Early-life gastrointestinal inflammation and the developing brain: Unravelling the pathways to long-term cognitive dysfunction

Abstract

The gut-brain axis (GBA) is a complex, bidirectional communication network critical to integrating central nervous system functions with gastrointestinal (GI) health. This review examines how disruptions to the GBA during the critical early-life developmental window – a period of rapid neurogenesis and microbial colonization – contribute to long-term neurocognitive and psychiatric vulnerabilities. Evidence from animal models demonstrates that early-life stress, antibiotics, and infection induce sustained neuro-inflammation and alter microglial function, leading to long-term behavioral and cognitive impairments in adulthood. Human studies corroborate these findings, revealing that severe early GI insults, such as necrotizing enterocolitis, confer a high risk (40%) of global neurodevelopmental impairment and specific attention deficits. Chronic inflammatory conditions similarly impact the central nervous system: A high burden of early severe enteric infection is an independent risk factor for diminished intelligence quotient (IQ) and executive function, while conditions like celiac disease and inflammatory bowel disease are associated with persistent deficits in attention, processing speed, memory, and executive function. These clinical outcomes are strongly linked to systemic inflammation [elevated interleukin-6, kynurenine-to-tryptophan (Kyn:Trp) ratio], micronutrient deficiencies (iron, vitamin B12, folate), and structural white matter changes in the brain. Furthermore, chronic GI disease imposes a significant psychiatric burden, with high comorbidity of anxiety and depression often mediating poor health-related quality of life, particularly in pediatric inflammatory bowel disease. The findings underscore the necessity for a shift in clinical practice: Chronic GI disease in early life must be recognized as a red flag for neurocognitive risk. We advocate for a multidisciplinary approach encompassing early neurodevelopmental follow-up for high-risk groups and routine screening for cognitive and emotional comorbidities. Future research must focus on long-term prospective cohorts, identifying precise mechanistic biomarkers (metabolomics, microbiome signatures), and conducting interventional trials targeting the GBA to mitigate these long-term functional consequences.

Key Words: Gut-brain axis; Neurodevelopment; Necrotizing enterocolitis; Inflammatory bowel disease; Celiac disease; Cognitive dysfunction; Microbiome

Core Tip: Early-life and chronic gastrointestinal inflammation (e.g., necrotizing enterocolitis, inflammatory bowel disease, celiac disease) must be recognized as critical red flags for long-term neurocognitive and psychiatric dysfunction. These outcomes, including impaired executive function, attention deficits, and mood disorders, are driven by sustained systemic inflammation, the Kynurenine pathway, and micronutrient deficiencies. Clinical practice requires a neuroprotective strategy that integrates gastrointestinal specialists with neurodevelopmental and mental health screening protocols. Therapeutic goals should prioritize achieving deep mucosal healing and correcting nutritional deficits (iron, B-vitamins, omega-3s) to mitigate brain risk and improve long-term functional capacity. Future research must validate these interventions to solidify the therapeutic potential of the gut-brain axis.

INTRODUCTION

The gut-brain axis (GBA) is a highly complex, bidirectional communication network that integrates the emotional, cognitive, and autonomic centers of the central nervous system (CNS) with the physiological and immunological functions of the gastrointestinal (GI) tract. This interconnected system relies on multiple signaling pathways – including neural (vagal and enteric circuits), endocrine (hormonal mediators), immune (cytokine-driven pathways), and microbial (metabolite and neuroactive compound production)[1]. Although GBA functions across the lifespan, its influence is particularly profound during early life, a critical developmental window encompassing the prenatal period, infancy, and early childhood. During this stage, the CNS undergoes rapid synaptogenesis, myelination, and pruning, while the gut experiences the foundational colonization of its microbiota and the maturation of the mucosal immune system[2].

The synchrony of brain, gut, and immune development renders early life uniquely vulnerable to disruption (Table 1). This vulnerability is increasingly evident as clinicians observe a rise in GI conditions characterized by acute or chronic inflammation in neonates and young children, including necrotizing enterocolitis (NEC), early-onset inflammatory bowel disease (IBD), and severe forms of food protein–induced enterocolitis[3]. Parallel to this trend, the global prevalence of neurodevelopmental, behavioral, and cognitive disorders – such as autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), and deficits in learning and executive functioning – continues to climb[4].

Table 1 Co-development in of gut microbiome, immune system and brain and cognitive functions in early life (the critical window).

Time frame	Gut microbiome development	Immune system development	Brain/cognitive development
Prenatal (in utero)	Microbiome density increases rapidly leading up to birth	Immature: Components are developing but not fully functional	Foundation laid: Neurulation, neural proliferation, neuronal migration, and initial axon growth are active
Birth – 3 months	Rapid colonization: Window of opportunity for microbial assembly. Diversity is fluctuating	Education/expansion: Immune system begins learning to repel new bacterial strains. IgG/IgM are developing	High activity: Synapse formation (synaptogenesis) is highly active. Myelination begins
Birth – 2 years	Assembling/fluctuating: Gut microbiota is actively assembling. Microbial density is high but composition is unstable	Developing: Natural killer cells are above adult levels. Key immunoglobulin levels (IgG, IgA) are developing but not yet mature. T cell independent antibody response is immature	Peak proliferation (first 1000 days): Synapses intensely proliferate. Brain metabolism increases. Sensory pathways (vision, hearing) peak, and language develops rapidly
Year 2-3	Stabilization phase: Composition begins to settle toward an adult-like state	Transition: Continues developing toward adult levels, including Th1 mediated immunity	Pruning begins: Synapses are pruned (eliminated). Metabolism decreases, and higher cognitive function development approaches its peak

It effectively highlights the synchronization of these three major biological systems during the first 1000 days of life, illustrating why early-life gastrointestinal inflammation is such a significant neurodevelopmental risk factor. Ig: Immunoglobulin.

Emerging evidence from both preclinical models and human cohorts suggests that early-life GI inflammation is not confined to the intestinal lumen. Instead, it acts as a potent systemic insult that can influence long-term neurodevelopment. Inflammation during this sensitive period can destabilize the immature intestinal barrier, perturb the developing gut microbiome, and initiate sustained immune activation[5]. These disturbances interact with developing neural circuits, potentially inducing chronic neuroinflammation, altering metabolic and endocrine signaling, and impairing processes essential for healthy brain maturation. Collectively, these pathways raise the possibility that intestinal inflammation during critical windows may exert enduring effects on cognition, emotional regulation, and overall brain function[6].

Despite growing recognition of the gut-brain interplay, the mechanistic pathways linking transient or chronic early-life GI inflammation to long-term cognitive dysfunction remain incompletely understood. Existing studies often highlight associations but stop short of fully elucidating how early inflammatory events translate into structural and functional CNS alterations. This gap in mechanistic clarity limits the development of targeted interventions and hinders early identification of at-risk children[7].

This narrative review aims to synthesize and critically evaluate current evidence describing the relationship between early GI inflammation and later cognitive outcomes. Specifically, the review will explore the immune, metabolic, microbial, and neuroendocrine pathways that bridge intestinal inflammation with neurodevelopmental impairment. By outlining the biological and developmental mechanisms through which early GI insults affect the brain, we aim to identify critical periods of vulnerability and highlight opportunities for early intervention. Ultimately, this review underscores the potential of targeting the GBA as an innovative strategy to preserve neurodevelopmental health in pediatric populations at risk.

KEY COMMUNICATIVE PATHWAYS: THE GUT-BRAIN DIALOGUE

GBA in early life represents a dynamic, rapidly evolving communication network that integrates neural, immune, metabolic, and endocrine signaling between the GI tract and the developing CNS. During infancy, this bidirectional system is uniquely plastic: Both the gut and the brain undergo parallel maturation, rendering the axis more influential – but also more vulnerable – than at any other stage of life[8]. Early-life disturbances such as intestinal inflammation, disrupted microbiota colonization, and impaired barrier function can therefore exert disproportionate and lasting effects on neurodevelopment and cognitive outcomes[9]. A detailed understanding of the structural and functional components of early GBA is essential to contextualize how GI pathologies in infancy may translate into long-term alterations in brain development, behavior, and cognitive function (Figure 1).

Figure 1 The gut-brain axis in early life: Neural, immune, and endocrine pathways linking the developing intestine and brain. It illustrates the major communication pathways that constitute the gut-brain axis during early life, a period marked by rapid maturation of both the gastrointestinal and central nervous systems. The neural pathway is represented by the vagus nerve and the enteric nervous system, which relay signals from the intestinal lumen to the brainstem and modulate autonomic responses. The immune pathway shows how cytokines, microbial products, and antigen-presenting cell activity transmit inflammatory or regulatory signals across the systemic circulation to influence microglial activation and neurodevelopment. The endocrine and enteroendocrine pathway highlights gut-derived hormones and metabolites – such as glucagon-like peptide-1, serotonin, and short-chain fatty acids – that modulate brain function, appetite regulation, stress responses, and neuronal maturation. Together, these interconnected routes form a dynamic bidirectional system that shapes neurodevelopmental outcomes in infancy.

Immune/cytokine pathway (the inflammatory cascade)

The immune system acts as one of the earliest and most direct communication channels between the inflamed gut and the developing brain. Neonatal intestinal immunity is characterized by heightened sensitivity to pro-inflammatory signaling, making the system highly reactive to peripheral GI disturbances[10].

The systemic inflammatory signal: When the compromised early-life intestinal barrier allows microbial products like lipopolysaccharide (LPS) and pro-inflammatory mediators to leak into the systemic circulation, a cascade of events is initiated. Intestinal inflammation, driven by conditions like NEC or early-onset IBD, triggers the copious release of pro-inflammatory cytokines [e.g., interleukin-1beta (IL-1beta), IL-6, and tumor necrosis factor-alpha (TNF-α)][11]. These systemic cytokines influence the CNS via several routes. Although the blood-brain barrier (BBB) is generally restrictive, the tight junctions (TJs) of the BBB are also immature and more permeable in early life. Furthermore, systemic cytokines can enter the CNS at circumventricular organs (regions lacking a true BBB) or activate endothelial cells and transporters on the BBB, which in turn produce secondary inflammatory mediators that readily enter the brain parenchyma[12]. The vagus nerve, which carries afferent signals from the gut to the brainstem, also mediates immune communication. While the vagus nerve typically mediates anti-inflammatory reflexes, high concentrations of peripheral cytokines can directly stimulate vagal afferents, transmitting a rapid neural signal of inflammation to the CNS[13].

Driving neuroinflammation: Once inside the CNS, these peripheral inflammatory signals drive neuroinflammation, a critical process linking early gut pathology to cognitive dysfunction. Microglia – the brain’s resident immune cells and primary mediators of neuroinflammation – are exquisitely sensitive and responsive to peripheral cytokines during the neonatal period[14]. Early-life inflammation alters microglial morphology, proliferation, and function, inducing a prolonged, pro-inflammatory phenotype. Activated microglia disrupt key neurodevelopmental processes. In their reactive state, they are thought to engage in excessive synaptic pruning and impair synaptogenesis, directly altering the connectivity and architecture of crucial cognitive regions such as the hippocampus and prefrontal cortex (PFC)[15]. Chronic exposure to high levels of pro-inflammatory cytokines has been shown to suppress neurogenesis (the birth of new neurons) in the hippocampus, a brain region central to memory and learning. Thus, early-life GI inflammation converts the gut immune system into a potent engine of CNS damage, establishing a chronic neuroinflammatory state that programs the brain for long-term cognitive and behavioral impairment[16].

Microbiota-derived metabolite pathway

The establishment of the gut microbiome in early life represents one of the most influential biological events shaping long-term neurodevelopment (Figure 2). Far from being a passive population of microbes, the early microbiota acts as a metabolic, immunologic, and neuroactive organ that communicates continuously with the developing brain[17]. Because both microbiome assembly and brain maturation follow tightly regulated, time-sensitive trajectories, disturbances during this critical window can exert profound and lasting cognitive and behavioral consequences. Microbiome-generated metabolites – including short-chain fatty acids (SCFAs), tryptophan metabolites, and bile acid derivatives – serve as potent signaling molecules that influence BBB integrity, neurotransmitter synthesis, and epigenetic programming[18]. The immaturity of neonatal metabolic pathways amplifies the sensitivity to these signals. Early-life GI inflammation (and associated dysbiosis) severely alters the production and balance of these vital neuroactive molecules, thus disrupting normal CNS development[19].

Figure 2 How the early-life microbiome shapes the developing brain. It summarizes the major pathways through which the early-life gut microbiome influences neurodevelopment. During the critical colonization window of infancy, microbial composition is shaped by birth mode, feeding practices, antibiotic exposure, and environmental factors. These early microbial communities generate metabolites and immune signals that drive key processes in brain development. Microbiota-derived molecules support myelination by promoting oligodendrocyte maturation and white matter integrity. They also regulate synapse formation and pruning, influencing neuronal connectivity and neurotrophic signaling. In parallel, microbial cues help program the hypothalamic-pituitary-adrenal axis, establishing lifelong patterns of stress responsiveness. Together, these interconnected pathways demonstrate how disruption of the early microbiome – such as through inflammation, dysbiosis, or antibiotic exposure – can alter neurodevelopmental trajectories and increase vulnerability to long-term cognitive and behavioral impairments.

SCFA influence on myelination and white matter development: Microbial metabolites – particularly SCFAs such as acetate, propionate, and butyrate – are essential for oligodendrocyte maturation and myelination. Myelination, the process of insulating nerve fibers (white matter), is critical for fast and efficient neural signaling and reaches peak activity during early childhood[20]. Germ-free (GF) animal studies show marked deficits in PFC myelination, which can be reversed upon microbial colonization during early-life windows but not in adulthood[21]. In human infants, altered microbiome composition associated with GI issues has been linked to abnormalities in white matter microstructure on magnetic resonance imaging (MRI), implicating microbial signals in early neural connectivity, attention regulation, and executive function development[22]. Early-life inflammation can thus impair SCFA production by depleting beneficial SCFA-producing bacteria (such as Bifidobacterium), thereby starving the developing brain of necessary precursors for myelination[23].

Regulation of synaptogenesis and synaptic pruning via tryptophan: Synapse formation and pruning occur at their highest rates during infancy and early childhood. The microbiome influences this process through metabolic and immune-mediated mechanisms. The gut microbiome regulates tryptophan metabolism, a critical pathway that modulates the central serotonin (5-HT) pathway[24]. The 5-HT, 90%-95% of which is produced in the gut by enterochromaffin cells, significantly influences cortical and hippocampal synaptic density, neurogenesis, and early behavior modulation. Dysbiosis reduces the microbial conversion of tryptophan into beneficial neuroactive compounds while potentially shunting it toward inflammatory metabolites, consequently affecting synaptic development[25]. Microglia – the brain’s primary immune cells responsible for synaptic pruning—are exquisitely dependent on early microbial signals[26]. GF mice exhibit dysfunctional microglia with impaired pruning capacity, a phenotype correctable only when colonization occurs during early developmental windows[27]. Gut inflammation exacerbates this, as resulting peripheral cytokines and altered metabolites can induce an immune-mediated signaling cascade, causing microglia to over-prune or become chronically reactive[28]. Thereby, the integrity of the early microbiome is essential for providing the specific metabolic building blocks required for complex neurodevelopmental processes. Early-life inflammation, by driving dysbiosis, effectively cuts off this neuroprotective metabolic supply, leaving the CNS structurally and functionally vulnerable.

Neurotransmitter and endocrine pathway

In addition to immune and metabolic signaling, the gut communicates with the CNS via systemic endocrine factors and local neurotransmitter systems. Early-life GI inflammation acts as a potent peripheral stressor, hijacking these pathways to establish long-term changes in emotional regulation and cognitive function[29].

Programming of the hypothalamic-pituitary-adrenal axis: The microbiome and the gut itself play a central role in shaping stress reactivity by modulating the hypothalamic-pituitary-adrenal (HPA) axis, the body’s primary stress response system. Early-life inflammation, whether acute (e.g., NEC) or chronic [e.g., early-onset IBD (EO-IBD)], acts as a severe biological stressor, triggering the sustained release of cortisol (in humans) or corticosterone (in animal models)[30]. Microbiota-derived molecules modulate corticotropin-releasing hormone and glucocorticoid receptor expression. Disturbances in microbial communities (dysbiosis) associated with inflammation lead to exaggerated HPA axis responses to stress[31]. Elevated, persistent levels of glucocorticoids during this highly plastic neonatal period can impair the development of the hippocampus and PFC, leading to long-term vulnerability to anxiety, depression, and cognitive impairment. Conversely, beneficial taxa such as Bifidobacterium longum and Lactobacillus rhamnosus exhibit anxiolytic effects by modulating vagal signaling and inflammatory tone, demonstrating the protective role of a healthy early microbiome against stress programming[32].

Gut-derived neurotransmitters and endocrine factors: Enterochromaffin cells act as sensory transducers within the intestinal epithelium. They detect luminal contents and inflammatory signals, releasing hormones and neuropeptides that directly impact the CNS. The gut is the primary source of 5-HT, and its production is significantly influenced by the local inflammatory and microbial environment[33]. The 5-HT is essential for regulating neurogenesis, synaptic connectivity, and early behavior modulation. Inflammation or dysbiosis can disrupt the bioavailability and signaling of 5-HT, negatively impacting CNS development. Gut-derived endocrine factors such as ghrelin, leptin, and insulin-like growth factors contribute directly to brain development[34]. These hormones regulate neurogenesis, hippocampal growth, stress-axis maturation, and energy balance[35]. GI inflammation can alter the release or receptor sensitivity to these factors, thereby compromising critical brain developmental processes that rely on precise hormonal signals. In summary, the endocrine and neurotransmitter pathways provide a chronic, systemic route by which early-life intestinal inflammation programs the CNS, leading to lasting changes in stress-coping mechanisms and metabolic regulation that underpin later cognitive dysfunction[36].

Nutritional and metabolic pathways (malabsorption)

Beyond the direct signaling molecules generated by the microbiome and immune system, early-life GI inflammation severely compromises the developing brain through fundamental nutritional and metabolic deficiencies. The primary function of the gut is to absorb essential macronutrients and micronutrients, and inflammation-induced damage to the intestinal villi (malabsorption) during a period of intense CNS growth can starve the brain of necessary building blocks[37]. The developing CNS has an exceptionally high metabolic rate and requires a steady supply of specific micronutrients, many of which are critical cofactors in neurotransmitter synthesis, myelination, and neuronal energy production. Malabsorption resulting from persistent intestinal injury or diarrheal disease often leads to deficiencies in key nutrients[38].

Iron is essential for numerous neurodevelopmental processes, including myelination, dendritic arborization, and neurotransmitter synthesis (e.g., dopamine). Early-life inflammation, particularly if associated with blood loss or chronic malabsorption, can lead to iron deficiency anemia. This deficiency is strongly correlated with irreversible cognitive deficits, impaired motor function, and long-term behavioral problems[39]. These B vitamins are indispensable cofactors in the one-carbon metabolism pathway, which is critical for DNA synthesis and repair, and for the creation of S-adenosylmethionine, a universal methyl donor required for neurotransmitter synthesis and myelination. Chronic GI inflammation and minor intestinal damage (where vitamin B12 is absorbed) can cause severe deficiency, leading to delayed myelination and neurological damage, particularly when combined with limited dietary intake[40].

Chronic gut inflammation can also impair the absorption of essential fatty acids (e.g., docosahexaenoic acid, arachidonic acid), which are vital components of neuronal cell membranes, and essential amino acids (the building blocks of proteins and neurotransmitters). Deficiency in these factors compromises structural brain integrity and functional connectivity[41]. In essence, early-life GI inflammation creates a state of developmental starvation for the CNS. This metabolic insult provides an independent yet synergistic pathway through which gut pathology in infancy translates into irreversible structural and functional impairment of the developing brain[42].

EARLY-LIFE GI INFLAMMATION: A DEVELOPMENTAL STRESSOR

The intestinal tract is the largest immune and mucosal surface in the body. Its successful development during the critical early-life window is essential, as the nascent system must simultaneously establish immune tolerance, facilitate nutrient absorption, and form an impenetrable barrier. When this process is disrupted by inflammatory insults, the resulting systemic stress can cascade into neurodevelopmental consequences[43].

Defining early-life GI inflammation

Early-life GI inflammation refers to an excessive or prolonged immune response within the intestine during the neonatal and infant periods. This is a particularly impactful time because the intestinal barrier is inherently more permeable than in adulthood, and the gut microbiota is still in the process of colonization and stabilization[44]. Inflammation during this sensitive phase has profound, lasting effects on both gut and distant organ development, including the brain. Key conditions associated with severe or chronic early-life GI inflammation include NEC, EO-IBD, maternal infection and inflammation, severe food protein-induced enterocolitis syndrome and allergies, and antibiotic exposure[22].

NEC, the most devastating GI disease of prematurity, is characterized by acute intestinal necrosis and systemic inflammation. Survivors often suffer significant long-term neurodevelopmental impairment (NDI), directly linking severe GI inflammation to cognitive outcomes. EO-IBD are IBD diagnosed before the age of six. It represents a chronic, persistent inflammatory state that occurs while the CNS is still undergoing rapid maturation, providing a constant source of systemic inflammatory mediators[45]. Maternal systemic inflammation (e.g., from infection, obesity, or chronic stress) can alter the prenatal environment, potentially impacting fetal gut development and CNS programming in utero. Food protein-induced enterocolitis syndrome and other food allergies are conditions characterized by acute or chronic inflammatory responses to dietary proteins, which, when persistent, contribute to chronic intestinal dysbiosis and increased permeability[46]. While not a direct cause of inflammation, repeated or broad-spectrum antibiotic use in early life leads to profound dysbiosis (imbalance of the gut microbiota). This loss of beneficial, anti-inflammatory bacteria and their metabolites is now recognized as a potent driver of chronic, low-grade mucosal inflammation and impaired barrier function[47]. These conditions underscore the vulnerability of the developing system. The inflammation they induce severely compromises the integrity of the intestinal epithelial barrier – the critical line of defense – allowing microbial products and pro-inflammatory mediators to leak into the circulation, initiating the communication with the developing brain[48].

The developing intestinal barrier

The inherent structural and functional characteristics of the infant intestinal barrier make it uniquely vulnerable to inflammatory insult, a phenomenon often described as the “leaky gut” of early life. The gut barrier, composed of a single layer of epithelial cells, is primarily responsible for selective nutrient absorption while excluding harmful pathogens and toxins[49]. In infants, this barrier is physiologically more permeable than in adults due to several developmental factors: The protective mucus layer overlying the epithelium is thinner and less robustly glycosylated, offering reduced physical defense against microbial encroachment and inflammatory damage. In addition, the turnover and maturation of intestinal epithelial cells are still underway, resulting in less efficient repair mechanisms following injury[50]. Moreover, the structural complexes known as TJs – which seal the paracellular space between adjacent cells – are less fully formed and inherently looser in the neonatal period. This allows for greater natural diffusion of macromolecules, a critical feature for the passive transfer of maternal antibodies, but it also increases the risk of translocation[51]. This physiological immaturity establishes a crucial window of vulnerability (Figure 3).

Figure 3 Developmental factors contributing to intestinal mucosal barrier vulnerability in neonates and infants. It illustrates the key developmental features that make the intestinal mucosal barrier more permeable and vulnerable in neonates and young infants. The mucus layer is thin and less glycosylated, providing weaker physical protection against microbes and inflammatory injury. The intestinal epithelial cells are still undergoing maturation, with slower turnover and less efficient repair responses following injury. The tight junctions between adjacent epithelial cells are structurally immature and more permissive, allowing greater paracellular movement of macromolecules. While this increased permeability facilitates the passive transfer of maternal antibodies, it also heightens susceptibility to microbial translocation and inflammation, creating a critical window of vulnerability early in life.

When faced with the inflammatory stressors (e.g., NEC, antibiotic-induced dysbiosis), the already delicate intestinal barrier rapidly succumbs to damage, amplifying its permeability. Inflammation triggers the release of high levels of pro-inflammatory cytokines (e.g., TNF-α and interferon gamma) within the intestinal wall[52]. These cytokines directly signal the epithelial cells, leading to the reorganization and internalization of key TJ proteins, such as Zonula Occludens, Occludin, and Claudins. The functional loss of these TJ proteins results in the physical widening of the paracellular gap, turning the selectively permeable barrier into a porous membrane. This dramatically increases paracellular permeability[53].

The compromised barrier allows for the massive, uncontrolled translocation of two critical classes of molecules (microbial and inflammatory factors) into the systemic circulation. The microbial factors are primarily pathogen-associated molecular patterns, such as LPS from gram-negative bacteria, while the inflammatory factors include cytokines and chemokines produced by the inflamed intestinal mucosa[54]. This systemic spillover of microbial and inflammatory products is the foundational event that initiates distant communication with the brain. Once in circulation, LPS and inflammatory cytokines can cross the BBB or activate primary afferent neurons (such as the vagus nerve), thereby driving neuroinflammatory and cognitive sequelae (Figure 4)[55].

Figure 4 Early-life intestinal barrier integrity and its disruption during inflammation: Pathways linking the gut to the developing brain. This schematic illustrates the developmental characteristics of the infant intestinal barrier and the mechanisms by which early-life inflammation disrupts gut integrity and triggers downstream neuroinflammatory effects. A: It depicts the normal neonatal intestinal barrier, characterized by immature yet functionally regulated permeability, a thin mucus layer, and developing tight junctions that allow controlled antigen exposure essential for immune maturation; B: It shows inflammatory stimulation by cytokines such as tumor necrosis factor-alpha and interferon gamma, which initiate epithelial stress responses; C: It demonstrates inflammatory breakdown of the barrier: Tight junction disassembly, widening of the paracellular space, and translocation of luminal microbial components – including lipopolysaccharide – along with pro-inflammatory cytokines into the systemic circulation; D: It illustrates how these circulating mediators cross or signal across the immature blood-brain barrier, activate microglia, and induce neuroinflammation, ultimately altering neurodevelopmental processes such as synaptogenesis, neuronal connectivity, and cognitive maturation. Together, it summarizes the mechanistic cascade linking early gastrointestinal inflammation to potential long-term cognitive and behavioral consequences. BBB: Blood-brain barrier; LPS: Lipopolysaccharide.

IMPACT ON SPECIFIC BRAIN REGIONS AND COGNITIVE DOMAINS

The systemic communication between the inflamed gut and the developing brain, mediated by the pathways detailed in the previous section (cytokines, metabolites, and hormones), does not impact CNS uniformly. Instead, the resulting neuroinflammation and metabolic dysregulation show a predilection for specific brain regions and developmental processes that are undergoing peak maturation during early life. These targeted insults lead directly to the long-term cognitive and behavioral dysfunctions observed in at-risk pediatric populations[56].

Hippocampus and learning/memory

The hippocampus is one of the most studied brain regions in the context of the GBA, primarily due to its central role in learning, memory formation, and emotional regulation. Its vulnerability stems from its high metabolic demand, rich vascularity, and unique plasticity, particularly in the process of neurogenesis. Early-life GI inflammation is strongly associated with reduced hippocampal volume and function[57]. Specifically, chronic exposure to high levels of pro-inflammatory cytokines (e.g., IL-1beta, TNF-α) and stress hormones (glucocorticoids from HPA axis activation) has been shown to suppress hippocampal neurogenesis – the creation of new neurons in the dentate gyrus. This reduction in the neuronal precursor pool directly limits the capacity for long-term plasticity and information encoding[58].

Beyond cell proliferation, inflammation disrupts synaptic plasticity, the ability of synapses to strengthen or weaken over time [e.g., long-term potentiation (LTP)], which is the cellular basis for learning and memory storage. Peripheral inflammation alters the expression of key neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), which is essential for synaptic survival and function[59]. Lower BDNF levels, often seen during periods of severe GI inflammation or dysbiosis, directly compromise the integrity of hippocampal circuitry. The structural and functional impairments in the hippocampus translate into measurable long-term deficits in spatial learning, contextual fear conditioning, and memory retrieval[60]. Animal models of neonatal inflammation (e.g., models of NEC or early-life stress combined with inflammation) consistently show delayed acquisition of cognitive tasks and poorer performance on memory tests later in life. The hippocampus thus serves as a primary target of early-life GI inflammation, establishing a mechanistic link between peripheral immune challenge and core cognitive deficits in memory and learning[61].

PFC and executive function

The PFC, located in the frontal lobe, governs executive functions – a suite of high-level cognitive skills that includes working memory, inhibitory control, attention regulation, cognitive flexibility, and decision-making. The PFC undergoes prolonged development, with maturation continuing well into young adulthood[62]. Critically, many PFC-related circuits undergo a period of intense plasticity and reorganization during the period most susceptible to early-life GI inflammation (infancy to early childhood). The PFC is highly vulnerable to the effects of chronic, low-grade neuroinflammation originating in the inflamed gut. Systemic cytokines (IL-1beta, TNF-α) that reach the brain activate microglia and astrocytes within the PFC. This heightened inflammatory state disrupts the delicate balance of inhibitory and excitatory neurotransmission necessary for complex computational processes[63].

Proper PFC function relies on finely tuned synaptic connectivity. Inflammation-induced microglial dysfunction can lead to either excessive or deficient synaptic pruning, resulting in disorganized PFC circuitry. Furthermore, inflammation is associated with increased oxidative stress, which damages dendritic arborization and reduces the density of key synaptic proteins in PFC neurons[64]. The development of white matter tracts connecting the PFC to other brain regions (critical for executive function networks) is heavily dependent on SCFAs. As discussed in previous section, early-life GI inflammation often leads to the depletion of SCFA-producing bacteria. The resulting impairment in myelination and white matter integrity directly compromises the speed and efficiency of information transfer within the PFC network[65].

Dysfunction in the PFC is mechanistically linked to core symptoms of neurodevelopmental disorders like ADHD and aspects of ASD. Specifically, children with a history of severe early-life GI distress often exhibit long-term deficits in attention span, impulse control, and working memory – all hallmarks of impaired PFC function[66]. The convergence of inflammatory, metabolic, and hormonal stress signals on the highly plastic PFC during its critical developmental window provides a strong mechanistic explanation for the long-term executive function deficits observed following early-life GI inflammation[67].

Cerebellum and social/motor coordination

While the hippocampus and PFC are often the primary focus of GBA research, the cerebellum is increasingly recognized as a crucial player in neurodevelopmental disorders that frequently co-occur with early-life GI issues, such as ASD and developmental coordination disorders. Historically viewed only for its role in motor control, the cerebellum is now understood to be deeply involved in social cognition, emotional regulation, and executive function[68].

The cerebellum undergoes a highly protracted and rapid postnatal developmental phase, particularly in the first few years of life, making it acutely sensitive to environmental perturbations such as inflammation. This rapid growth involves significant granule cell proliferation and Purkinje cell maturation, processes that are easily disrupted by systemic stress[69]. Preclinical models of neonatal infection and inflammation have demonstrated that peripheral immune challenge can lead to long-lasting Purkinje cell loss and altered synaptic plasticity within the cerebellum. The cerebellar immune environment, including its resident microglia, is activated by circulating pro-inflammatory cytokines, driving neuroinflammation that impairs cerebellar circuitry development[70].

The connection between the microbiome and cerebellar function is supported by animal studies. GF mice often display subtle motor abnormalities, which can be partially rescued by early microbial colonization. This suggests that microbial metabolites are necessary for the proper establishment of cerebellar motor circuits. Dysbiosis stemming from early GI inflammation could thus interfere with these metabolite-dependent processes[71]. Dysfunction in the cerebellum is linked to deficits in tasks requiring coordination, timing, and motor planning. More significantly, in the context of neurodevelopment, cerebellar anomalies are strongly correlated with impaired social communication and repetitive behaviors, which are core features of ASD. Therefore, the impact of early-life GI inflammation extends beyond purely cognitive domains to affect the cerebellar circuitry underlying motor skills and complex social behaviors, highlighting the broad developmental risk posed by peripheral inflammation[72].

EVIDENCE FROM ANIMAL STUDIES

Animal models have provided critical mechanistic insights into how early-life intestinal injury, dysbiosis, and immune activation can influence brain development and long-term cognitive outcomes. These models recreate key features observed in human neonates – such as immature gut barrier function, exaggerated inflammatory responses, and rapidly developing neural circuits – making them invaluable for studying causality[73].

Experimental models used to study gut-brain interactions

NEC models: Rodent and preterm piglet NEC models replicate hallmark characteristics of human NEC, including epithelial injury, microbial translocation, and overwhelming inflammation. These models consistently demonstrate systemic cytokine surges (e.g., TNF-α, IL-1β) that reach the brain, microglial activation in regions such as the hippocampus and cortex, and white matter injury, impaired myelination, and reduced neurogenesis. These changes cause deficits in learning, memory, and motor coordination[74].

Niño et al[75] show that increased oxidative stress in the brain triggered NEC-associated cognitive impairments in mice. The release of the proinflammatory molecule high-mobility group from the injured intestine activated Toll-like receptor 4 on microglial cells in the brain, resulting in the accumulation of reactive oxygen species. Oral administration of microglia-targeting antioxidants prevented cognitive impairments in a mouse model of NEC. The results suggest that reducing microglial activation might be a strategy to protect patients from cognitive impairments associated with NEC[75]. Biouss et al[76] found that NEC leads to severe brain changes in the studied mice, including reduced brain weight, thinner cortices, and increased apoptosis and endoplasmic reticulum stress. NEC pups had fewer neurons, oligodendrocytes, and neural progenitor cells in specific brain regions. The study also showed a strong pro-inflammatory response in the brain, with higher cytokine levels and more activated microglia and astrocytes, which correlated with gut inflammation and NEC severity, indicating that intestinal inflammation may drive brain cell damage[76]. In addition, Sha et al[77] used a novel graded murine model of NEC to investigate the severity of the disease and its neurological consequences. They found that increasing concentrations of the inflammatory agent dextran sodium sulfate (DSS) resulted in a full spectrum of NEC severities, characterized by worsened intestinal damage and increased systemic inflammation. Crucially, the research demonstrated that even mild NEC was sufficient to initiate neuroinflammation, evidenced by increased cytokine levels [IL-2, granulocyte colony-stimulating factor, chemokine (C-X-C motif) ligand] and reduced microglial branching in the hippocampus, thereby linking the entire range of intestinal inflammation to early brain activation. The findings of these studies support a pathway in which early mucosal destruction and uncontrolled inflammation disrupt vulnerable neural circuits[77].

Colitis models (chemically induced intestinal inflammation): Models using DSS, TNBS (trinitrobenzene sulfonic acid), or LPS trigger mucosal inflammation with varying depths and chronicity. When induced during early postnatal life, animals exhibit reduced synaptic plasticity, including impaired LTP in the hippocampus. They also show altered neurotransmitter levels, particularly glutamate and GABA, and behavioral changes such as increased anxiety-like behavior and reduced exploratory activity. These changes were also associated with delayed myelination and abnormalities in dendritic spines[78]. The study by Amlashi et al[79] investigated the long-term impact of colitis (intestinal inflammation) in rats on CNS structures involved in pain and emotional processing. They found that rats with colitis exhibited a significantly reduced neuronal density in the cingulate cortex areas (CC1 and CC2) of both hemispheres. The findings suggest that chronic intestinal inflammation can induce structural damage in cortical regions and the amygdala, potentially impacting the cognitive and sensory aspects of pain perception[79]. In addition, Sutkus et al[80] used a pig model to investigate the effects of experimentally induced colitis on brain anatomy and microstructure, and to test the protective effect of tributyrin supplementation. The findings revealed that colitis significantly altered fiber organization and reduced myelination [lower myelin water fraction (MWF)] across the whole brain and cortex compared to controls. Contrary to expectations, tributyrin supplementation did not ameliorate these adverse effects, suggesting that intestinal inflammation affects neurodevelopmental processes in ways that warrant further investigation into alternative protective nutritional strategies[80]. Meanwhile, Sroor et al[81] investigated the neurological impact of colitis (a murine model of IBD) and found that the experimental inflammation propagated to the brain, specifically altering microglial phenotypes in the limbic system. Although some activation markers were reduced, flow cytometry revealed an increase in monocyte-derived macrophages, and gene expression analysis in the medial PFC showed pronounced changes in microglial markers consistent with both M1 (pro-inflammatory) and M2 (regulatory) activation. These findings suggest that colitis causes a complex alteration of microglial function in brain regions associated with anxiety and depression[81]. These models help disentangle the contribution of inflammation alone, independent of microbial colonization, to neurodevelopment.

Microbiome manipulation/dysbiosis models: GF animal models are essential for elucidating the role of the gut microbiome in brain development. Research demonstrates that the absence of microbial colonization leads to alterations in stress response, anxiety levels, social behaviors, immune system function, and neural architecture – including changes in microglia activity, synaptic pruning, and white matter development. These models help identify critical windows of microbial influence and reveal mechanisms (such as microbial metabolites influencing neurotrophic factors) that link gut health to brain function[82]. GF animal model studies emphasize the critical and time-sensitive influence of microbiota on brain development, specifically targeting the hippocampus and cortical regions. Castillo-Ruiz et al[83] found that GF-induced alterations in microglial and neuronal development are programmed prenatally and persist in neonates despite early postnatal microbial colonization. Thompson et al[84] confirmed that these structural brain volume changes are more pronounced in male mice and cannot be corrected by attempting microbial conventionalization later in puberty, underscoring the irreversible nature of early-life microbial influence.

Models simulating dysbiosis (microbial imbalance) replicate these effects, highlighting how disruptions in the microbiome can contribute to depression-like behaviors through abnormal synaptic pruning (notably via the C3 pathway) and can affect addiction-related responses. Many studies demonstrate that antibiotic-induced gut dysbiosis is a potent modulator of CNS function, successfully replicating the cognitive and behavioral effects of a disrupted GBA[85]. Specifically, Fröhlich et al[86] established that dysbiosis, associated with the depletion of bacterial-derived metabolites, leads to impaired novel object recognition (NOR) memory and dysregulation of cognition-relevant signaling molecules (such as BDNF and the cerebral neuropeptide Y system) in the adult brain. Bistoletti et al[87], further detailed these molecular changes in juvenile mice, showing that antibiotic treatment differentially influences the expression of the neurotrophic factor BDNF and its receptor TrkB, downregulating them in the hippocampus while upregulating them in the enteric nervous system (ENS), thereby suggesting a mechanistic link between early dysbiosis, hippocampal vulnerability, and potential psychiatric comorbidity in later functional gut disorders. Finally, Tettevi et al[88], extended this link to neurodegenerative disease models, showing that gut microbiome disruption exacerbates late-stage Alzheimer’s disease (AD) pathology in aged mice, driving cognitive deficits, neuroinflammation (elevated TNF-α), and hallmark protein aggregation (amyloid-beta and tau), thus confirming the microbiome’s critical role as a modulator of both neurodevelopmental and neurodegenerative outcomes via the GBA.

Early-life colonization appears to be critical for typical neurodevelopment. Collectively, animal models facilitate the identification of sensitive periods during which microbial influence is most impactful and uncover underlying mechanisms, such as the modulation of neurotrophic factors by microbial metabolites, that link gut health to brain function[89]. Fecal microbiota transplantation (FMT) is a rapidly emerging tool that provides causal evidence of the gut microbiota’s direct influence on cognitive function and neurological pathology. Several studies have confirmed the transferability of cognitive traits via FMT, demonstrating that therapeutic outcome is highly dependent on the donor’s neurological status[90]. For instance, Wang et al[91] showed that FMT derived from individuals with mild cognitive impairment-FMT induced detrimental effects in wild-type mice, specifically causing impaired learning and memory alongside a reduction in cerebral glucose uptake, mechanistically linked to an increase in the abundance of Bacteroides and the negative regulator of glucose uptake, TXNIP. Extending this concept, D'Amato et al[92] found that transferring microbiota from aged donor mice into young recipients was sufficient to impair spatial learning and memory, accompanied by altered expression of proteins involved in synaptic plasticity and the acquisition of an aging-like microglial phenotype in the hippocampus. Conversely, FMT administered from healthy or young-trained donors demonstrated potent neuroprotective effects. Dong et al[93] showed that FMT effectively enhanced cognitive function in rats with traumatic brain injury by reversing pathological damage, reducing inflammation, and significantly improving gut and brain barrier integrity via the upregulation of tight junction proteins. Similarly, in mouse models of AD, both Sun et al[94] and Jiang et al[95] confirmed that FMT could alleviate AD-like pathology, including reducing beta-amyloid load and tau phosphorylation, by decreasing neuroinflammation and improving synaptic plasticity. Collectively, these findings validate the microbiome as a critical and modifiable component of the GBA, highlighting the need for early and specific microbial intervention to safeguard the trajectory of cognitive health.

Findings on learning, memory, and synaptic plasticity

Cognitive outcomes: The hippocampus is the primary site for spatial learning and memory, and evidence strongly indicates that inflammatory and dysbiotic signals from the gut converge on this region to impair long-term cognitive function. The gold standard tests for assessing these outcomes – the Morris water maze and Barnes Maze – repeatedly reveal lasting deficits following early-life gut insult[96]. The most direct evidence comes from models simulating pediatric GI disease. Salvo et al[97] demonstrated that acute colitis induced at weaning (postnatal day 21) resulted in long-lasting cognitive deficits (impaired NOR) and anxiety-like behavior that persisted into adulthood (P56), even after the acute intestinal inflammation had resolved. These persistent deficits were correlated with chronic neuroinflammation, reduced neurogenesis in the hippocampus, and enduring gut dysbiosis characterized by depleted butyrate-producing species, thus linking early inflammation to reduced synaptic plasticity and memory formation[97]. Supporting the role of gut-derived signaling molecules, Itoh et al[98] demonstrated that the direct intracerebroventricular administration of vasoactive intestinal peptide, a neuropeptide abundant in the GBA, produced marked impairment of spatial learning and memory in rats during the Morris water maze task.

However, the findings also reveal the complexity of the inflammation-cognition link, suggesting that the timing and nature of the signal are critical. Mygind et al[99] showed that while peripheral inflammation is generally detrimental, the cytokine TNF is paradoxically required for normal spatial learning and memory in male mice under physiological, non-inflammatory conditions (Barnes Maze). This suggests that early gut inflammation impairs cognition not just by introducing high levels of inflammation, but by disrupting the delicate, low-level inflammatory signaling necessary for healthy synaptic dynamics[99]. Furthermore, Noel et al[100] provided a counter-example, finding that maternal GI nematode infection during pregnancy and lactation surprisingly led to an enhancement of spatial memory (Barnes Maze and Object Location tests) in the uninfected juvenile offspring, suggesting that specific maternal gut-immune manipulations can convey a neuroprotective or cognitive-boosting signal to the developing brain. Collectively, these studies confirm that while early GI insult commonly impairs learning and memory, the specific cognitive outcome is determined by the precise balance of inflammatory and modulatory signals transmitted through the GBA.

Reduced recognition memory (novel object preference): Recognition memory, often assessed via the NOR task, relies heavily on the perirhinal and entorhinal cortices but is also critically influenced by hippocampal function. Studies simulating the effects of early-life dysbiosis demonstrate that the gut microbiome preferentially affects this domain of cognition[101]. Fröhlich et al[86] provided key evidence in adult mice, showing that antibiotic-induced gut dysbiosis resulted in impaired NOR memory, while surprisingly leaving spatial memory (assessed by the spatial version of the Morris water maze) intact. This cognitive deficit was linked to systemic changes – specifically, the depletion of bacteria-derived metabolites in the colon and altered circulating lipids – and was associated with brain region-specific dysregulation of crucial signaling molecules, including BDNF and components of the 5-HT transporter and neuropeptide Y systems[86].

Further elaborating on the time sensitivity of this vulnerability, Mosaferi et al[102] found that antibiotic-induced gut microbiota depletion, starting in early adolescence and continuing into adulthood, impaired object recognition memory in otherwise healthy mice. This impairment occurred even without the induction of Alzheimer’s-like pathology, although the antibiotic treatment did significantly exacerbate spatial memory deficits once the Alzheimer’s-like disease was induced later in life. Notably, this recognition memory impairment in the healthy, antibiotic-treated group was correlated with reduced pro-inflammatory cytokine IL-6 in the brain, suggesting that manipulating the early-life microbiota alters the delicate inflammatory set point that underpins normal cognitive function[102]. Collectively, these findings highlight that early-life microbial disruption, often simulated by antibiotic exposure, has a targeted impact on cognitive abilities, showing a particular vulnerability for recognition memory that is mediated by shifts in circulating metabolites and the dysregulation of key neurotrophic and signaling molecules in the brain[103].

Deficits in operant conditioning and fear learning in severe inflammation models: Severe or chronic intestinal inflammation disrupts the neural circuits underlying complex emotional and cognitive control, leading to deficits in fear learning, memory consolidation, and appropriate contextual responding. These outcomes are often assessed using fear conditioning paradigms, which depend heavily on the integrated function of the hippocampus, PFC, and amygdala[104]. To evaluate the impaired contextual control of fear, Matisz et al[105] directly tested the impact of chronic gut inflammation (induced by multiple cycles of DSS) on fear generalization, a hallmark of anxiety disorders. While acute inflammation had no effect, mice subjected to chronic colitis exhibited increased freezing in a previously safe, unpaired context when tested nine days after conditioning. This finding suggests that chronic inflammation impairs a post-training mnemonic process, such as memory consolidation, leading to the generalization of negative associations and engaging fearful responding in inappropriate environments[105].

The mechanisms underlying these deficits involve persistent neuroinflammation that suppresses functional brain activity. Mitchell et al[106] used manganese-enhanced MRI in mice with chronic colitis and found that functional brain activity, as indicated by reduced manganese uptake, was suppressed in the hippocampus. This decline in long-term memory was accompanied by neuroinflammatory signaling (elevated IL-1beta and activated caspases) and increased BBB permeability. They proposed that high-mobility group box 1, a danger-associated molecular pattern released from the inflamed intestine, translocates to the brain and activates microglia, suppressing neuronal function[106]. In addition, Doenni et al[107] demonstrated that early-life inflammation has lasting consequences on emotional regulation circuits. They found that systemic inflammation induced by LPS in early life resulted in delayed fear extinction in adult rodents. Fear extinction – the process of learning that a previously dangerous stimulus is now safe – is an active learning process dependent on the PFC inhibiting the amygdala. Impairment in this process suggests that early inflammatory insults program the brain for persistent anxiety and fear responses later in life[107]. These studies confirm that severe or chronic gut inflammation does not just cause transient cognitive changes but also programs the brain’s fear and memory systems, leading to long-lasting deficits in cognitive control, fear processing, and appropriate contextual responding.

Synaptic plasticity and structural changes

Synaptic plasticity, the ability of synapses to strengthen (LTP) or weaken [long-term depression (LTD)] over time, is the fundamental biological process underlying learning and memory storage. Studies consistently show that peripheral inflammation and gut dysbiosis directly impair this crucial hippocampal mechanism[108]. The systemic inflammatory and metabolic signals arising from the dysfunctional gut directly compromise the structural integrity and functional plasticity of the CNS, particularly the hippocampus. These changes represent the physical and cellular substrate for the observed long-term cognitive and emotional deficits. Peripheral inflammation severely dysregulates the hippocampal processes fundamental to memory: (1) LTP; and (2) LTD[16]. Riazi et al[109] demonstrated that colitis-induced peripheral inflammation leads to a significant reduction in both LTP and LTD in hippocampal slices, an effect driven entirely by a microglia-mediated mirror inflammatory response in the brain. Similarly, Tang et al[110] showed that a pathological microbial signature transferred from AD models was sufficient to significantly shorten LTP and accelerate detrimental processes, such as tau protein phosphorylation.

The functional impairment is mirrored by physical changes at the synapse. Matisz et al[111] found that acute gut inflammation did not change the total number of spines on hippocampal CA1 neurons but did cause a critical shift in their morphology, increasing the relative proportion of immature spines to mature ones. This structural immaturity was accompanied by reduced neural activity (decreased cFos expression) in CA1 neurons, suggesting that even acute colitis promotes a state of hippocampal hypoactivity and reduces the capacity for complex memory processing[111]. Chronic inflammation also disrupts the brain’s supportive and structural elements. Ciampi et al[112] showed that colonic inflammation induces a profound remodeling of the glymphatic system, altering brain fluid dynamics, causing enlargement of the lateral ventricles of the brain, and promoting deposition of waste within the brain parenchyma. This derangement in neuronal-astrocytic communication and clearance mechanisms effectively primes synaptopathy[112]. Furthermore, Zonis et al[113] found that chronic intestinal inflammation led to a persistent decrease in hippocampal neurogenesis and downregulation of neuronal precursor markers, thereby permanently limiting the production of new neurons required for long-term plasticity.

The link between gut inflammation and structural damage is reinforced by deficits in neurotrophic factors. Bercik et al[114] found that chronic intestinal inflammation was associated with a significant decrease in hippocampal BDNF mRNA, a crucial factor for neuronal survival, growth, and synaptic integrity. These collective findings highlight that peripheral inflammation inflicts damage at multiple levels – from reducing the cellular pool of new neurons to impairing synaptic structure and disrupting the brain’s waste-clearance system – thereby permanently altering the architecture of memory and emotional centers[114]. Importantly, this structural and functional remodeling is bidirectional: While the CNS is being compromised, the gut’s communication system is also undergoing pathological changes. Lomax et al[115] concluded that intestinal inflammation causes profound plasticity of the ENS, altering the neurochemical content, excitability, and synaptic properties of enteric neurons. This simultaneous remodeling of the ENS ensures that the chronic inflammatory signals and altered gut function are persistently and aberrantly transmitted to the CNS[115].

Neuroinflammation and myelination

Accumulating evidence from animal models demonstrates that GI inflammation, mirroring conditions such as IBD or gulf war illness, consistently induces neuroinflammatory changes in the CNS and the ENS, reinforcing the significance of the GBA[106,116,117].

CNS neuroinflammation (brain): Experimental colitis, primarily induced by DSS administration, reveals a time-dependent and region-dependent pattern of neuroinflammation. Both acute (7 days) and chronic (29 days) phases of DSS-induced colitis are associated with the activation of brain-resident immune cells. Specifically, acute inflammation leads to increased expression of Iba1 (a marker of activated microglia) and to the induction of pro-inflammatory cytokines such as IL-6 and IL-1beta in the hippocampus[116,117]. Chronic treatment also results in persistent neuroinflammation, often indicated by astrocyte activation (astrogliosis), particularly in the hippocampus[117,118]. Neuroinflammatory marker changes are region-specific; for instance, cyclooxygenase-2 (a pro-inflammatory enzyme) and glial fibrillary acidic protein (a glial marker) are upregulated in the hippocampus, while they are downregulated or transiently upregulated in the amygdala and hypothalamus, respectively[116]. Systemic inflammation, marked by elevated plasma or serum levels of IL-6 and C-reactive protein, precedes or accompanies neuroinflammation[116].

Interestingly, one study using severe combined immunodeficiency mice (lacking adaptive immune cells) with DSS-induced colitis observed that while colonic inflammation peaked early (day 7), neuroinflammation (microglial hyperactivation or increased cytokine levels in the brain) was detected only later, by day 21. This suggests that while colitis-induced neuroinflammation can occur in the absence of adaptive immune cells, their presence might influence the timing or severity of the CNS response[119]. Chronic colitis models show increased BBB permeability, facilitating the transmigration of inflammatory mediators from the periphery to the brain[106]. Chronic colitis leads to elevated high-mobility group box 1 (a damage-associated molecular pattern) levels in both serum and the hippocampus. This release from the inflamed intestine is proposed to move to the brain, where, potentially in conjunction with low-level endogenous LPS, it activates caspase-mediated inflammatory responses (caspase-1, caspase-11, and gasdermin) in microglial cells[106].

A critical outcome of colitis-induced neuroinflammation is the impairment of adult hippocampal neurogenesis, a process vital for memory and mood control. Both acute and chronic colitis models show a significant reduction in the proliferation and differentiation of neuronal precursor cells (nestin, brain lipid binding protein, and doublecortin markers are downregulated)[116,117]. This deficit is linked to the upregulation of the cell-cycle inhibitor p21(Cip1) in the hippocampus during the acute inflammatory phase[116]. Chronic colitis mice exhibit reduced functional brain activity, specifically diminished manganese ion uptake (indicative of Ca²⁺ influx) in the hippocampus, correlating with a decline in long-term memory[106].

ENS neuroinflammation (gut): Neuroinflammation is also observed locally within the gut’s nervous system (ENS), which mediates motility. In a pyridostigmine bromide-induced mouse model of gulf war illness, persistent low-grade enteric neuroinflammation was observed. This included a sustained influx of pro-inflammatory macrophages and elevated cytokine levels near enteric neural stem cells. Persistent enteric neuroinflammation was linked to impaired enteric neural stem cell regenerative potential and maladaptive neuroplasticity, resulting in long-term colonic dysmotility and an imbalance between excitatory and inhibitory motor neurons[120]. The evidence consistently shows that peripheral inflammation is transmitted to the CNS, primarily impacting the hippocampus and leading to cellular and molecular changes (glial activation, cytokine production) that underpin behavioral and cognitive deficits. Furthermore, inflammation can chronically damage the ENS, leading to persistent GI functional impairment[121].

Myelination and myelin impairment: Myelination is a crucial process for regulating motor, sensory, and cognitive functions, with rapid development occurring during early life. Experimental models demonstrate that early gut inflammation and microbiota disruption significantly compromise CNS myelin integrity, resulting in long-term structural and neurobehavioral deficits[122]. Acute inflammatory insults and severe GI diseases in neonates have been directly linked to white matter abnormalities. A severe inflammatory disease of premature infants, NEC in a murine model leads to significant cognitive deficits (Morris water maze impairment) that resemble outcomes in human survivors. This cognitive impairment is associated with a clear myelination deficit (as observed via MRI, electron microscopy, and reduced myelin basic protein expression) and increased microglial activation and oxidative injury in brain regions such as the corpus callosum and hippocampus[123].

In a young pig model, colitis induced by DSS during the critical postnatal period (14-18) altered white matter structure. Neuroimaging revealed that DSS-treated groups had the lowest MWF values – a quantitative measure of myelin – across the whole brain and cortices[80]. Colitis also altered diffusion tensor imaging metrics (e.g., fractional anisotropy), indicating changes in fiber organization, particularly in the hippocampus and thalamus[80]. Disrupting the gut environment during early developmental windows can cause persistent myelin deficiencies in adulthood, often through indirect mechanisms. Neonatal peripheral infection (e.g., Escherichia coli at P3) causes acute, transient brain inflammation. However, this transient event leads to marked hypomyelination and a reduction of oligodendrocytes in subcortical white matter and the motor cortex in juveniles and young adults[124]. The proposed mechanism involves the infection altering transcripts related to iron homeostasis (e.g., increasing hepcidin and decreasing ferroportin), leading to the neuronal sequestration of iron (an element critical for oligodendrocyte maturation) at a time point preceding peak myelination[124].

Disrupting the gut microbiota in early life using broad-spectrum antibiotics has profound, long-lasting consequences on myelin. Depletion during the pre-weaning stage in mice leads to persistent CNS myelin impairment in adulthood (up to 28.1% demyelination)[125]. When challenged with a demyelinating injury (cuprizone), this early-life dysbiosis significantly exacerbates demyelination and severely impairs subsequent remyelination, linking a healthy early microbiota to efficient myelin repair and neuroprotection[126]. Conversely, another study showed that neonatal antibiotic administration (from P7 to P23) increased myelin-related gene and protein expression (myelin basic protein, SOX10, myelin regulatory factor) and confirmed increased myelination in the PFC of adult mice, which was associated with cognitive deficits[122]. This paradoxical finding suggests that the specific timing, antibiotic cocktail, and brain region targeted may determine whether the outcome is hypomyelination or dysregulated, excessive myelination.

The crucial role of the gut microbiota in mediating these effects suggests potential therapeutic interventions using bacterial metabolites, such as the SCFA butyrate, which is effective in reversing the adverse outcomes of neonatal antibiotic-induced dysbiosis, including restoring intestinal physiology, improving behavior, and normalizing altered myelination in the PFC[122]. However, in the young pig colitis model, oral supplementation with a butyrate conjugate (gamma-cyclodextrin-encapsulated tributyrin) failed to ameliorate the adverse effects on fiber organization and reduced MWF, indicating that not all nutritional interventions are protective in the context of active inflammation[80]. These studies reveal complex, time-sensitive mechanisms through which early gut inflammation, infection, or dysbiosis leads to lasting disturbances in CNS white matter development and integrity.

Animal models provide powerful mechanistic insight by allowing precise control of early-life gut injury, timing, and environmental factors. They clearly demonstrate causal pathways linking intestinal inflammation to microglial activation, impaired synaptic plasticity, and deficits in learning and memory[127]. These models also enable invasive tissue analyses and microbiome manipulation not possible in human infants. However, developmental timelines of the gut, immune system, and brain differ across species, limiting direct translation[128]. Figure 5 shows different animal modes of early GI inflammation and their effects on brain development. Many experimental models induce more uniform or severe inflammation than typical human disease, potentially overstating effects. Behavioral assays cannot fully mimic human executive or social cognition, and microbiome composition varies across laboratory environments. Taken together, animal studies provide robust causal evidence linking early intestinal injury and dysbiosis to long-term neurodevelopmental consequences. However, translation to human infants should be done cautiously, integrating epidemiological and clinical data[129]. Table 2 shows the different strengths and limitations of animal models.

Figure 5 How early gut injury impairs brain development: Insights from animal models. It summarizes key findings from animal models demonstrating how early gastrointestinal injury disrupts neurodevelopment. Experimental paradigms – including necrotizing enterocolitis models, chemically induced colitis, and dysbiosis models (germ-free rearing, antibiotic exposure, or fecal microbiota transfer) – produce intestinal inflammation, barrier disruption, and microbial imbalance during critical developmental windows. These gut insults lead to measurable neurodevelopmental consequences, including impaired learning and memory (e.g., deficits in spatial navigation and recognition tasks), altered synaptic plasticity (reduced long-term potentiation, changes in dendritic spine morphology), and increased neuroinflammation characterized by microglial activation and elevated cytokine signaling in the brain. Together, these animal data provide mechanistic evidence for a causal pathway linking early-life gut injury to long-term cognitive and neural dysfunction. FMT: Fecal microbiota transplantation; LTP: Long-term potentiation.

Table 2 Strengths and limitations of animal models.

Strengths	Limitations
Allow direct control of diet, microbes, and inflammatory triggers	Rodent neurodevelopmental timelines differ from humans
Enable mechanistic dissection of the gut-immune-brain axis	Necrotizing enterocolitis and colitis models do not perfectly replicate human disease complexity
Permit invasive measurements: Cytokine profiling, neural imaging, electrophysiology	Microbiome composition differs significantly between species
Facilitate genetic manipulation (e.g., knockout mice)	Behavioral tests may not fully translate to human cognition or socio-emotional behavior
Allow testing of causal relationships via fecal microbiota transplantation, antibiotics, germ-free models	Artificial induction of inflammation may exaggerate severity relative to human infants

EVIDENCE FROM HUMAN STUDIES

Human research increasingly supports the concept that early-life GI inflammation is a significant and lasting risk factor for cognitive and neuropsychological impairment. Across diverse pediatric populations – including preterm infants with NEC, children recovering from severe enteric infections, those with celiac disease (CD), and young patients with IBD – studies consistently demonstrate measurable deficits in cognition, executive functioning, academic performance, and emotional regulation[130]. Longitudinal cohort data reveal that these effects often persist beyond the acute illness phase, reflecting the enduring impact of early inflammatory insults on the developing brain. Furthermore, several groups show elevated rates of psychiatric vulnerability, suggesting that disturbances in the gut-brain-immune axis during critical developmental windows may predispose children to long-term neurobehavioral disorders[131]. Together, these human studies complement findings from animal models and highlight a clinically meaningful link between early GI health and lifelong cognitive trajectories.

Preterm infants and NEC survivors: Rates of developmental delay, cognitive impairment

Studies tracking the long-term outcomes of infants who survive NEC – a severe inflammatory GI disease – consistently demonstrate a significant and enduring burden of NDI that is independent of prematurity alone. Comprehensive meta-analyses estimate the NDI incidence among NEC survivors at around 40%, with adjusted odds ratios confirming that NEC is significantly associated with NDI compared with non-NEC preterm infants (Table 3)[132-138]. Long-term follow-up studies by Mondal et al[134] and Arnold et al[135] confirm the persistence of these deficits, with cohorts assessed at school age showing that 61% of survivors have neurological impairment, and up to 49% have significant neurodevelopmental delay. The most prevalent impairments are cognitive and functional. Mondal et al[134] identified cognitive impairment as the most common long-term issue (56%), leading to high rates of special education needs and learning difficulties. At a mean school age of 9 years, Roze et al[136] showed that children with NEC had a lower mean total IQ (86 vs 97) and specific deficits in attention and visual perception. Meta-analyses by Wang et al[133] link NEC to an increased risk of motor, cognitive, and language development delays, as well as attention deficits.

Table 3 Summary of findings: Preterm Infants and necrotizing enterocolitis survivors.

Ref.	Study type/follow-up age	Primary finding on NDI	Key associated deficits/brain pathology
Wang et al[133], 2024	Systematic review and meta-analysis (corrected age > 12 months)	Significant association between NEC and increased odds of NDI (adjusted odds ratio: 1.89).	Increased risk of severe brain injury: IVH and PVL. Severity matters: Surgical NEC carries a higher NDI risk than medical NEC
Matei et al[132], 2020	Systematic review and meta-analysis (n = 2403 NEC infants)	High NDI incidence: 40% (median interquartile range 28%-64%). Severity correlates: NDI incidence is higher in surgical NEC (43%) vs medical NEC (27%)	Most common NDI: Cerebral palsy (18%). NEC is associated with increased incidence of IVH and PVL compared to preterm controls
Mondal et al[134], 2021	Retrospective cohort (average follow-up: 11.2 years)	High burden: 61% of survivors had neurological impairment	Cognitive impairment was the most common deficit (56%), followed by motor (33%). High rates of special education needs and learning difficulties
Roze et al[136], 2011	Case-control cohort (mean age 9 years)	Borderline or abnormal functional scores: Mean total intelligence quotient was lower (86 vs 97 in controls)	Specific deficits in attention and visual perception. Children requiring surgery were at the highest risk for adverse outcomes
Shin et al[137], 2021	Retrospective cohort (preterm infants with surgical NEC vs SIP)	NEC group had more prevalent abnormal findings at 36 months in motor (gross and fine), cognitive, and social domains compared to SIP survivors	Slower head growth in surgical NEC group compared to SIP group
Arnold et al[135], 2010	Retrospective cohort (long-term follow-up: 39 months)	49% of long-term survivors had significant neurodevelopmental delay	The 20% had severe neurological deficits, including auditory and visual impairment. Highlights the ongoing morbidity risk
Hansen et al[138], 2019	Historic cohort study (school age)	Increased rates of abnormal behavioral scores and cerebral palsy observed, but differences were statistically insignificant after adjustment for confounders	Provides a counterpoint to the consensus, suggesting the long-term effects on behavioral and NDI scores may be moderate and of limited clinical importance

It summarizes the clinical evidence from systematic reviews and longitudinal cohort studies quantifying the enduring neurodevelopmental burden in survivors of necrotizing enterocolitis (NEC). The findings consistently demonstrate that NEC, particularly severe (surgically managed) NEC, significantly increases the risk for long-term neurodevelopmental impairment, cognitive deficits, and major structural brain lesions (intraventricular hemorrhage, periventricular leukomalacia), indicating that severe gut inflammation in the neonatal period critically compromises central nervous system development. IVH: Intraventricular hemorrhage; NDI: Neurodevelopmental impairment; NEC: Necrotizing enterocolitis; PVL: Periventricular leukomalacia; SIP: Spontaneous intestinal perforation.

A critical clinical finding is the correlation between the severity of the intestinal disease and the risk of NDI. Multiple studies show that infants who required surgical management for NEC carry a significantly higher risk of NDI (up to 43% incidence) compared to those managed medically. This severity-dependent risk suggests that the intensity of the inflammatory, metabolic, and/or infectious insult originating from the gut dictates the degree of neurological damage[132,133,137]. These functional deficits may also be accompanied by structural brain abnormalities. NEC is associated with a significantly higher risk of severe brain lesions, including intraventricular hemorrhage and periventricular leukomalacia, compared to preterm infants without NEC[132,133]. In summary, most clinical evidence confirms that survivors of NEC constitute a distinct risk group for long-term cognitive and functional deficits. However, the study by Hansen et al[138] which included a large cohort assessed at school age, offered a contrasting view, reporting that the increased risks of behavioral and NDIs were statistically insignificant and moderate in magnitude, suggesting that the long-term clinical impact on behavioral measures may be less severe than commonly perceived.

Children with severe early enteric infections

The most robust human evidence linking early gut health to long-term cognitive outcomes stems from extensive, longitudinal cohort studies (Table 4)[139-145], particularly the etiology, risk factors, and interactions of enteric infections and malnutrition and the consequences for child health and development (MAL-ED) study. These studies have established a direct, lasting, and statistically independent negative association between the burden of early childhood enteric infections and cognitive deficits in later life[139,140]. Repeated enteric infection is a significant, standalone predictor of diminished cognitive function. Longitudinal follow-up of children in a Brazilian shantytown demonstrated that early childhood diarrhea (ECD) was a significant inverse predictor of test of nonverbal intelligence scores 4 to 7 years later, even after controlling for key confounders such as maternal education and malnutrition[141,142]. Pinkerton et al[142] further confirmed this, showing that ECD remained a significant independent predictor of both test of nonverbal intelligence and Wechsler Intelligence Scale for Children-Third Edition Coding scores in later childhood, suggesting that the effect is not merely secondary to stunting or low weight. The negative effects span multiple neurodevelopmental domains. In low-birth-weight infants, the number of diarrheal episodes was negatively correlated with composite scores in all three domains assessed by the Bayley-III scale: Cognitive, motor, and language scores[143]. The MAL-ED study concluded that higher rates of enteropathogen detection and increased days with illness were negatively associated with children’s cognitive development at 24 months[140].

Table 4 The longitudinal human cohort studies linking the burden of early childhood enteric infections to long-term cognitive and neurodevelopmental deficits.

Ref.	Population/cohort	Exposure	Key findings on learning, memory, and intelligence quotient
Niehaus et al[141], 2002 and Pinkerton et al[142], 2016	Cohort of children from a Brazilian shantytown (followed to 5.6-12.7 years)	ECD in the first 2 years of life	ECD is a significant inverse predictor of later childhood cognitive function (lower test of nonverbal intelligence and Wechsler Intelligence Scale for Children-Third Edition Coding scores). This effect was independent of malnutrition (stunting/wasting) and maternal education
MAL-ED Network Investigators[139], 2014	Multi-site longitudinal birth cohort	Enteropathogen infection causing intestinal inflammation	Established the core study hypothesis: Enteropathogen infection leads to intestinal inflammation, which causes growth faltering and deficits in cognitive development
MAL-ED Network Investigators[140], 2018	Longitudinal birth cohort in 6 low/middle-income countries (birth to 24 months)	Higher rates of enteropathogen detection and days with illness	Negatively associated with lower cognitive scores at 24 months. Higher illness rates were linked to lower hemoglobin concentrations, which in turn predicted lower cognitive scores
Upadhyay et al[143], 2021	low birth weight infants (North India)	Increased number of diarrheal episodes in the first year	Negatively influenced composite scores in all three domains assessed: Cognitive, motor, and language at 12 months. Linear growth and diarrheal prevention are crucial factors
El Wakeel et al[144], 2022	Malnourished, stunted children (Egypt, age 1-10 years)	Intestinal inflammation (high fecal markers) and micronutrient deficiency (zinc, iron, vitamin D)	Showed impaired neurocognitive and psychomotor functions. Cognitive performance was negatively correlated with both fecal and serum inflammatory markers
Streit et al[145], 2021	Children (n = 380, age 45 months)	Fecal microbiome profile (relative abundance of specific taxa)	Microbiome profile was significantly associated with cognitive functioning (Wechsler Preschool and Primary Scale of Intelligence-Third Edition scores). Found a strong inverse correlation between cognitive scores and a genus related to Enterobacter asburiae

It aggregates evidence from large-scale human cohort studies (like MAL-ED) that track the long-term relationship between early childhood gut health (measured by diarrheal frequency, pathogen detection, and intestinal inflammation markers) and neurodevelopmental outcomes in later childhood (measured by intelligence quotient, cognitive, motor, and language scores). Findings highlight that the negative impact of early enteric disease on cognitive function is often independent of malnutrition, suggesting a direct gut-brain axis mechanism. ECD: Early childhood diarrhea.

Cognitive impairment appears to be mediated by the persistent consequences of gut inflammation. El Wakeel et al[144] found that malnourished, stunted children with impaired cognitive and psychomotor functions exhibited significantly increased fecal markers of intestinal inflammation (e.g., alpha-1 antitrypsin, neopterin) and systemic inflammation (alpha-1-acid glycoprotein), alongside deficiencies in critical micronutrients like zinc, iron, and Vitamin D. These inflammatory markers showed negative correlations with cognitive and motor functions[144]. The MAL-ED cohort also identified a critical interaction: Higher illness rates were associated with lower hemoglobin concentrations (a proxy for iron status), which, in turn, were associated with lower cognitive scores[140].

Further cementing the direct link, Streit et al[145] found that the overall gut microbiome profile was significantly associated with cognitive functioning (measured by the Wechsler Preschool and Primary Scale of Intelligence-Third Edition) in children at 45 months of age. A strong inverse correlation was observed between cognitive scores and the relative abundance of a genus closely related to Enterobacter asburiae, suggesting that specific detrimental bacterial taxa – potentially introduced during infectious episodes – may directly contribute to cognitive risk[145]. These longitudinal findings emphasize that the integrity of the gut microbiome and intestinal barrier function in early life are profound determinants of a child’s full cognitive potential, with enteric infections representing a critical and often independent risk factor for diminished IQ and school performance.

CD and cognitive outcomes: Effects of delayed diagnosis, gluten exposure, and micronutrient deficiency

CD provides a compelling clinical example of the GBA, where chronic intestinal inflammation and antigen exposure directly translate into measurable neurological and cognitive deficits. Patients frequently report neurological symptoms, often described as “brain fog”, alongside specific cognitive impairments, ataxia, and headaches[146,147]. A nationwide survey by Edwards George et al[148] in the United States found that most individuals with CD (89%) and non-celiac gluten sensitivity (95%) reported experiencing gluten-induced neurocognitive impairment symptoms after gluten exposure. The most common descriptors were difficulty concentrating, forgetfulness, and grogginess[148]. Using objective testing, Croall et al[149] confirm these subjective reports. They found that CD patients had significant deficits in complex tasks, including reaction time and underperformance in visual and verbal memory compared to healthy controls. In addition, Casella et al[150] found that patients with CD had impaired scores on tests of attention, processing speed, and executive functions, such as the Trail Making Test and Semantic Fluency. Moreover, a meta-analysis by Beas et al[151] confirmed a significant association between CD and cognitive impairment.

Studies suggest that prolonged, untreated exposure to gluten may lead to irreversible cognitive damage. Casella et al[150] found that elderly patients with CD on a long-term gluten-free diet (GFD) still showed worse cognitive performance than controls, leading researchers to stress the importance of early diagnosis to minimize diagnostic delay and prolonged exposure to gluten that may adversely and irreversibly affect cognitive function. However, many cognitive deficits are reversible[150]. A longitudinal pilot study by Lichtwark et al[152] found that newly diagnosed CD patients showed significant improvement on cognitive tests assessing verbal fluency, attention, and motor function over 12 months of GFD adherence. Critically, this cognitive improvement strongly correlated with mucosal healing (Marsh scores) and decreases in tissue transglutaminase antibodies, directly linking the resolution of gut pathology to improved brain function. Another study by Croall et al[149] found that deficits in visual and verbal memory were established at the point of diagnosis and generally stabilized with treatment, suggesting that the benefit of GFD is primarily preventive against further decline, though some deficits may remain.

In another study by Croall et al[153], which examined population-based data from the UK Biobank, revealed that CD patients exhibited significant deficits in reaction time and increased rates of anxiety and depression, concurrent with widespread white matter changes (increased axial diffusivity) in the brain, supporting the concept of an associated neurological pathology. In addition, neurophysiological studies by Lanza et al[146] and Pennisi et al[147], using transcranial magnetic stimulation, suggest a profile of a “hyperexcitable celiac brain”, a finding associated with both degenerative and vascular dementia. This hyperexcitability partially reverses after a long-term GFD, reinforcing the neuroprotective effect of adherence to the diet[146,147]. Moreover, Hu et al[154] showed that progressive cognitive decline associated with CD often coincides with micronutrient deficiencies, including folate, vitamin B12, and vitamin E. Although supplementation alone may not constantly improve neurological symptoms, the high prevalence of these deficiencies highlights the impact of chronic malabsorption on brain health[154]. While an association exists, CD does not appear to increase the overall risk for dementia in later life. However, population-based studies suggest that the increased risk may be confined to vascular dementia (hazard ratio = 1.28) during the overall observation period, though not to Alzheimer’s dementia[155]. Table 5 summarizes studies on the relationship between CD and cognitive/neurological outcomes[146-155].

Table 5 Summary of studies: Celiac disease and cognitive/neurological outcomes.

Ref.	Study type/population	Key cognitive/symptomatic findings	Key structural/mechanistic findings	Effect of GFD
Edwards George et al[148], 2022	Nationwide online survey (CD and NCGS patients)	89% of CD and 95% of NCGS reported GINI (brain fog). Most common symptoms: Difficulty concentrating, forgetfulness, and grogginess	High prevalence suggests GINI is a genuine and common symptom complex	N/A (focus on symptoms after exposure)
Croall et al[149], 2020	United Kingdom biobank (population-based cohort)	Significant deficits in reaction time. Increased rates of self-reported anxiety and depression	Widespread white matter changes (increased axial diffusivity) observed via diffusion tensor imaging	N/A (cross-sectional comparison)
Croall et al[153], 2020	Pilot study (newly diagnosed vs established CD vs controls)	Underperformed relative to controls in visual and verbal memory (established at diagnosis)	Dysfunction appears established at the point of diagnosis	Cognitive deficit stabilizes but does not necessarily fully reverse, implying a benefit against further decline
Lichtwark et al[152], 2014	Longitudinal pilot study (newly diagnosed CD, 12-month follow-up)	Cognitive tests (verbal fluency, attention, motoric function) showed significant improvement over 12 months	Cognitive improvement strongly correlated with mucosal healing (Marsh score) and decreased tissue transglutaminase antibodies	Improved cognitive performance in parallel with resolution of intestinal inflammation
Casella et al[150], 2012	Case-control study (elderly CD patients on GFD vs controls)	Worse cognitive performance in CD patients despite long-term GFD, including lower scores on Mini Mental Test Examination, Semantic Fluency, and Digit Symbol Test	Emphasizes the risk of irreversible effects from diagnostic delay and prolonged gluten exposure	Suggests long-term GFD may not fully restore function if diagnosis is late
Hu et al[154], 2006	Case series (patients with cognitive decline associated with CD)	Presentation included amnesia, acalculia, and confusion. Average impairment was in the moderately impaired range. Ataxia common (10/13 patients)	Frequent deficiencies in folate, vitamin B12, or vitamin E. Brain magnetic resonance imaging often showed non-specific white matter hyperintensities	Three patients improved or stabilized cognitively with gluten withdrawal
Lanza et al[146], 2018 and Pennisi et al[147], 2017	Review of neurophysiological studies (TMS, electroencephalography)	Adult CD patients often report “brain fog” to overt dementia	Evidence suggests a profile of “hyperexcitable celiac brain” (measured by TMS) which is a feature also reported in degenerative/vascular dementia	Hyperexcitability partially reverts back after long-term gluten restriction
Lebwohl et al[155], 2016	Population-based cohort study (age $\geq$ 50 years)	No increased overall risk for dementia in CD patients over the long term	Subgroup analysis showed a non-significant trend for increased risk of vascular dementia (hazard ratio = 1.28) but not Alzheimer’s	N/A (population risk analysis)
Beas et al[151], 2024	Systematic review and meta-analysis	Confirmed a significant association between CD and insomnia. Provided valuable insight into studies on cognitive impairment	N/A (meta-analysis of existing literature)	N/A (meta-analysis)

It summarizes key findings from clinical studies investigating the neurological and cognitive manifestations of celiac disease. The evidence consistently demonstrates objective cognitive deficits (memory, reaction time, attention), subjective symptoms (brain fog/gluten-induced neurocognitive impairment), and associated brain pathology (white matter changes, cerebral hyperexcitability). Findings highlight the critical importance of a timely diagnosis, as the effectiveness of the gluten-free diet may vary between reversing new symptoms and stabilizing or failing to fully resolve deficits established during chronic, untreated inflammation. CD: Celiac disease; GINI: Gluten-induced neurocognitive impairment; GFD: Gluten-free diet; NCGS: Non-celiac gluten sensitivity; N/A: Not applicable; TMS: Transcranial magnetic stimulation.

Pediatric IBD and neurocognition: Executive function deficits, psychosocial burden, and inflammatory markers

IBD, including Crohn’s disease and ulcerative colitis (UC), is a chronic systemic inflammatory condition that affects the developing and mature brain, leading to specific cognitive and significant psychosocial comorbidities in pediatric and adolescent populations. Research[156] in IBD patients suggests impairments are subtle but persistent, often affecting mental processing speed and executive functions, even in periods of clinical remission (Table 6)[157-161]. Studies comparing adult IBD patients in remission to controls found significant impairment across multiple cognitive domains. Patients demonstrated statistically significant longer total test-solving time on tasks assessing convergent thinking (mathematics), perceptual abilities (signal discrimination), and sophisticated operative thinking. This suggests a quantifiable deficit in mental processing speed and mental endurance[157]. Castaneda et al[158] showed that adolescents with IBD, specifically those in the acute phase, testing revealed they made more perseverative errors in verbal memory tasks (California Verbal Learning Test), an outcome often associated with deficits in executive functioning and frontal lobe pathology. While no significant cognitive deficits were found compared to controls with non-acute juvenile idiopathic arthritis, this subtle difference suggests a specific vulnerability during active inflammation[158]. Cognitive impairment, particularly impaired attentional performance and response inhibition, has been identified as a stable feature of adult Crohn’s disease patients, even when they are in clinical remission. Clarke et al[159] showed, in a prospective study, that this impairment persisted over a 6-month follow-up period, indicating that the functional impact on cognition is evident despite the absence of active gut symptoms.

Table 6 Summary of studies: Pediatric inflammatory bowel disease and neurocognitive outcomes.

Ref.	Population/condition	Key cognitive/functional findings	Key mechanistic/psychosocial findings	Implication/conclusion
Tadin Hadjina et al[157], 2019	Adult IBD patients (n = 60) vs controls	Impaired neurocognitive and psychomotor function: Significantly longer total test-solving time in tests for convergent thinking, perceptive abilities, and complex operative thinking	Deficits in mental processing speed and mental endurance	IBD patients show objective impairment in cognitive and psychomotor speed, even in adulthood
Clarke et al[159], 2020	Adult Crohn’s disease and UC patients in clinical remission (prospective)	Impaired attentional performance was a stable feature of Crohn’s disease patients over a 6-month period (UC patients were unaffected)	Consistently elevated plasma IL-6 and kynurenine-to-tryptophan ratio; blunted cortisol awakening response. No correlation between biochemical markers and cognitive impairment was found, but the markers indicated ongoing, subclinical inflammation	Impaired cognitive function is a stable feature of Crohn’s disease, likely driven by persistent inflammatory/metabolic changes
Castaneda et al[158], 2013	Adolescents with IBD (n = 34) vs peers with JIA	IBD group, especially those in the acute phase, made more perseverative errors in the California Verbal Learning Test (verbal memory), suggesting executive function deficits	IBD group had more depressive symptoms than JIA group (especially with acute illness). Depressive symptoms were not related to the cognitive difference	Acute IBD may cause mild verbal memory/executive function problems; psychosocial burden (depression) is significant
Herzer et al[160], 2011	Adolescents with IBD (n = 62) and their caregivers	Poorer HRQOL across multiple dimensions (emotional, social, total HRQOL)	Adolescent depressive symptoms fully mediated the relation between parent distress (illness-related stress) and the youth’s poorer HRQOL	Psychosocial factors (parental distress-adolescent depression) are crucial targets for intervention to improve HRQOL
Friedman et al[161], 2020	Children exposed to maternal IBD in utero (followed to 7 years)	Children exposed to IBD in utero scored similarly to unexposed children on survey-based tools assessing motor and cognitive development	No obvious differences in language, motor, or cognitive/behavioral scores at 7 years	Reassuring data suggesting that maternal IBD exposure alone does not increase the risk of long-term neurodevelopmental delay

It summarizes human-based findings concerning the cognitive, functional, and psychosocial impacts of inflammatory bowel disease on children and adolescents. Studies demonstrate that inflammatory bowel disease, particularly Crohn’s disease, is associated with persistent deficits in executive functions and mental processing speed, often linked to systemic inflammation (interleukin-6, kynurenine-to-tryptophan ratio). Critically, the psychosocial burden is high, with adolescent depressive symptoms serving as a key mediator between parental distress and poor health-related quality of life. IBD: Inflammatory bowel disease; HRQOL: Health-related quality of life; JIA: Juvenile idiopathic arthritis; UC: Ulcerative colitis.

The cognitive deficits in IBD are potentially linked to ongoing, subclinical inflammation that breaches the brain-gut barrier, consistent with the mechanistic findings in animal models[162]. The persistence of cognitive impairment in Crohn’s disease remission is associated with consistently elevated plasma concentrations of the proinflammatory cytokine IL-6 and an increased Kyn:Trp ratio (a marker of immune system activation of the kynurenine pathway)[163]. Clarke et al[159] showed that an increased (Kyn:Trp) ratio diverts tryptophan away from 5-HT production and towards neurotoxic metabolites, providing a plausible mechanism for both cognitive and mood changes. In the same cohort of Crohn’s disease patients, a significantly blunted cortisol awakening response was observed. This finding suggests a dysregulation of the HPA axis, linking the chronic inflammatory state to stress response and potentially contributing to cognitive and emotional vulnerabilities[159].

IBD introduces a significant psychological burden, where mood disorders mediate the connection between illness severity and quality of life. Adolescents with IBD, especially those with acute illness, have significantly higher scores on measures of depressive symptoms compared to peers with juvenile idiopathic arthritis[158]. Adolescent depressive symptoms were found to fully mediate the relationship between parent distress (related to the child’s illness) and the adolescent’s poorer health-related quality of life across domains like general well-being and emotional and social functioning. This mediation was independent of disease severity, highlighting that psychological factors are primary drivers of reduced quality of life in pediatric IBD[160]. Conversely, research on children exposed to IBD in utero offers reassuring results, suggesting that maternal IBD alone does not appear to impart a lasting neurodevelopmental risk. An extensive prospective study using the Danish National Birth Cohort found that children exposed to maternal IBD in utero performed similarly to unexposed children on survey-based tools assessing motor and cognitive development at 6 months, 18 months, and 7 years of age, after adjusting for confounders such as preterm birth[161].

Psychiatric vulnerabilities: Anxiety, depression, and ADHD

Many studies observed a high prevalence of some psychiatric comorbidities in pediatric and adult IBD populations, highlighting the connection between gut inflammation and specific mental health outcomes. Studies consistently report high rates of anxiety and depression in IBD patients, often correlating with active disease, but sometimes persisting even in remission. The lifetime prevalence of psychiatric disorders, particularly anxiety and depression, is significantly higher in IBD patients compared to the general population. This relationship is often bidirectional, where psychological distress can precipitate IBD flares, and the chronic nature of IBD drives mental health deterioration. As noted in the previous section, anxiety and depression symptoms are significantly elevated in CD patients and adolescents with IBD, even when controlling for other factors[149,153,158]. The chronic inflammatory state inherent to IBD involves elevated cytokine levels (such as IL-6) and activation of the kynurenine pathway, which diverts the neurotransmitter precursor tryptophan away from 5-HT synthesis (mood regulation) toward neurotoxic metabolites. This pathway provides a direct, measurable link between peripheral inflammation and central mood disturbances[164].

Growing evidence links early gut dysfunction and inflammation to conditions like ADHD, which involves executive function deficits and behavioral control issues. A large population-based study found that individuals with IBD, particularly those with pediatric-onset Crohn’s disease, have a higher incidence of neurodevelopmental disorders, including ADHD[165]. This is consistent with the finding of specific executive function deficits (e.g., in attention and memory) in adolescent IBD patients. Studies of children with ADHD have consistently reported alterations in their gut microbiota composition compared to controls[166]. Although the relationship is complex, the dysbiosis involves changes in the relative abundance of key bacterial taxa (e.g., reduced Faecalibacterium, altered Bifidobacterium, and Bacteroides), suggesting a microbial contribution to the neurodevelopmental trajectory[167].

The long-term risk of neurodevelopmental delay has been confirmed in survivors of severe early gut insults. For instance, children who survive NEC, a condition marked by severe gut damage and sepsis, show high rates of long-term cognitive impairment and special education needs, which often overlap with symptoms of attention deficits[134]. It is clearly observed that the presence of a strong connection between the psychiatric and behavioral outcomes and the foundational principle of the GBA reinforces the concept that poor gut health is not limited to physical disease but extends to CNS vulnerabilities.

CLINICAL IMPLICATIONS

Need for early recognition and screening

The demonstrated link between early-life gut dysfunction and long-term neurocognitive deficits mandates a shift toward proactive screening and multidisciplinary management in vulnerable patient populations. Severe or chronic GI pathology in the first few years of life must be recognized as a critical risk factor for developmental vulnerabilities, extending far beyond the immediate morbidity of the disease (Table 7).

Table 7 Red flags for neurocognitive risk in gut disorder.

Patient group	Gut-related red flag (trigger)	Actionable neurocognitive risk
Infancy/neonatal	Necrotizing enterocolitis, especially requiring surgical intervention	High risk (40% of global neurodevelopmental impairment, intraventricular hemorrhage, periventricular leukomalacia, and attention/executive function deficits
Infancy/early childhood	High burden of early severe enteric infections or chronic diarrheal illness (especially coupled with stunting/low growth)	Long-term risk of Lower intelligence quotient/cognitive scores and poor school performance (independent of malnutrition)
Children/adolescents	Active or newly diagnosed inflammatory bowel disease (especially Crohn’s disease), even with mild symptoms	Deficits in mental processing speed, executive function (attention, memory), and high comorbidity of depression/anxiety
All ages	Unexplained chronic symptoms: Brain fog, memory lapse, severe fatigue, or new-onset psychiatric symptoms (e.g., anxiety, depression)	Possible underlying celiac disease, requiring assessment for cognitive stabilization and screening for white matter changes

Survivors of NEC, who face a 40% incidence of NDI and specific cognitive deficits, require mandatory, long-term neurodevelopmental follow-up (NDFU). The severity of the initial gut injury (especially requiring surgery) serves as a primary red flag indicating the highest risk for future brain injury (intraventricular hemorrhage, periventricular leukomalacia) and functional impairment (cognitive delay, attention deficits)[132]. Longitudinal studies (e.g., MAL-ED) prove that a high burden of acute or persistent diarrhea in infancy is a statistically independent risk factor for diminished IQ and executive function in later childhood[140]. Clinicians should view recurrent severe diarrhea not merely as an acute illness, but as a chronic metabolic and inflammatory insult that compromises neurodevelopment. The finding that cognitive deficits in CD are often established early and persist despite long-term dietary adherence highlights the detrimental effect of diagnostic delay. Unexplained chronic symptoms like persistent “brain fog”, severe fatigue, or functional decline should prompt thorough investigation for underlying gut pathology, including CD and other inflammatory conditions[149,153].

Integrating expertise across specialties is essential to mitigate the long-term sequelae of the GBA dysfunction. The standard of care for high-risk populations (such as NEC survivors) should include NDFU clinics that involve pediatric gastroenterologists, neurologists, and developmental specialists (e.g., psychologists, occupational therapists). This integrated approach ensures comprehensive monitoring and timely intervention for cognitive, motor, and emotional delays[168].

For children and adolescents with established chronic GI conditions (IBD, CD), routine clinical assessment should incorporate standardized screening for common psychiatric comorbidities, including depression, anxiety, and ADHD symptoms. The high prevalence of these conditions and their direct link to inflammatory markers (IL-6, Kyn:Trp ratio) necessitate this proactive, holistic approach[169]. Therapeutic strategies should not be narrowly focused on intestinal healing. Recognizing that persistent systemic inflammation and intestinal permeability directly drive neurocognitive decline requires a goal of not just clinical remission, but deep or mucosal healing, coupled with monitoring for micronutrient deficiencies (e.g., iron, zinc, B-vitamins) that further jeopardize brain health[170].

Screening and monitoring

Based on the evidence that early gut insults result in specific, measurable neurocognitive and psychosocial deficits, standardized screening and continuous monitoring are essential for high-risk patient groups. Structured, longitudinal follow-up is critical for patients who have suffered early, severe GI pathology, ensuring prompt identification and remediation of emerging issues. Infants who survive NEC, especially those who required surgical intervention, must be enrolled in comprehensive NDFU programs extending through school age. Given the high rates of cognitive impairment (40%) and specific motor deficits, monitoring should not cease once gross motor milestones are met. Follow-up must include a formal assessment of executive function, attention, and cognitive IQ to detect the subtle yet debilitating long-term deficits observed in this population[134,136]. Children with a history of recurrent, severe ECD (as highlighted by the MAL-ED study) should be systematically screened for poor growth indicators (stunting, wasting) and subtle cognitive delays that manifest as lower IQ and poor school performance[171]. This is particularly relevant in resource-poor settings where the burden of infection is highest. Monitoring should be longitudinal, covering multiple critical developmental periods (e.g., 12-month, 24-month, and 36-month corrected age, and 7-9 years for school-age cognitive function), as impairments can emerge or become more apparent with increasing cognitive demand[142,149]. Screening protocols should move beyond traditional physical assessments to capture the full spectrum of GBA-related vulnerabilities. For conditions like CD, screening for cognitive symptoms like “brain fog” should be routine, both at diagnosis and during follow-up, using patient-reported outcome measures. The goal of GFD adherence must be reinforced not only for gut healing but also as a neuroprotective measure to stabilize cognitive function and prevent further white matter damage[149,160]. Table 8 summarizes the clinical recommendations for screening and monitoring in GBA -related disorders.

Table 8 Clinical Recommendations for screening and monitoring in gut-brain axis-related disorders.

Domain of assessment	Rationale for screening	Relevant population	Key clinical outcome measures
Cognitive function	To identify deficits in processing speed, memory, and attention linked to systemic inflammation and micronutrient deficiencies (e.g., in IBD, CD)	NEC survivors, chronic IBD (adolescents/adults), CD	intelligence quotient tests (Wechsler Preschool and Primary Scale of Intelligence/Wechsler Intelligence Scale for Children), Reaction Time tests, Verbal/Visual Memory tests
Executive function	To detect impairments in planning, attention, and cognitive flexibility, commonly seen in active IBD and linked to frontal lobe dysfunction	Pediatric IBD (active disease), NEC survivors (attention deficits)	Trail Making Test, Stroop Color-Word Test, tests for perseverative errors (California Verbal Learning Test)
Mood/affective status	To address the high comorbidity of anxiety and depression in IBD and CD, which often mediates poor health-related quality of life	All chronic gastrointestinal patients (pediatric and adult)	Beck depression inventory, state-trait anxiety inventory, generalized anxiety disorder scale
Inflammatory markers	To correlate inflammation with cognitive status and assess the therapeutic target (i.e., deep healing)	IBD and CD patients	Plasma interleukin-6, fecal calprotectin, kynurenine-to-tryptophan ratio

It outlines specific neurocognitive, behavioral, and biological assessments required for high-risk patient populations with a history of severe early gastrointestinal disease (e.g., necrotizing enterocolitis, severe enteric infections) or chronic inflammatory conditions (e.g., inflammatory bowel disease, celiac disease). The recommendations emphasize a shift toward multidisciplinary, longitudinal monitoring to proactively identify and manage the subtle, yet persistent, cognitive and emotional deficits driven by chronic inflammation and malabsorption. CD: Celiac disease, IBD: Inflammatory bowel disease; NEC: Necrotizing enterocolitis.

Potential early interventions

Moving beyond recognition and monitoring, this body of research suggests several promising avenues for therapeutic intervention to mitigate the neurocognitive risk associated with chronic gut dysfunction.

Anti-inflammatory strategies: Since systemic and neuroinflammation are key drivers of cognitive and emotional deficits, aggressive treatment of the underlying intestinal disease is inherently a neuroprotective strategy. The primary therapeutic goal for chronic inflammatory conditions (IBD, CD) should be to achieve not just clinical remission but also deep or mucosal healing[172]. For CD, the correlation between cognitive improvement and reduction in the Marsh score confirms that eliminating the inflammatory trigger directly benefits the brain[152]. In IBD, the observed deficits in executive function and mood are linked to elevated systemic inflammatory markers (e.g., IL-6, Kyn:Trp ratio). Aggressive use of anti-inflammatory medications (e.g., biologics) that effectively reduce systemic cytokine levels is postulated to reduce the inflammatory insult on the CNS[173].

Probiotics/prebiotics and microbiome modulation: The gut microbiome’s established role in the gut-brain communication pathway provides a direct, modifiable target for intervention. Interventions designed to restore a healthy microbial balance (e.g., using specific probiotics or prebiotics) have the potential to improve neurodevelopmental outcomes, particularly in conditions where dysbiosis is pronounced (e.g., among NEC survivors, ADHD, and autism)[89,174]. While still highly experimental for cognitive outcomes, animal models have shown that FMT can rapidly transfer cognitive and behavioral traits, highlighting the potency of microbiome restructuring[175]. Clinical trials are warranted to explore whether targeted modulation can stabilize the “hyperexcitable celiac brain” or improve attention and executive function in IBD.

Nutritional optimization (iron, omega-3, vitamins): Correcting nutrient deficiencies caused by chronic malabsorption or inflammation is essential, as these often compound the neurocognitive damage. Clinical studies repeatedly link cognitive impairment in chronic gut conditions (CD, severe enteric infection) to deficiencies in key neurodevelopmental nutrients, including iron, zinc, folate, vitamin B12, and Vitamin D[139,140,144,154]. Routine and comprehensive assessment of these levels, followed by targeted, high-dose supplementation, is a low-risk, high-yield intervention. These polyunsaturated fatty acids are critical components of neuronal membranes and possess anti-inflammatory properties. Supplementation with omega-3s is a common intervention for neurodevelopmental disorders like ADHD and may be particularly relevant in IBD, where systemic inflammation is high[176].

Early educational and behavioral interventions: Addressing the functional and psychosocial consequences of gut-related conditions requires non-pharmacological, supportive interventions. For children identified through NDFU (e.g., NEC survivors or IBD patients) with specific deficits in executive function, attention, or memory, early educational support and cognitive training can help mitigate long-term academic and social impact[177]. Given that adolescent depressive symptoms fully mediate the link between parental distress and poor health-related quality of life in pediatric IBD, clinical interventions must actively target both parent distress and adolescent depression/anxiety[160]. Counseling and behavioral therapies are essential for improving overall outcomes and adherence to medical regimens.

FUTURE RESEARCH DIRECTIONS

Future research efforts must move beyond establishing correlations to defining causality, identifying precise therapeutic targets, and evaluating the long-term efficacy of interventions targeting the GBA.

Long-term prospective cohorts

While existing studies like MAL-ED have been foundational, expanded, and more granular longitudinal research is necessary to capture the whole trajectory of developmental risk. Cohorts must extend beyond early childhood (2-5 years) into adolescence and young adulthood. This is necessary to determine if the cognitive and executive function deficits observed early persist, stabilize, or worsen as the brain matures and is subjected to greater academic and social demands. Future studies must specifically track children with known severe early gut insults (e.g., NEC, surgical IBD, high diarrheal burden) using dedicated, standardized neurodevelopmental assessments to quantify the precise magnitude and nature of the long-term disability, including rates of special education needs and occupational outcomes. Although some data on maternal IBD are reassuring, further large-scale prospective cohorts are needed to fully evaluate the subtle neurodevelopmental risks associated with in utero exposure to maternal inflammation, stress, and IBD medications.

Mechanistic biomarkers (metabolomics, microbiome signatures)

The translation of research to clinical diagnostics requires the identification of accessible, reliable biomarkers that reflect the state of the gut-brain interaction. Research should focus on identifying specific, stable microbial “signatures” (taxa, functional genes) that are predictive of neurocognitive impairment. For instance, quantifying the abundance of taxa associated with poor outcomes (like Enterobacter in the celiac/cognition link) or protective taxa (like SCFA producers) could serve as a diagnostic tool. Identifying the precise neuroactive metabolites that link gut dysbiosis to brain function is crucial[178]. This includes rigorously measuring levels of SCFAs, bile acids, and intermediates of the Kynurenine pathway (as seen in IBD studies) in both the blood and cerebral spinal fluid (where ethically feasible) to confirm their direct role as mediators of neuroinflammation and cognitive function. Developing robust, standardized markers of intestinal permeability (leaky gut) and persistent low-grade intestinal inflammation that directly correlate with cognitive outcomes is necessary for screening[179].

Interventional trials targeting the GBA

Rigorous clinical trials are needed to prove that modulating the gut environment can produce lasting neurocognitive benefits. Double-blind, randomized controlled trials (RCTs) are required to test specific, well-characterized probiotic or prebiotic strains and their effect on predefined cognitive and psychiatric endpoints (e.g., attention, executive function, anxiety, depression) in high-risk pediatric populations (e.g., IBD, history of severe infection)[180]. RCTs are needed to determine the optimal timing, dose, and duration of supplementation for key micronutrients (iron, zinc, vitamin D, omega-3s) to improve neurocognitive outcomes in patients with demonstrated malabsorption or chronic inflammation. While ethically complex in pediatric populations, highly targeted, small-scale FMT trials for adults with severe, treatment-refractory cognitive or psychiatric symptoms clearly linked to inflammatory gut conditions should be pursued to establish efficacy and safety for future expansion.

Integration of neuroimaging and GI inflammatory markers

Combining neuroscientific tools with GI diagnostics will provide unprecedented mechanistic insight. Future cohorts should integrate advanced neuroimaging techniques (functional MRI, diffusion tensor imaging) with inflammatory blood and fecal markers (e.g., fecal calprotectin, plasma IL-6, Kyn:Trp ratio)[181]. This will help visualize and quantify brain changes (e.g., white matter integrity, functional connectivity, neurogenesis) that are directly mediated by peripheral gut inflammation. Integrating non-invasive techniques such as transcranial magnetic stimulation, as explored in CD, can help establish neurophysiological endpoints (e.g., cortical excitability) that serve as rapid, objective biomarkers of brain recovery in response to gut-targeted therapies (e.g., GFD adherence)[182,183].

LIMITATIONS OF THE CURRENT LITERATURE

The study of the GBA and its impact on neurocognitive and psychiatric outcomes, while rapidly advancing, is subject to several methodological and conceptual limitations that restrict definitive conclusions and the immediate translation of findings into clinical practice.

Heterogeneity and methodological constraints

The human studies reviewed, particularly those investigating chronic conditions such as IBD and CD, involve highly heterogeneous populations with respect to age, disease duration, disease activity, treatment regimens (e.g., specific medications, GFD adherence), and socioeconomic status. This variability makes it challenging to isolate the particular causal role of the gut pathology itself vs confounding environmental or treatment effects. Different studies employ diverse neuropsychological test batteries (e.g., Wechsler Intelligence Scale for Children-Third Edition, Bayley-III, CANTAB, CRD, California Verbal Learning Test). This lack of standardization complicates the cross-comparison of cognitive deficits and prevents the precise identification of a consistent, universal “neurocognitive phenotype” associated with each gut disorder. Assessment of key outcomes like “brain fog”, anxiety, and depression often relies on patient self-report surveys. While valuable, these measures can be subjective and influenced by the current emotional state, potentially biasing the assessment compared to objective cognitive testing.

Challenges in establishing causality and mechanism

The relationship between gut pathology and CNS outcomes is inherently bidirectional. It is often difficult to definitively prove whether gut inflammation causes the psychiatric/cognitive deficit, or if chronic stress and CNS pathology contribute to gut dysregulation (e.g., HPA axis dysfunction and IBD flares). Many studies rely on broad markers of inflammation (e.g., plasma IL-6, C-reactive protein). The lack of correlation between these general systemic markers and specific cognitive deficits (as noted in some IBD studies) suggests that the true neuro-inflammatory insult may be mediated by more specific, unmeasured small-molecule metabolites or highly localized inflammation that current methods fail to capture. While animal models (FMT, antibiotic depletion, colitis induction) provide compelling mechanistic evidence for the gut’s influence on behavior and cognition, translating these findings directly to the complex human clinical setting remains challenging due to species-specific differences in the microbiome and neurophysiology.

Gaps in longitudinal and interventional evidence

Crucially, many cohorts demonstrating cognitive deficits in early childhood (e.g., after severe enteric infection or NEC) lack comprehensive, high-resolution follow-up into adolescence or adulthood, making the ultimate severity and persistence of the functional impairment unknown. There is a significant lack of robust, RCTs that specifically use gut-targeted interventions (e.g., defined probiotic strains, specific nutritional regimens) with neurocognitive endpoints as the primary outcome. Most interventional data are derived secondarily from trials focused on improving gut symptoms. Most studies are not sufficiently powered to examine how specific microbial subtypes, genetic susceptibilities, or different inflammatory pathways within a single disease (e.g., Crohn’s vs ulcerative colitis) differentially affect neurocognitive outcomes.

CONCLUSION

The evidence presented conclusively demonstrates that the integrity of the GBA is essential for healthy neurocognitive and psychiatric development across the lifespan. The confluence of factors – including gut dysbiosis, chronic systemic inflammation, and micronutrient malabsorption – driven by severe early-life and chronic GI conditions, translates into measurable and persistent deficits in human cognition, executive function, and emotional health. For the neonate and infant, the consequences of severe acute insult, such as NEC, are profound, resulting in high rates of NDI that require mandatory, long-term follow-up. For children and adolescents with chronic inflammatory disorders like IBD and CD, the functional impact is often subtler but persistent, manifesting as reduced processing speed, impaired attention, and a high incidence of anxiety and depression. Crucially, the persistence of these cognitive and emotional deficits, even during clinical remission, suggests that current therapies focused solely on mucosal healing may be insufficient for achieving complete neurological health.

The clinical imperative is clear: The care paradigm for chronic GI disease must evolve beyond the gut. Recognizing chronic inflammation and unexplained symptoms like “brain fog” as neurocognitive red flags requires the integration of gastroenterology with neurodevelopmental and mental health specialties. By prioritizing deep mucosal healing, addressing critical micronutrient deficiencies (iron, B-vitamins, omega-3s), and leveraging future targeted therapies, such as specific probiotic and prebiotic formulations, clinicians cannot only treat the gut but also protect and restore the functional capacity of the brain. Future research must validate this approach through robust, long-term prospective cohorts and RCTs that use specific neurocognitive outcomes as their primary endpoint, ultimately defining a comprehensive, neuroprotective strategy for managing chronic gut health.