The Weaning of Pigs: Mechanistic Outcomes to Intestinal Immunity and Integrity

ABSTRACT

Weaning is a critical stage in pigs characterized by abrupt maternal separation, dietary transition, social mixing, relocation, and increased exposure to environmental and pathogenic challenges. These stressors occur during a period of physiological immaturity and collectively disrupt intestinal homeostasis. Dysbiosis alters luminal signaling and microbial metabolite profiles, contributing to activation of innate immune pathways, oxidative imbalance, increased intestinal permeability, and impaired capacity for epithelial maintenance, ultimately compromising nutrient absorption and growth performance. Despite their diverse origins, these stressors converge on shared biological pathways, including pattern‐recognition receptor activation, cytokine amplification, redox imbalance, and tight junction disruption, which sustain mucosal inflammation and barrier dysfunction during the post‐weaning period. This review combines the current understanding of how weaning‐associated and dietary stressors, including dietary antigens, non‐starch polysaccharides, enteric pathogens, and mycotoxins, affect intestinal immunity and epithelial integrity in pigs. Nutritional interventions are evaluated based on their ability to influence common epithelial–immune pathways. Functional feed additives can regulate host–microbe interactions, dampen inflammatory signaling, enhance redox homeostasis, and maintain epithelial integrity. Overall, post‐weaning intestinal dysfunction should be considered a multifactorial condition driven by interconnected stress pathways, and nutritional approaches targeting these shared mechanisms may enhance intestinal function and growth performance in weaned pigs.

1. Introduction

Weaning is a period of immense physiological transition for young pigs. Abrupt separation from the sow and dietary transition from milk to solid feeds, combined with social mixing and relocation stress, all occur within a developmental window in which overall physiological development remains immature (Colson et al. 2006; Moeser, Vander Klok, et al. 2007; Li et al. 2017). This combination of stressors results in coordinated alterations in mucosal immune activation, epithelial turnover, redox balance, and barrier integrity (McLamb et al. 2013; Cao et al. 2018). The alterations directly influence intestinal immunity and epithelial structure, leading to reduced nutrient absorption and the mobilization of nutrients toward immune responses, which ultimately suppresses growth performance during the post‐weaning period (Zheng et al. 2021; Chang et al. 2024).

The intestinal mucosa acts as a dynamic link between the external environment and the intestinal epithelium and associated immune cells (McLamb et al. 2013; Kim and Duarte 2021). Enterocytes, immune cells, and microbiota communicate through signaling cascades primarily initiated through interactions with pattern recognition receptors (PRR; Rakoff‐Nahoum et al. 2004). Under homeostatic conditions, these pathways maintain selective permeability, controlled immune activation, and efficient nutrient absorption. However, weaning‐associated stress disrupts establishment of a favorable microbial composition, thereby modifying epithelial signaling (Kim and Duarte 2021; Garavito‐Duarte, Bonetti, et al. 2025). Importantly, stress‐induced immune activation in weaned pigs is not exclusively driven by microbial interactions and can be compounded by additional stressors that further exacerbate the inflammatory response. Dietary antigens, soybean meal–derived antinutritional factors (Friesen et al. 1993; Deng et al. 2025), mycotoxins (Holanda and Kim 2021a), and oxidative metabolites (Kerr et al. 2020) can also stimulate PRR‐mediated pathways. Although these stressors differ in origin, they converge at shared physiological endpoints, including pro‐inflammatory cytokine production, oxidative imbalance, and disruption of epithelial barrier proteins (Hu et al. 2013; Deng et al. 2025).

Because these diverse stressors converge on shared signaling pathways, nutritional strategies can be categorized according to their regulatory effects on epithelial–immune interactions. Targeting PRR activation dynamics, cytokine amplification cascades, redox balance, epithelial repair processes, and microbiota‐mediated immune modulation provides several opportunities to support intestinal function in weaned pigs. Therefore, this review aims to integrate current mechanistic knowledge on how weaning stress coupled with additional dietary or environmental challenges influence epithelial and immune pathways, identify shared physiological end points, and discuss nutritional strategies organized by regulatory function to clarify how dietary interventions can support intestinal immunity and barrier integrity during the post‐weaning period.

2. Stressors in Weaned Pigs

The weaning transition exposes pigs to multiple stressors that collectively disrupt intestinal homeostasis. These include abrupt maternal separation, social mixing stress, dietary transitions, and increased exposure to environmental pathogens (Moeser, Vander Klok, et al. 2007; Smith et al. 2010; Zheng et al. 2021). These challenges interact to alter endocrine signaling, feeding behavior, and intestinal physiology, often resulting in inflammation and compromised barrier integrity (Holanda and Kim 2022; Garavito‐Duarte, Duarte, and Kim 2025; Pasquetti et al. 2025). Understanding how these stressors mechanistically influence intestinal physiology is necessary in the development of targeted nutritional interventions to support intestinal health post‐weaning.

2.1. Weaning‐Associated Stress

The weaning period represents a critical transition in which young pigs are exposed to multiple simultaneous changes, including maternal separation, transport, environmental shifts (housing and temperature), dietary transition, and social mixing, all of which collectively contribute to a multifactorial stress response (Telkänranta and Edwards 2018). Separation from the sow, transport, and establishment of a new social hierarchy induce stress characterized by elevated circulating cortisol, which contributes to increased intestinal permeability and activation of immune signaling pathways (Moeser, Ryan, et al. 2007; Li et al. 2017; Escribano et al. 2019). These endocrine responses contribute to villus atrophy and crypt hyperplasia, resulting in impaired intestinal function in the early nursery period (Meddings and Swain 2000; Barreau et al. 2007; Shen et al. 2012). Reduced feed intake is a common consequence of weaning, reflecting the combined effects of physiological stress and the behavior adjustments associated with transitioning from suckling to consuming solid feeds (Spreeuwenberg et al. 2001). This reduction in intake suppresses the activation of nutrient‐sensing pathways, including mechanistic target of rapamycin (mTOR) and insulin‐like growth factor, that are critical for stimulating epithelial growth and barrier maintenance (Ben‐Sahra and Manning 2017; Dou et al. 2023; Fabà et al. 2025).

Compromised barrier integrity increases mucosal exposure to luminal antigens and microbial‐associated molecular patterns (MAMP; Turner et al. 1997; Zhao et al. 2021). This elevated antigen load induces immune stimulation at the epithelial surface and contributes to the initiation of inflammatory responses (Zhao et al. 2021). Increased translocation of luminal components further amplifies antigen exposure, creating a feed‐forward condition that predisposes the intestinal mucosa to sustain immune activation during the post‐weaning period (Smith et al. 2010).

2.2. Diet Transition and Composition Stress

The transition from sow milk to a plant‐based diet is a significant challenge from a physiological perspective. Milk‐derived substrates and bioactive compounds support epithelial development and immune regulation (Hurley 2015; Gormley, Garavito‐Duarte, and Kim 2024). In addition, milk‐derived immunoglobulins provide passive luminal protection (Lallès et al. 2007; Declerck et al. 2016). The rapid dietary transition to nursery diets introduces complex carbohydrates, storage proteins, and non‐starch polysaccharides (NSP) that are much less digestible relative to milk‐derived nutrients, which subsequently alters substrate availability and microbial fermentation dynamics (Zhang et al. 2020; Fabà et al. 2024; Baker et al. 2025). Incomplete digestion of dietary proteins and carbohydrates in the small intestine increases mucosal exposure to undigested substrates and antigenic compounds, which can disturb epithelial metabolic homeostasis and promote reactive oxygen species (ROS) formation (Huang et al. 2022; Li, Ding, et al. 2022).

Soybean meal is commonly used in the diets of pigs as a source of protein; however, soybean meal contains antigenic peptides that interact with mucosal immune cells of the lamina propria, promoting cytokine secretion and activation of adaptive immune responses (Li et al. 1990; Zhao et al. 2015; Zheng et al. 2023). In particular, β‐conglycinin and glycinin act as antinutritional factors that disrupt intestinal homeostasis through immune and cellular stress mechanisms. Dietary exposure to these proteins induces endoplasmic reticulum stress in the jejunal mucosa, a condition characterized by the accumulation of misfolded proteins within enterocytes that activates cellular stress responses and can impair epithelial function (Wang et al. 2023). In addition, antigenic soybean proteins activate c‐Jun N‐terminal kinase (JNK), p38 MAPK, and NF‐κB signaling pathways in the small intestine (Peng et al. 2018), which promote pro‐inflammatory responses (Lee and Lee 2012; Park et al. 2020). These stress responses impair goblet cell function and reduce tight junction protein expression (zonula occludens‐1, claudin‐1, and occludin), thereby compromising barrier integrity and intestinal morphology (Zhao et al. 2015; Wang et al. 2023; Choi et al. 2025).

Cereals are widely used in pig diets as primary energy sources. However, cereals and their coproducts contain variable concentrations of NSP, which can influence nutrient digestibility and intestinal function. According to Jaworski et al. (2015), corn contains approximately 8.1% total NSP, whereas wheat contains about 9.5%. In contrast, sorghum contains a substantially higher concentration, with approximately 44.3% total NSP. Because pigs do not produce endogenous enzymes capable of hydrolyzing NSP, these carbohydrates may interfere with the digestion and absorption of other nutrients (Bach Knudsen and Hansen 1991; Gutierrez et al. 2014; Baker et al. 2025). Consequently, excessive NSP inclusion is generally discouraged, particularly in diets formulated for nursery pigs. High concentrations of NSP increase digesta viscosity and modify nutrient digestion (Kim et al. 2003; Passos et al. 2015), serve as fermentable substrates for intestinal microbiota, and change the physiological function of the intestine (Kiarie et al. 2013). Changes in the physical characteristics of the digesta generated by NSP, along with reduced nutritional digestibility, can increase bacterial fermentation and the proliferation of harmful bacteria, such as Escherichia coli ( E. coli ), in the intestine (McDonald et al. 2001; Hopwood et al. 2002). Increased pathogen load and MAMP may subsequently promote PRR activation, cytokine secretion, and further disruption of epithelial barrier proteins, increasing inflammatory signaling within the post‐weaning period.

2.3. Enteric Pathogenic Challenge

Because of the stressor generated during the weaned period, the susceptibility to enteric pathogens increases (Lallès et al. 2004; Zheng et al. 2021). The occurrence of post‐weaning diarrhea (PWD) remains a major health challenge in pigs (Duarte et al. 2023). Among the etiological agents associated with this condition, enterotoxigenic E. coli is recognized as the primary pathogen responsible for PWD and edema disease (ED) in young pigs (Luppi et al. 2016; Duarte et al. 2023). Enteric pathogens represent potent activators of innate immune signaling in nursery pigs. Bacterial components such as lipopolysaccharide (LPS) from Gram‐negative bacteria such as E. coli act as potent inflammatory stimulators (Liu et al. 2019). The LPS binds to TLR4 in association with CD14 and myeloid differentiation factor‐2 (MD‐2), initiating MyD88‐dependent signaling cascades (Guo et al. 2015). This activation leads to NF‐κB translocation and transcription of pro‐inflammatory cytokines and chemokines that recruit neutrophils and monocytes to the intestinal mucosa (Régnier et al. 1997; Chen et al. 2024).

E. coli colonization of the intestinal epithelium depends on specific fimbrial adhesins that mediate attachment to enterocytes in the small intestine. Among these, F18 fimbriae are strongly associated with PWD in recently weaned pigs (Zhang et al. 2007; Duarte et al. 2023). This adhesion facilitates bacterial colonization and the subsequent release of enterotoxins. The F18⁺ E. coli strains commonly produce heat‐stable enterotoxins A and B (STa and STb). These toxins disrupt epithelial ion transport pathways, leading to increased chloride secretion and water efflux into the intestinal lumen (Gonçalves et al. 2008; Bessone et al. 2017). Moreover, pathogen‐induced inflammatory signaling alters the distribution of tight junction proteins, increasing paracellular permeability and further amplifying luminal antigen exposure (Nighot et al. 2017; Duarte et al. 2023).

Weaned pigs are also vulnerable to Salmonella infection (Stevens and Gray 2013; Casanova‐Higes et al. 2019), which is associated with a reduction in beneficial bacterial populations driven largely by the dietary transition from sow milk to solid feed (Konstantinov et al. 2006). In contrast to E. coli , Salmonella spp. are invasive pathogens that translocate across the intestinal epithelium via M cells and enterocytes, triggering intracellular PRR such as NOD1 and NOD2 (Geddes et al. 2010). This invasion activates NF‐κB–dependent inflammatory responses and promotes the recruitment of immune cells, leading to mucosal inflammation and epithelial damage. The resulting disruption of epithelial integrity further enhances pathogen translocation and amplifies intestinal immune activation (Le Negrate et al. 2008). In addition, Lawsonia intracellularis represents a distinct class of enteric pathogen, characterized by its obligate intracellular lifecycle within ileal enterocytes. L. intracellularis infection induces proliferative enteropathy through dysregulated epithelial cell proliferation, resulting in crypt hyperplasia, impaired absorptive capacity, diarrhea, reduced growth performance, and, in severe cases, sudden death (Lawson and Gebhart 2000; Boesen et al. 2004).

2.4. Mycotoxins Contamination

Mycotoxins are secondary metabolites synthesized by filamentous fungi that frequently contaminate feed ingredients during both pre‐harvest and post‐harvest stages, including storage (Patriarca and Fernández Pinto 2017; Gruber‐Dorninger et al. 2019). It has been demonstrated that a substantial proportion of feedstuffs contain at least one mycotoxin, with deoxynivalenol (DON), aflatoxins, and fumonisins being among the most commonly detected contaminants (Gruber‐Dorninger et al. 2019; Holanda and Kim 2021a). Nursery pigs are particularly vulnerable to mycotoxin exposure during the post‐weaning period due to limited detoxification capacity and the presence of concurrent physiological stressors (Kim, Holanda, et al. 2019).

Exposure to mycotoxins disrupts both intestinal and systemic physiology through multiple pathways. For instance, aflatoxins and DON interfere with protein synthesis and cellular homeostasis, which can impair enterocyte proliferation and compromise epithelial integrity (Swamy et al. 2003; Cheng et al. 2006). In addition, these toxins promote oxidative stress and inflammatory responses, as evidenced by increased ROS production and upregulation of pro‐inflammatory cytokines (Hu et al. 2013; Holanda and Kim 2021b). Specifically, DON has been shown to elevate IL‐8 expression and oxidative stress‐related enzymes, whereas reducing intestinal surface area and nutrient absorptive capacity (Lessard et al. 2015; Kim, Holanda, et al. 2019). Beyond direct epithelial effects, mycotoxins also influence the intestinal microbiota, altering its composition and metabolic activity and thereby disrupting mucosal immune homeostasis (Kim, Holanda, et al. 2019; Wang et al. 2019). These changes are associated with reduced nutrient digestibility and energy utilization, ultimately leading to decreased feed intake and impaired growth performance (Jo et al. 2016; Clarke et al. 2018; Holanda et al. 2020). Prolonged exposure to mycotoxins such as DON and aflatoxins may further result in hepatic dysfunction and systemic toxicity, exacerbating negative impacts on overall animal health and productivity (Ytrebø et al. 2006; Weaver et al. 2013; Son et al. 2024). Collectively, mycotoxins act as both nutritional and environmental stressors that interact with weaning‐associated challenges, intensifying intestinal dysfunction, inflammatory signaling, and growth suppression during the weaned phase.

3. Shared Immune and Barrier Disruption Pathways Across Stressors

Although stressors in weaned pigs originate from behavioral, dietary, toxicological, or infectious sources, their biological consequences frequently converge on a limited number of conserved signaling pathways. These pathways coordinate innate immune activation, cytokine amplification, oxidative stress, epithelial permeability changes, and microbiota alterations (Lopez‐Rincon et al. 2026).

3.1. Pattern‐Recognition Receptor Activation

Pattern‐recognition receptors constitute the primary interface between luminal stimuli and mucosal immune activation (Fukata and Arditi 2013). In weaning pigs, receptors such as TLR2, TLR4, and nucleotide‐binding oligomerization domain proteins (NOD1, NOD2) detect MAMP, dietary antigens, and endogenous danger signals derived from epithelial stress (Oliveira‐Nascimento et al. 2012; Hu et al. 2013; Gormley et al. 2025). Activation of these receptors triggers signaling pathways that converge on NF‐κB and MAPK, inducing pro‐inflammatory mediators that coordinate immune cell recruitment and mucosal defense (Guo et al. 2015). The PRR activation is influenced not only by pathogen exposure but also by epithelial permeability and microbial composition (Fukata and Arditi 2013). Increased paracellular flux exposes basolateral receptors to luminal ligands, amplifying immune activation. Disruption of the epithelial barrier and PRR signaling reinforce one another, driving persistent mucosal inflammation (Budikhina et al. 2021).

3.2. Cytokine Cascades and Immune Amplification

Following PRR activation, cytokine production amplifies and propagates the inflammatory response. Pro‐inflammatory cytokines such as TNF‐α, IL‐1β, IL‐6, and IL‐8 are produced by epithelial and immune cells, reinforcing immune signaling and modifying epithelial function (Menendez et al. 2013; Zheng et al. 2021). The pro‐inflammatory cytokines activate downstream transcriptional pathways and promote recruitment of immune cells to the intestinal mucosa. In addition, IL‐6 and IL‐8 contribute to epithelial proliferation, immune cell recruitment, and increased oxidative activity, linking immune activation with tissue remodeling (De Simone et al. 2015). In the context of weaning stress, repeated or combined stimuli may maintain elevated cytokine expression, prolonging epithelial dysfunction.

Pro‐inflammatory cytokines, TNF‐α and IL‐1β, activate additional NF‐κB signaling in both epithelial and immune cells, increasing expression of inflammatory mediators and tight junction regulatory proteins (Ma et al. 2004; Al‐Sadi et al. 2010). Additional pro‐inflammatory cytokines further propagate mucosal inflammation; for example, IL‐6 and IL‐8 further propagate mucosal inflammation by activating STAT3 signaling that regulates epithelial proliferation and immune differentiation and simultaneously promoting neutrophil recruitment to the intestinal mucosa (De Simone et al. 2015), increasing ROS production. This cytokine amplification system can shift from controlled defense to sustained inflammatory signaling if regulatory feedback mechanisms, including IL‐10 and TGF‐β signaling, are insufficient.

3.3. Oxidative and Redox Stress

A central feature of post‐weaning intestinal dysfunction is the interaction between inflammatory signaling and redox imbalance. Increased ROS, generated during immune activation and mitochondrial perturbation, amplify inflammatory signaling and compromise epithelial integrity (Luo et al. 2016; Cao et al. 2018; Bellanti et al. 2025). The ROS are produced from multiple cellular sources, including mitochondrial respiration, NADPH oxidase activity, and infiltrating immune cells (St‐Pierre et al. 2002; Dennis et al. 2009), whereas antioxidant systems such as glutathione, superoxide dismutase, and catalase maintain redox balance under physiological conditions (Venditti et al. 2013). Excessive ROS production increases inflammatory signaling and disrupts tight junction assembly, contributing to barrier dysfunction. The Nrf2 pathway plays a central role in maintaining redox homeostasis by regulating antioxidant responses (Kobayashi and Yamamoto 2006). Impaired antioxidant capacity or excessive oxidative stress shifts the balance toward epithelial damage and immune dysregulation (Cao et al. 2018).

3.4. Intestinal Integrity

Epithelial tight junctions regulate paracellular permeability through coordinated expression of claudins, occludin, and zonula occludens proteins (Kuo et al. 2022). Under inflammatory conditions, cytokines and oxidative stress alter their expression and cellular distribution, leading to increased intestinal permeability. These changes facilitate the translocation of luminal antigens across the epithelial barrier, which further stimulates immune activation in the lamina propria (Awad et al. 2017; Szabó et al. 2023). Consequently, barrier disruption functions both as a consequence of inflammation and as a driver of further immune activation. Inflammatory signaling also alters epithelial turnover dynamics. Increased epithelial apoptosis, together with compensatory crypt cell proliferation, disrupts villus–crypt architecture, reducing absorptive capacity and modifying epithelial–immune interactions within the intestinal mucosa (Xu et al. 2022).

4. Nutritional Modulations of Stress‐Activated Immune Pathways

Nutritional interventions targeting intestinal health in weaning pigs can be conceptualized as the regulation of several interconnected biological pathways, including alteration of intestinal microbiota, PRR activation, cytokine amplification responses, immune cell metabolic activity, redox balance, and epithelial repair mechanisms. Functional dietary compounds can influence these pathways by modulating host–microbe interactions and immune signaling within the intestinal mucosa.

4.1. Intestinal Host–Microbe Interactions

Dietary interventions can influence intestinal health by influencing the intestinal microbiota and the production of microbial metabolites that regulate immune and metabolic processes (Table 1). Feed additives such as NSP‐degrading enzymes play a key role in shaping the intestinal environment by reducing digesta viscosity and generating fermentable oligosaccharides during NSP hydrolysis (Choi et al. 2024). The enzymatic breakdown of arabinoxylans and β‐glucans produces xylooligosaccharides and gluco‐oligosaccharides, which act as prebiotic substrates for beneficial bacteria in the small intestine (Bindelle et al. 2009; Lærke et al. 2015; Yan et al. 2018). In vivo studies have confirmed increased release of these oligosaccharides following supplementation with xylanase (Passos et al. 2015; Petry et al. 2021; Moita et al. 2022) and β‐glucanase (Jensen et al. 1998; Tsai et al. 2017).

TABLE 1.

Mechanistic effects of nutritional additives on intestinal microbiota composition in weaned pigs.

Additive	Mechanistic target	Type	Key responses	References
Enzymes	Substrate hydrolysis driving shifts in microbial fermentation and community structure.	Amylase + Xylanase + Protease	In rectum feces: ↑ Lactobacillus spp. ↑ Bacillus subtilis spp. ↓ Salmonella spp. ↓ Escherichia coli spp.	Zhang et al. 2014
		Amylase + Protease + Xylanase + Glucanase	In caecum: ↑ Lactobacillus ↑ Lachnospiraceae ↓ Oscillibacter ↓ Veillonella	Long et al. 2021
		Xylanase	In jejunum mucosa: ↑ Massilia indica ↑ Telluria mixta ↓ Cupriavidus necator	Moita et al. 2022
		Xylanase and β‐glucanase	In jejunum mucosa: ↑ Lactobacillus johnsonii ↓ Helicobacter rappini	Choi et al. 2024
		Xylanase + β‐glucanase + β‐mannanase	In jejunum mucosa: ↑ Lactobacillus delbrueckii ↑ Lactobacillus reuteri‐vaginalis ↓ Helicobacter ganmani	Baker et al. 2025
Phytobiotics	Direct antimicrobial effects altering microbial community composition and suppressing pathogenic taxa.	Essential oil‐based phytobiotics (cinnamaldehyde, eugenol, carvacrol, and thymol)	In feces: ↑ Lactobacillus	Montoya et al. 2021
		Plant extracts, and green propolis extract (curcuminoids, carnosic derivatives, naringin flavonoids, salicylic derivatives, and artepillin‐C)	In fecal samples: ↑ Lactobacillus	Montoya et al. 2021
		Origanum vulgare , Crithmum maritimum, Allium sativum , Camelina sativa	In caecum: ↓ Enterobacteriaceae ↓ Lactobacillaceae	Magklaras et al. 2025
		Blend of botanical (Terpenes and terpenoids)	In jejunum mucosa: ↑ Lactobacillus mucosae ↓ Staphylococcus kloosii ↓ Helicobacter rappini ↓ Staphylococcus saprophyticus‐xylosus ↓ Prevotella copri	Garavito‐Duarte, Bonetti, et al. 2025
		Ground herb‐based phytobiotics ( Holarrhena antidysenterica , Berberis aristate, Punica granatum , Aegle marmelos , Woodfordia fruticose)	In jejunum mucosa: ↑ Staphylococcus saprophyticus ↑ Staphylococcus cohnii‐nepalensis ↓ Helicobacter equorum ↓ Dialister succinatiphilus	Garavito‐Duarte, Duarte, and Kim 2025
		Essential oil‐based phytobiotics ( Ocimum sanctum , Mentha piperita, Eucalyptus globulus , Trychyspermum ammi)	In jejunum mucosa ↓ Syntrophococcus spp.	Garavito‐Duarte, Duarte, and Kim 2025
Prebiotics	Selective stimulation of beneficial microbial populations and fermentation pathways	Saccharomyces cerevisiae yeast cell wall	In fecal samples: ↑ Lactobacillus ↑ Limosilactobacillus ↓ Olsenella spp.	Huaman et al. 2024
		Manno‐oligosaccharide	In caecum: ↑ Lactobacillus ↑ Bifidobacterium ↑ Bacillus ↓ Escherichia coli	Yu, Chen, Yu, et al. 2021
		β‐glucan	In colon digesta: ↑ Lactobacillus ↑ Bacillus ↓ Escherichia coli ↑ Total bacteria	Luo et al. 2019
		Yeast β‐glucan	In colon digesta: ↓ Bifidobacterium spp.	Mukhopadhya et al. 2019
Probiotics	Introduction of beneficial microorganisms promoting competitive exclusion and stabilization of microbial communities.	blend of spore‐forming strains ( Bacillus subtilis , Bacillus coagulans , Clostridium butyricum , Bacillus licheniformis )	In fecal samples: ↑ Lactobacillus ↑ Bulleidia ↑ Limosilactobacillus ↓ Olsenella sp.	Huaman et al. 2024
		Enterococcus faecium , Bacillus subtilis , Saccharomyces cerevisiae	In fecal samples: ↑ Ruminococcaceae ↑ Prevotella ↑ Eubacterium coprostanoligenes	Park et al. 2024
Postbiotics	Provision of microbial‐derived metabolites modulating microbial interactions and community dynamics.	Saccharomyces yeast postbiotic	In jejunum mucosa: ↓ Corynebacterium sp. ↓ Sharpea sp. ↓ Bifidobacterium sp.	Gormley, Duarte, et al. 2024
		Lactobacillus reuteri postbiotics	In fecal samples: ↑ Firmicutes ↓ Bacteroidetes	Sun et al. 2025

The generation of fermentable substrates contributes to the modulation of mucosa‐associated microbiota; for example, enzyme supplementation, including xylanase, β‐glucanase, and β‐mannanase, has been associated with increased abundance of beneficial taxa such as Lactobacillus spp. and Bifidobacterium dentium , alongside reductions in opportunistic pathogens including Helicobacter rappini in the jejunal mucosa of weaned pigs (Duarte et al. 2021; Moita et al. 2022; Baker et al. 2025). Additional increases in species such as Lactobacillus delbrueckii and Lactobacillus reuteri–vaginalis further support a shift toward a more favorable microbial ecosystem (Baker et al. 2025). In contrast, elevated digesta viscosity induced by high NSP in diets has been associated with increased proliferation of E. coli in the small intestine (McDonald et al. 2001; Montagne et al. 2004), highlighting the importance of viscosity control in maintaining microbial balance. Enzyme supplementation has also been linked to changes in fibrolytic bacterial populations such as Prevotellaceae, which are associated with enhanced production of short chain fatty acids (Amat et al. 2020; Choi et al. 2024). These microbial metabolites serve as energy substrates for epithelial cells and play a central role in immune regulation within the gastrointestinal tract (Chen et al. 2017).

Phytogenic compounds may further influence microbial ecology and intestinal homeostasis in pigs. Phytobiotics containing terpenes and terpenoids have been shown to reduce the relative abundance of bacterial taxa such as Staphylococcus saprophyticus , Staphylococcus kloosii , and Helicobacter rappini during F18⁺ E. coli challenge (Garavito‐Duarte, Bonetti, et al. 2025). These antimicrobial effects are likely mediated by interactions between terpenes and bacterial lipid membranes, leading to increased membrane permeability and leakage of intracellular components (Guimarães et al. 2019; Di Matteo et al. 2024). Gram‐negative bacteria, characterized by an outer membrane rich in LPS and phospholipids, may be particularly susceptible to these compounds (Beveridge 1999; Połeć et al. 2022).

4.2. Regulation of PRR Activation and Pathogen Recognition

Dietary bioactive compounds can regulate intestinal immune responses by influencing the interaction between luminal microbes and host PRR (Table 2). Some dietary glycans, such as milk oligosaccharides (MO), can modulate host–microbial interactions by interfering with pathogen adhesion and reducing excessive activation of innate immune receptors (Newburg et al. 1990; Xu et al. 2004). These structures like fucosylated MO have been reported to inhibit the attachment of enteric pathogens such as Helicobacter pylori , E. coli , and Campylobacter jejuni (Newburg et al. 1990; Ruiz‐Palacios et al. 2003; Xu et al. 2004). By reducing pathogen–epithelium interactions, these compounds may indirectly limit excessive stimulation of PRR pathways such as TLR4‐mediated responses to LPS, thereby contributing to immune homeostasis in the intestinal mucosa.

TABLE 2.

Mechanistic effects of nutritional interventions on intestinal immunity and integrity in weaned pigs.

Additive	Mechanistic target	Type	Key responses a	References
Enzymes	Attenuation of inflammatory signaling and oxidative stress through reduced luminal antigenic stimulation.	β‐glucanase	In jejunum mucosa: ↓ TNF‐α (17.3%) ↓ IL‐6 (11.2%)	Duarte et al. 2021
		Amylase + Protease + Xylanase + Glucanase	In serum: ↑ IgA (17.7%) ↑ IgG (12.4%) ↑ Superoxide dismutase (15.2%) ↓ Malondialdehyde (26.5%)	Long et al. 2021
		Xylanase	In jejunum mucosa: ↓ Malondialdehyde (41.4%) ↓ Protein carbonyl (20.5%)	Moita et al. 2022
		Xylanase + β‐glucanase + β‐mannanase	In jejunum mucosa: ↓ IL‐6 (23.7%) ↓ Protein carbonyl (13.5%)	Baker et al. 2025
Phytobiotics	Direct modulation of inflammatory signaling pathways and oxidative balance, with antimicrobial activity.	Blend of Cinnamomum zeylanicum and Trachyspermum copticum essential oils; plant extracts of Mikania micrantha and Garcinia lanceifolia	In serum: ↓ IL‐1β (6.54%) ↑ IgM (12.0%)	Samanta et al. 2021
		Premixture of grape seed and grape marc extract, green tea, and hops	In serum: ↓ TNF‐α (58.3%) ↓ IL‐6 (17.5%)	Chang et al. 2022
		Blend of botanical (Terpenes and terpenoids)	In jejunum mucosa: ↓ TLR4 (45.3%) ↓ IL‐6 (40.1%) ↓ IgG (31.9%)	Garavito‐Duarte, Bonetti, et al. 2025
		Ground herb‐based phytobiotics	In jejunum mucosa: ↓ Protein carbonyl (44.4%)	Garavito‐Duarte, Duarte, and Kim 2025
Prebiotics	Regulation of PRR‐mediated signaling, cytokine production, and epithelial barrier function.	β‐glucan	In jejunum mucosa: ↑ Glutathione peroxidase (30.8%) ↑ Catalase (44.2%) ↓ Malondialdehyde (55.5%)	Luo et al. 2019
		β‐glucan	In ileal mucosa: ↓ TNF‐α (26.0%) ↓ IL‐6 (22.0%) ↓ IL‐1β (22.0%)	Kim, Ehrlich, et al. 2019
		Manno‐oligosaccharide	In serum: ↓ TNF‐α (6.0%) ↓ IL‐6 (26.5%) ↓ IL‐1β (24.1%) ↑ IgA (33.3%) IgG (9.5%)	Yu, Chen, Yu, et al. 2021
		β‐glucan	In jejunum mucosa: ↓ TNF‐α (25.0%) ↓ IL‐6 (28.0%) ↓ TLR4 (50.0%) ↓ NF‐κB (60%) ↓ Malondialdehyde (50.8%)	Zhou et al. 2022b
		Saccharomyces cerevisiae yeast cell wall	In serum: ↓ IL‐1β (44.7%) ↓ IL‐8 (59.5%)	Huaman et al. 2024
		β‐glucan	In jejunum mucosa: ↑ Glutathione peroxidase (30.8%) ↑ Catalase (44.2%) ↓ Malondialdehyde (55.5%)	Luo et al. 2019
Probiotics	Modulation of host immune responses and maintenance of epithelial integrity.	Lactobacillus delbrueckii	In serum: ↑ IgG (31.4%) ↑ IL‐10 (33.3%) ↓ IL‐1β (3.7%) In jejunum mucosa: ↑ Glutathione peroxidase (36.4%)	Li, Hou, et al. 2019
		L. acidophilus, L. casei, B. thermophilum and E. faecium	In serum: ↓ TNF‐α (35.8%)	Sun et al. 2021
		Lactobacillus rhamnosus	In serum: ↓ TNF‐α (31.9%) ↓ TGF‐β1 (25.4%)	Kang et al. 2021
		Lacticaseibacillus rhamnosus	In serum: ↓ TNF‐α (33.9%) ↓ IL‐6 (31.4%)	Shin et al. 2025
Postbiotics	Direct regulation of host signaling pathways involved in immune activation, oxidative status, and epithelial metabolism.	Saccharomyces yeast	In jejunum mucosa: ↓ TLR4 (71.3%) ↓ Protein carbonyl (54.1%) ↑ mTOR (32.0%)	Gormley, Duarte, et al. 2024
		Lactobacillus reuteri postbiotics	In plasma: ↑ Superoxide dismutase (33.3%) ↓ Malondialdehyde (7.1%)	Sun et al. 2025

^a IgG, immunoglobulin G; IgM, immunoglobulin M; IL‐1β, interleukin‐1β; IL‐6, interleukin‐6; IL‐8, interleukin‐8; mTOR, mechanistic target of rapamycin; NF‐κB, nuclear factor kappa‐light‐chain‐enhancer of activated B cells; TLR4, toll‐like receptor 4; TNF‐α, tumor necrosis factor alpha.

Phytogenic compounds may also regulate PRR activation. For example, increasing dietary levels of a microencapsulated blend of phytobiotics containing terpenes and terpenoid molecules, with thymol as the major constituent, reduced relative TLR4 expression in the jejunum of weaned pigs challenged with F18⁺ E. coli . This suggests that phytobiotics may help to control the excessive PRR‐mediated inflammatory activation during enteric infection (Garavito‐Duarte, Bonetti, et al. 2025).

In addition to glycans and phytogenic compounds, postbiotic‐derived components have also been reported to modulate PRR signaling. Yeast‐derived products, particularly Saccharomyces cerevisae cell wall fractions rich in β‐glucans and mannan‐oligosaccharides, exhibit immunomodulatory properties that can influence intestinal immune responses in monogastric animals (Bortoluzzi et al. 2018; Yan et al. 2024). In weaned pigs challenged with F18⁺ E. coli , dietary supplementation with Saccharomyces cell wall reduced jejunal TLR4 gene expression by 29%, suggesting a mitigation of excessive innate immune activation (Gormley, Duarte, et al. 2024). This result may be partially explained by shifts in mucosa‐associated microbiota, including a reduced population of gram‐negative bacteria in the mucosa‐associated microbiota, thereby lowering stimulation of TLR4 by LPS. Different mechanisms may explain the reduction in Gram‐negative bacteria by β‐glucans. The supplementation of β‐glucans can enhance mucosal immune function through interaction with pattern‐recognition receptors such as dectin‐1 on macrophages and dendritic cells, promoting phagocytosis and bacterial clearance (Stothers et al. 2021; Yu, Chen, Du, et al. 2021). Additionally, β‐glucans may indirectly modulate intestinal microbiota by promoting the growth of beneficial commensal bacteria, which competitively exclude Gram‐negative pathogens through nutrient competition and production of antimicrobial metabolites (Zhou et al. 2022a; Rhayat et al. 2023; Gormley, Duarte, et al. 2024).

4.3. Modulation of Cytokine Amplification

Dietary bioactive compounds can regulate intestinal immune responses by attenuating cytokine amplification cascades triggered by microbial stimuli (Table 2). Beyond preventing pathogen adhesion, some dietary glycans can directly influence inflammatory signaling pathways. For example, fucosylated MO have been shown to reduce LPS‐induced inflammation in intestinal epithelial cells. In vitro studies demonstrated that fucosylated MO decreased IL‐8 secretion by approximately 45% in T84 cells in a dose‐dependent manner up to 4 mg/mL (Xu et al. 2004). Similarly, fucosylated and other MO including lacto‐N‐neotetraose, 6‐sialyllactose, 3‐sialyllactose, and free sialic acid reduced circulating inflammatory cytokines and exerted systemic immunomodulatory effects associated with MO exposure early in life in human infants and newborn pigs (Marriage et al. 2015; Comstock et al. 2017). These findings indicate that dietary glycans can attenuate cytokine amplification pathways activated during microbial challenges.

Marine‐derived polysaccharides may also regulate inflammatory responses in the intestinal mucosa. Dietary supplementation with macroalgae sulfated polysaccharides has demonstrated anti‐inflammatory effects in nursery pigs. Inclusion of 0.2% mixed macroalgae sulfated polysaccharides reduced TNF‐α by 27% and tended to decrease IL‐8 by 28% in the jejunal mucosa in weaned pigs (Cheng et al. 2026). The anti‐inflammatory effects of these compounds are largely attributed to ulvan and carrageenan, sulfated polysaccharides derived from macroalgae species such as Ulva spp. and Solieria chordalis (Corino et al. 2021). These bioactive compounds can regulate intracellular inflammatory pathways by inhibiting NF‐κB activation and MAPK signaling cascades, which are key regulators of cytokine transcription (Berri et al. 2017; Flórez‐Fernández et al. 2023).

Phytogenic compounds can also influence cytokine signaling during intestinal infection. A blend of phytobiotics containing terpenes and terpenoid, with thymol as the major constituent, reduced jejunal IL‐6 concentrations on Day 7 after F18⁺ E. coli challenge in weaned pigs (Garavito‐Duarte, Bonetti, et al. 2025). This attenuation of early inflammatory responses may be attributed to phytochemical compounds such as terpenes and terpenoids, including thymol, which have been shown to suppress inflammatory cytokine expression by inhibiting IL‐6 mRNA transcription (Li, Zhang, et al. 2019).

Yeast‐derived postbiotics components may also contribute to the reduction of cytokine amplification during intestinal stress. Supplementation with Saccharomyces cerevisiae cell wall extracts has been shown to reduce systemic inflammatory markers, with a reported 16% decrease in circulating TNF‐α in weaned pigs exposed to mycotoxin‐contaminated diets including aflatoxin B1, DON, and fumonisin B1 (Kim, Holanda, et al. 2019). This effect is likely associated with the presence of β‐glucans and mannan‐rich fractions, which can modulate immune signaling by interacting with immune cell receptors and reducing pro‐inflammatory cytokine production (Davis et al. 2004; Sweeney et al. 2012). Additionally, yeast‐derived components have demonstrated the ability to bind mycotoxins under in vitro conditions (Yiannikouris et al. 2004; Yiannikouris et al. 2006). This binding capacity has been associated with reduced tissue deposition of mycotoxins and enhanced elimination from the body (Firmin et al. 2010).

Dietary bioactive compounds can influence adaptive immune responses by modulating immune cell activation and antibody production in the intestinal mucosa. For example, MO has been shown to regulate immune responses during early life. In neonatal pig models, supplementation with sialyllactose increased serum IgG concentrations whereas reducing circulating TNF‐α, suggesting that dietary MO may support immune regulation and enhance humoral immune responses (Li, Chen, et al. 2022). Additionally, neutral oligosaccharides such as lacto‐N‐neotetraose (LNnT) have been reported to promote balanced immune responses by stimulating T helper cell activity and increasing interferon‐γ, whereas simultaneously supporting anti‐inflammatory signaling through IL‐10 production in the ileum (Li et al. 2014). Phytogenic compounds may also influence immune cell responses in the intestine. Supplementation with a microencapsulated blend of phytobiotics containing terpenes and terpenoids produced a quadratic effect on jejunal IgG concentrations in weaned pigs challenged with F18⁺ E. coli , suggesting that phytobiotic compounds may modulate B‐cell‐mediated immune responses (Garavito‐Duarte, Bonetti, et al. 2025). Terpenes and terpenoids have been proposed to interact with immune B cells and influence antibody production, which may help support host defense whereas limiting excessive inflammatory responses (Nimmerjahn and Ravetch 2010).

According to Holanda et al. (2020), supplementation of a multi‐component additive containing modified clay minerals, inactivated Saccharomyces cerevisiae , fermentation extracts, calcium propionate, and botanicals ( Silybum marianum , Rosmarinus officinalis , licorice root, and Peumus boldus ) reduced jejunal IgA by 43% in weaned pigs fed DON‐contaminated diets. This response likely reflects reduced antigenic stimulation, as toxin adsorption and improved epithelial integrity limit luminal antigen exposure and subsequent mucosal immune activation (Holanda et al. 2020).

4.4. Intestinal Redox Balance and Antioxidant Response

In addition to modulating microbial populations and metabolites, dietary interventions can influence intestinal redox balance. Enzyme supplementation, such as xylanase, has been shown to reduce oxidative stress in the jejunum of weaned pigs by decreasing concentrations of MDA, a marker of lipid peroxidation (Duarte et al. 2019; Moita et al. 2022). Similarly, yeast‐derived components have been associated with reduced oxidative damage, as evidenced by an 11% decrease in protein carbonyl concentrations in the jejunal mucosa of weaned pigs exposed to mycotoxins including aflatoxins and fumonisins (Kim, Holanda, et al. 2019). Consistent with these findings, Saccharomyces yeast postbiotics have been reported to further reduce protein carbonyl levels by up to 46%, indicating a substantial reduction of oxidative protein damage and improved redox homeostasis (Gormley, Duarte, et al. 2024).

Under mycotoxin‐induced stress, multi‐component interventions targeting both toxin sequestration and antioxidant capacity appear particularly effective. Supplementation of a blend containing modified clay minerals, inactivated Saccharomyces cerevisiae , fermentation extracts, calcium propionate, and botanicals ( Silybum marianum , Rosmarinus officinalis , licorice root, and Peumus boldus ) reduced malondialdehyde (or MDA) concentrations by 40% in weaned pigs fed DON‐contaminated diets (Holanda et al. 2020). This response suggests a combined effect of mycotoxin adsorption and enhancement of antioxidant defenses, which may facilitate cellular repair processes and reduce lipid peroxidation (Holanda et al. 2020). In addition, supplementation with 6% spray‐dried plasma reduced serum 8‐hydroxy‐deoxyguanosine by 28%, indicating attenuation of oxidative DNA damage, likely associated with the immunomodulatory and anti‐inflammatory properties of bioactive components present in plasma proteins (Weaver et al. 2014).

Marine‐derived polysaccharides may also enhance antioxidant defense mechanisms. Supplementation with seaweed‐derived polysaccharides increased the activity of key antioxidant enzymes, including glutathione peroxidase (24%), superoxide dismutase (29%), and catalase (35%) in weaned pigs (Zou et al. 2021). These enzymatic systems are critical for neutralizing ROS and maintaining cellular redox homeostasis (Yin et al. 2013). In addition, sulfated polysaccharides from macroalgae may further reduce oxidative stress by downregulating inflammatory mediators such as inducible nitric oxide synthase and cyclooxygenase‐2, thereby limiting the production of nitric oxide and prostaglandin E2 (Shen et al. 2024).

4.5. Epithelial Repair and Barrier Integrity

Maintenance of epithelial structure and function is essential for preserving intestinal barrier integrity and supporting efficient nutrient absorption during the post‐weaning period. Certain dietary bioactive compounds, including MO, have been shown to influence epithelial turnover and intestinal development. In piglets, supplementation with sialyllactose increased villus height and the villus‐to‐crypt depth ratio in the jejunum, suggesting improvements in epithelial maturation and absorptive capacity (Li, Chen, et al. 2022). Evidence also indicates that fucosylated MO supplementation may stimulate expansion of ileal crypt and increase sucrase activity, reflecting improved enterocyte differentiation and functional maturation (Coppa et al. 2006; Daniels et al. 2022).

Enteric infections can compromise epithelial barrier integrity. For example, F18⁺ E. coli challenge reduced the expression of tight junction proteins such as occludin and zonula occludens‐1 during the acute post‐challenge phase, indicating disruption of intestinal barrier function (Duarte et al. 2023). However, dietary interventions may help preserve epithelial integrity during intestinal stress. Phytogenic compounds rich in terpenes and terpenoids have been shown to support intestinal barrier function during enteric challenges. Supplementation with these compounds increased the expression of tight junction proteins in pigs exposed to F18⁺ E. coli , indicating enhanced epithelial integrity (Garavito‐Duarte, Bonetti, et al. 2025). In addition to reinforcing barrier structure, phytobiotics may also promote epithelial renewal. The inclusion of phytobiotics has reported increased crypt cell proliferation (Garavito‐Duarte, Bonetti, et al. 2025), as indicated by a greater abundance of Ki‐67⁺ cells during the early post‐challenge period, suggesting an improved regenerative response of the intestinal mucosa (Chen et al. 2019; Gāliņa et al. 2020; Duarte and Kim 2024). Supplementation with Saccharomyces yeast postbiotics reduced cell proliferation count (Ki‐67⁺ cells) by 31% in weaned pigs challenged with F18⁺ E. coli (Gormley, Duarte, et al. 2024), suggesting that Saccharomyces yeast postbiotics can support cell proliferation in challenged weaned pigs. The supplementation of β‐glucan increased villus height in the duodenum by 18% in weaned pigs (Manzke et al. 2025). Furthermore, yeast‐derived postbiotics have been associated with increased expression of mTOR, with reported upregulation of 32% under challenge conditions (Gormley, Duarte, et al. 2024). Activation of mTOR signaling is closely linked to cellular protein synthesis, epithelial turnover, and maintenance of intestinal integrity (Wang et al. 2020), indicating that these compounds may not only attenuate inflammatory signaling but also support mucosal recovery and functional homeostasis.

Marine‐derived polysaccharides may also contribute to the preservation of epithelial structure during the post‐weaning period. Dietary supplementation with seaweed‐derived polysaccharides has been shown to markedly increase jejunal villus height, with reported improvements of up to 42% in weaned pigs, indicating enhanced absorptive surface area (Zou et al. 2021). In parallel, these compounds upregulated the expression of tight junction proteins, including zonula occludens, claudin‐1, and occludin, suggesting improved barrier integrity.

Pigs exposed to diets contaminated with DON showed a reduction in proliferative Ki‐67⁺ cells in the jejunum when supplemented with a blend of adsorbent agents (bentonite and calcareous algae) and Saccharomyces cerevisiae –derived β‐glucans (Holanda et al. 2020). This response suggests that these additives may attenuate epithelial proliferation and modulate mucosal turnover under mycotoxin‐induced stress. The adsorptive capacity of bentonite is attributed to its negative charge (Chaytor et al. 2011), whereas β‐glucans from yeast cell walls have also been shown to contribute to mycotoxin binding (Sabater‐Vilar et al. 2007; Kong et al. 2014). In addition, the supplementation of calcareous algae ( Lithothamnium calcareum ) has shown adsorption capacity with mycotoxins in vitro and broilers (Perali et al. 2020).

5. Implications

Post‐weaning intestinal dysfunction in pigs is driven by multiple stressors that converge on common biological pathways, including microbiota imbalance, PRR activation, cytokine amplification, oxidative stress, and epithelial barrier disruption. This convergence indicates that nutritional strategies should be evaluated according to their regulatory effects on these shared pathways rather than only by ingredient class. Functional compounds such as milk oligosaccharides (MO), phytobiotics, yeast‐derived postbiotics, macroalgae sulfated polysaccharides, and NSP‐degrading enzymes can modulate different components of this network, including host–microbe interactions, inflammatory signaling, redox balance, and epithelial repair.

These observations suggest that the most effective nutritional interventions may be those that combine complementary mechanisms of action and are matched to the predominant stressors present in a given production setting. Accordingly, nursery diet formulation should place greater emphasis on preserving mucosal function and reducing the metabolic cost of immune activation, in addition to supporting growth performance. Future research should integrate microbiota, immune, oxidative biomarkers, and epithelial morphology with functional outcomes to better define how dietary interventions improve intestinal health.

Funding

This work was supported by North Carolina Agricultural Foundation, 10.13039/100009591, 660101, 665164; USDA‐NIFA Hatch, 02893; Nagoya University and NC State Seed Funds.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.