Moeko Noguchi-Shinohara et al. conducted a 2-year follow-up survey of 490 subjects over 60 years of age with cognitive performance and blood tests. Even after correcting for potential confounders, drinking green tea was found to significantly reduce the chance of cognitive deterioration (91). In a questionnaire-based study of 1,003 Japanese participants aged 70 or older, Shinichi Kuriyama et al. discovered a correlation between higher green tea drinking and a lower prevalence of cognitive impairment (32). A brief analysis of tea consumption and prevalence of AD in different country regions by Fernando et al. revealed that countries with higher intake of tea, such as Japan, China, and India, had lower prevalence of AD, whereas European and American countries with lower intake of tea had higher prevalence of AD (92). Although epidemiological data favorably show a negative relationship between drinking tea and the preponderance of AD in that part of the country, any correlation between tea consumption and AD prevalence should be evaluated with caution because the effects of racial differences, dietary preferences, and lifestyle cannot be excluded (92). Yang Yuhuan et al. conducted a questionnaire survey to gauge the cognitive function of seniors 60 years of age and older in the Huangshi community in order to better understand the prevalence of mild cognitive impairment (MCI) and its influencing factors (93). The survey data were tested by chi-square test and it was concluded that the prevalence of MCI was lower in occasional tea drinkers, which may be related to the caffeine and catechins contained in tea, caffeine can reduce the level of Aβ in the brain, which is beneficial for improving cognitive function, while catechins have strong antioxidant capacity, but the study did not prove the relationship between tea drinking and AD prevalence. Wang, Ziqi et al. performed the Mini-Mental State Examination (MMSE) for the assessment of cognitive function in 870 people aged 90 years or older, and cardinality testing of the collected data revealed that the mild cognitive index was significantly different from normal in those who regularly consumed animal oils and legumes (94). In contrast, no significant differences were found for the other 10 foods, including tea, in both the unadjusted and adjusted models (94). Numerous studies have demonstrated the potential of tea consumption to mitigate cognitive decline in older adults; however, experimental evidence supporting its efficacy in AD is lacking (95). Controlled studies examining AD cases have not yielded significant findings regarding tea consumption, thus limiting the inference of beneficial effects of green tea catechins solely based on AD pathogenesis and in vitro studies (96). Despite this, the observed efficacy of green tea in AD surpasses initial expectations, warranting further investigation into the specific role of catechins in AD patients.

Given that Aβ aggregation is recognized as a pivotal factor in the pathogenesis of AD and its impact on the human nervous system, Mahsa Amirpour et al. investigated the neuroprotective potential of green tea in a streptozotocin (STZ)-induced AD model. Their study examined the effects of green tea on cognitive decline, inflammation, and oxidative stress (97). The findings demonstrated that the active compounds present in green tea could mitigate cognitive impairment and ameliorate learning and memory deficits associated with STZ injection (81). Furthermore, green tea may reduce the risk of AD through antioxidative and anti-inflammatory pathways, thus positioning it as a potential preventive intervention (90) (Table 1).

Tingting Chen et al. used mice as a model to demonstrate that the polyphenolic compounds EGC and ECG effectively alleviated Aβ40 aggregation and protofibrillar toxicity by chelating Cu²⁺ and Zn²⁺ and reduced ROS production, thereby mitigating Cu²⁺-Aβ40 and Zn²⁺-Aβ40induced neuronal toxicity (111). The results showed that tea polyphenols had significant beneficial effects on different aspects of AD pathology (112). Among them, catechin ECG had the most significant effect due to the therapeutic effect of ECG through the BBB, reducing Aβ plaques in the brains of APP/PS1 mice and thus protecting neurons from damage (111). Therefore, the potential of catechins to prevent or improve AD symptoms was laterally demonstrated (111). Lee JW et al. found that EGCG reduced Aβ1-42-induced memory dysfunction by altering the secretion of α-secretase, in addition to EGCG inhibiting Aβ1-42-induced apoptosis (113). These findings imply that EGCG may be a useful tool for delaying the start or progression of AD (Figure 3).

Neuroinflammation as a pathogenesis of AD has been confirmed by numerous studies. It has been found that cerebrospinal fluid levels of pro-inflammatory factors such as IL-1β, IL-6, and TNF-α are high in AD patients and increase with disease progression (114, 115). In addition, microglia, which play an important role in chronic neuroinflammation, are also involved in this process. Microglia resist the onset and progression of AD by degrading Aβ and tau. However, Aβ in turn activates microglia through TLRs to release pro-neuroinflammatory mediators. In the early stages of AD development, neuroprotective phenotypic microglia appear around Aβ plaques (116, 117). However, in late AD pathogenesis, elevated expression of proinflammatory factors will result in the emergence of microglia with a proinflammatory phenotype and a decrease in their phagocytic activity (118, 119). Pro-inflammatory microglia drive tau proliferation and toxicity by promoting neuroinflammation, such as activation of NLRP3 inflammasomes or induction of NF-kB signaling (23). Defective microglial autophagy leads to dysregulation of lipid metabolism, which increases the pathology of tau within neurons further exacerbating AD (23).

Numerous studies have shown that EGCG treatment of AD is associated with chronic neuroinflammation induced by microglia of anti-inflammatory phenotype (105). Wei et al. conducted in vitro experiments demonstrating that EGCG effectively suppressed the expression of TNFα, IL-1β, IL-6, and iNOS while concurrently restoring intracellular antioxidant levels, including Nrf2 and HO-1. These actions counteracted the pro-inflammatory effects of microglia (120). Furthermore, EGCG inhibited the secretion of pro-inflammatory factors from Aβ-induced pro-inflammatory microglia phenotypes and attenuated microglial neurotoxicity (121). Importantly, EGCG also mitigated Aβ-induced cytotoxicity by attenuating ROS-mediated NF-κB activation and MAPK signaling pathways, including JNK and p38 signaling (121). In vitro investigations have demonstrated that Aβ deposition significantly diminishes following intraperitoneal injection of EGCG at a dose of 20 mg/kg or oral administration of EGCG at 50 mg/kg in drinking water (109, 122). Similarly, Li et al. observed a substantial reduction in Aβ deposition in the frontal cortex (60%) and hippocampus (52%) following oral administration of EGCG at a dose of 20 mg/kg/day for 3 months in an AD mouse model (123). Furthermore, recent findings by Lee et al. revealed that EGCG attenuated LPS-induced memory impairment and neuronal apoptosis, concomitant with a reduction in the expression of inflammatory cytokines TNF-α, IL-1β, and IL-6 (105). These results align with in vitro observations, suggesting that EGCG holds promise as a therapeutic agent for neuroinflammation-associated AD.

The brain is particularly vulnerable to oxidative damage due to its high content of easily oxidizable lipids, elevated oxygen consumption rates, and limited antioxidant defense mechanisms. Age-related increases in brain oxidation contribute to the recognized risk of AD (124). Under normal physiological conditions, SOD catalyzes the conversion of superoxide anions to hydrogen peroxide, thereby safeguarding cells against free radical assault. However, in the presence of elevated levels of certain metal ions such as Fe and Cu, SOD can convert hydrogen peroxide to the more hazardous hydroxyl radical (125). Notably, AD patients exhibit heightened SOD activity, diminished glutamine synthetase activity, and elevated lipid peroxidation, collectively resulting in heightened oxidative stress and accumulation of free radicals. Free radicals inflict damage upon biofilms, disrupting the intracellular milieu and precipitating cellular senescence and demise (126). Peroxidation of impaired lipids results in ribonucleic acid inactivation, prompting DNA and RNA cross-linking and instigating DNA mutations (127). Decomposition of peroxidized lipids yields aldehydes, such as acrolein, which react with phosphoric acid and proteins to generate lipofuscin (128). Accumulation of lipofuscin in the brain contributes to cognitive impairment (129). Furthermore, mitochondrial dysfunction and oxidative stress in AD patients are intricately intertwined, with evidence indicating mutual exacerbation, culminating in AD pathogenesis (130).

Numerous studies have delineated the involvement of increased oxidative stress in AD pathogenesis, and highlighted the potential of EGCG’s antioxidant properties in mitigating this process (131, 132). Abdul M. Haque et al. observed that long-term administration of green tea catechins to AD model mice significantly ameliorated cognitive impairment, accompanied by reduced ROS levels and enhanced antioxidant capacity in the hippocampus and cortex (133). Similarly, Regina Biasibetti et al. investigated the effects of oral EGCG administration (10 mg/kg/day) for 1 month in a rat model of dementia, revealing cognitive deficits reversal and notable reductions in ROS levels and NO production (110). Catechins exert their antioxidative effects by scavenging free radicals and chelating metal ions such as Fe and Cu, thereby reducing ROS production. This dual action mitigates oxidative stress in both peripheral and brain tissues, thereby inhibiting further deterioration of cognitive deficits-associated behaviors (134). Mitochondrial dysfunction enhances ROS generation via the NADPH oxidase pathway (135). EGCG reinstates mitochondrial respiration rate, ATP levels, ROS levels, and membrane potential (102). Its antioxidant properties scavenge ROS production and safeguard against mitochondrial damage (136). Furthermore, EGCG treatment mitigates neuronal apoptosis triggered by endoplasmic reticulum stress subsequent to Aβ exposure. The inflammatory response to neuronal injury induced by various stimuli culminates in the release of pro-inflammatory cytokines and cytotoxins, further exacerbating oxidative stress (137). Numerous studies have demonstrated EGCG’s protective effects against lipopolysaccharide-induced memory impairment and inflammatory responses (105, 138). Through mechanisms associated with protein kinase C (PKC), which facilitates the generation of nontoxic soluble peptide APPβ (sAPPβ) and cell survival, catechins may exert an influence on AD (139, 140). Levites et al. reported that EGCG (1–5 μM) enhances sAPPβ production from PC12 and human neuroblastoma cells (141).

In order to determine the relationship between PD incidence and tea consumption, Quintana et al. examined a total of 12 studies from 1981 to 2003, comprising 2,215 cases and 145,578 controls. Their analysis revealed that tea consumption can prevent PD and that this protective effect is more pronounced in the Chinese population (147). In order to study the non-hereditary factors associated with PD, Hosseini Tabatabaei N. et al. used a sample of 150 people, including 75 PD patients and 75 people as controls, and showed that tea intake was protective against PD and that adherence to daily tea consumption reduced the risk of PD by 80% (148). A case–control study was conducted by Harvey Checkoway et al. By studying and counting PD cases (n = 210) and controls (n = 347), it was found that people who drank two or more cups of green tea per day had a reduced incidence of PD compared to those who did not drink green tea (149). According to research by E-K Tan and colleagues, drinking one unit of tea (3 cups per day for 10 years) would result in a 28% decrease in the incidence of PD (150). The effects of tea consumption on 60 patients with idiopathic PD were examined by Chahra CD et al. According to the study’s findings, PD patients who drank tea in addition to traditional medication experienced improvements in their non-motor symptoms and depression (143). Boris Kandinov et al. also demonstrated that drinking tea and smoking delayed the age of PD attacks, while drinking coffee may have the opposite effect (151). Observational epidemiological studies in PD have more experimental data demonstrating a protective effect of green tea compared to AD, and even though epidemiological findings support the beneficial effects of tea consumption, some have not yet provided clear evidence. Therefore, more research is required to determine the connection between drinking tea and the risk of PD.

Pathological accumulation of metal ions or a rapid increase in monoamine oxidase B (MAO-B) activity can induce endogenous dopamine (DA) oxidation, leading to α-synuclein aggregation, mitochondrial dysfunction, and other factors contributing to the heightened incidence of PD. Consequently, mitigation strategies involve the use of ROS scavengers, DA oxidation inhibitors, MAO-B inhibitors, and DA quenchers (152). Zhou et al. demonstrated that catechins can impede DA oxidation by inhibiting enzymes and metal ions. Furthermore, they inhibit MAO-B activity, detoxify ROS, DA quenchers, and harmful DA oxidation byproducts, while regulating the Nrf2-Keap1 and PGC-1 pathways. These findings underscore the inhibitory effects of tea polyphenols on DA-related toxicity (153). In a study by Shyh-Mirnin Ph.D. et al., the influence of EGCG on MAO-B enzyme activity in the adult rat brain was investigated, revealing a decrease in MAO-B enzyme activity (154).

PD primarily affects dopaminergic neurons in the substantia nigra pars compacta (SNpc) region of the brain (155). The neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6-hydroxydopamine (6-OHDA) specifically damage this brain region, resulting in the loss of dopaminergic neurons (156). This neuronal loss leads to disrupted neural firing patterns and impaired motor control (157). Weinreb et al. investigated the impact of pretreatment with tea extract (0.51 mg/kg) and the tea polyphenol EGCG (2.10 mg/kg) on dopamine neurogenesis loss in the substantia nigra of MPTP-induced PD mouse models (158). Their study revealed a considerable mitigation of neurogenesis loss (158). Siddique Y. H. et al. examined the effects of EGCG in an α-synuclein (h-αS) transgenic Drosophila model of PD, analyzing statistical data and markers of changes in climbing capacity, lipid peroxidation, and apoptosis (159). Their findings demonstrated that various concentrations of EGCG (0.25, 0.50, and 1.0 g/mL) substantially delayed the loss of climbing ability in Drosophila, while reducing oxidative stress and apoptosis (159).

In a study by Tingting Zhou et al., a PD mice model induced by MPTP was utilized to investigate the potential therapeutic effects of EGCG for PD. The results demonstrated that EGCG administration ameliorated impaired locomotion behavior in MPTP-treated mice and protected tyrosine hydroxylase-positive cells in the substantia nigra pars compacta from MPTP-induced toxicity (160). Additionally, following EGCG treatment, flow cytometric analysis revealed an increase in the CD3 + CD4+ to CD3 + CD8+ T cell ratio in peripheral blood of MPTP-treated mice. Furthermore, EGCG appeared to downregulate the expression of inflammatory mediators such as TNF and IL-6 in serum (160). These findings suggest that EGCG may confer neuroprotective effects in MPTP-induced PD mice models, potentially by modulating peripheral immune responses.

Current understanding of PD pathogenesis implicates neurofilaments, synaptic vesicle proteins, and ubiquitinated α-synuclein as primary contributors to the disease pathology (161). Additionally, Lewy bodies may exacerbate the release of free radicals, excessive nitric oxide synthesis, microglia-mediated inflammation, and disruption of protein degradation pathways, further exacerbating the pathophysiology (162). Specific beneficial effects and mechanisms of action of EGCG in PD are summarized in Table 2. In conclusion, EGCG exhibit diverse pharmacological activities in PD by modulating gene expression and interfering with signaling pathways (172). Despite substantial experimental evidence supporting this notion, challenges such as low solubility, limited bioavailability, and BBB impermeability hinder efficient delivery of EGCG to the brain and impede clinical translation (173). Overcoming these obstacles necessitates cross-sectional studies aimed at elucidating chemical modification strategies and optimizing drug delivery mechanisms to enhance their therapeutic efficacy.

Numerous studies have established neuroinflammation as a significant etiological factor in PD, playing a pivotal role in its early pathogenesis. Notably, activated microglia make substantial contributions to this process. Evidence supporting the involvement of activated microglia-mediated chronic neuroinflammation in PD includes: (1) Pro-inflammatory effects are often observed in activated microglia surrounding dopaminergic neurons, with the degree of microglial activation correlating with dopaminergic endings loss in PD (174, 175). (2) Injured neurons release excessive α-synuclein, activating proinflammatory factors like TNF-α, NO, and IL-1β produced by microglia, thereby modulating chronic neuroinflammation in PD (176, 177). (3) Jmjd3, critical for microglial cell phenotype expression, when inhibited, leads to overactivation of pro-inflammatory microglial responses, exacerbating neuroinflammation and neuronal cell death (178). Additionally, it has been proposed that α-synuclein aggregates exert toxicity on neurons only in the presence of microglia (179, 180). In PD patients, misfolded α-synuclein is released from injured neurons into the extracellular fluid, where it binds to Toll-like receptors (TLRs), Fcγ receptors (FcγR), or nucleotide-binding oligomerization domain-like receptors (NLRPs), further activating microglia (181–184). The proinflammatory cytokines released by activated microglia subsequently activate protein kinase R (PKR), leading to phosphorylation of α-synuclein at Ser129, a process considered of significant pathological importance, particularly in Lewy bodies of PD patients. Moreover, microglia are involved in the clearance of protein deposits, including α-synuclein and Aβ, from astrocytes (185–187). Activation of microglia upregulates MHC I expression on neurons, promoting neuronal presentation of α-synuclein antigen. Subsequently, these neurons are targeted and eliminated by α-synuclein-reactive T cells (187). The emergence of α-synuclein pathology follows microglial activation, suggesting α-synuclein’s pivotal role in PD progression, albeit not as an initiator. Similarly, mounting evidence suggests that the immune response contributes to neuronal death as a cause rather than a consequence (22).

A growing body of evidence suggests that EGCG may impede or postpone the progression of PD by targeting chronic neuroinflammation. EGCG exhibits potent anti-inflammatory activity both in vitro and in vivo, primarily attributed to its ability to inhibit microglia-induced cytotoxicity (120). In vitro studies have demonstrated that EGCG suppresses the secretion of pro-inflammatory factors from LPS-activated microglia by downregulating the expression of iNOS and TNF-α (188). Furthermore, EGCG has been shown to inhibit microglial activation and reduce neuronal damage in SH-SY5Y and rat mesencephalic cultures (188). Gülşen Özduran et al. reported that EGCG restored viability in PD model cells, inhibited apoptosis, and enhanced survival by attenuating 6-OHDA-induced expression of TNF-α and IL-1β in SK-N-AS cells (189). The findings from the in vivo study corroborate those observed in vitro, further substantiating the potential of EGCG to mitigate the inflammatory response associated with microglia-mediated damage to dopaminergic neurons. Al-Amri et al. demonstrated that EGCG significantly increased the number of TH-immunoreactive neurons in the midbrain of PD model rats by reducing the production of TNF-α and NO (170). Similarly, EGCG liposomes alleviated symptoms in a PD rat model by suppressing the expression of NO and TNF-α in microglia exhibiting an LPS-induced inflammatory phenotype (165). In summary, EGCG shows promise as a therapeutic and prophylactic agent for PD, exerting neuroprotective effects both in vivo and in vitro through the inhibition of neuroinflammation (Figure 4).

PD patients commonly exhibit reduced mitochondrial complex I activity and increased ROS production (190). This diminished function of proton pumps on mitochondria, coupled with decreased membrane voltage and the opening of permeability channels, initiates the apoptotic process. Deficiency in mitochondrial complex I can result in oxidative stress, heightening neuronal susceptibility to excitotoxic injury. The densely packed substantia nigra is particularly vulnerable to elevated oxidative stress compared to other brain regions. Under normal conditions, H₂O₂ generated by dopamine toxicity is neutralized by reduced glutathione, mitigating potential harm. However, in the remaining dopamine neurons of PD patients, ineffective scavenging of H₂O₂ may occur due to compensatory mechanisms, including accelerated toxicity production in dopamine metabolism, heightened monoamine oxidase (MAO)-B activity, and reduced glutathione levels (191). Excessive H₂O₂ reacts with Fe²⁺ via Fenton chemistry, yielding highly toxic hydroxyl radicals, culminating in lipid peroxidation and apoptosis of nigral neurons. This oxidative stress and mitochondrial dysfunction form a reciprocal relationship, perpetuating a vicious cycle.

The neuroprotective effects of EGCG, attributed to its antioxidant properties, have been observed in PD (Figure 4). Typically, α-synuclein localizes to the mitochondria-associated membrane, and its presence may disrupt mitochondrial function by promoting the formation of the mitochondrial permeability transition pore (mPTP), leading to mitochondrial membrane potential (MMP) loss, subsequent mitochondrial degradation, and ultimately cell death (192). Compounds capable of preserving mitochondrial activity are therefore deemed invaluable in combating PD. EGCG has been shown to safeguard mitochondrial function by preventing Ca2+ influx through voltage-gated calcium channels and mitochondrial Ca2+ uptake via the mitochondrial Ca2+ uniporter (159, 193). Furthermore, in vivo studies have demonstrated EGCG’s ability to reduce oxidative stress by decreasing serum protein carbonyls and mitigating neurotoxicity in the MPTP-induced mouse model of PD (166). Similarly, Pinto et al. reported that EGCG improved cognitive dysfunction induced by 6-OHDA in male Wistar rats. 6-OHDA is known to induce ROS generation. EGCG treatment reversed striatal oxidative stress and attenuated immunohistochemical alterations (194). In conclusion, EGCG has been shown to alleviate PD by inhibiting neurotoxin-induced oxidative stress injury both in vitro and in vivo.

The liver is known to be the major drug metabolizing organ in the human body. Initially, K Nakagawa et al. examined the distribution of EGCG (500 mg/kg body weight) in the body after 1 h of oral administration in rats (227). They observed that EGCG concentrations were highest in the intestine, followed by the liver, with plasma levels approximately one-fourth of those in the liver and notably lower concentrations in the brain (227). Autopsy findings further confirmed EGCG induced hepatotoxicity, correlating the extent of liver damage with dosage, route, and duration of EGCG administration (228). Studies on oral EGCG toxicity have documented varying degrees of hepatotoxicity, ranging from mild elevation in liver enzymes (alanine aminotransferase (ALT) and aspartate aminotransferase (AST)) to severe hepatocellular necrosis and bile duct hyperplasia as therapeutic doses increased (229). Thus, it is evident that the liver is a significant target organ for EGCG toxicity.

Animal studies have shown that the severity of liver injury produced by EGCG treatment is related to dose, administration route, and treatment duration. Balaji Ramachandran et al. investigated the relationship between the adverse effects of EGCG treatment with dose and administration route by giving EGCG (108, 67.8, 21.1, and 6.6 mg/kg/d) orally or intraperitoneally to mice (230). Subcutaneous injection of 108 mg/kg EGCG resulted in severe hepatic parenchymal congestion, hepatocellular balloon-like degeneration, kupffer cell hyperplasia, and calcification (acute hepatitis); serum levels of bilirubin, AST, ALT, and ALP were markedly elevated, leading to mortality by the 8th day of treatment (230). Mice injected with 67.8 mg/kg EGCG subcutaneously exhibited moderate hepatic peritoneal and mild lobular inflammation; elevated serum AST and ALT levels were observed, with mortality occurring by day 16 of the experiment (230). In comparison to subcutaneous injection, oral administration of EGCG resulted in lower hepatotoxicity, with significant liver damage observed only in mice receiving 108 mg/kg EGCG orally. Notably, increasing EGCG doses correlated exclusively with hepatic toxicity, ranging from mild periportal inflammation to severe hepatitis (230). Similarly, Dongxu Wang et al. investigated the dose-dependent hepatotoxic effects of subcutaneously injected EGCG (55, 70, and 125 mg/kg/day) in mice (229). Their findings revealed that all mice injected with 125 mg/kg or 70 mg/kg EGCG succumbed within 2 days, showing severe hepatotoxicity characterized by elevated serum levels of ALT, AST, and 4-HNE, along with increased expression of Nrf2 target genes in the liver. Mice injected with 55 mg/kg EGCG exhibited hepatotoxic effects but survived the duration of the study (229). It has also been demonstrated that subcutaneous injection of 45 mg/kg/day of EGCG represents the maximum tolerated dose in mice, with long-term administration at this dose showing no impact on the body’s oxidative defense mechanisms (231). However, injections of 55 or 75 mg/kg/day of EGCG induced hepatotoxicity in mice, accompanied by inhibition of hepatic antioxidant enzymes and increased nuclear distribution of Nrf2 (231). Furthermore, repeated injections of 75 mg/kg/day of EGCG altered the oxidative defense mechanism, significantly reducing levels of SOD, catalase, and GPX (231). Subcutaneous injection of EGCG in mice at doses exceeding 100 mg/kg/day induces severe hepatotoxicity and dose-dependent mortality, with higher concentrations leading to accelerated death. This treatment also inhibits Nrf2 target gene expression and diminishes antioxidant defense capacity (231). Similarly, gavage administration of EGCG yielded comparable results: mice exhibited hepatic congestion and a slight elevation in ALT levels after receiving 750 mg/kg/day of EGCG for 5 consecutive days (228). Following gavage of 750 mg/kg/day of EGCG for 7 consecutive days, mice exhibited a significant increase in ALT, MDA, MT, and γH2AX levels in the liver, along with hepatocyte degeneration, resulting in a mortality rate of 75% (232). single gavage of 1,500 mg/kg of EGCG led to a 108-fold increase in ALT levels and an 85% mortality rate among mice (232). Metabolites EGCG-2′-cysteine and EGCG-2″-cysteine were detected in urine following high-dose gavage of EGCG (233). Notably, EGCG administered via diet was well tolerated and demonstrated reduced hepatotoxicity compared to gavage administration in animals (233). Studies administering EGCG to Beagles indicated that fasting increased the likelihood of hepatotoxicity compared to animals that were fed prior to treatment (234). These findings underscore the influence of dose, route of administration, treatment duration, and nutritional status on EGCG-induced hepatotoxicity.

Animal experiments have shown that EGCG induced hepatotoxicity correlates with changes in several oxidative stress markers in the body, including MDA, 4-HNE, MT, γH2AX, and Nrf2 (228, 229, 231, 235). MDA and 4-HNE are products of lipid peroxidation and serve as biochemical indicators of oxidative stress (228). MT and γH2AX are molecular markers associated with oxidative stress. All these biomarkers suggest that hepatotoxicity induced by EGCG treatment is largely induced by oxidative stress (201). Nrf2 functions as a crucial transcription factor in antioxidant defense. Under normal physiological conditions, Nrf2 is sequestered by Keap1; however, during oxidative stress, Nrf2 dissociates from Keap1 and translocates to the nucleus where it binds to antioxidant response elements. This activation of the Nrf2-ARE signaling pathway upregulates the expression of various antioxidant genes such as HO-1, GST, and NADP (H): NQO1 (231). The Nrf2-ARE signaling pathway activates and enhances the expression of downstream antioxidant enzymes, serving as a critical cellular defense mechanism against oxidative stress (236). This pathway, particularly in the liver, is pivotal in mitigating EGCG-induced hepatotoxicity (236). Animal studies have shown that subcutaneous injection of EGCG at 45 mg/kg/day in mice does not impair major hepatic antioxidant defenses but modestly increases hepatic expression of Nrf2 target genes (231). Conversely, injection of 75 mg/kg/day of EGCG inhibits major hepatic antioxidant enzymes while significantly elevating Nrf2 expression and its target genes (231). Injection of 100 mg/kg/day of EGCG notably suppresses the hepatic Nrf2 pathway (231). These findings indicate a biphasic response of Nrf2 to different EGCG doses. In summary, EGCG-induced hepatotoxicity involves the inhibition of major antioxidant enzymes, with the Nrf2 salvage pathway playing a crucial role in mitigating toxicity. However, this pathway becomes inhibited at higher concentrations of EGCG.

Nora O. Abdel Rasheed et al. investigated potential nephrotoxic effects of EGCG treatment in diabetic mice, a crucial concern due to the kidney’s vulnerability in diabetes (237). Diabetic mice injected with 100 mg/kg EGCG daily for 4 days exhibited decreased resistance to oxidative stress, as indicated by elevated NADPH oxidase levels and reduced expression of Nrf2, HO-1, and HSP90 (237). Serum levels of CYS-C and NGAL were significantly elevated, and histopathological analysis confirmed EGCG-induced renal injury in diabetic mice (237). Similarly, another study demonstrated nephrotoxicity in colitis mice treated with green tea extract containing 35% EGCG, evidenced by increased serum creatinine levels (a nephropathy biomarker), and elevated expression of antioxidant enzymes (HO-1 and NQO1) and HSP 90 (238). These findings collectively underscore the potential nephrotoxic effects of EGCG treatment, exacerbated by oxidative stress implicated in diabetes and its complications (239). Thus, caution is advised when considering EGCG supplements for diabetic patients, particularly at high doses.

In addition to nephrotoxicity and hepatotoxicity, numerous studies have documented gastrointestinal toxicity associated with EGCG administration, whether by gavage or in diet, in animal models (240–242). The severity of gastrointestinal effects varied with dosage, ranging from mild gastric erosion and vomiting to severe ulceration, hemorrhage, and epithelial necrosis. Notably, gastrointestinal toxicity was more pronounced in animals administered EGCG via gavage or when fasted, whereas administration via diet, water, or capsule resulted in milder effects (234, 243). In conclusion, treatment with EGCG at high doses or for prolonged duration may have adverse effects, and the above data suggest that the boundary between protective and toxic doses of EGCG may be narrow.