A systematic literature search was performed to identify all relevant studies published in recent 10 years on HT, especially in the past 5 years. In the review, we added 71 references in total to strength. The following electronic databases were queried such as PubMed, Web of Science and Google Scholar. The search strategy utilized a combination keywords related to hydroxytyrosol, livestock, gut health, oxidative stress, liver protection and feed additive.

Studies were included if they met the following criteria: firstly, the paper should be an original paper with a well-designed experiment and statistically significant results for data supporting. Secondly, the research had to focus on the application of hydroxytyrosol in livestock and poultry production. Thirdly, the research should be conducted within 5 years and the experiments should be reproducible. Conversely, studies were excluded if they related to human. Secondly, review articles were not searched for in this paper as references. Finally, articles with insufficient experimental design and sample size, or articles not in English were all not chose as references.

Reactive Oxygen Species (ROS) are highly reactive molecules containing oxygen, generated through both normal cellular metabolism and exposure to external factors. This complex interplay of metabolic byproducts, enzymatic reactions, conversions, and environmental exposures constitutes the major pathways of ROS formation within biological systems. When the generation and clearance of ROS are not balanced, it often results in damaging to organs. Therefore, inhibiting ROS is fundamental to protecting cellular molecules (such as lipids, proteins or DNA) and avoiding the development of degenerative diseases (23, 24). The imbalance of ROS is due to the fact that the clearance capacity of the antioxidant system in vivo is less than the rate of ROS generation, which leads to a large accumulation of ROS and causes oxidative stress damage. At this time, antioxidants need to be supplemented in vitro. Its hydroxyl group (OH) has electron-donating ability at the adjacent position and can form stable hydrogen bonds with phenoxy groups, thereby reducing the generation of free radicals and inhibiting oxidative stress reactions (25–27). In swine, dietary HT supplementation significantly alleviates intestinal oxidative damage induced by herbicides like diquat. It directly activates the PI3K/Akt-Nrf2 signaling pathway in porcine intestinal epithelial cells (IPEC-J2), boosting the production of antioxidant enzymes to scavenge reactive oxygen species (ROS) (11). Concurrently, HT promotes mitophagy to remove damaged mitochondria, reducing ROS generation. The synergistic interaction between Nrf2 activation and mitophagy preserves gut barrier integrity, thereby improving nutrient absorption and growth efficiency (28). In broilers, HT (0.5 mg/g in feed) elevates serum antioxidant capacity, evidenced by increased catalase (CAT) activity and reduced malondialdehyde (MDA), a lipid peroxidation marker. It also down-regulates pro-inflammatory cytokines such as IL-1β, IL-6, TGF-β and suppresses the TLR4/NF-κB pathway in the jejunum, alleviating intestinal inflammation (29). These effects collectively enhance growth metrics: HT-supplemented broilers showed a 3.7% increase in final body weight and improved average daily feed intake (ADFI) compared to controls, nearing the performance of antibiotic-fed groups. HT can activate antioxidant enzymes that scavenge ROS and promote the expression of Nrf2, including γ-Glutamyl Cysteine Ligase (γ-GCL), heme oxygenase-1 (HO-1), NAD(P)H:quinone oxidoreductase (NQO1) and thioredoxin reductase (TrxR), inhibit the generation of ROS and vascular damage, and actively regulate the antioxidant defense system in vascular endothelial cells (30, 31). In addition to the above antioxidant mechanisms, HT can also directly scavenge free radicals and destroy free radical chain reactions, preventing the generation of a large number of new free radicals due to chain reactions, thereby reducing oxidative stress damage caused by excessive free radicals (32). HT can also bind to unstable metal ions in cells through hydroxyl groups, thereby protecting cells from DNA damage or cell apoptosis caused by oxidative stress (33).

In terms of anti-inflammation, Yao et al. (34) showed that the protective effect of HT on vascular endothelial cell inflammation is achieved through the PKM2 signaling pathway mediated by the SIRT6. In addition, studies have shown that HT has a therapeutic effect on COX-2-mediated inflammation and can be used in combination with traditional short-acting anti-inflammatory drugs (35). Scoditi et al. (36) found in an in vitro experiment that HT can inhibit the peroxisome proliferator-activated receptor γ (PPARγ) signaling pathway in mouse 3 T3-L1 adipocytes, thereby showing anti-inflammatory activity. There are also relevant reports on in vivo studies. Liu et al. (37) added 50 mg/(kg·d) HT to the diet of C58BL/6 J mice for 8 weeks, it was shown that the levels of liver inflammatory markers including interleukin (IL)-1β and IL-6 were significantly decreased, the expression of tumor necrosis factor-α (TNF-α), IL-1β, Toll-like receptor 4 (TLR4) and phosphorylated phosphokinase (p-JNK) in the liver were reduced, meanwhile the release of lipopolysaccharide (LPS) in the blood was inhibited. Richard et al. (38) established an inflammatory model by stimulating mouse macrophages with LPS and evaluated the effects of HT on inflammatory mediators, cytokines and chemokines. The results showed that HT reduced the secretion of cytokines including IL-1α, IL-1β, IL-6, IL-12, TNF-α and decreased the gene expression of nitric oxide synthase (iNOS), IL-1α, CXCL10/IP-10, macrophage inflammatory protein-1β (MIP-1β), matrix metalloproteinase 9 (MMP-9), clarifying the molecular basis of HT in the treatment of inflammation. Zhang et al. (39) also determined the anti-inflammatory mechanism of HT by detecting the expression of iNOS, cyclooxygenase 2 (COX-2), the formation of TNF-α and the release of NO. The results showed that HT inhibited the expression of COX-2 gene in LPS-stimulated monocytes and significantly reduced the secretion of TNF-α and the release of NO, providing an effective theoretical basis for the treatment of inflammation. The previous research has found that the metabolite Tyr produced by the interaction of HT with intestinal microorganisms can prevent excessive production of nitric oxide (NO) by regulating p38 and ERK1/2MAPK, and concentrate it in the intestine, thereby significantly enhancing its protective activity against inflammation (40). In terms of antibacterial, a large number of studies have shown that HT has good antibacterial activity (41, 42). Rodríguez-Morató et al. (43) found that low concentrations of HT have antibacterial activity against respiratory and gastrointestinal pathogens such as Vibrio enteritidis, Vibrio cholerae, Salmonella typhi, Haemophilus influenzae, Staphylococcus aureus, and the inhibitory concentration is even lower than that of some antibiotics such as ampicillin. However, Medina-Martínez et al. (44) found that low concentrations of HT had a poor ability to inhibit bacterial growth, requiring 400 μg/mL to inhibit the growth of Escherichia coli strains. This phenomenon may be due to the oxidation of HT in nutrient-rich culture media, which weakened its antibacterial effect. Some studies have also shown that HT can effectively destroy fungal cell membranes and thus exhibit anti-fungal effects (45, 46). Therefore, HT has strong anti-inflammatory and antibacterial effects and can be used for diarrhea caused by intestinal inflammation in animals.

Studies have reported that HT can reduce the differentiation and proliferation of adipocytes and reduce the number of lipid droplets in adipocytes (47–49). Garcia-Contreras et al. (50) added 1.5 mg/(kg·d) HT to the diet of pregnant sows. The results showed that HT did not affect the deposition of fetal fat, but significantly increased the synthesis of essential fatty acids in the fetus and significantly affected the ratio of ω-6 and ω-3 fatty acids, thereby improving the symptoms of intrauterine growth retardation in animals. This suggests that there are differences in the effects of HT on fat metabolism in vivo. In addition, experiments conducted by Echeverria et al. (51) on rats showed that the combined use of HT and eicosapentaenoic acid (EPA) can significantly reduce the occurrence of non-alcoholic fatty liver disease (NAFLD). The main effect of HT is that it enhances the antioxidant capacity and inhibits liver fatty degeneration. HT has antioxidant activity both in vitro and in vivo, not only can scavenge reactive oxygen free radicals, but also reduce the number of lipid droplets in hepatocytes, thereby reducing the accumulation of triglycerides in cells, and significantly reduce lipid synthesis (52). HT can also improve insulin resistance by regulating endoplasmic reticulum stress and prevent hepatic steatosis in diet-induced obese mice (53). In summary, on one hand HT can reduce the differentiation and proliferation of adipocytes to reduce the number of lipid droplets in adipocytes; on the other hand, it can reduce lipid synthesis and deposition by regulating the expression of inflammatory-related factors and strong antioxidant capacity. The mechanism action of HT under different conditions was shown in Table 2.

Hydroxytyrosol (HT), a natural phenolic compound primarily derived from olives and olive oil, has garnered significant attention in poultry production due to its potent antioxidant, anti-inflammatory and antimicrobial properties. Recent research highlights its potential as a green feed additive alternative to antibiotics, particularly in broiler chickens (54). Studies demonstrated that dietary supplementation of HT in broiler diets significantly enhanced serum antioxidant capacity by increasing catalase (CAT) activity and reducing malondialdehyde (MDA) levels, thereby mitigating oxidative stress (55). Additionally, HT down-regulated key inflammatory pathways such as TLR4/NF-κB and reduces pro-inflammatory cytokines such as IL-1β, IL-6, TGF-β in intestinal mucosa, improving gut health (56). Although HT boosted average daily feed intake (ADFI), its effects on growth performance remain statistically insignificant compared to antibiotics, suggesting nuanced benefits (29). Emerging mechanisms reveal HT activated the Nrf2 antioxidant pathway and modulates gut microbiota, further enhancing antioxidant defenses and metabolic health (57). In summary, HT is a viable, natural strategy to promote poultry health in antibiotic-free farming, though further research is needed to optimize its application and assess long-term impacts.

HT is also widely used as a feed additive in pigs. Recent research highlighted its efficacy in alleviating intestinal oxidative stress, a common issue in pigs caused by factors like weaning stress, mycotoxins, and high stocking densities (58). Studies demonstrated that dietary supplementation with HT significantly enhances antioxidant capacity by activating the PI3K/Akt-Nrf2 signaling pathway and promoting mitophagy in intestinal epithelial cells, thereby reducing oxidative damage and improving gut barrier function. For instance, HT was shown to increase serum levels of catalase (CAT) and superoxide dismutase (SOD) while decreasing malondialdehyde (MDA), a marker of lipid peroxidation (59). Additionally, HT modulated bile acid metabolism, further contributing to its antioxidative effects (13). In weaned piglets, HT supplementation improved growth performance, as evidenced by increased average daily gain (ADG) and reduced feed conversion ratio (F/G) (60). Moreover, HT enhanced intestinal health by up-regulating tight junction proteins such as ZO-1 and occludin, reducing pro-inflammatory cytokines such as IL-1β and IL-6 (61). Recent findings also indicated that HT benefits boar semen quality by improving gut microbiota and blood metabolome, increasing beneficial bacteria like Bifidobacterium and reducing harmful bacteria such as Streptococcus (62). Therefore, hydroxytyrosol had been widely used in swine industry. It is a new feed additive to replace antibiotics and has great potential in livestock production.

Hydroxytyrosol (HT) as a natural polyphenol primarily derived from olives, shows emerging potential in ruminant nutrition, particularly for its antioxidant and anti-inflammatory benefits. However, direct evidence of its impact on methane emissions in cattle remains limited. Current research indicated that HT enhances antioxidant capacity in ruminants by activating the Nrf2 signaling pathway and improving gut microbiota composition such as increasing Lactobacillus and Firmicutes abundance, thereby reducing oxidative stress and supporting metabolic health (63). Although no studies explicitly link HT to methane reduction in ruminants, studies had shown that HT influences the rumen microbiota by selectively promoting beneficial bacteria and suppressing methanogenic archaea. Similarly, other plant-derived polyphenolic extracts have demonstrated positive effects on ruminal fermentation and degradability in vitro, supporting the potential of phenolic compounds to modulate microbial activity in ruminants (64). For example, studies indicated that HT supplementation increases the abundance of Ruminococcaceae and Prevotella which were the key taxa involved in fiber degradation and propionate production while reducing Desulfovibrionaceae, which is associated with sulfate reduction and methane formation. Shifts in microbial composition toward propionate-producing pathways to enhance energy harvest and reduce hydrogen availability for methanogenesis (65–67). Recent advancements in methane mitigation had focused on direct interventions like feed additives such as Plantain (68), but HT remains under-explored in this context. Future research should investigate the role of HT in modulating rumen fermentation and its synergy with established methane-reducing strategies. In conclusion, while hydroxytyrosol is not yet a proven methane-reducing agent in cattle, its strong antioxidant and microbiota-modulating effects position it as a promising supportive tool for improving ruminant health and potentially contributing to broader sustainable farming practices.

Hydroxytyrosol (HT) has gained significant attention in aquaculture for its potent antioxidant, anti-inflammatory and metabolic regulatory properties. Recent research highlighted its multifaceted benefits across various aquatic species. In rainbow trout, HT supplementation effectively mitigated Aeromonas hydrophila-induced kidney damage by reducing pathological changes and modulating immune gene expression, thereby strengthening antioxidant defenses and immune response (69). In blunt snout bream, HT alleviated high-fat-diet-induced fatty liver by activating AMPK signaling, enhancing mitochondrial function and regulating lipid metabolism such as promoting fatty acid oxidation via PPAR-α pathways (70). Similarly, in gilthead sea bream, HT-rich extract reduced plasma free fatty acids and muscle lipid peroxidation, improved liver lipid metabolism and enhanced antioxidative capacity, demonstrating its role in optimizing lipid utilization and health under high-fat diets (71). Compared to its application in terrestrial species, HT is more easily utilized by aquatic organisms and increase the utilization rate. Therefore, hydroxytyrosol emerges as a promising natural additive in aquaculture, offering solutions to oxidative stress, infectious diseases and metabolic disorders like fatty liver. However, optimal dosing and species-specific responses require further investigation to maximize its benefits in sustainable aquaculture practices. The summary of potential applications of hydroxytyrosol in livestock and poultry production was shown in Figure 3.