Although the avian brain is relatively small, it achieves highly complex physiological regulation through specialized modular structures (48). As the central hub of neuroendocrine integration, the hypothalamus precisely regulates metabolic homeostasis through the neuropeptide network of the arcuate nucleus. Because feeding is essential for animals to maintain metabolism and growth, the hypothalamus plays a key role in body weight regulation, feeding behavior, and energy balance (49).

In regard to the feeding behavior of birds, the hypothalamus contains various neuropeptides involved in appetite regulation, which can be categorized based on their effects as appetite-promoting or appetite-suppressing neuropeptides (50). Appetite-promoting neuropeptides include neuropeptide Y (NPY), agouti-related peptide (AGRP), and orexin, while appetite-suppressing neuropeptides include pro-opiomelanocortin (POMC) and its derivative α-melanocyte-stimulating hormone (α-MSH) (51). The arcuate nucleus of the hypothalamus is a key region for regulating feeding behavior. Neuropeptides such as NPY and AGRP are expressed in neurons within this region and modulate feeding behavior by projecting to secondary neurons in areas such as the paraventricular nucleus, lateral hypothalamus, and periventricular zone. Secondary neurons located in the lateral hypothalamus primarily express orexin and melanin-concentrating hormone (MCH), both of which promote feeding (52, 53). In addition to NPY/AGRP, POMC and cocaine- and amphetamine-regulated transcript (CART) are also expressed in arcuate nucleus neurons. Unlike NPY/AGRP, POMC and CART are expressed in different neurons within the arcuate nucleus. POMC/CART primary neurons can also project to secondary neurons located in the paraventricular nucleus, lateral hypothalamus, and periventricular zone of the hypothalamus (54).

Studies have found that under HSD conditions, the expression levels of NPY and AGRP in broilers are closely related to changes in food intake, with NPY expression reduced and AGRP showing a downward trend, while POMC gene expression decreases, in opposition to its appetite-suppressing effects (55). In addition, under restraint stress, both body weight and food intake in mice significantly decreased along with the ghrelin mRNA level in the hypothalamus, while the POMC mRNA level significantly increased (56). Ghrelin is an appetite-promoting factor, while POMC, through its product α-MSH, inhibits appetite and activates melanocortin receptors to suppress feeding. These results suggest that broilers in high-density rearing environments may face stronger competition for resources or the environment, which further exacerbates the occurrence of stress responses.

In growth regulation, the hypothalamus-pituitary-growth axis promotes the secretion of growth hormone (GH) through growth hormone-releasing hormone (GHRH), which is strictly regulated by somatostatin (SST). GH further induces the synthesis of insulin-like growth factor 1 (IGF-1) in the liver through growth hormone receptors (GHR), thereby promoting the development of bones and muscles (57). Numerous studies have shown that high-density rearing increases inflammation in broilers, and the release of pro-inflammatory factors like TNF-α, IL-1β further triggers leptin resistance, weakens AGRP neuron function, and induces metabolic reprogramming, ultimately leading to reduced food intake, nutritional imbalance, and stunted growth (58, 59). In addition, chronic stress activates the HPA axis, resulting in sustained elevation of glucocorticoids, which suppresses the GH/IGF-1 signaling pathway and reduces GHRH sensitivity (60). This structural-functional cascade reveals the systemic impact of high-density rearing on poultry production performance through the hypothalamus-mediated neuroendocrine network.

The liver is not only the largest digestive gland in an animal’s body, but also the core organ for redox regulation, energy metabolism conversion, and the synthesis of various substances in the body for maintaining physiological homeostasis (61). The liver receives dual blood supply from the systemic circulation (hepatic artery) and digestive system (portal vein), executing critical functions including nutrient metabolism, xenobiotic detoxification, and immunoregulation (62). The portal venous system serves not only as a conduit for nutrient transport but also delivers gut-derived exogenous substances, digestive metabolites, senescent erythrocytes, and microbial components with immunogenic potential (63).

The central nervous system (CNS) regulates hepatic immunity under stress through two primary pathways. First, the HPA axis releases glucocorticoids, which bind to hepatic glucocorticoid receptor alpha (GRα), influencing glucose metabolism and inflammation (64). Second, the sympathetic nervous system (SNS) is activated via splanchnic nerves, releasing norepinephrine that activates β2-adrenergic receptors on hepatocytes and Kupffer cells (65). These mechanisms collectively modulate the liver’s immuno-metabolic responses during stress.

Oxidative stress and inflammation are closely related in a variety of pathological processes; they occur simultaneously and promote each other, especially at damage sites (66). Studies have shown that HSD induces oxidative damage in the liver and triggers chronic inflammation by increasing reactive oxygen species (ROS) in broilers (67). A similar phenomenon has also been reported in laying ducks, where HSD significantly weakened their antioxidant capacity, thereby affecting egg production performance and egg quality (68). Similar hepatic lesions and oxidative-stress signatures have been described in rodent restraint-stress models, which helps contextualize stress-related inflammatory and redox pathways in the liver (69, 70).

In fish, density stress similarly disrupted the dynamic balance of energy reserves and expenditure, requiring the body to reallocate metabolic resources to meet the increased energy demands (71). Similarly, HSD can cause lipid metabolic disorders in poultry by affecting lipid synthesis, degradation, and transport. Excessive lipid accumulation can cause liver toxicity, leading to liver dysfunction and inflammation. This sustained metabolic load disrupts the balance between lipid synthesis and breakdown in liver cells, ultimately leading to pathological fat deposition (72, 73).

Mitochondria play a crucial role in cellular energy production through oxidative phosphorylation, which supports essential biological processes (74, 75). External stimuli can inhibit the replication of mitochondrial DNA, reduce mitochondrial biogenesis, and ultimately mitochondrial numbers (76). HSD environments increase markers of oxidative damage in liver mitochondria, such as elevated MDA levels, reduced GSH content, and decreased ATP levels. These changes affect mtDNA replication and significantly disrupt mitochondrial morphology and function in the livers of broilers (77). These results confirm the negative impact of HSD on liver energy metabolism.

The liver has traditionally been considered a sterile organ, but recent studies have challenged this view by demonstrating the presence of microorganisms in the livers of various animals (78). Research in cattle, dogs, and mice has identified bacterial communities within the liver parenchyma, suggesting that the liver may possess its own microbiome (79–81). Another study found an increased abundance of Pseudomonas aeruginosa, particularly associated with weakened hepatic antioxidant capacity, in the livers of broilers raised under HSD (67). This finding suggests a potential link between the hepatic microbiome and antioxidant defense. P. aeruginosa has been reported to induce oxidative stress in host cells by producing ROS through virulence factors (82, 83). Across poultry species, HSD is consistently linked to oxidative/metabolic disturbance, but the dominant physiological trade-offs appear to differ. In broilers and laying ducks, reduced antioxidant capacity aligns with impaired production traits, whereas geese show elevated citrate and L-malate under HSD, suggesting a stronger shift in TCA-cycle–related energy metabolism.

The avian intestinal system is a highly specialized and efficient tubular structure for digestion. Based on the order of food passage, it can be divided into the crop, proventriculus, gizzard, small intestine, large intestine, and cloaca (84). The crop serves as a temporary storage organ, where mucus is secreted to soften the ingesta. In some species, such as pigeons, it can also secrete crop milk to feed the young. The food then enters the proventriculus for chemical digestion, where strong acidic gastric juices and proteases initiate protein breakdown. The gizzard, with its thick muscular layer and grit, mechanically grinds hard food particles within 2–4 h, while a keratinized lining prevents tissue damage. The small intestine (comprising the duodenum, jejunum, and ileum) completes nutrient breakdown with the help of pancreatic enzymes and bile, while densely packed villi efficiently absorb nutrients like monosaccharides and amino acids. The paired ceca in the large intestine ferment cellulose through microbial action, while the rectum concentrates feces. Ultimately, all metabolic waste is expelled through the cloaca, which consists of three chambers (fecal, urinary, and rectal passages) (85). This short yet efficient intestinal structure, along with a rapid transit rate, meets the high metabolic demands of birds.

When food enters the small intestine, fine, microscopic intestinal villi on the surface of the intestinal mucosal epithelial cells increase the absorptive surface area, enhancing nutrient absorption efficiency, while also protecting the intestinal mucosa from damage and irritation. Villus height (VH), crypt depth (CD), and the ratio of villus height to crypt depth (VCR) are key indicators of intestinal morphological structure. VH represents the number and absorptive area of mature intestinal villus cells, while CD indicates the maturity of crypt cells; thus, the shape of the intestinal villi can influence the growth and development of the organism. Research has found that HSD at 21 days of age reduced the VH, the VCR, and the villus surface area in the duodenum, while increasing the number of intraepithelial lymphocytes (IELs) in the ileum (86). The impact of HSD on morphological parameters was even more pronounced at 42 days, affecting the villus surface area across all segments of the small intestine. These findings are consistent with the study by Kridtayopas et al. (87), which reported a decrease in villus height in broiler chickens due to HSD. Kamel et al. (88) also observed negative effects of HSD on the morphology of duodenal tissue. At the same time, studies have found that the feed conversion ratio (FCR) in broiler chickens was positively correlated with the VCR. A decrease in villus height is accompanied by a significant decline in FCR (89). In addition, villus damage leads to a decrease in disaccharidase activity, and undigested lactose ferments at the rear end of the intestine, causing osmotic diarrhea (90).

Tight junction proteins, including claudins, occludin, and adhesion molecules, are structures located between the epithelial cells of the intestinal wall that regulate paracellular permeability and the exchange of substances between cells, serving as important components of the intestinal epithelial barrier (91). These tight junction proteins form a robust barrier that prevents bacteria, toxins, and large molecules from indiscriminately entering the intestinal tissue while allowing water, nutrients, and certain small molecules to pass through (92). An increase in stocking density leads to broiler chickens being in a prolonged state of HSD stress, which in turn causes persistent activation of the HPA axis, resulting in elevated serum corticosterone levels. This suppresses the mRNA transcription of claudin-1, occludin, and ZO-1, while accelerating the degradation of tight junction proteins through the ubiquitin-proteasome system, and impairing the binding of ZO-1 to the cytoskeleton (25). Consequently, the intestinal barrier structure is compromised, leading to increased intestinal permeability and chronic intestinal inflammation. Under stress conditions, the production of ROS in the intestine exceeds the body’s regulatory capacity, causing an imbalance in redox homeostasis and leading to the oxidation of thiol groups in claudin and occludin, which results in a loss of activity (93).

The occurrence of intestinal barrier damage is often accompanied by dysbiosis of the gut microbiota (94). The primary function of gut microorganisms is to provide nutrients to the intestinal epithelial cells and the host through their metabolic products, enhance host immunity, help the host resist foreign invasions, and perform various functions (95, 96). For example, gut microbiota can not only influence the formation of intestinal blood vessels but also provide essential short-chain fatty acids (SCFAs) and vitamins, as well as assist in the digestion of dietary fiber (97). However, HSD not only affects the intestinal morphology of broiler chickens but also alters the composition of the gut microbiota. Lactobacillus has been confirmed as a beneficial bacterium in the intestine, competitively hindering the colonization of pathogenic bacteria in the gut, promoting the development of intestinal villi, and enhancing nutrient absorption (98). It plays an important role in maintaining the functional integrity of the intestinal microbial barrier. Studies have found that when the stocking density reaches 39 kg/m², the population abundance of Lactobacillus is significantly reduced, indicating a decline in this beneficial bacterial group under HSD conditions (99). In a study on the gut microbiota of ducks at different stocking densities, it was found that excessive HSD significantly increased the relative abundance of Firmicutes in the gut, while the ratio of Firmicutes to Bacteroidetes, which is associated with energy metabolism, is also significantly elevated (100). Research has found that in the intestines of broiler chickens under HSD stress, the population levels of Faecalibacterium increase, whereas those of Lactobacillus and Bifidobacterium decrease, reflecting shifts in key microbial populations. These changes collectively contribute to dysbiosis of the intestinal microbiota, which may impair nutrient utilization and reduce production performance (101, 102). Disruption of the structure of the gut microbiota in poultry can reduce the efficiency of nutrient absorption and decrease feed utilization, thereby adversely affecting poultry production performance (103). Across poultry, higher stocking density is consistently associated with gut microbiota disruption, though the specific signatures vary by species. Broilers commonly show reduced beneficial taxa (e.g., Lactobacillus, Bifidobacterium) with enrichment of potentially pro-inflammatory microbes, ducks tend to exhibit an increased Firmicutes/Bacteroidetes ratio, and geese show altered cecal fermentation metabolites consistent with shifts in fiber and lipid utilization.

Overall, the brain, liver and intestine emerge as major target organs of HSD in poultry. Hypothalamic dysregulation alters feed intake and growth hormone signaling, hepatic oxidative injury and lipid dysmetabolism compromise nutrient handling, and intestinal barrier disruption together with dysbiosis undermines digestion and immune defense. By integrating these organ-level responses, HSD can be understood as a systemic condition in which central and peripheral dysfunctions.

Glycolysis is the metabolic process through which glucose is broken down into pyruvate or lactate via enzyme-catalyzed reactions, generating ATP (138). As one of the fundamental energy pathways, glycolysis can maintain cell function under hypoxic conditions by rapidly producing energy (139). During inflammatory responses, immune cells upregulate glycolytic activity to meet their high energy demands, and the enhanced glycolytic metabolism can amplify the inflammatory cascade by increasing the synthesis of pro-inflammatory cytokines and ROS (140). Under acute heat stress conditions, significant disturbances occur in the metabolic and endocrine systems of broilers, characterized by accelerated plasma protein breakdown, increased glucose levels, and decreased circulating levels of triiodothyronine (T3) (141). These changes are closely related to the activation of inflammatory responses and glycolytic pathways.

Long-term exposure of poultry to stress can affect their muscle metabolism. For example, heat stress accelerates muscle glycolysis, leading to a rapid decrease in pH and reduced water-holding capacity of the muscles, thereby increasing the risk of pale, soft, exudative (PSE) meat (142). Research by Wu et al. (143) found that, compared to a low-density group, broilers in a HSD group exhibited significantly higher cooking loss rates, greater decreases in pH, and increased lactate dehydrogenase activity in their breast muscles. Transcriptomic analyses further indicated that under HSD conditions, the expression of genes related to protein hydrolysis, glycolysis, and immune stress was upregulated in the muscles of broilers, while the expression of genes associated with muscle growth, cell adhesion, and collagen synthesis was downregulated (144). The study by Ebeid et al. (145) confirmed that HSD significantly reduced the sensory quality scores of broiler meat, suggesting that abnormal activation of glycolysis under HSD may be the primary cause of lower meat quality in broilers.

The TCA cycle is an intermediate pathway in the metabolism of carbohydrates, fats, and amino acids (146). This cycle begins with the oxidative decarboxylation of pyruvate, a product of glycolysis, to form acetyl-CoA, which then reacts with oxaloacetate to produce citrate (147). During this cycle, citrate is isomerized to isocitrate, then undergoes two oxidative decarboxylation reactions to generate α-ketoglutarate and succinyl-CoA, accompanied by substrate-level phosphorylation to produce one molecule of guanosine triphosphate (GTP) (148). Ultimately, succinyl-CoA completes the cycle by regenerating oxaloacetate. Throughout this process, the TCA cycle generates three molecules of nicotinamide adenine dinucleotide (NADH), one molecule of flavin adenine dinucleotide (FADH2), and one molecule of GTP, which provide a significant amount of reducing equivalents for ATP synthesis through the mitochondrial respiratory chain (149).

Under stress conditions, changes in metabolite levels in poultry can disrupt the TCA cycle, thereby affecting metabolic balance (150). For instance, under heat stress in broilers, the levels of key intermediate metabolites in the TCA cycle (such as L-malate and citrate) and the microbial metabolic product isobutyrate in the cecum are significantly reduced, indicating that TCA cycle activity is suppressed, which may lead to insufficient energy and impaired intestinal barrier function (151). In contrast, in geese under HSD stress environments, the concentrations of citrate and L-malate increase, while ribonucleic acid levels decrease, indicating enhanced TCA cycle activity. These metabolites may be converted into glucose through the TCA cycle to meet high energy demands but may also trigger inflammatory responses due to the accumulation of pro-inflammatory substrates (152). Therefore, the regulation of the TCA cycle under stress has a dual nature. Thus, although broilers, ducks and geese all exhibit TCA-cycle perturbations under density-related stress, the direction and extent of metabolite changes differ by species, suggesting that quantitative thresholds for HSD and optimal mitigation strategies may need to be tailored to each poultry species rather than extrapolated directly from Gallus-based data.

The mitochondrial oxidative phosphorylation system is a core component of cellular metabolism (153). The protein complexes, I-IV, coenzyme Q, and cytochrome c distributed on the mitochondrial inner membrane cristae must assemble into super-complexes to maintain normal function. Together with complex V (F1F0-ATP synthase), they complete the oxidative phosphorylation apparatus for ATP production, which is the primary energy carrier in nearly all cellular processes (154).

The mitochondrial membrane potential, driven by the formation of a proton gradient, is a critical factor for ATP synthesis, its decline typically indicating damage to the oxidative phosphorylation system (155). Evidence shows that HSD significantly reduces the mitochondrial membrane potential in the liver of broilers and inhibits the activity of Na⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase. These changes inhibit ATP synthesis by disrupting ion gradients and reducing oxidative phosphorylation efficiency (102). Furthermore, the activity of the electron transport chain complexes (I-IV) directly influences oxidative phosphorylation efficiency, and their reduced activity typically impedes normal electron transfer, resulting in increased superoxide formation (156). Yang et al. (77) found that, compared to a low-density group, broilers in the HSD group exhibited decreased activity of complexes I and III in liver tissue, accompanied by elevated levels of MDA, and reduced levels of glutathione (GSH) and ATP. This confirms the importance of complexes I and III in oxidative stress, which increases superoxide leakage, adversely affecting cellular energy metabolism and redox status. Thus, HSD has a significant impact on mitochondrial oxidative phosphorylation and overall cellular metabolism.

Figure 2 summarizes the metabolic consequences of HSD by integrating glycolysis, the TCA cycle, and mitochondrial oxidative phosphorylation into a unified model. HSD-induced activation of the HPA axis elevates corticosterone, which redirects nutrient allocation toward stress adaptation and enhances glycolytic flux. Concurrently, impaired TCA cycle turnover and reduced activities of electron transport chain complexes I and III compromise mitochondrial ATP generation, increasing ROS leakage and amplifying oxidative damage. By visually linking these pathways, Figure 2 clarifies how HSD disrupts energy metabolism at multiple regulatory nodes.

Chlorogenic acid (CGA) is a bioactive dietary polyphenol, widely distributed in medicinal plants such as honeysuckle (Lonicera japonica) and Eucommia (Eucommia ulmoides). Its biosynthesis involves the shikimate pathway (157). Because of its multiple functions, including antibacterial, anti-inflammatory, and heat-clearing properties, CGA has shown broad potential applications in medicine, food, healthcare, and organic syntheses. In recent years, CGA has been referred to as “plant gold,” and its use as a feed additive for animal health has attracted considerable attention, especially in response to the reduction or ban of antibiotic usage in agriculture. The pharmacokinetics of CGA have been elucidated in many different species. In rat models, Lafay et al. (158) confirmed that a small amount of the parent compound is directly absorbed in the stomach, while most CGA is hydrolyzed by esterases into caffeic acid and quinic acid once it reaches the intestines (159). These metabolites are then transported into the bloodstream through intestinal epithelial cells. Plasma primarily contains metabolites such as hydroxycinnamic acid derivatives, rather than CGA itself (160).

In HSD poultry farms, the addition of CGA to feed significantly improved the height of the ileal villi and the villus height-to-crypt depth ratio in broiler chickens. It also restored the expression of tight junction proteins (OCLN and ZO-1), reduced the levels of TNF-α, IL-1β, and IL-6, and decreased the abundance of harmful bacteria in the gut (161). CGA also mitigated oxidative stress by restoring SOD/GSH-Px activity, enhanced intestinal barrier function by upregulating tight junction protein expression, and restored balance of cecal microbiota by enriching beneficial bacteria such as Blautia (14). CGA also significantly lowered oxidative stress, which improved the meat quality of HSD broiler chickens. By activating the Nrf2 pathway, CGA significantly improved meat quality under oxidative stress, by restoring normal muscle pH, water-holding capacity, and color. Research has shown that the antioxidant effects of CGA are closely related to its regulation of metabolites. Four potential biomarkers—pyrimidine/purine metabolism, propionate metabolism, phenylalanine metabolism, and lysine metabolism—have provided new directions for future research on meat quality assessment (162).

In studies on the effects of heat stress on chicken embryos, eggs were injected with different doses of CGA into the amniotic cavity of chicken embryos, followed by heat stress treatment. Eggs injected with 4 mg of CGA exhibited significant antioxidant benefits, with a marked reduction in MDA levels, and a significant increase in the activity of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) (163). Moreover, CGA significantly increased intestinal microbiota diversity, promoted the production of short-chain fatty acids, and enhanced the expression of the immune-related proteins, thymosin β and legumain, by activating PPAR and MAPK signaling pathways. It also elevated the levels of health-promoting metabolites such as 2,4-dihydroxybenzoic acid, providing new insights into the application of CGA in improving immune function and gut health in broiler chickens (164).

In recent years, the application of vitamins as feed additives has gained increasing attention. Vitamin E, comprising tocopherols and tocotrienols, is a fat-soluble nutrient with potent antioxidant properties (165). It protects cellular membranes and tissues from free radical-induced lipid peroxidation while modulating enzyme activity (166). Among its isoforms, α-tocopherol, the most biologically active form, participates in the glutathione peroxidase pathway to combat oxidative damage. Due to its lipid solubility, vitamin E serves as an ideal membrane-bound antioxidant. Poultry require dietary vitamin E supplementation as they cannot synthesize it endogenously. Under heat stress, elevated levels of corticosterone and catecholamines induce lipid peroxidation in cellular membranes. Studies demonstrate that vitamin E enhances the survival, proliferation, and functionality of lymphocytes, macrophages, and plasma cells, protecting them from oxidative damage and enhancing immune responses (167).

Selvam et al. (168) investigated the effects of vitamin E and stocking density on broiler performance and antioxidant capacity. Results showed that HSD groups given vitamin E had significant improvement in body weight, FCR, European production efficiency factor (EPEF), heterophil-to-lymphocyte (H/L) ratio, and hepatic GSH and MDA levels compared to non-supplemented HSD groups. The study concluded that adding 70 g/ton of vitamin E to HSD broiler diets effectively mitigate the adverse effects of overcrowding, enhancing both productivity and antioxidant status (168). Shehata evaluated LSD, MSD, and HSD in broilers, with vitamin E and zinc supplementation. They measured growth, hormones, gene expression, and economic outcomes. MSD and HSD impaired these parameters compared with LSD. HSD had the strongest negative effect. Vitamin E alleviated HSD-related impairments and increased profitability (169). It downregulated IL-1β, interferon-γ, mucin 2, and trefoil factor family 2. It also upregulated IL-4 and IL-10. Overall performance improved accordingly. These findings underscored vitamin E’s anti-inflammatory efficacy in broilers, particularly during coccidiosis vaccination-induced inflammation (170). Additional studies showed that water-soluble vitamin E (WVE) more effectively downregulated pro-inflammatory cytokine and upregulated anti-inflammatory cytokine gene expression in the jejunum compared to fat-soluble forms. Increasing WVE dosage further suppressed jejunal inflammatory markers, highlighting its superior role in modulating intestinal immune responses (171).

As an essential trace element for animals, selenium (Se) plays a vital role in maintaining organism health and antioxidant defense systems (172). This micronutrient not only participates in forming core components of antioxidant systems, but also scavenges ROS and helps to maintain redox homeostasis. The biological functions of selenium are primarily mediated through its incorporation into selenoproteins. Twenty-four selenoproteins have been identified in broilers that perform specialized roles across various cellular organelles and tissues (173). Mitochondrial selenoproteins including glutathione peroxidase 4 (GPX4), selenoprotein O (Seleno-O), and thioredoxin reductase 2 (TXNRD2) are crucial for maintaining mitochondrial function and preventing ROS-induced damage (174, 175). Endoplasmic reticulum-localized selenoproteins such as deiodinase 2 (DIO2), selenoprotein F (Seleno-F), K (Seleno-K), M (Seleno-M), N (Seleno-N), S (Seleno-S), and T (Seleno-T) play key roles in protein folding, calcium homeostasis, and cellular stress responses (176, 177). Additionally, cytoplasmic selenoproteins including glutathione peroxidase 1–3 (GPX1), GPX2, GPX3), and selenoprotein W (Seleno-W) work synergistically with their mitochondrial and endoplasmic reticulum counterparts to eliminate excess ROS, restore cellular redox balance, and protect against oxidative damage (178).

In practical poultry production, maintaining optimal Se status is critical. Deficiency impairs antioxidant defenses, increases oxidative stress susceptibility, and adversely affects growth performance and immune function. Conversely, excessive Se intake may induce selenosis characterized by growth retardation, feather loss, and neurological damage (179). Optimal Se supplementation becomes particularly important under HSD rearing or stress conditions. Organic Se forms like selenomethionine (SeMet) have enhanced bioavailability compared to inorganic sources, enabling more effective antioxidant and immunomodulatory actions (180).

Recent studies showed that SeMet effectively protected broiler liver against oxidative damage and metabolic disorders under chronic heat stress conditions. By increasing hepatic Se concentrations and upregulating key selenoproteins (GPX4, TXNRD2, etc.), SeMet enhanced antioxidant capacity while alleviating mitochondrial dysfunction, TCA cycle abnormalities, and ER stress. This treatment also helped to normalize hepatic lipid and glycogen concentrations (181). SeMet also modulates AMPK signaling pathways to inhibit lipid/glycogen synthesis while promoting their breakdown, offering potential preventive and therapeutic strategies against heat stress-induced hepatic metabolic disturbances. Comparative studies in the LMH chicken hepatoma cell line revealed superior antioxidant activity of SeMet over sodium selenite. SeMet enhanced mRNA stability and protein synthesis rates for glutathione peroxidase (GPx) and thioredoxin reductase (TrxR), and in H₂O₂-induced oxidative stress models, SeMet provided stronger protection through ROS/MDA/NO reduction and antioxidant enzyme activation; Nrf2 pathway activation and upregulation of antioxidant selenoenzymes further contribute to its protective effects. These findings highlight SeMet’s potential as a feed additive for preventing or mitigating oxidative damage in poultry. SeMet also exhibits significant anti-inflammatory properties. Compared to normal diets, Se supplementation significantly inhibited LPS-induced hepatic inflammatory damage by reducing oxidative stress, inflammatory cytokines, and heat shock proteins, and downregulating NLRP3 and caspase-1 expression. Mechanistically, SeMet suppresses TLR4/NF-κB/NLRP3 signaling pathways to counteract LPS-induced hepatic inflammation (182).

In conclusion, anti-inflammatory and antioxidant feed additives including CGA, VE, and SeMet show promise in mitigating HSD stress in broilers (Table 2). These compounds not only improve growth performance and feed efficiency but also enhance immune function and overall health status, effectively counteracting the negative impacts of high-density rearing on hepatic energy metabolism. Future research should focus on investigating nutrient synergies, optimizing application protocols, elucidating mechanistic pathways, and evaluating long-term efficacy and environmental impacts to develop optimal nutritional interventions.