Green tea (GT) is a perennial, cross-pollinated shrub with evergreen leaves, white blooms, and green fruits that was discovered by Emperor Shennong in China in 2737 BC [41]. The order Ericales and family Theaceae are both home to the green tea plant. Due to its health benefits, GT herbs are used to produce a variety of tea beverages for humans. Tea plants intended for human and animal consumption must be planted on good quality soil that is free of heavy metals, metalloids, and other pollutants, which are reported to cause morphological and physiological alterations, such as reduced growth rate, nutritional imbalance, and photosynthetic suppression [42]. The processing of the tea by fermentation is known to change its phenolic composition, with the concentration of some compounds increased after [43], whilst some compounds are reduced (e.g., theanine) [44].

Green tea products contain considerable amounts of important nutrients for poultry nutrition. The crude protein content of GT leaves ranges from 18.15% to 22.9% [47,48] and can meet the protein requirements of laying and breeding birds (20% CP) as recommended by the National Research Council [49]. In addition, the leaves contain varying amounts of amino acids, including methionine, threonine, leucine, and L-theanine (Figure 1) occurring in largest amounts. L-theanine (γ-glutamylethylamide) is a non-protein water-soluble amino acid that is the most abundant of all amino acids (~50%) in GT. It is reported to have pharmacological properties that are beneficial in poultry production [41], including serving as an antioxidant, growth promoter, immune booster, antimicrobial, and anti-inflammatory agent [50].

Abdo et al. [47] reported that GT leaves can provide between 11.3 and 14.6 MJ/kg metabolisable energy (Table 1), which is adequate for growing and laying quail (11.7 MJ/kg and 10.7 MJ/kg, respectively) [52]. The leaves also contain high concentrations of minerals and trace elements, vitamins (A, C, E, K, and B complex), lipids (α-linolenic and linoleic acids), and pigments (carotenoids and chlorophyll) [53], which could be the reason why most research focused on its potential as a source of dietary nutrients. Moreover, the leaves also possess complex carbohydrates, such as pectins, cellulose, hemicellulose, and lignin [54], which limit their utilisation in poultry, especially at higher dietary levels [55]. This necessitates the need to determine the optimum dietary inclusion level of GT products in each poultry species so that the wellbeing of the birds is not compromised [3].

Green tea is a rich source of biologically active compounds, such as catechins, epicatechins, l-theanine, theaflavins, flavonol glycosides (quercetin, kaempferol, and myricetin), theobromine (methylxanthine), caffeine, and volatile organic substances [57]. It is also a rich source of polyphenols, such as flavanols, flavandiols, flavonoids, and phenol acids, which have antioxidant and antimicrobial activities [47]. The antioxidant effects of GT are due to its ability to limit the number of free radicals by binding to reactive oxygen species [58]. Green tea catechins are the main polyphenols that are responsible for its health benefits linked to GT consumption [59]. Catechins are made up of three hydrocarbon rings and are classified into non-ester catechins (epicatechin (EC) and epigallocatechin (EGC)) as well as ester catechins (epicatechin gallate (ECG) and epigallocatechin gallate (EGCG)). Catechins such as EC, gallocatechin (GC), ECG, EGC, gallocatechin gallate (GCG), and EGCG [60] are monomeric units of condensed tannins with low molecular weights [59], as indicated in Figure 2. Farahat et al. [61] reported that GT leaves contain 0.7% catechin, 1.6% EC, 1.5% GC, 3.4% EGC, 7.15 EGCG, 3.8% ECG, and 1.6% GCG (w/w) polyphenolic components.

Generally, these compounds are colourless and water-soluble with a bitter and astringent taste. Green tea catechins have antimicrobial, antioxidant, hypoallergic, anticarcinogenic, and hypoglycaemic effects [62]. The most abundant catechin type found in GT is EGCG, which constitutes about 50 to 80% of total catechins in the leaves [63]. The effectiveness of antioxidant activities among GT catechins is greatest in EGCG, followed by ECG, EGC, and EC [64]. These polyphenols can help reduce the risks of stroke, cancer, and other cardiac-related diseases [65], and can reduce plasma and meat cholesterol in chickens [66].

The number and position of hydroxyl groups in the molecular structure of these bio-compounds influence their antioxidative effects such that catechins have a strong capacity to supply hydrogen on the B and C rings [68]. The unsaturated 4-oxo group and the 2,3-double bond in the C-ring stimulate the delocalization of electrons of the ortho-dihydroxy catechol in the B-ring [68]. In addition, the antioxidant action of these bioactive compounds is associated with molecular structure, microenvironment, and starting conditions of the reaction medium. Since all these compounds are made of a 4-oxo 3-hydroxy C ring structure, they possess oxidation resistance properties. Furthermore, GT consists of a wide range of antimicrobial activities due to the presence of catechins, particularly epigallocatechin gallate [69]. Hara [70] demonstrated the ability of GT polyphenols to effectively inhibit bacterial growth, especially Vibrio cholera, Staphylococcus aureus, and Clostridium botulinum species.

Green tea products can be used as alternatives to antibiotics to improve poultry performance and reduce enteric pathogenic microbial loads, thus improving nutrient digestion and absorption [71]. This is attributed to the presence of catechins, phenolic acids, and flavonols with antimicrobial, antioxidant, and anti-inflammatory effects [69]. These compounds can selectively alter gut microbiota through antimicrobial activity by affecting the survivability of microorganisms (mainly Gram-positive bacteria, Eimeria parasites, and avian influenza subtype viruses) through increased hydrophobicity [68]. This changes the characteristics of cell membranes causing ion leakage, thus making microbes less virulent [72] and resulting in improved feed efficiency, nutrient utilisation, and stimulation of the immune system. Improved nutrient utilisation is also a result of protection from the oxidative damage of lipids and improved absorption of nutrients in the gut [73]. Indeed, improvements in the feed conversion ratio, growth performance, and meat quality have been reported in broiler chickens reared on GT-containing diets [74]. Polyphenols such as catechins, phenolic acids, and flavanols found in GT have hypolipidemic, antimicrobial [75], and anticoccidial effects [28], which have been reported to improve the growth performance of broiler chickens [74]. Mahlake et al. [3] reported that replacing zinc-bacitracin antibiotic with GT leaf powder in Jumbo quail diets increased the overall feed intake but not the weight gain or feed conversion efficiency. This demonstrates the potential of green tea products to act as alternatives to in-feed antibiotics in poultry production.

In the interest of environmental sustainability and human health, the use of natural phytogenics like GT products in poultry nutrition is largely influenced by the demand for food products that are devoid of antibiotic residues. Indeed, Tuzcu et al. [76] reported that supplementing fattening quail diets with 200 or 400 mg/kg synthetic epigallocatechin gallate increased the carcass weight and dressing percentage under stress conditions (at 34 °C), which could be due to the antioxidant effects of GT polyphenols, as observed in previous studies with chickens [47,63,77]. Furthermore, the feeding of GT products to poultry birds has shown great potential to enhance the quality of meat for consumers. Kara et al. [78] observed a significant increase in water-holding capacity and antioxidant capacity whilst decreasing serum glucose and total cholesterol levels of breast meat in Japanese quail fed diets supplemented with 2.50 g/kg catechin. This improvement could be due to the catechins’ antioxidant effects that protect the cell walls against lipid peroxidation. It is evident that the use of GT products in poultry can improve carcass and meat quality parameters. This practice would encourage organic farming and production of poultry products that are free of antibiotics. Unfortunately, limited information is available regarding the effect of GT products as alternatives to antibiotics on carcass characteristics, meat quality, and stability traits.

Green tea bioactive compounds are known to improve immune responses in poultry birds by acting against Eimeria [28], a causative agent for coccidiosis, which is an economically important disease in the poultry industry. Infection by Eimeria destructs the intestinal epithelium cells, resulting in cell permeability, nutrient and plasma protein leakage, impaired nutrient absorption, and thus contributes to poor bird health [79]. Interestingly, supplementation of chicken diets with 5 and 20 g/kg GT products decreased Eimeria maxima faecal oocysts shedding by 38.5% and 51.5%, respectively, at five weeks of age [28]. Similarly, in vitro studies have reported that GT constituents have anti-parasitic activities by inhibiting egg hatching and larval development of Trichostrongylus colubriformis and Teladorsagia circumcincta [80]. Furthermore, Song et al. [69] studied catechin derivatives and their in vitro anti-influenza viral activity and reported inhibitory effects of the derivatives for avian influenza virus by inhibiting the absorption of the viruses in red blood cells.

The consumption of tea leaves has been shown to lower sugar uptake and reduce sugar levels in the blood by suppressing the glucose transporter activity in the intestinal epithelium in rats [81]. Green tea catechins also inhibit digestive lipases and interfere with the formation of lipid micelles in the intestine, resulting in lower fat absorption [82]. Kara et al. [83] further reported a decrease in serum triglyceride levels when layer quail diets were supplemented with 4 g/kg catechin. In addition, GT extracts can improve antibody responses against Newcastle disease virus [61], suggesting that GT bioactive compounds can be exploited to boost immunity in poultry birds [61].

Microbial resistance to conventional antibiotics has become a global problem, which has led to the need to search for novel phytogenic plants as potent antimicrobial agents. A study by Zhao et al. [84] suggested that GT leaf meal polyphenols not only have antimicrobial effects on intestinal bacterial pathogens but also have effects on a wide range of viral and fungal pathogens. According to Zhao et al. [84], EGCG found in GT leaf meal is capable of cross-linking with proteins, causing damages to microbial cytoplasmic lipids and proteins, which results in a broad spectrum of antimicrobial activities. The main antibacterial mechanism of EGCG lies in its capacity to attach itself directly to the peptidoglycan layer of bacterial cell walls, causing damage to the cross-linking peptides and resulting in the destruction of cell walls [85]. Zhao et al. [84] also stated that EGCG damages bacterial cytoplasmic lipids, membrane proteins, or cytoplasmic enzymes, thus exerting antibacterial effects. As a result, EGCG weakens bacterial resistance to antibiotics and, in doing so, increases the sensitivity of bacteria to antibacterial agents. In an in vivo study by Bakkir et al. [86], the presence of green tea extract was associated with a reduction in the severity of necrosis and swelling in rabbits subcutaneously inoculated with methicillin resistant S. aureus (MSRA). Moreover, Al-Kayali et al. [87] reported that 10% aqueous GT extract and various antibiotics had similar antimicrobial effects against the antibiotic resistant S. pyogenes, P. mirabilis, and S. aureus species that cause diseases.

Gut microbiota play a vital role in poultry nutrition and health. They increase the digestion and absorption of nutrients, the supply of nutrients (secondary metabolites), as well as protect the GIT mucosa. Unlike conventional antibiotics, GT products may stimulate the growth of beneficial microbes while reducing pathogenic ones [88]. Catechins compounds have antimicrobial activities against both Gram-negative and Gram-positive bacteria [89]. For instance, studies have reported that GT could inhibit the synthesis of PBP2 protein in methicillin-resistant Staphylococcus aureus and Gram-negative bacteria [6,34]. The reduction in pathogenic microbes may decrease microbial competition in the GIT and, thus, enhance the growth of beneficial microbes. Moreover, GT products may contain other components (secondary metabolites) that can be utilised by beneficial microbes to synthesise their proteins, fatty acids, and vitamins.