How Diet Affects the Gut Nervous System: From Basic Processes to Possible Treatments

The ENS constitutes a sophisticated cellular network, exhibiting a composition similar to that of the CNS. It predominantly consists of neurons and glial cells that are dispersed throughout the GI tract, where they oversee local functions and react to sensory stimuli originating both locally and from distant sources [25]. Neurons within the ENS are categorized according to various characteristics, such as morphology, neurotransmitter expression, cell surface receptors, electrophysiological properties, connectivity patterns, their specific location within the intestinal wall, and their functional roles.

Based on the morphological characteristics of ENS neurons, particularly the shape and length of their dendrites, Dogiel classified these neurons into types Dogiel I-III [28]. With the advent of advanced technologies, notably the application of single-cell RNA sequencing, a more intricate heterogeneity of neurons within the ENS has been uncovered. Through single-nucleus analysis of the human intestine, researchers have identified 436,202 nuclei and successfully isolated 1445 neurons [29]. The functionality of the ENS is dependent on the coordinated action of diverse neurotransmitters, which can be categorized according to their chemical properties and functions. Among cholinergic neurotransmitters, acetylcholine (ACh) is the most prevalent and plays a pivotal role in the ENS [22]. It is predominantly synthesized and released by excitatory motor neurons and certain interneurons. ACh binds to nicotinic acetylcholine receptors (nAChRs) on the postsynaptic membrane, initiating action potentials and subsequently inducing contraction of intestinal smooth muscle [30].

Monoaminergic neurotransmitters: Serotonin is a crucial monoamine neurotransmitter with dual roles within the ENS. It is released by enterochromaffin cells (ECs) and engages 5-HT3 receptors, a class of ionotropic receptors, to induce pronounced contractions of intestinal muscles [31]. Conversely, the activation of 5-HT4 receptors facilitates intestinal peristalsis and stimulates glandular secretion [32]. Additionally, 5-HT is integral to the modulation of intestinal inflammatory responses, with its dysregulation being strongly linked to inflammatory bowel disease (IBD) [33]. Amino acid neurotransmitters: Gamma-aminobutyric acid (GABA) functions as a major inhibitory neurotransmitter in the ENS, playing a critical role in maintaining intestinal immune homeostasis and regulating gut functions [34]. Peptidergic neurotransmitters play a crucial role in the ENS by mediating a variety of physiological functions. Vasoactive intestinal peptide (VIP), a key inhibitory neurotransmitter, is extensively distributed across the intestinal wall, with a particular concentration in the submucosal and mucosal layers. VIP exerts its effects by binding to specific receptors, resulting in the inhibition of smooth muscle contraction and a consequent decrease in intestinal motility [35]. Furthermore, VIP exhibits anti-inflammatory and immunomodulatory properties, which may have implications for the modulation of intestinal barrier function [36]. Substance P (SP), another significant peptidergic neurotransmitter, is predominantly found in the myenteric plexus and facilitates intestinal muscle contraction through its interaction with neurokinin-1 receptors [37]. Neuropeptide Y (NPY) also plays a role in modulating intestinal motility and secretion by inhibiting smooth muscle contraction and reducing glandular secretion [38]. In addition to peptidergic neurotransmitters, nitrogenic neurotransmitters such as nitric oxide (NO) are integral to ENS function. NO is a gaseous neurotransmitter synthesized primarily by nitric oxide synthase (NOS). It promotes smooth muscle relaxation and vasodilation through the activation of guanylate cyclase, which leads to an increase in intracellular cyclic guanosine monophosphate levels [39].

EGCs are critical components of the ENS and, in conjunction with enteric neurons, constitute a sophisticated neural network responsible for regulating a variety of gastrointestinal functions [40]. Initially identified by Dogiel in the late 19th century, EGCs were not formally characterized as specialized glial cells with distinct functional roles until the 1970s by Gabella [41]. In contrast to glial cells found in the central and peripheral nervous systems, EGCs possess unique morphological and functional attributes [42]. Within the ENS, EGCs are significantly more numerous than neurons, with ratios ranging from 3:1 to 10:1. They are extensively distributed throughout both the submucosal and myenteric plexuses, extending processes that form complex interaction networks with neurons, epithelial cells, immune cells, and the vascular system [43]. The morphology and function of EGCs are modulated by the local microenvironment, exhibiting considerable heterogeneity not only among different EGC subtypes but also in their distribution and functional specialization across various regions of the GI tract [26].

The heterogeneity of EGCs is predominantly manifested at three distinct levels: spatial, regional, and functional. Spatial heterogeneity pertains to the variations in morphology and function of EGCs within a specific intestinal region. For example, glial cells located in the myenteric plexus may display functional differences compared to those in the submucosal plexus [40]. Regional heterogeneity is evidenced by the differences in gene expression and functionality of EGCs across various segments of the GI tract, such as the small intestine and colon [44]. Recent single-cell transcriptomic analyses have identified substantial differences in gene expression profiles between EGCs in the human colon and small intestine, which likely reflect adaptations to region-specific functional requirements [45]. Additionally, the functional heterogeneity of EGCs is intricately linked to their local microenvironment. For instance, submucosal glial cells may play pivotal roles in regulating intestinal barrier function and immune responses, whereas intramuscular glial cells are primarily involved in modulating the motor function of intestinal smooth muscle [46]. This functional diversity enables EGCs to adapt effectively to their specific physiological contexts.

The ENS is integral to GI regulation, orchestrating the transit and mixing of intestinal contents to accommodate varying physiological conditions. During fasting, the GI tract remains relatively inactive, prompting the ENS to initiate the migrating motor complex (MMC) to maintain intestinal cleanliness and prepare for subsequent food intake [47]. The MMC is characterized by a cyclic, propulsive contractile pattern that originates in the gastric body and progresses along the small intestine toward the colorectum, with a periodicity of approximately 90–120 min. Its generation and regulation are predominantly mediated by the neuronal network within the myenteric plexus [47]. Following gastric emptying, cholinergic neurons within the myenteric plexus become activated, releasing ACh to induce intestinal contraction. Concurrently, nitrergic motor neurons release NO, which inhibits smooth muscle contraction in subsequent intestinal segments, thereby creating a lumen-opening effect that facilitates the propagation of the contractile wave. This alternating excitatory and inhibitory mechanism ensures the directional migration of the MMC [48].

The myenteric plexus plays a crucial role in the direct regulation of GI motility, whereas the submucosal plexus is essential for the perception of peristaltic stimuli. Upon the introduction of food into the GI tract, luminal distension and chemical signals activate sensory neurons within the submucosal plexus. This activation initiates ENS-mediated reflexes that modify intestinal motor patterns, resulting in a combination of segmental and peristaltic contractions [49]. Segmental contractions, which are primarily observed in the small intestine, are governed by local reflex circuits within the myenteric plexus. Conversely, peristaltic contractions, characterized by their propulsive nature, can occur throughout the entire gastrointestinal tract [50]. When chyme interacts with the intestinal wall, sensory neurons in the submucosal plexus transmit signals to interneurons located within the myenteric plexus. These interneurons, in turn, activate inhibitory motor neurons in the segment anterior to the chyme, leading to smooth muscle relaxation, and excitatory motor neurons in the segment posterior to it, resulting in smooth muscle contraction. This orchestrated pattern of "relaxation ahead and contraction behind" effectively facilitates the distal propulsion of chyme [51]. In response to fasting and feeding regimens, guinea pigs exhibit adaptive modifications in their myenteric neurons, which in turn produce specific gastrointestinal motility responses aligned with their nutritional state. Additionally, fasting-mimicking diets have been observed to retard tumor progression in murine models [52,53].

The regulation of the equilibrium between intestinal secretion and absorption represents a critical function of the ENS in sustaining internal homeostasis and enhancing nutrient absorption. This regulatory process is predominantly mediated by the submucosal plexus, which orchestrates the activities of intestinal epithelial cells, enteroendocrine cells, and vascular smooth muscle tissue [54]. In the context of secretion regulation, the ENS governs the release of digestive enzymes, mucus, and electrolytes, thereby establishing an optimal environment for food digestion [54]. Upon contact with intestinal epithelial cells, chemical constituents of food (e.g., fatty acids, amino acids) or microbial metabolites (e.g., SCFAs) trigger downstream signaling pathways, leading to the release of molecules such as 5-HT and ATP. These signaling molecules activate sensory neurons within the submucosal plexus, which subsequently—via interneurons—stimulate cholinergic motor neurons. The ACh released by these neurons acts on enteroendocrine cells, thereby promoting the secretion of digestive enzymes, mucus, and electrolytes [55].

In the regulation of absorption, ENS neurons influence the expression and activity of intestinal epithelial cells through neurotransmitter release [56]. Research indicates that ACh activates nicotinic receptors on intestinal epithelial cells, initiating calcium ion currents. These calcium signals propagate across the epithelial layer via gap junctions, facilitating cellular maturation while diminishing proliferation and inflammatory responses. Disruption of this process may result in chronic damage, characterized by ion imbalance, cell death, and an increase in inflammatory cytokines, akin to the pathological features observed in IBD [57].

The regulation of intestinal immune equilibrium constitutes a vital function of the ENS in sustaining gut homeostasis. Through bidirectional communication with immune cells, the ENS not only provides defense against pathogenic invasion but also mitigates excessive immune responses that could precipitate inflammation. This interaction involves intricate communication among ENS neurons, EGCs, and intestinal immune cells, including macrophages, dendritic cells, T cells, and mast cells [58]. Research has shown that helminth infection elevates intestinal levels of interferon-gamma (IFN-γ), which activates EGCs and stimulates the secretion of CXCL10. This chemokine subsequently recruits CD8⁺ T cells that produce additional IFN-γ, thereby facilitating IFN-γ-mediated tissue repair and damage control [59]. In a mouse model of colitis induced by dextran sulfate sodium (DSS), the ablation of choline acetyltransferase-positive (ChAT⁺) neurons aggravated colitis and resulted in motor dysfunction. In contrast, optogenetic activation of cholinergic neurons alleviated colitis, maintained smooth muscle contractility, prevented neuronal loss, and diminished the production of pro-inflammatory cytokines [60].

Immune cells are capable of negatively regulating ENS function through the secretion of cytokines and inflammatory mediators, thereby establishing a bidirectional immune-to-neuro signaling loop. In response to pathogen invasion or inflammatory stimuli, macrophages and dendritic cells secrete pro-inflammatory cytokines, including IL-6, IL-17, and TNF-α, which influence the activity of enteric neurons and EGCs [61]. With advancing age, macrophage polarization transitions from the M2 phenotype (anti-inflammatory) to the M1 phenotype (pro-inflammatory). This transition is associated with elevated cytokine levels and increased infiltration of immune cells within the ENS, which correlates with heightened apoptosis, neuronal and EGC loss, and delayed intestinal transit [62].

The interaction between the ENS and the gut microbiome constitutes a dynamic, bidirectional relationship. The microbiome significantly influences the development and function of the ENS, while the ENS, in turn, plays an active role in shaping the microbial ecosystem. Specifically, the ENS indirectly regulates microbial composition by modulating critical aspects of the intestinal environment. For instance, the ENS can affect the structure of microbial communities and regulate intestinal inflammatory responses by altering the pH of the microbial habitat [63]. In zebrafish with congenital agaglionosis resulting from mutations in the sox10 gene, which lead to the absence of an ENS, microbiota-dependent inflammation occurs. Restoration of ENS function in these genetically deficient zebrafish can ameliorate the disordered gut microbiota and pathological inflammation [64].

In contrast, the establishment of this bidirectional circuit occurs early in life. Recent research indicates that the diversification of the gut microbiota, which accompanies the postnatal maturation of the ENS, may play a critical role in the development and functionality of the ENS [10]. Studies have demonstrated that vaginal dysbiosis associated with Ureaplasma parvum infection leads to a reduction in enteric neurons and glial cells within the ovine fetal ENS [65]. The mode of delivery, whether vaginal or cesarean section, influences the initial microbial colonization of the infant; the gut microbiota of infants delivered vaginally more closely mirrors that of their mothers [66]. Additionally, breast milk, which influences the composition of the infant's gut microbiota, contains neurotrophic factors and cytokines that support the survival and growth of enteric neurons in vitro [67].

Consequently, the microbiome–ENS interaction established during early life not only establishes the foundational developmental framework for intestinal physiology but also, when disrupted, predisposes individuals to a range of gastrointestinal and neurological disorders in later stages of life. This highlights the critical lifelong importance of maintaining homeostasis within this bidirectional axis.

Tryptophan is an essential aromatic amino acid distinguished by its β-carbon linked to the 3-position carbon of an indole ring. Among the 20 standard amino acids, tryptophan possesses the largest molecular mass and exhibits the lowest abundance in proteins and cells. Despite this scarcity, it functions as a crucial biosynthetic precursor for a diverse array of microbial and host metabolites, thereby imparting significant physiological importance. The capacity of certain microorganisms to synthesize tryptophan has been extensively harnessed in industrial applications. As cells are incapable of synthesizing tryptophan de novo, humans must acquire it from external sources, predominantly through dietary intake. Common natural dietary sources include oats, bananas, dried plums, milk, tuna, cheese, bread, poultry, peanuts, and chocolate [75].

Tryptophan is metabolized in vivo predominantly via three principal pathways: the kynurenine pathway, the serotonin pathway, and the gut microbiota-mediated metabolic pathway. The equilibrium among these pathways is crucial for the maintenance of systemic homeostasis [76]. The kynurenine (KYN) pathway constitutes the primary route for tryptophan catabolism, accounting for approximately 95% of total tryptophan degradation. The enzymes indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase serve as the rate-limiting factors in this pathway. This metabolic route yields various biologically active metabolites, including KYN, kynurenic acid (KYNA), and quinolinic acid (QA). Notably, KYNA possesses neuroprotective properties, whereas excessive levels of QA may lead to neuro-excitotoxicity [77]. The serotonin pathway is responsible for approximately 1–2% of tryptophan metabolism, with 5-HT as its primary end product. Serotonin is a crucial neurotransmitter involved in the regulation of mood, sleep, and appetite. Furthermore, 5-HT can be converted into melatonin, which plays a role in the regulation of circadian rhythms [78]. The microbial metabolic pathway is predominantly influenced by gut microbiota, leading to the production of indole derivatives, including indole-3-lactic acid (ILA) and indole-3-acetic acid (IAA). These metabolites have the capacity to modulate a range of host physiological processes through the activation of the aryl hydrocarbon receptor (AhR) [79].

GABA is a naturally occurring non-protein amino acid that functions as the primary inhibitory neurotransmitter in the CNS and as a critical signaling molecule within the ENS [80]. In contrast to polyphenols, the regulatory impact of GABA on the ENS is characterized by neurotransmitter homology, as its chemical structure is identical to that of GABA endogenously synthesized and secreted by ENS neurons. Consequently, GABA can directly bind to GABA receptors in the ENS without necessitating structural modification or metabolic conversion, thereby exerting its physiological regulatory effects. This attribute positions GABA as one of the few dietary bioactive molecules capable of directly engaging in neural signaling within the ENS [81].

GABA is prevalent in a variety of plant-based foods and fermented products, with its concentration being significantly affected by the type of food, its maturity, and the methods of processing employed. Under conditions of stress, plants activate the enzyme glutamate decarboxylase (GAD), which catalyzes the conversion of glutamate to GABA, thereby enhancing stress tolerance. As a result, plant-based foods that undergo stress treatments, such as germinated grains and stress-exposed vegetables, frequently exhibit increased levels of GABA. Additionally, during the fermentation process, microorganisms, such as lactic acid bacteria, can express GAD, which converts glutamate in the substrate into GABA, thereby promoting its accumulation. This biochemical process renders fermented foods, including kimchi, yogurt, and cheese, significant dietary sources of GABA [80].

Soluble dietary fiber encompasses a class of fibers that dissolve in water, forming gel-like substances. A key characteristic of soluble dietary fiber is its fermentability by gut microbiota, which positions it as a crucial element of prebiotics. By providing a carbon source for beneficial intestinal bacteria, soluble dietary fiber influences the composition and metabolic activity of the microbiota, thereby affecting ENS function via the microbiota–gut–brain axis [82]. These fibers are prevalent in plant-based foods and can be categorized based on their chemical structures into non-starch polysaccharides (such as inulin, pectin, and β-glucan), resistant oligosaccharides (such as fructooligosaccharides and galactooligosaccharides), and gums (such as guar gum and arabic gum). The diverse molecular structures of various soluble dietary fibers result in distinct selective regulatory effects on gut microbiota and fermentation properties [83]. The primary mechanism by which prebiotics exert their influence through the microbiota–gut–brain axis is rooted in the dual outcomes of their microbial fermentation. Firstly, prebiotics selectively enhance the growth of beneficial bacterial species, such as Bifidobacterium and Lactobacillus, thereby optimizing the microbial community structure. Secondly, the bioactive metabolites produced during fermentation, notably SCFAs, indirectly modulate the development and functionality of the ENS [82].

Probiotics are live microorganisms that, when administered in sufficient quantities, provide health benefits to the host. Their regulatory effects on the ENS are entirely contingent upon the "microbiota–gut–brain axis" mechanism. This mechanism indirectly influences the structure and function of the ENS through intricate interactions among probiotics, gut microbiota, intestinal epithelial cells, and immune cells [15]. The mechanisms by which probiotics regulate the ENS via the microbiota–gut–brain axis can be delineated into three levels: microbial interactions, host cell interactions, and metabolic products. At the microbial interaction level, probiotics primarily enhance gut microbiota composition through strategies such as "competitive exclusion" and "microbial remodeling." At the host cell interaction level, probiotics directly engage with intestinal epithelial cells, EGCs, and immune cells, secreting signaling molecules to modulate their functions, thereby affecting the ENS. At the metabolic level, bioactive metabolites produced by probiotics—including SCFAs, bacteriocins, vitamins, and indole derivatives—function as crucial signaling molecules that establish a connection between probiotics and the ENS [15,83]. This complex, multi-pathway regulatory mechanism highlights the substantial potential of probiotics in modulating ENS-related disorders, thereby positioning them as a promising strategy for dietary intervention.

Distinct dietary compounds influence smooth muscle contractility through highly varied mechanisms by interacting with specific receptors located on ENS neurons. Within the murine colonic myenteric plexus, GABAA receptors are composed of multiple subunits, including α1–5 and γ2, with the α4 and α5 subtypes predominantly found on motor neurons. The binding of GABA to these receptors facilitates the release of NO, thereby substantially inhibiting the contractility of the longitudinal smooth muscle [84]. Notably, the expression of the α3 subunit increases with age, reaching its peak in 18-month-old mice, which results in a decreased sensitivity to the contractility-inhibiting effects of GABA in older mice [85]. In an acute stress model, the GABAA receptor positive allosteric modulator alprazolam counteracted stress-induced increases in colonic contractions, an effect that was dependent on the activity of NOS [84].

SCFAs, particularly butyrate, are essential energy substrates for ENS neurons, meeting over 70% of their energy demands. Upon uptake through the monocarboxylate transporter MCT1 into myenteric plexus neurons, butyrate undergoes β-oxidation to produce ATP, thereby maintaining the neuronal capacity to regulate smooth muscle function [73]. Research indicates that intracolonic perfusion of butyrate in neonatal rats leads to an increase in the number of cholinergic neurons (ChAT⁺) within the myenteric plexus, enhances the efficiency of neuromuscular transmission, and improves smooth muscle contractility [86]. Moreover, butyrate significantly increases the proportion of cholinergic neurons through a mechanism mediated by monocarboxylate transporter 2 (MCT2), as silencing MCT2 with siRNA negates the pro-proliferative effects of butyrate on ChAT⁺ neurons [87]. Conversely, Lactobacillus rhamnosus interacts with ENS neurons through its surface adhesion protein, leading to the activation of neuronal formyl peptide receptor 1 (FPR1). This activation induces the production of reactive oxygen species (ROS), augments neuronal excitability, and indirectly facilitates smooth muscle contraction. Consequently, this mechanism elevates GI peristaltic frequency in mice by approximately 18%, an effect that can be entirely suppressed by FPR1 antagonists [88].

The stability of intestinal motility frequency is dependent on the rhythmic firing activity of ENS neurons and the pacemaker function of interstitial cells of Cajal, which are responsible for generating slow-wave potentials. Dietary components can sustain intestinal motility homeostasis by modulating these two critical elements. Research indicates that the expression of GABAA receptor subunits in the mouse colon exhibits stage-dependent variations throughout development. Notably, the expression of the α1, α2, α5, and γ2 subunits peaks at postnatal day 10 (P10), while α2 and γ2 subunits remain highly expressed at postnatal day 60 (P60). At both developmental stages, GABA significantly enhances colonic peristaltic frequency. However, this regulatory effect is substantially reduced or absent at postnatal day 15 (P15), attributed to a temporary downregulation of the α3 subunit, and in 18-month-old mice due to its excessive upregulation [85]. The modulation of the ENS by GABA presents a multifaceted scenario, with its effects varying across different age groups. This variability is attributed to the dynamic alterations in the composition of GABA receptor subtypes within the gastrointestinal tract over time. Such age-dependent effects imply that dietary intervention strategies should be tailored specifically for infants, adults, and the elderly. Furthermore, colonization with tryptophan-synthesizing bacteria, such as Bacillus subtilis R0179, has been shown to increase colonic 5-HT levels in mice. The neurotransmitter 5-HT activates 5-HT4 receptors located on neurons within the myenteric plexus, facilitating the release of ACh and subsequently increasing the frequency of peristalsis. Exogenous administration of 5-hydroxytryptophan (5-HTP) can replicate this effect, thereby alleviating symptoms of constipation [89,90]. SCFAs, which are primary metabolites produced by gut microbiota, have been shown to promote neuronal recovery following antibiotic-induced damage, highlighting their capacity to stimulate enteric neurogenesis in vivo [91]. Additionally, maternal gut dysbiosis prior to conception results in reduced SCFA levels in the offspring's colon, which suppresses GPR41 signaling and leads to dysfunction of interstitial cells of Cajal pacemakers, manifesting as decreased motility frequency. These adverse effects can be mitigated through butyrate supplementation [92]. Furthermore, prolonged adherence to a diet devoid of fiber, such as one lacking guar gum, diminishes ACh release from cholinergic neurons in the rat myenteric plexus, thereby reducing motility frequency. The introduction of guar gum into the diet restores neuronal signaling and enhances colonic peristalsis [93].

Butyrate functions as a crucial energy substrate for ENS neurons, and an inadequate metabolic supply of butyrate may result in neuronal dysfunction. Research has demonstrated that myenteric neurons in the neonatal rat colon exhibit a significantly greater uptake efficiency for butyrate compared to other short-chain fatty acids, with uptake rates approximately three times higher. Intracolonic perfusion of butyrate significantly elevates intracellular ATP levels in neurons and mitigates apoptosis induced by energy failure, underscoring its vital role in maintaining energy homeostasis within the ENS [86].

Intestinal inflammation can inflict direct damage on ENS neurons, whereas certain dietary components exhibit neuroprotective properties by modulating immune-inflammatory responses. Research indicates that GABA, released by GABAergic neurons in the gut, can activate Gabbr1/2 receptors, thereby inhibiting the proliferation of group 3 innate lymphoid cells (ILC3s) and the secretion of interleukin-17A, ultimately mitigating local inflammation [94]. Additionally, SCFAs, such as butyrate and propionate, in the murine colon activate myenteric plexus neurons via the GPR43 receptor, leading to the release of the anti-inflammatory cytokine IL-10. IL-10 subsequently acts on lamina propria macrophages, suppressing their production of the pro-inflammatory cytokine TNF-α and diminishing the neurotoxic effects of inflammatory factors on the ENS. Supplementation with SCFAs in microbiota-depleted mice has been shown to significantly reduce the expression of inflammatory stress markers, such as inducible nitric oxide synthase (iNOS), in neurons [95].

The structural integrity of the ENS is contingent upon the maintenance of neuronal populations and synaptic connectivity. Dietary components can indirectly enhance the morphology and functionality of enteric plexuses by optimizing the intestinal microenvironment. Studies have demonstrated that antibiotic administration in juvenile mice results in a disruption of gut microbiota, accompanied by a reduction in neuronal numbers within the colonic myenteric plexus. Colonization with Lactobacillus rhamnosus has been shown to effectively counteract microbiota disruption, exhibiting more significant neuroprotective effects in male mice [96]. Research has demonstrated that essential vitamins, such as vitamin D, have the capacity to modulate enteric neurons. For example, supplementation with vitamin D has been shown to confer protection against enteric neuronal loss induced by a high-fat diet and palmitic acid in mice. This protective effect is mediated through the attenuation of oxidative stress [97,98].

Key pathological changes in the ENS associated with functional constipation predominantly include a reduction in neuronal density within the myenteric plexus, dysfunction of nitrergic neurons, and delayed intestinal motility. Dietary interventions may alleviate these pathological conditions by providing energy substrates for the ENS and modulating neurotransmitter signaling. A study conducted on 460 Chinese women with varying bowel movement frequencies employed metagenomic sequencing to analyze gut microbiota and measured serum SCFA levels. The study revealed a positive correlation between the abundance of the butyrate-producing bacterium Fusobacterium varium and bowel movement frequency, while serum butyrate concentration was significantly negatively correlated with bowel movement frequency. Further in vitro experiments indicated that low concentrations of butyrate (0.5 mM) promoted enteric neuronal proliferation, whereas high concentrations had inhibitory effects, suggesting a concentration-dependent role of butyrate in the pathogenesis of constipation [101]. Furthermore, the findings suggest that the regulation of the ENS by butyrate exhibits a distinct dose–response relationship, highlighting the importance of precise dosing in its therapeutic application.

In a model of slow-transit constipation, supplementation with a specific probiotic mixture—comprising Bifidobacterium bifidum W23, Bifidobacterium lactis W51, Bifidobacterium lactis W52, Bifidobacterium longum W108, Lactobacillus casei W79, Lactobacillus plantarum W62, and Lactobacillus rhamnosus W71—was found to ameliorate constipation phenotypes through multiple mechanisms. This probiotic combination upregulates the c-Kit/SCF signaling pathway, enhances the survival of interstitial cells of Cajal, and inhibits their apoptosis. Additionally, it upregulates TPH1 expression, thereby increasing the biosynthesis of 5-HT and inhibiting its degradation, which promotes intestinal peristalsis. Moreover, the intervention modulated the mRNA and protein expression levels of NOS1, BDNF, TRPV1, and GDNF, suggesting a comprehensive enhancement of enteric nervous system function [102]. While probiotics demonstrate potential in modulating intestinal motility and ENS function, it is crucial to acknowledge that their effects are not universally applicable. Probiotic strains that are effective in animal models may produce inconsistent outcomes in human clinical trials, emphasizing the need for personalized probiotic therapies in future research.

In chronic colitis, exemplified by IBD, the excessive release of intestinal inflammatory mediators can result in damage to both ENS neurons and EGCs. Dietary compounds have the potential to mitigate inflammatory responses and safeguard neural structures by modulating the "microbiota–immune–ENS" network.

In a TNBS-induced colitis model, Akkermansia muciniphila demonstrated protective effects by decreasing intestinal permeability and inhibiting the NF-κB signaling pathway. This was evidenced by reduced myeloperoxidase activity, a decrease in goblet cell numbers, and lower levels of pro-inflammatory cytokines. Additionally, Akkermansia muciniphila alleviated intestinal neuroinflammation and glial cell hyperplasia [103]. In the same model, butyrate supplementation mitigated the loss of nNOS⁺, ChAT⁺, and GPR41⁺ neurons in the colon and reduced the number of GFAP⁺ glial cells. Further analysis indicated that GPR41 co-localized with nitrergic (nNOS⁺) and cholinergic (ChAT⁺) neurons, but not with GFAP⁺ glial cells, suggesting that butyrate primarily exerts its effects directly on enteric neurons via the GPR41 receptor to alleviate colitis-associated neural injury, rather than through glial pathways [104]. Another study found that a 5-HT7 receptor antagonist reduced neural innervation in the mucosal plexus of an IBS-like mouse model by inhibiting the overproduction of neurotrophic factors in the submucosal plexus, highlighting the significant role of 5-HT signaling in the regulation of enteric neuroinflammation [105].

A prominent mechanistic hypothesis suggests that pathologically misfolded α-synuclein may initially aggregate within susceptible enteric neurons. This aggregation is proposed to propagate in a prion-like fashion, ascending from the gastrointestinal tract to the brainstem and eventually reaching the substantia nigra through the vagus nerve and other neural pathways. Therefore, dietary interventions aimed at reducing oxidative stress or glial activation in the enteric nervous system may not only alleviate local symptoms but also target the initial or early propagation stages of the fundamental pathological process in Parkinson's disease. In a rotenone-induced mouse model of PD, there was a significant elevation in intestinal levels of ROS, which was associated with a reduction in tyrosine hydroxylase-positive (TH⁺) dopaminergic neurons in the myenteric plexus and the accumulation of α-synuclein within ENS neurons. Supplementation with chlorogenic acid (CGA, 50 mg/kg) resulted in the upregulation of metallothionein (MT-1/2) expression in both astrocytes and EGCs. MT-1/2, functioning as endogenous antioxidant proteins, directly scavenged ROS and inhibited abnormal α-synuclein aggregation. In vitro experiments further corroborated that pretreatment with CGA significantly enhanced the survival of primary enteric neurons exposed to rotenone and induced the upregulation of MT expression [106].

In cases of spinal cord injury (SCI), the primary CNS lesion impairs descending autonomic regulation, resulting in unopposed sympathetic activity and diminished vagal influence on the gastrointestinal tract. This disruption in 'top-down' autonomic control directly contributes to ENS dysfunction and subsequent intestinal dysmotility. The aforementioned content has been incorporated into the revised version [107]. Research indicates that dietary supplementation with inulin can prevent ENS atrophy and dysmotility in mice with spinal cord injuries, as demonstrated by a marked restoration in the expression of the pan-neuronal marker PGP9.5 and the nitrergic neuronal enzyme (nNOS). Additionally, interventions involving microbiota-derived SCFAs have been shown to mitigate ENS dysfunction following spinal cord injury. Importantly, the neuroprotective effects of both inulin and SCFAs are contingent upon the activation of the IL-10 signaling pathway [108].

In murine models of depression, a reduction in central 5-HT levels is frequently accompanied by disrupted 5-HT signaling within the ENS, which is evidenced by a decrease in colonic peristaltic frequency and diminished neuronal activity in the myenteric plexus. In a model of depression induced by chronic unpredictable stress, the expression of tryptophan hydroxylase 1 (TPH1)—the enzyme crucial for 5-HT synthesis—was markedly downregulated in intestinal EC cells, resulting in a decreased colonic 5-HT concentration. This downregulation led to impaired activation of 5-HT3 receptors on ENS sensory neurons and weakened peristaltic reflexes [109]. Supplementation with sustained-release 5-hydroxytryptophan (5-HTP, 10 mg/kg) provided a continuous supply of precursor for 5-HT synthesis in EC cells, thereby restoring local colonic 5-HT levels and activating 5-HT receptors in the myenteric plexus. This activation facilitated the release of ACh and increased the frequency of colonic motility. This intervention not only enhanced intestinal motor function but also ameliorated GI dysfunction associated with depression [109]. In conclusion, dysfunction of the ENS in depression affects the brain through specific pathways. Reduced production of gut-derived 5-HT diminishes vagal signaling to the brain, thereby impairing the transmission of essential homeostatic signals. Additionally, the accompanying intestinal dysfunction induces a state of systemic inflammation. The resultant circulating cytokines have the capacity to directly penetrate the brain, causing damage to normal neurons and aggravating depressive symptoms. Consequently, dietary interventions targeting the ENS may partially ameliorate these disrupted pathways.

In a rat model of diabetes, there was a significant reduction in the total number of neurons (HuC/D⁺) and nitrergic neurons (nNOS⁺) within the ileal myenteric plexus. This neuronal loss was associated with increased levels of lipid peroxidation products, indicating that oxidative stress may play a contributory role in the damage to the ENS [110,111].

Research has demonstrated that administering quercetin microcapsules at a dosage of 10 mg/kg effectively mitigates the loss of nitrergic neurons and significantly reduces oxidative damage in diabetic rats [110]. Conversely, a separate study employing the same model revealed that a higher quercetin dosage (100 mg/kg) diminished the density of interstitial cells of Cajal in the jejunum of healthy rats, suggesting that elevated doses of this compound should be avoided in non-pathological conditions [111]. Therefore, the neuroprotective properties of polyphenols do not consistently exhibit a linear relationship. The efficacy of quercetin is significantly influenced by both dosage and the specific pathological context, necessitating careful consideration before advocating its use as a universal dietary recommendation. Additionally, resveratrol supplementation at 10 mg/kg was found to enhance systemic antioxidant capacity by increasing superoxide dismutase (SOD) activity and glutathione (GSH) levels, thereby neutralizing ROS. It also inhibited the expression of iNOS, which reduced the loss of HuC/D⁺ neurons in the myenteric plexus and improved intestinal motility [112]. In a type 2 diabetic murine model, prebiotic intervention was observed to modulate the gut microbiota and affect enteric neuronal function, thereby inhibiting duodenal hypercontractility and ultimately ameliorating hyperglycemia. This process was contingent upon the synthesis of the bioactive lipid 12-hydroxyeicosatetraenoic acid and involved signaling through mu-opioid receptors located on enteric neurons [113].

In a rat model of intestinal I/R injury, there was a significant reduction in the density of HuC/D⁺ neurons within the ileal myenteric plexus. Additionally, nNOS⁺ neurons displayed hypertrophic morphology, and there was a marked increase in the number of S100⁺ EGCs, indicating structural abnormalities in the ENS and heightened glial cell reactivity [114,115]. Pretreatment with resveratrol (10 mg/kg), administered from four days prior to surgery until seven days post-surgery, effectively ameliorated I/R-induced morphological abnormalities in nitrergic neurons and inhibited the hyperproliferation of EGCs [114]. Furthermore, another study demonstrated that oral administration of resveratrol enhanced the reduced/oxidized GSH ratio, improved antioxidant capacity, and facilitated the clearance of accumulated lipid peroxides during I/R injury. These effects significantly contributed to the recovery of HuC/D⁺ neuronal density, thereby corroborating the neuroprotective effects of resveratrol through the alleviation of oxidative stress [115].

The studies reviewed in this chapter unequivocally illustrate the therapeutic potential of dietary interventions for various ENS-related disorders, while concurrently highlighting a significant translational gap between foundational research and clinical application. In animal models of constipation, colitis, and Parkinson's disease, substantial and promising proof-of-concept data have been obtained. However, rigorously controlled clinical trials in human subjects remain relatively limited, leaving the true therapeutic efficacy of these interventions yet to be conclusively established. This translational challenge arises from multiple factors. Firstly, human diseases such as IBD and PD typically present with complex and chronic etiological characteristics that are challenging to fully replicate using the acute or genetically simplified animal models commonly employed in laboratory research. Secondly, a particularly notable challenge is individual variation. For example, probiotic combinations that demonstrate efficacy in animal studies often produce variable outcomes in human applications, likely due to the unique gut microecological environment and ENS phenotype of each individual. These findings collectively suggest a need for further investigation into personalized approaches and more sophisticated models to bridge the translational divide.