuclear receptor Peroxisome Proliferator-Activated Receptor gamma (PPARγ), thereby promoting mitochondrial proliferation and activation of the respiratory complex. Decanoic acid also enhances the anticonvulsant effect of valproate and, when combined with octanoic acid, can produce synergistic benefits [165,166]. Less frequently described mechanisms include the activation of voltage-dependent potassium channels by polyunsaturated fatty acids, increased production of brain-derived neurotrophic factor (BDNF) mediated by AMP-activated protein kinase (AMPK)/mTOR pathways, and remodeling of the gut microbiota, which increases the GABA/glutamate ratio in the hippocampus. The AMPK/mTOR pathway acts as a key metabolic regulator, with AMPK detecting low energy levels and subsequently inhibiting mTOR, which promotes growth. AMPK activation leads to the direct inhibition of mTORC1. This occurs via TSC2 phosphorylation or Raptor inhibition, subsequently activating autophagy and suppressing anabolic metabolism [167,168,169,170]. Clinical and observational studies confirm the efficacy of KD variants in adults with drug-resistant epilepsy. The modified Atkins diet (MAD) has demonstrated a moderate reduction in seizures (>25%) in patients with refractory focal epilepsy, as well as improvements in quality of life despite its demanding regimen [171,172]. The KD has also proven effective in cases of super-refractory status epilepticus, with seizure resolution rates of up to 82% and discontinuation within 3–25 days. With the classic KD, MCT, MAD, and LGIT, numerous studies have consistently documented decreased seizure frequency and intensity, as well as enhanced mood and quality of life [171,173]. Additionally, the MAD, MCT, and LGIT diets are more appealing and easier to follow than the classic KD, leading to fewer dietary limitations and less severe digestive issues. However, the links between these metabolic changes and treatment success are still unclear, and further studies are needed to clarify the correct mechanisms underlying these findings.

Several studies have shown that the GM plays a crucial role in regulating various processes of the CNS via the MGBA. The GBA involves various ascending and descending pathways connecting the CNS, enteric nervous system, gut, and its microbiota. Additionally, the MGBA regulates gastrointestinal homeostasis and influences higher emotional and cognitive functions [174,175]. Based on current understanding, the brain creates and sends substances and signals affecting the GM, regulating the gastrointestinal tract, while neurotrophic substances from the gut may impact brain functions and behaviors [176]. According to new data, a connection between the GM and epilepsy has been demonstrated, suggesting a potential impact on neuron overactivity, seizures, and epileptogenesis. This promotes epileptogenesis and modifies inflammation’s pro-excitation via peripheral inflammation, which may manifest as neuroinflammation in the CNS [174,177]. Research on patients suffering from epilepsy (PWE) indicates a shared pathological link between epilepsy and its comorbidities. Intestinal dysbiosis may be a common factor, making it a possible treatment target [178]. Several preclinical studies found that treating animals with probiotics and prebiotics can enhance their cognitive function by strengthening gut barriers and the BBB, also by managing gut inflammation. These compounds rebalance the GM and promote the proliferation of SCFA-producing species, resulting in an increase in their systemic levels [179]. Reducing systemic LPS and enhancing biological barriers could decrease peripheral inflammation and glial activation, which may reduce the progression of neurodegeneration [180]. In addition, probiotics and prebiotics play a role in the restoration of neurotransmitter systems, with beneficial effects observed in several models of neurodegenerative diseases. They also play a crucial role in two main psychiatric effects, depression and anxiety [53,54,181,182,183,184,185,186].

Polyphenols, which are phytonutrients from plants, have garnered considerable interest because of their potential to treat neurological conditions and neuroinflammation. These compounds show multiple neuroprotective features, such as antioxidant, anti-inflammatory, and anti-amyloid qualities, that help to reduce the progression of neurodegenerative diseases, and they have been intensely investigated for their capacity to control inflammation by changing pro-inflammatory gene activity and impacting signal pathways, thus decreasing neuroinflammation and neuron death (Table 1). Polyphenols have also shown potential to affect cellular signals linked to neuron health, synaptic changes, and thought processes. Polyphenols primarily offer neuroprotection by influencing oxidative stress, a key factor in neuroinflammation [210]. Due to their strong antioxidant activity, polyphenols can neutralize ROS and reduce oxidative damage to neurons [211,212].

Researchers have analyzed the role of polyphenols, in particular flavonoids, positive allosteric modulators of GABA receptors, and found that polyphenols’ affinities for the benzodiazepine site are beneficial in enhancing neuronal inhibition and reducing the excitability associated with epileptic seizures [223]. These compounds appear to increase GABAergic transmission or reduce glutamatergic activity, thus contributing to a beneficial neurochemical balance. These findings confirm that research on flavonoids, as a potential treatment for epilepsy, particularly drug-resistant cases, is a growing trend of exploring natural compounds [223]. A recent study showed that Lippia origanoides essential oil (LOEO) treatment is effective in treating pentylenetetrazol-induced epileptic seizures in rats. Furthermore, LOEO exhibited a synergistic anticonvulsant effect combined with diazepam (DZP), enhancing efficacy while minimizing side effects [213]. More research highlighted the neuroprotective properties and cognitive-enhancing activities of benzyl isothiocyanate (BITC), a natural compound found in cruciferous vegetables. These effects have been observed in mice with chronic temporal lobe epilepsy induced by lithium-pilocarpine, specifically concerning learning, memory, and spatial cognition. Treatment with BITC increased the antioxidant capacity of hippocampal tissue by activating the nuclear factor E2-related factor/haem oxygenase 1 (NRF2/HO-1) signaling pathway. The Nrf2/HO-1 signaling pathway plays a crucial role in defending cells from oxidative stress, inflammation, and metabolic dysfunction. Nrf2 is a transcription factor that, upon activation, increases the production of HO-1, an enzyme involved in heme degradation and providing antioxidant, anti-inflammatory, and cytoprotective functions. Additionally, an increase in glutathione peroxidase (GSH-Px) activity and a reduction in malondialdehyde (MDA) content were observed [214]. A study found that tetrahydrocurcumin, a metabolite of curcumin, plays a neuroprotective role in neurodegenerative disorders by reducing oxidative stress, modulating neuroinflammation, activating autophagy, and inhibiting the mitochondrial apoptotic pathway [224]. In fact, curcumin can prevent Parkinson’s disease from progressing. Consequently, curcumin may also have anti-epileptic and neuroprotective effects by controlling the GBA’s equilibrium [220]. Resveratrol’s regulatory role in the GBA is backed by increasing evidence, and its mechanisms involve glucagon-like peptide-1 (GLP-1), the 5-HT system, and gut microbiome diversity. GLP-1 protects against neurodegenerative diseases like AD, PD, and stroke [225,226]. Resveratrol could enhance GLP-1 effects in the CNS and intestine by boosting silent information regulator 1 (SIRT1) and forkhead box, sub-group O (FOXO) gene activity. To enhance resveratrol’s efficacy in preventing amyloid-beta (Aβ) accumulation in the hippocampus and to address GM imbalance by controlling Alistipes, Helicobacter, Rikenella, Desulfovibrio, and Faecalibaculum, a small resveratrol–selenium–peptide nanocomposite was created using an ADs mouse model [215]. Epigallocatechin-3-gallate, a catechin (EGCG), showed a protective effect by changing the GM of Drosophila melanogaster with Phosphatase and Tensin Homolog (PTEN)-induced kinase 1 (PINK1) mutations in a prototype PD model [216]. Quercetin-3-O-glucuronide (Q3G), a flavonol, may counteract cognitive impairment caused by Aβ by reversing brain insulin resistance [221]. Silibinin and silymarin might improve memory issues and decrease amyloid plaques in precursor protein- presenilin- 1 (APP/PS1) mice. These polyphenols changed microbiota diversity and impacted the levels of specific AD-linked bacteria, suggesting silibinin/silymarin may fight AD via GM control [217]. Luteolin, found in celery, parsley, and thyme, inhibits microglial activation and lowers pro-inflammatory cytokines [218]. According to Charrière et al., apigenin from chamomile and parsley changes GABAergic activity, decreases amyloid plaques, and has anti-inflammatory properties [227]. Furthermore, anthocyanins, a type of water-soluble flavonoid that acts as a natural color pigment in colorful plants like berry fruits and vegetables, have garnered considerable interest for their neuroprotective effects on the CNS. In particular, berries such as blueberries and raspberries contain anthocyanins, which alleviate oxidative stress, lessen neuroinflammation, and improve synaptic plasticity, thus reducing both neural cell apoptosis and neuronal inflammation and improving microglia vitality [228].

Current reports indicate anthocyanins may impact the gut microbiota, subsequently influencing the CNS. Indeed, gut neurotransmitter synthesis pathways might impact brain neuronal activity and cognitive function [229].

Further research indicates that anthocyanins impact host tryptophan metabolism, generating metabolites that could modulate CNS inflammation. Consequently, anthocyanins can function as microbe–gut–brain axis mediators, thereby controlling neuroinflammation through gut microbial modification. Studies have also shown that anthocyanins can influence the gut microbiota, which in turn affects the production of metabolic molecules (e.g., tryptophan, SCFAs) and harmful substances (e.g., LPS), enhancing BBB integrity and reducing neuroinflammation in nerve cells to treat neurodegenerative diseases [228].

Another study revealed that BB supplements enhanced spatial memory in underperforming animals while maintaining it in high performers. Latency was where these effects were clearest: BB-fed poor learners found the platform faster, BB-fed good learners did not slow down, and control-fed rats took longer. Average performers fed BBs had increased latency from pre- to post-test. The fact that raspberry and BB improved performance in those with poor results is expected, as this group had the most room to improve [222]. In addition, pterostilbene in grapes and blueberries has been observed to boost antioxidants, affect sirtuins, and lower brain inflammation [230].

The hesperidin in citrus fruits (oranges, lemons) elevates antioxidant defenses, fights neuroinflammation, and improves cognitive function [231]. Strawberries, apples, and onions contain Fisetin, a compound that acts as an antioxidant, reduces inflammation, and affects aging processes [232]. Oxidative stress is lessened, estrogen receptors are adjusted, and neuroinflammation is stopped by genistein, which comes from soybeans and legumes [233]. Citrus naringenin reduces inflammation, oxidative stress, and prevents Aβ aggregation [219]. Epileptic activity is accompanied by multiple neurological comorbidities, whereas secondary seizures are commonly observed in multiple neuropathological contexts. The anti-inflammatory and antioxidant properties of polyphenols make them potentially attractive in the treatment of both epilepsy and epilepsy-associated disorders [234]. These bioactive compounds might regulate several molecular cascades tied to diverse neurological disorders. As a result, investigating the potential of polyphenols to prevent symptomatic seizures is important [235]. Changes in epileptic patients’ microbiota could encourage seizures. More research is needed to understand how polyphenols directly affect the gut and benefit epilepsy. However, various anti-epileptic polyphenols affect the GBA, altering the human microbiota related to epilepsy [236]. Furthermore, the gut–brain axis highly influences the effect of polyphenols on neurotrophic factors [237]. Polyphenols are changed by the GM into active compounds, which then cross the blood–brain barrier and impact the CNS [238]. These interactions might improve cognitive function and mood by boosting BDNF and reducing neuroinflammation. Despite this enhanced range of traditional pharmaceutical treatments, the prevalence of refractory epilepsy in patients diagnosed with epilepsy remains at a high level, accounting for around 30% of cases. Gut microbiome dysbiosis is seen in epilepsy patients and animal models, implying that treatments that restore GM balance may be anti-epileptic candidates thanks to their antioxidant and anti-inflammatory activity. Therefore, a new method for treating epilepsy might involve using nutraceuticals to affect, primarily, gut microbiome dysfunction. Animal models are the preferred method for epilepsy studies, and even with advancements in studying epileptogenesis in animals, it is essential to remember the differences in brain damage between humans and animal models. Consequently, it is important to consider the translational limitations of animal model findings, which may not accurately reflect human conditions due to differences in anatomy, physiology, and pharmacology. Overall, supplementation with nutraceuticals could help bridge the gap until innovative therapeutic methods are developed [239].