The Power of Antioxidant: Tea Catechin and Body Oxidative Stress

Abstract

Green tea is rich in catechins, particularly (−)-epigallocatechin-3-gallate (EGCG), which act as potent antioxidants and can help to prevent oxidative stress-related diseases. This article revealed the importance of green tea catechins in mitigating the risk of complex diseases such as cardiovascular disease, cancer, and neurological diseases. It also highlighted the potential side effects of excessive green tea consumption, emphasising the need for moderation. The review covered a wide range of potential health benefits of green tea, including its effects on weight loss, diabetes, metabolic syndrome, and cognitive decline. Additionally, the collection of research articles elaborated on the antioxidant and neuroprotective properties of green tea, as well as its potential role in preventing skin cancer and improving cognitive function. Overall, the evidence presented underscores the potential of green tea as a valuable dietary component in inhibiting diseases such as diabetes, cardiovascular, cancer, and infectious illness, while also emphasising the importance of green tea consumption in a balanced manner.

1. Introduction

Oxidative stress occurs when there is an imbalance between antioxidants and free radicals in the human body. Imagine antioxidants and free radicals as players in a delicate game. Free radicals, molecules with an uneven number of electrons, act quickly, creating large chemical chains through a process called oxidation. While oxidation is essential for generating energy and aiding in chemical reactions, it is not always beneficial. Picture antioxidants as the team that donates electrons to stabilise the free radicals, making them less reactive without losing their own stability [1].

Free radicals can be naturally produced from various environmental sources, like pollution, radiation, ozone, smoking, or exposure to chemical substances such as cleaners and pesticides. They can also originate from a diet high in sugar, fat, and alcohol. Imagine the imbalance between free radicals and antioxidants like a seesaw. When free radicals outweigh antioxidants, they can damage three major components of the human body: fats, DNA, and proteins. This damage can lead to various health issues, including diabetes, high blood pressure, atherosclerosis, heart diseases, cancer, and neurodegenerative diseases [1].

To reduce the risk of oxidative stress, we can boost our antioxidant levels, which act as defenders against the formation of harmful free radicals. Including antioxidant-rich foods in our diet, such as tea, can be an effective strategy to prevent various complex illnesses [1]. Notably, green tea, abundant in catechins—a potent antioxidant—holds the potential to combat degenerative diseases like cardiovascular diseases (CVD) [2].

Tea polyphenols, serving as antioxidants, perform various protective roles by scavenging reactive oxygen and nitrogen species, binding to transition metal ions, and stimulating antioxidant enzymes [3]. These actions contribute to safeguarding tissues, cells, and plasma from potential oxidative damage [4, 5].

While there are numerous health benefits associated with consuming antioxidants from tea in general, this article will specifically delve into the antioxidant mechanisms of catechins in preventing diseases related to oxidative stress by incorporating green tea (GT), such as diabetes, cardiovascular disease, cancer, and infectious diseases.

2. Green tea catechin and diseases correlation

There are two types of tea plants namely Camellia sinensis and Camellia assamica which exhibit distinct characteristics in terms of hardiness and leaf size [6]. Camellia sinensis has a robust, small-leaf plant, and resilient to cold weather, while Camellia assamica is more delicate, large-leafed, and thrives in tropical regions. These plants are then processed into the most common four primary tea types—black, green, white, and oolong—that undergo diverse fermentation processes, leading to variations in polyphenol composition [7, 8]. Notably, black tea (BT) is the most produced tea with 78%, while GT makes up about 20% [9]. The fermentation levels also distinguish these teas, with GT being non-fermented, oolong tea semi-fermented, and BT fully fermented. White tea, with a 10–20% fermentation level, stands between green and black teas [10].

This fermentation disparity results in crucial differences in polyphenol balance and sensory characteristics [11] which are indicated by the production of oxidised polyphenolic compounds like theaflavins and thearubigins in BT [12, 13]. In white tea, the least processed one, is often considered the oldest form of tea and also contains unique components like gamma-amino butyric acid (GABA) and L-theanine [14]. The polyphenols found in black and green tea are depicted in Figure 1 [15, 16].

Figure 1.

Polyphenols in black and green tea.

The physiological effects of tea can be attributed to specific compounds found in different types of tea. BT contains thearubigins and theaflavins, while green tea is rich in catechins. These compounds have various health benefits, including the inhibition of cancer cell growth [17], reduction of inflammation, prevention of platelet aggregation, regulation of glucose and lipid metabolism, DNA repair, stimulation of immune function, and modulation of detoxification enzymes [18, 19]. The antioxidant properties of catechins depend on factors such as the hydroxyl group at the C-3 position or the degree of hydroxylation of the B ring [19]. Among green tea catechins, (−)-epigallocatechin-3-gallate (EGCG) stands out as the most abundant and potent antioxidant, with a notable reputation for its role in cancer prevention [20, 21, 22]. In addition to EGCG, that acts as a powerful scavenger of reactive oxygen species [23], caffeine and theanine in tea have stimulating and relaxing properties, working together for improved mental alertness [11]. In various plants, flavonoids with hydroxyl groups attached to the ring structure contribute to antioxidant activity, as seen in compounds like vitamin C, β-carotene, and carotenoids [24, 25].

Table 1 outlines the physical properties of key catechins, typically appearing as colourless crystalline substances with a bitter, astringent taste. Their solubility in polar solvents like water and methanol is influenced by factors such as extraction temperature, time, and the solvent type used [28, 29, 30]. In green tea, EGCG is the most concentrated, followed by (−)-Epigallocatechin (EGC), (−)-Epicatechin gallate (ECG), and (−)-Epicatechin (EC) [31]. Esterified catechins, contributing to astringency and a bitter taste, differ from free catechins, which are less astringent and slightly sweet [32]. Those with ester binding, like EGCG and ECG, exhibit a greater tendency to form precipitates with enzymes, leading to cream formation [29].

	EC	EGC	ECG	EGCG	Reference
Molecular structure	C15H14O6	C15H14O7	C22H18O10	C22H18O11	[26]
Molecular mass	290	306	442	458	[26]
Absorbance peak (in nm)	280	269	280	273	[26]
Melting point (C)	242	218	257	224	[26]
Dissolution	Time/Temperature/ Solvent Dependent	Time/Temperature/ Solvent Dependent	Time/Temperature/ Solvent Dependent	Time/Temperature/ Solvent Dependent	[27]
Solvents Water/methanol/ethanol	Soluble	Soluble	Soluble	Soluble	http://Chemicalland21.com
Chloroform	Not Soluble	Not Soluble	Not Soluble	Not Soluble	http://Chemicalland21.com
Taste	Bitter with sweet after taste	Bitter with sweet after taste	Bitter with sweet after taste	Bitter with sweet after taste	[27]
Appearance	Yellow to greenish powder	Light yellow powder	Light yellow powder	White to pink powder	http://Chemicalland21.com

Table 1.

Physical attributes of four catechins.

Extraction time, solvent type, and temperature play crucial factors in obtaining higher catechin content such as EGC and EGCG in green tea [27, 33], the same goes with BT in obtaining theaflavin and any compound within [34]. Although theaflavin is the most abundant component in BT, catechin is also available in this type of tea but only a small amount of component. When the catechin is consumed, it has a different function in the body system [35]. In laboratory-based observation showed that catechin in black and green tea could prevent hyperglycaemia by inhibiting the damage of β-cells and increasing insulin activity respectively [35, 36]. Hyperglycaemia refers to elevated blood sugar levels, and these findings suggest that catechins in tea may play a role in managing blood glucose levels.

To measure and identify catechins, UV light absorption is used with the most absorption occurring for EGCG, EGC, ECG, and EC between wavelengths of 210 and 280 nm [37]. Techniques such as liquid chromatography (LC) and capillary electrophoresis (CE) are commonly used to separate, identify, and measure catechins. LC systems often use detectors like UV–vis, diode array (DAD), mass spectrometry (MS), fluorescence (FD), and photodiode array detectors [37].

Not only in green tea, high concentrations of catechins are also present in various sources, including red wine, broad beans, black grapes, apricots, and strawberries. Epicatechin (EC), found in apples, blackberries, broad beans, cherries, black grapes, pears, raspberries, and chocolate, exhibits notable concentrations. The skin of fruits like apples holds flavanols, and peeling reduces catechin levels [38]. Apples contain diverse polyphenols, including quercetin, catechin, phloridzin, and chlorogenic acid, with variations among fruit varieties and changes during maturation. Gallic acid esters of catechin are rare in fruits, found mainly in berries, black currants, and grapes [39]. Strawberries, for instance, possess a complex catechin mixture, including catechin, ECG, EGC, and GC [30]. Table 2 presents the total catechins found in diverse food sources, while Table 3 provides a detailed breakdown of the catechin types present in selected food items.

Food sources	Catechinsa	Catechinsb
Apple	20–86	10-43
Apricot	20–50	10
Beans	70–110	25
Black tea	12–100	35–55
Blackberry	9–11	6–50
Cherry	10–44	9–11
Chocolate	23–30	5–22
Cider	8	46–61
Grape	6–35	3–17
Green tea	20–160	10–80
Peach	10–28	5–14
Red raspberry	2–48	2–48
Red wine	8–30	8–30
Strawberry	2–50	2–50

Table 2.

Distribution of catechins in food sources.

^a

Value in mg/ serving.

^b

Value in mg/ 100 g sample.

Source: [40, 41].

	C	EGC	EC	EGCG	ECG	Total catechins
Black current	ND^a	1.3 ± 0.01^b	3.0 ± 0.01^c	6.1 ± 0.01^d	3.6 ± 0.03^e	13.9 ± 0.02^f
Black tea	0.7 ± 0.01^a	3.0 ± 0.05^b	2.3 ± 0.01^c	24.9 ± 0.20^d	5.9 ± 0.01^e	36.9 ± 1.8^f
Cherry	ND^a	1.1 ± 0.03^b	2.3 ± 0.02^c	4.0 ± 0.01^d	2.7 ± 0.01^e	10.1 ± 0.02^f
Forest fruit	ND^a	0.9 ± 0.01^b	2.0 ± 0.02^c	3.3 ± 0.01^d	2.3 ± 0.02^e	8.5 ± 0.01^f
Green tea	1.3 ± 0.02^a	19.0 ± 0.09^b	4.4 ± 0.03^c	23.4 ± 0.05^d	5.6 ± 0.03^e	53.5 ± 1.5^f
Lemon	ND^a	1.1 ± 0.01^b	2.0 ± 0.03^c	3.4 ± 0.01^d	2.7 ± 0.01^e	8.6 ± 0.01^f
Orange	ND^a	1.0 ± 0.01^b	2.5 ± 0.01^c	4.2 ± 0.01^d	2.9 ± 0.01^e	10.6 ± 0.01^f
Strawberry	ND^a	1.0 ± 0.02^b	2.0 ± 0.01^c	3.9 ± 0.01^d	2.2 ± 0.02^e	9.6 ± 0.01^f

Table 3.

Catechins in selected food sources.

Source: [42].

ND: not detected.

^a,b,c,d,e,f values are expressed in mg of dry matter.

Vegetables like onions, broccoli, kale, lettuce, and tomatoes are rich in flavonols like quercetin and kaempferol. Some vegetables, such as celery, sweet peppers, and lettuce, contain flavones [43]. Catechins and type B procyanidins are absent in leafy greens or root vegetables but present in legumes like broad and green beans [44]. Tomatoes uniquely contain flavanones, naringenin, and hesperetin [45].

It is noticeable that green tea has higher (compared to other teas) antioxidant content because the process does not include fermentation or enzyme activation. It is purposed to prevent autolysis and oxidation of constituents and aid stabilising the components during storage. Compared to BT, the process undergoes fermentation through the enzymatic process, which stimulates oxidation, so most of the catechin is oxidised in this tea [46].

EGCG as an essential constituent in green tea has multiple functions as a strong antioxidant [47] such as reducing agents, hydro-donating antioxidants, singlet oxygen quenchers, and metal chelation. It is classified as an antioxidant because, in low concentration, it prevents autooxidation or free radical-mediated oxidation [25, 48]. Also, during further oxidation, it can stabilise the radical formation after scavenging the electron through intramolecular hydrogen bonding [25, 49].

According to [25], the recommended doses of this component in our dietary is about one gram. However, green tea catechin (GTC) is not a single component that can give health benefits to a human’s body. The study conducted by [50] showed that the combination of a small amount of caffeine from green tea itself gives a mild effect to stimulate exercise performance. This combination aids fat burning by lowering fat absorption and reducing adipocyte lipogenesis, which affects the increase of thermogenesis and fat oxidation [50, 51].

EGCG in green tea is the most potential catechin derivates in suppressing norepinephrine enzyme called COMT (catechol-o-methyl-transferase). Norepinephrine is a neurotransmitter that helps the body’s sympathetic nervous system control processes like generating heat (thermogenesis) and burning fat for energy. When we consume catechins in green tea that block the COMT enzyme, it can enable norepinephrine to have a longer-lasting impact on these processes, promoting continued fat burning and heat production [52, 53].

Besides, as a donor for hydrogen bonding, the phenolic groups in catechins bind firmly to proteins, lipids, and nucleic acids. For instance, the binding between EGCG and protein, such as 67-kDa laminin receptor and prolyl cis/trans isomerase, is a mechanism of anti-cancer [46]. Therefore, drinking green tea can reduce the risk of several oxidative stress conditions. However, excessive tea consumption (more than 200–400 mg or 4–8 cups a day) can cause side effects such as headache or nausea due to its function in lowering blood sugar [1].

3. Green tea catechin with diabetes

Emerging diseases such as diabetes and obesity become a global health concern, and they are closely associated with metabolic syndrome (MetS) [54]. MetS is a symptom that includes an increase in serum triglyceride, elevated blood pressure, and reduced high-density lipoprotein-associated cholesterol (Ford, 2005). Therefore, the consumption of green tea catechin will be beneficial in countering health effects by reducing body weight and alleviating MetS.

Some studies explained that the consumption of EGCG in green tea as anti-diabetic could significantly reduce adipose tissue, lowering blood glucose or insulin levels, and stimulate insulin sensitivity or glucose tolerance. Insulin resistance is the condition when the body cannot use the hormone insulin properly to metabolites sugar and lead to further weight gain. Hence, by reducing body weight, people who are severe in diabetes can improve glucose tolerance [46, 50, 54].

This evidence was validated in an earlier in-vitro study by using genetically obese/diabetic samples [55], showing that EGCG intake can reduce not only body weight but also visceral fat weight. A similar result was also obtained in the experiment using western-style diet, where EGCG attenuated insulin resistance, plasma cholesterol, and monocyte chemoattractant protein concentrations [52, 55].

Also, tea catechin can deal with obesity because it can alter bile acids (BAs) as the regulator of body cholesterol. BAs are steroid molecules produced in the liver, essential for digesting and absorbing dietary fats. They aid in emulsifying fats in the small intestine, facilitating enzymatic digestion. After digestion, BAs are reabsorbed in the small intestine and recycled to the liver, contributing to efficient digestion. Beyond their digestive role, BAs also serve as signalling molecules, influencing physiological processes such as lipid metabolism and glucose homeostasis. Abnormalities in BA metabolism can lead to disorders like gallstones and liver diseases.

So, when there is an oral exposure to catechin, the major proportion of physiological cholesterol will be cascaded in the system through the metabolic process in hepatocytes [56, 57]. Complex diseases can be significantly induced by gut microflora through metabolic processes that involve BAs and the intervention of bioactive component like catechin can either directly affect BAs synthesis in the liver or disturb microflora by increasing the level of BAs [58]. Due to its role in metabolism, high levels of BAs can remove fat and then suppress the diet-induced obesity [59] and are responsible for inclining total body expenditure and dissipation of energy in the form of heat [57].

On the other hand, the study about diabetes is also associated with the reduction of BMD (bone mineral density), which seems contradictive with the way catechin aids in reducing body weight. The potential effects of EGCG on BMD may be linked to its interaction with various biological pathways. On one hand, EGCG has antioxidant and anti-inflammatory properties, which could theoretically be beneficial for bone health by reducing oxidative stress and inflammation. These factors are known to influence bone metabolism. On the other hand, certain studies have reported that high doses of EGCG may have adverse effects on bone health. One possible explanation is that, in excess, EGCG might interfere with the activity of osteoblasts, cells responsible for bone formation. Additionally, EGCG may affect the absorption of minerals like calcium, which is essential for bone mineralisation [13].

The contradictory results in different studies may be attributed to variations in study design, dosage, duration, and the specific populations being studied. Factors such as the source of EGCG, individual variations in response, and the presence of other compounds in green tea may also contribute to the inconsistent findings. Therefore, while some studies suggest that EGCG may have a positive impact on BMD due to its antioxidant and anti-inflammatory properties, conflicting results and potential adverse effects at high doses indicate that more research is needed to understand the complex relationship between EGCG and bone health.

In general, the prevention of diabetes can be achieved by controlling body weight, particularly addressing obesity. One effective approach is supplementing with EGCG, known to attenuate insulin resistance, lower blood sugar, and increase the level of BAs. This is crucial for enhancing insulin sensitivity, allowing cells to respond more efficiently to insulin and improving glucose uptake. EGCG has additional benefits, such as inhibiting the absorption of glucose in the intestine, contributing to more stable blood sugar levels post-meals. Its anti-inflammatory properties also play a role in alleviating chronic inflammation, a factor linked to insulin resistance. Furthermore, EGCG shows a protective effect on pancreatic beta cells, essential for insulin production, and may activate AMP-activated protein kinase (AMPK), an enzyme associated with increased glucose uptake and improved insulin sensitivity. While these mechanisms suggest a potential role for EGCG in managing insulin resistance, further research, including clinical trials, is necessary for concrete recommendations. Individuals with diabetes or insulin resistance should seek guidance from healthcare professionals before making significant dietary changes or incorporating supplements [36, 52].

4. Green tea and cardiovascular disease

Cardiovascular disease is a significant global health concern, with lifestyle factors, including dietary choices, playing a crucial role in its development [60, 61]. The latest research highlights the potential protective effects of tea against cardiovascular issues, specifically through tea polyphenols. These compounds exhibit various cardioprotective properties, such as reducing cholesterol absorption, lowering blood cholesterol levels, and mitigating risks of thrombosis, inflammation, and oxidative damage [62, 63].

Tea polyphenols go beyond by regulating vascular reactivity, influencing plasma lipid profiles, and demonstrating anti-thrombotic, antioxidant, anti-inflammatory, and anti-proliferative activities, all contributing to improved vascular function [64, 65, 66]. Green tea polyphenols, in particular, have shown promise in preventing various cardiovascular diseases. Meanwhile, BT polyphenols have potential benefits, such as reducing postprandial hypertriacylglyceremia and enhancing endothelial function [67, 68].

First and foremost, these compounds act as potent antioxidants, neutralising free radicals that can otherwise damage blood vessels and contribute to cardiovascular ailments. Furthermore, EGCG demonstrates anti-inflammatory properties, countering chronic inflammation associated with heart-related conditions. By positively influencing lipid metabolism, EGCG aids in improving the lipid profile, reducing levels of low-density lipoprotein (LDL) cholesterol and triglycerides. Notably, it enhances endothelial function, promoting optimal blood vessel dilation and contraction. Additionally, studies suggest a potential mild hypotensive effect, contributing to overall blood pressure regulation [67].

Polyphenols in BT offer their own set of advantages. Notably, they play a role in reducing postprandial hypertriacylglyceremia, addressing elevated triglyceride levels after meals. This effect contributes to a more stable post-meal lipid profile. Additionally, BT polyphenols have been associated with enhancing endothelial function, crucial for maintaining healthy blood vessels and optimal blood flow. This suggests that incorporating tea into one’s lifestyle may play a positive role in cardiovascular health. Thus, tea polyphenols present a compelling strategy for lowering the risk of cardiovascular disease, making them a valuable inclusion in discussions about maintaining a heart-healthy lifestyle [68].

5. Green tea catechin with the cancers

Tea polyphenols, known for their ability to prevent cancer, protect against different stages of cancer development by reducing inflammation and slowing down tumour growth. They act as antioxidants, neutralising harmful free radicals, which help to induce cell death and stop the growth of cancer cells. Both black and green tea polyphenols are important in blocking cancer-causing enzymes, activating protective enzymes, and preventing damage to DNA, cell death, blood vessel formation, and the spread of cancer, providing defence against various types of cancers [69, 70, 71, 72].

Ovarian cancer, ranking seventh in global cancer-related deaths, is influenced by familial, age, genetic, dietary, and lifestyle factors. Some conflicted results exist regarding tea consumption, especially green tea, with ovarian cancer risk. In general, green tea polyphenols demonstrate promise by reducing protein expression linked to ovarian cancer and inducing apoptosis, but BT polyphenols show the contrary effect [73, 74].

While some studies suggest a reduction in risk, epidemiological studies indicate an inverse relationship between tea consumption and ovarian cancer [75, 76, 77]. This lack of clarity may arise from variations in tea-drinking habits, types of tea consumed, and the duration of consumption across different study populations, as well as a failure to control for potential confounding factors. Existing literature exploring the relationship between the type and duration of tea consumption and ovarian cancer risk has not provided consistent findings. However, a particular study [78] suggests that an increase in the frequency and duration of tea drinking, especially green tea, may reduce the risk of ovarian cancer. It is important to note that further investigation is required to determine the protective effects of BT and oolong tea.

It is proven in another study [77] that the correlation between caffeinated beverages including green tea and ovarian cancer remains inconclusive. Interestingly, there was no elevated risk observed for coffee or caffeinated soft drinks. While overall tea consumption did not show an association with ovarian cancer risk, a nuanced analysis considering the type of tea revealed an increased risk among exclusive BT drinkers. However, no excess risk was found for those exclusively consuming green tea. These patterns persisted when examining post-menopausal women, with the association with BT consumption mainly noted in the endometrioid histotype.

In addition to that, there is a positive effect of oestrogen modulation, which is the risk factor for ovarian cancer, when reacts with green tea polyphenols, impacting oestrogen metabolism. Meanwhile, BT consumption is associated with increased oestrogen levels in postmenopausal women, influencing ovarian cancer risk. Unless the TF3 (theaflavin-3) in BT, synergistically works with CDDP (cisplatin, a chemotherapy drug commonly used for ovarian cancer treatment). TF3 and CDDP together exerted synergistic cytotoxicity against cisplatin-resistant ovarian cancer cells. TF3 and CDDP synergistically induced apoptosis (programmed cell death) and G1/S cell cycle arrest in ovarian cancer cells. Additionally, the combination treatment with TF3 and CDDP synergistically down-regulated Akt phosphorylation (a signalling pathway associated with cell survival) in ovarian cancer cells [76].

Lung cancer, the primary cause of global cancer-related deaths, is potentially influenced by dietary factors, including tea consumption. Tea polyphenols exhibit protective effects against lung cancer by inducing apoptosis, inhibiting tumour progression, and regulating key factors in lung cancer cells. Observational studies suggest a dose-dependent protective effect of green tea against lung cancer, while BT shows no significant impact [79]. White tea is proposed to have chemo-preventive properties against lung tumour formation due to its anti-carcinogenic attributes [80].

Skin cancer, with an alarming increase in incidence globally, is linked to solar ultraviolet radiation. Polyphenols in black and green tea emerge as potential chemo-preventive agents, inhibiting UV radiation penetration and reducing DNA damage by countering skin inflammation through antioxidant properties. Green tea, specifically, offers skin photoprotection, preventing photo-carcinogenesis, inducing DNA repair, stimulating immune responses, inhibiting angiogenesis, and influencing various UVB-carcinogenesis biomarkers. Some studies find no association between tea consumption and skin cancer risk, while others suggest an inverse association [81, 82].

Breast cancer, the most frequent cancer in women, exhibits lower incidence in Asian countries, prompting hypotheses linking higher green tea consumption to reduced risk. Green tea polyphenols impact oestrogen biosynthesis enzymes, which are significant in breast cancer development. Despite findings supporting green tea’s protective role, some studies report weak or no relationships between green tea and breast cancer risk, occasionally noting positive correlations with BT [83, 84].

Colorectal cancer, a prevalent and deadly form of cancer, has drawn attention regarding potential protective effects from dietary and lifestyle factors. Findings on tea consumption’s impact present contradictions, with some studies indicating no association between green tea and colorectal cancer risk and others reporting a reduced risk with regular tea consumption. EGCG, a green tea polyphenol, plays a role in inhibiting key factors integral to colon cancer development [85].

Lastly, endometrial cancer, a prevalent gynaecological cancer, is influenced by dietary habits that can either increase or decrease its long-term risk factors. Studies present conflicting findings on the association between tea consumption and endometrial cancer risk. Some indicate no or minimal association, while others suggest a reduced risk with tea consumption. In contrast to the protective effect of green tea, BT consumption is positively associated with the risk of endometrial cancer [86].

The lack of clear evidence for black tea’s cancer-reducing potential might be due to variations in the types and amounts of compounds present in BT compared to green tea. Both teas come from the same plant, Camellia sinensis, but they undergo different processing methods. Green tea is minimally processed, preserving more of its natural compounds, including polyphenols like EGCG. BT, on the other hand, undergoes fermentation, altering its composition [18].

Polyphenols, known for their potential health benefits, are more abundant in green tea. These compounds have antioxidant properties and may play a role in reducing the risk of certain cancers. The fermentation process in BT might result in lower levels of specific polyphenols compared to green tea, potentially affecting its observed protective effects against cancer [20].

It’s essential to note that research in this area is ongoing, and factors like individual variations, study designs, and lifestyle differences can contribute to the complexity of the findings [85, 86].

6. Tea catechin and neurological disease

Tea polyphenols demonstrate significant neuroprotective effects against major neurological diseases, such as Alzheimer’s and Parkinson’s disease [87]. Lifestyle factors, particularly dietary choices, play a crucial role in maintaining central nervous system health and combating neurological diseases [17]. Studies suggest that regular tea consumption is associated with improved cognitive function [16, 88], better verbal fluency scores [89], and a reduced risk of cognitive decline [90] and impairment [91].

Bioactive components in green tea, such as catechins and theanine, have been linked to improved cognition in rodent models. Their anti-oxidative and anti-inflammatory properties contribute to cognitive benefits. Green tea’s neuroprotective mechanisms include inhibiting acetylcholinesterase, regulating stress hormone secretion, and modulating neurotransmission in the brain. Human studies reported enhanced parieto-frontal connectivity, crucial for working memory processing, as a potential mechanism behind green tea’s positive impact on cognitive function, proven by the improved cognitive dysfunction scores. However, further clarification requires long-term, large-scale controlled studies [88]. Also, by exhibiting anti-inflammatory and antioxidant activities, tea polyphenols induce iron-chelating effects, modulate cell survival and signalling pathways, regulate stress hormone secretion, enhance neurotransmitter system levels, and reduce oxidative stress linked to age-related brain disorders [92, 93].

A small amount of caffeine in tea also contributes to protective effects against neurological diseases like Alzheimer’s and Parkinson’s by stimulating the central nervous system [94]. Recent research suggests that caffeine or caffeine-containing products may protect against the degeneration of dopaminergic neurons, impacting the progression of Parkinson’s Disease (PD). Furthermore, these findings indicate that caffeine could improve motor function in PD patients. Additionally, drugs like istradefylline, which block adenosine A2A receptors, have shown promise in reducing OFF time and dyskinesia linked to standard dopamine treatments for PD [95]. The caffeine content in green tea can vary, but on average, it typically contains about 20 to 45 mg of caffeine per 8-ounce cup. These values can differ based on factors such as the type of green tea, brewing time, and other variables. It’s essential to note that green tea generally has lower caffeine content compared to BT and coffee [94].

Furthermore, in PD, tea polyphenols offer neuroprotective effects with anti-aggregation, anti-chelating, anti-inflammatory, and antioxidant properties, inhibiting a-synuclein aggregation and modulating intracellular signalling pathways [70, 96]. Tea consumption is associated with a lower risk of PD and delays its onset, making it a promising dietary modification to slow down age-related neurodegenerative diseases [27, 97].

In Alzheimer’s disease, characterised by progressive neurodegeneration, tea polyphenols, particularly in green tea, reduce the risk by lowering toxic levels of brain Amyloid β (Aβ) peptide and inhibiting its production, a trigger for Alzheimer’s disease. Amyloid-β (Aβ) is associated with problems in the energy-producing part of cells, known as mitochondria, which are linked to the onset and progression of Alzheimer’s disease (AD). Seeking to enhance mitochondrial function as a therapeutic approach, researchers have explored plant-derived compounds, particularly flavonoids found in green tea like EGCG and luteolin. These compounds not only lower harmful Aβ levels in the brain but also show potential in safeguarding the energy production of nerve cells affected by Alzheimer’s [9].

Tea polyphenols also exhibit protective effects against associated conditions like diabetes, depression, and hypercholesterolemia. During depression, there is an activation of inflammatory pathways, dysfunction in mitochondria, an increase in indicators of oxidative stress, and a decline in the body’s antioxidant capacity. Around 30% of depressed individuals do not respond well to traditional medications. Preclinical studies suggest that phenolic compounds found in green tea may have the potential to alleviate depressive symptoms by influencing factors linked to oxidative stress, neuroinflammation, and the balance of intestinal microorganisms. These polyphenols can impact the makeup of gut bacteria, which is associated with health advantages, and the gut microbes can transform polyphenols into bioactive substances that offer therapeutic effects [98].

7. Green tea and infectious diseases

As effective antimicrobial agents, tea polyphenols demonstrate a broad-spectrum antimicrobial effect against various microorganisms both gram-positive and gram-negative bacteria, viruses, fungi, and parasites. Notably, strains such as Methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum beta-lactamases (ESBLs), which pose significant health threats, have demonstrated vulnerability to green tea extracts. Another study revealed that the efficacy of (−)-epigallocatechin (EGC) in green tea worked against multi-drug-resistant Escherichia coli strains from urinary tract infections. However, it is crucial to consider variations in green tea types, preparations, and compositions when interpreting these results [99, 100].

Green tea catechins exert their direct effects by interacting with the lipid bilayer cell membrane of bacteria. This interaction causes damage to the membrane, leading to antimicrobial effects. When Escherichia coli is exposed to green tea polyphenols, it results in changes to the regulation of 17 genes, causing significant damage to the bacterial cell membrane. This effect is more noticeable in gram-negative bacteria due to the negatively charged lipopolysaccharide (LPS) outer membrane in gram-negative bacteria. The compromised bacterial cell membrane disrupts essential functions, such as binding to host cells and forming biofilms, which play a crucial role in the pathogenesis of infections. Additionally, the damage to the bacterial membrane prevents the secretion of toxins, further contributing to the antimicrobial action of green tea catechins [101].

EGCG also has shown antiviral effects against various virus families, including Retroviridae, Orthomyxoviridae, and Flaviviridae. This includes well-known human pathogens like HIV [102], influenza A [103], and hepatitis C [104]. Additionally, EGCG disrupts the replication cycle of DNA viruses, such as hepatitis B [105], herpes simplex, and adenovirus [106]. The compound exhibits different modes of action against these viruses, highlighting its potential as a broad-spectrum antiviral agent. In general, to prevent infective diseases, EGCG exhibits antiviral properties by acting in various ways. It serves as a protective barrier, preventing viruses like HIV from entering our cells. Additionally, it disrupts the virus’s ability to replicate, essentially hindering its reproduction process. EGCG also plays a role in enhancing our immune system’s response to infected cells, aiding in their identification and elimination. Moreover, it helps to reduce inflammation, preventing an exaggerated response to the infection. Furthermore, EGCG can directly interact with viruses, rendering them ineffective in infecting cells. While promising in laboratory studies, more research is needed to determine its potential as a real treatment, and its effectiveness may vary depending on the specific virus [107].

In influenza virus, catechins work by changing how the influenza virus infects cells, specifically by interacting with viral hemagglutinin (HA) and affecting the synthesis of viral RNA in cells. Additionally, catechins hinder the activity of viral RNA polymerase, a key enzyme for the virus. The galloyl group in catechins plays an important role in this process. This suggests that green tea and its catechins could be useful in protecting against influenza. Even the leftover parts of green tea after extraction, known as by-products, can still be a strong source of anti-influenza properties due to the substantial amount of catechins they contain. Specifically, catechins highly concentrated in the ethyl acetate-soluble fraction show effective inhibitory effects against various stages of the influenza virus lifecycle, including transcription and release [108].

Furthermore, the catechin has the ability to fight against harmful fungi like Candida albicans, which can cause infections in humans. While the exact way EGCG works is not completely clear, some evidence suggests that it attaches to the protective outer layer of fungi, disrupting their function. Additionally, EGCG seems to interfere with the way bacteria and fungi manage folic acid, a vital nutrient for their growth, by inhibiting a key enzyme called dihydrofolate reductase. This disruption can weaken the fungi and make them less harmful. However, more research is needed to fully understand these processes and their effectiveness in real-life situations [109].

Additionally, EGCG shows promise in addressing anthrax infections caused by Bacillus anthracis, a spore-forming gram-positive bacterium that is associated with high mortality [110]. This is considered one of the most powerful biological warfare agents because its spores are highly resistant to natural conditions and can persist for many decades in the environment. B. anthracis spores can enter the body through a skin lesion (cutaneous anthrax), the lungs (pulmonary anthrax), or the gastrointestinal route (gastrointestinal anthrax). They then germinate, leading to the development of the active vegetative form. Anthrax is a public health concern in many countries, especially those relying on agriculture as their primary source of income [111].

EGCG from green tea works as a potent inhibitor of anthrax lethal factor by preventing LF-induced death of cells. It does this by interfering with the activity of LF, which is a major contributor to the development of anthrax. EGCG has been found to protect cells and animals from LF-induced toxicity, making it a valuable tool for studying the mechanisms of LF action and potentially preventing and treating anthrax. The exact mechanism of how EGCG achieves this inhibition is not fully understood, but it is thought to involve the direct interaction between EGCG and LF, leading to the prevention of LF-induced cytotoxicity. The multifaceted antimicrobial properties of tea polyphenols underscore their potential as therapeutic agents against a wide range of infectious diseases [110].

8. Conclusion

One of the key takeaways is the potential of green tea catechins, particularly EGCG, in combating oxidative stress. Oxidative stress, characterised by an imbalance between antioxidants and free radicals in the body, is implicated in the development of numerous health issues. By acting as powerful antioxidants, green tea catechins can help to restore this balance and thereby prevent oxidative stress-related diseases. This highlights the importance of incorporating antioxidant-rich diets, such as green tea, into daily consumption patterns to promote overall health and well-being.

Furthermore, the multifaceted benefits of green tea catechins extend beyond oxidative stress prevention. These benefits include the potential to reduce body weight, alleviate MetS, improve glucose tolerance, and regulate BAs. Additionally, the protective effects of green tea catechins against cardiovascular disease, cancer, and neurological diseases are underscored, indicating the wide-ranging impact of green tea consumption on human health.

It is crucial to mention that ongoing and detailed research is being conducted to delve deeper into the specific mechanisms and outcomes of green tea consumption in relation to the diseases discussed. Researchers are exploring nuanced aspects, such as optimal dosage, variations in individual response, and the long-term effects of sustained green tea intake.

It is important to note, however, the potential side effects of excessive green tea consumption, such as headaches or nausea. This highlights the need for moderation in consumption (around 200–400 mg a day) and the importance of seeking a balanced approach to reaping the benefits of green tea without experiencing adverse effects.

In conclusion, the scientific evidence presented in the passage strongly supports the notion that green tea catechins, particularly EGCG, can play a significant role in reducing the risk of various complex diseases by acting as potent antioxidants. The findings underscore the potential of green tea as a valuable dietary component in promoting overall health and well-being. However, it is essential to approach consumption in moderation and in conjunction with a balanced diet to maximise the benefits while minimising potential adverse effects. Ongoing research endeavours aim to provide more nuanced insights into the specific effects of green tea on various diseases, refining our understanding and optimising its integration into preventive healthcare strategies.

Acknowledgments

I extend my sincere gratitude to the IntechOpen for providing me with the opportunity to undertake and publish my previous writing. Also, the support and resources offered by the university have been instrumental in the successful completion of this work.

Conflict of interest

The authors declare no conflict of interest.