What Are The Effects of Catechins?

Academic Food 17(3) (2019) 417-428, DOI: 10.24323/academic-food.647733

Review Paper

Composition and Health Effects of Tea Plant (Camellia sinensis)

Cemre Elmas, Ceren Gezer

Eastern Mediterranean University, Faculty of Health Sciences, Department of Nutrition and Dietetics, Famagusta, Northern Cyprus Turkish Republic

Received: 27.04.2018, Accepted: 09.10.2018

SELF

The effect of tea, a widely consumed beverage worldwide, on health is mostly associated with the phytochemicals found in its composition. The fermentation process during tea production leads to changes in the variety and quantity of bioactive components present in tea. It is stated that catechins, polyphenols from tea, contribute to the prevention of diseases such as obesity, diabetes, cardiovascular diseases, and cancer through their antioxidant activity, and these components also have protective effects against viral, bacterial, and neurological diseases. This review aims to examine the composition of tea and its effects on health.

Keywords: Camellia sinensis, Tea, Flavonoid, Epigallocatechin gallate

ABSTRACT

Tea is one of the most widely consumed beverages worldwide. The effect of tea on human health is often related to phytochemicals in its composition. During tea production, fermentation may produce variations by the amount and type of bioactive components present in the tea content. Catechins in the tea polyphenols show antioxidant activity. These components contribute to the prevention of diseases such as obesity, diabetes, cardiovascular diseases and cancer. It is also stated that these components may have protective effects against viral, bacterial and neurological diseases. In this study, it is aimed to review the bioactive constituents of tea and their effect on human health.

Keywords: Camellia sinensis, Tea, Flavonoid, Epigalloatechin gallate

ENTRANCE

Tea ranks among the most popular beverages consumed worldwide, both hot and cold. Different tea products such as black tea, oolong tea, green tea, and white tea are produced from the leaves of the Camellia sinensis plant through various harvesting and processing methods. This diversity is associated with the degree of oxidation of polyphenols in the fresh leaves, depending on the fermentation process during processing [1]. In the production steps of black tea, leaves are curled to undergo oxidation with polyphenol oxidase, leading to a decrease in flavan-3-ol (catechin) levels. Green tea, on the other hand, is a rich source of unoxidized catechins as it is produced without oxidation. Catechins are the most important phenolic compounds found in tea [2]. Recent studies have shown that tea plays a significant role in the prevention and treatment of cardiovascular diseases, obesity, diabetes, oxidative and inflammatory diseases, bacterial and viral infections, cancer, and neurological disorders, mainly due to these bioactive components [3-5]. This review aims to examine the composition of tea and its effects on health.

GENERAL INFORMATION ABOUT TEA

The tea plant belongs to the Camellia genus of the Theaceae family (Camellia sinensis or Thea sinensis) [6]. It is a perennial shrub that grows in moist climates, remains green throughout the year, and is of short stature. The tea plant is cultivated in approximately 40 countries and can be considered a semi-tropical plant depending on climatic conditions. Significant tea production occurs in China, Sri Lanka, Indonesia, Japan, India, Taiwan, and central African countries [7-9].

TEA PRODUCTION AND PROCESSING TECHNIQUES

Tea production generally prefers using the tip bud and two leaves (referred to as two and a half leaves) from the shoot tip of the plant. This is because important components contributing to quality are concentrated in these two and a half leaves [10]. Basically, three types of tea are produced from the tea plant. These types are black tea, green tea, and oolong tea. All these teas are derived from Camellia sinensis and differ in the oxidation process during production [11-13].

According to the Turkish Food Codex, Tea Regulation (Regulation No: 2015/30), black tea is defined as a product obtained through processing tip buds and fresh leaves of different varieties of Camellia sinensis, including fresh single, double, and triple leaves, along with fresh stem parts connecting them, through production stages such as withering, rolling, cutting, oxidation, and drying [14]. Green tea, on the other hand, is defined as a non-oxidized product obtained through processing tip buds and fresh leaves of different varieties of Camellia sinensis, including fresh single, double, and triple leaves, along with fresh stem parts, through production stages such as enzyme inactivation, rolling, cutting, and drying [14]. Accordingly, black tea is obtained through full fermentation, while green tea is not fermented. Oolong tea, however, is partially fermented [15].

Looking at the production stages of tea, the harvested leaves are first withered in trays or drying racks in drying rooms. This process also includes dehydration. The moisture content of fresh leaves decreases from approximately 75% to 55-65%, and the leaves begin to soften. This process prepares the leaves for the next stage, rolling, and creates a suitable environment for the subsequent fermentation process. Enzyme activity begins for aroma development and fermentation. In the next stage, withered leaves are rolled, and fermentation begins. The rolled teas are broken into small pieces in machines. In the fermentation stage, which is the stage that provides diversity in teas, steam heat is applied to ensure fermentation occurs within 2-3 hours. To stop the fermentation process, tea leaves are dried to reduce the moisture content to a certain level. The prepared leaves are then separated according to specific characteristics and packaged under suitable conditions [16].

Apart from black tea, green tea, and oolong tea, a very small amount of white tea is produced. This type of tea is obtained by drying the leaves under the sun for a long time while the buds are still closed during the spring months, and the tea is slightly oxidized without undergoing the leaf rolling process [1].

TEA COMPOSITION

Nutrient Content

The chemical composition of tea products can vary depending on the origin, age, and processing of the tea [16]. Table 1 shows the amounts of components per 100 grams of tea consumed after adding 15 grams of tea leaves to one liter of water, steeping for five minutes, and then straining. While most macro and micronutrients are present in trace amounts, potassium stands out among them [17].

Polyphenol Content

The tea plant has over 4000 chemical components and is one of the plants with the highest flavonoid content by dry weight [18]. Fresh tea leaves contain a wide range of phenolic compounds, including catechins, flavonols, proanthocyanidins, and phenolic acids. Depending on the production of black, oolong, or green tea, the fermentation of catechins is carried out by endogenous enzymes polyphenol oxidase and peroxidase. During fermentation, catechins oxidize to form dimeric and oligomeric compounds, including theaflavins, theasinensins, theanaptokinons, and thearubigins [13, 19, 20]. While green tea contains higher levels of catechins, fermentation in black tea replaces these catechins with theaflavins and thearubigins. These compounds also impart characteristic aroma and color to the tea. Thearubigins are among the most abundant phenolic compounds in black tea [1, 13, 19, 20]. In green tea, the most abundant is epigallocatechin gallate (EGCG), followed by epicatechin gallate (ECG), epigallocatechin (EGC), and epicatechin (EC). Additionally, tea leaves contain hydrolyzable tannins of phenolic acid derivatives (ellagitannins and gallotannins), which are associated with antioxidant properties and linked to cancer and cardiovascular diseases. Another phenolic acid derivative, strictinin, is an important compound associated with allergic diseases. Strictinin is found in both green and black tea and its concentration varies depending on the maturity of the leaves. Trigalloyl glucose, theogallin, and gallic acid are other phenolic acid derivatives present in tea. Theanine is also a significant amino acid found in tea [13, 19, 20].

Caffeine Content

According to the Turkish Food Codex, Tea Regulation (Regulation No: 2015/30), caffeine-free black/green tea is defined as ‘tea with a caffeine content not exceeding 0.1% by weight of the dry matter’ [14]. Caffeine is one of the main alkaloids found in tea and its content can vary between 1.5% to 5% [13]. The caffeine content in a cup (237 ml) of green tea is approximately 30 mg, while in black tea, it is approximately 50 mg [21].

Table 1. Components of 100 grams of tea consumed

Components	Amount
Protein (g)	0.1
Fat (g)	Trace amount
Carbohydrate (g)	Trace amount
Sodium (mg)	Trace amount
Potasium (mg)	27
Calcium (mg)	Trace amount
Magnesium (mg)	2
Phosphorus (mg)	2
Iron (mg)	Trace amount
Copper (mg)	0.01
Zinc (mg)	Trace amount
Chlorine (mg)	1
Manganese (mg)	0.15
Selenium (mcg)	Trace amount
Lodine (mg)	Trace amount
Thiamine (mg)	Trace amount
Riboflavin (mg)	0.02
Niacin (mg)	0.2
Vitamin B6 (mg)	Trace amount
Vitamin B12 (mcg)	0.2
Folate (mcg)	3
Pantothenate (mg)	0.04
Biotin (mcg)	1

TEA AND HEALTH RELATIONSHIP

The belief in the health benefits of tea dates back to the 19th century. The health benefits of tea are generally associated with its various polyphenols [18, 22]. It has been suggested that tea plays an important role in the prevention and treatment of cardiovascular diseases, obesity, diabetes, oxidative and inflammatory diseases, bacterial and viral diseases, cancer, and neurological diseases [3-5] (Figure 1). Studies regarding these health effects are summarized in tables 2, 3, and 4 after discussing the health effects.

Figure 1. Tea and health relationship

The Effect of Tea on Cardiovascular Diseases

Cardiovascular diseases (CVD) encompass a wide range of diseases affecting the heart and blood vessels, including coronary heart disease, stroke, rheumatic heart disease, cerebrovascular disease, peripheral artery disease, hypertensive diseases, and arrhythmias [23]. Generally, tea consumption is known to reduce the risk of cardiovascular disease [24-28]. In a meta-analysis, it was indicated that the relationship between tea consumption and coronary heart disease and stroke was inconclusive, but it showed a protective effect against myocardial infarction [29]. The positive effect of tea consumption on these diseases is explained by the antioxidants in its composition. It is reported that flavonoids in tea reduce the risk, but the exact mechanism is still not fully understood. Flavonoids are reported to inhibit low-density lipoprotein (LDL) oxidation, reduce cholesterol absorption from the intestine, and decrease platelet aggregation, thereby affecting blood pressure [24, 30]. In a randomized, double-blind, placebo-controlled study involving 48 healthy adults, individuals were divided into 4 groups. The first group received 250 mg of caffeine, the second group received 200 mg of theanine, and the third group received caffeine and theanine in capsule form. The fourth group was the placebo group. It was found that the theanine in tea works as an antagonist to caffeine, lowering blood pressure [31].

The Effect of Tea on Weight Management and Body Composition

Obesity and its associated health problems continue to be major global health issues. Tea consumption is one of the factors that affect appetite, nutrient absorption, and thermogenesis, all related to body weight loss. Epidemiological and randomized controlled intervention studies have shown an inverse relationship between tea consumption and body fat percentage and waist circumference [20, 32, 33]. While the relationship between green tea and obesity is associated with the effects of catechins on thermogenesis and substrate oxidation, changes in appetite control, regulation of hepatic lipid metabolism enzymes, and decreased nutrient absorption are possible other mechanisms of action. The sympathetic nervous system (SNS) plays a major role in regulating energy expenditure and lipolysis. Norepinephrine (NE), a key player in the SNS, increases energy expenditure and fat oxidation. Catechins can prolong the action of NE released sympathetically into the synaptic cleft by inhibiting catechol O-methyltransferase (COMT), an enzyme that reduces norepinephrine. Additionally, caffeine found in green tea can also affect the SNS and exhibit similar effects. Therefore, the combination of catechins and caffeine can affect energy expenditure and lipolysis synergistically [34-38]. Low hepatic fatty acid oxidation increases appetite. Catechins, on the other hand, are reported to increase hepatic fat oxidation and may therefore affect appetite. However, experimental studies in this regard have yielded different and conflicting results [39-42]. Additionally, catechins can affect α-amylase and α-glucosidase activities, thereby inhibiting glucose absorption [43]. It is reported that the amount of plant material in prepared tea, brewing time, and water temperature can affect the composition of bioactive components. It is stated that these bioactive components may have different effects depending on whether tea is consumed as a beverage or taken as a supplement. Therefore, more research is needed on the relationship between the amount of bioactive components in green tea and obesity [20].

The Effect of Tea on Diabetes

The prevalence of diabetes continues to increase worldwide. However, it is known that this condition can be prevented by adopting healthy lifestyle behaviors [44]. It is reported that both coffee and tea consumption are inversely associated with the risk of diabetes. Tea consumption is not only associated with reducing the risk of diabetes related to low body mass index (BMI) but also affects amylase. Tea polyphenols can reduce sudden increases in serum glucose by slowing down starch digestion [45, 46]. A study conducted in Singapore showed that regular consumption of coffee and black tea is associated with a lower risk of diabetes, while no association was found with green tea. It is believed that this effect is due to the caffeine content in black tea [47]. Another retrospective cohort study showed that green tea intake was associated with a lower risk of diabetes compared to black tea [48]. In a study examining the relationship between coffee and tea consumption and the risk of type 2 diabetes in 5,823 non-diabetic adults, it was concluded that coffee and tea consumption was not associated with diabetes incidence [49].

The Effect of Tea on Inflammation

Many pathological conditions with acute or chronic inflammation cause significant changes in metabolic and biochemical processes in the body. Nutrition plays an important role in the development and resolution of these processes [50]. Flavonoids found in tea, including flavonols, anthocyanins, and isoflavones, are known to be negatively associated with the concentration of C-reactive protein (CRP), a significant marker of inflammation [30, 51]. An in vivo study in mice with spinal cord injury showed that EGCG administration suppressed inflammatory cytokines and had a protective effect in this regard [52]. Another study in mice showed that 10 mg/kg/day EGCG treatment for 30 days significantly reduced the levels of inflammatory cytokines and oxidative stress [53].

The Effect of Tea on Cancer

In the relationship between tea and cancer, EGCG and theaflavin, which are components of tea and exhibit antioxidant properties, are prominent. These components can prevent the formation, growth, and proliferation of cancer cells, mainly due to the antioxidant property of tea [46, 54, 55]. Particularly, the polyphenols found in green tea are reported to inhibit angiogenesis [56, 57]. While tea polyphenols have these effects on cancer cell cycles, it is noted that these effects are not observed in normal cells. The pathways through which EGCG is effective in cancer cells include mitogen-activated protein (MAP) kinases and activator protein-1 (AP-1), nuclear factor-kappa B (NF-kB) signaling pathway, epidermal growth factor receptor (EGFR) mediated pathways, insulin-like growth factor (IGF) -1 mediated signal transduction pathways, proteasome activities, matrix metalloproteinase (MMP) activities, urokinase-plasminogen activator activities, and induction of apoptosis, leading to cell cycle arrest [58]. Studies examining the relationship between green tea and cancer have found that increased consumption of green tea is associated with a decreased risk of various types of cancer such as prostate, breast, throat, and stomach cancer [59-61]. A study conducted on human prostate cancer cells (PC-3) showed that applying 0-50 mM EGCG for 48 hours inhibited cell proliferation [62]. In another study using human breast cancer cell line MDA-MB-231 cells, it was observed that applying 50-80 mg/mL EGCG for 24 hours reduced MMP-9 expression and induced cell apoptosis [63]. Another study with gastric cancer cells also found that EGCG application induced cell apoptosis [64].

The Antiviral and Antibacterial Effects of Tea

As previously mentioned, the potential health effects of tea stem from its phenolic compounds [65]. Particularly, green tea rich in EGCG and EGC has been shown to have antimicrobial, antifungal, and antiviral effects [66, 67]. In a study, green tea was found to have potential antimicrobial effects, especially against Escherichia coli [68]. Another study testing the antibacterial activity of green tea against various bacteria isolated from environmental sources found that green tea exhibited antibacterial activity [69]. A study investigating the role of non-polymeric phenolic and polymeric tannin components of green, black, and different herbal teas derived from Camellia sinensis revealed that these components had strong antioxidant and antibacterial properties [70]. In another study, it was found that green tea catechins could inhibit influenza virus replication, demonstrating antiviral effects [71]. Additionally, green tea catechins are known to have inhibitory effects on Herpes simplex virus and Helicobacter pylori infection [3].

The Effect of Tea on Oral Health

Dental caries and other oral diseases are common chronic illnesses, especially in children. The main bacterial agents in the development of caries are Streptococcus mutans (S. mutans) and Lactobacillus. Especially green tea is reported to be an effective substance in preventing dental caries. Like in other health issues, catechins, especially in oral health (dental caries, oral mucosa), are responsible for many beneficial properties. In many studies, green tea has been shown to inhibit the growth of streptococcal agents, prevent bacterial adhesion to tooth enamel, and prevent dental caries formation as a glucosyl transferase and amylase inhibitor [72-75]. A study found that green tea increased oral peroxidase activity and provided greater protection against oxidative stress in the oral cavity [76]. In a study comparing the effect of green tea mouthwash with sodium fluoride mouthwash on salivary Streptococcus mutans and Lactobacillus levels in children, it was observed that green tea mouthwash significantly reduced salivary S. mutans and Lactobacillus colony counts to a greater extent than sodium fluoride, with fewer side effects [77]. Similarly, other studies have shown that tea consumption, as well as coffee, significantly inhibits dental plaque formation and reduces S. mutans and Lactobacillus in dental plaque and saliva [78, 79]. Another study indicated a relationship between green tea consumption and reduced tooth loss rate [80]. In conclusion, green tea products can be effectively used in the prevention and treatment of many oral and periodontal diseases [4].

The Effect of Tea on Bone Health

Genetic factors, early menstruation, nutrition, low levels of physical activity, and deficiencies in calcium and vitamin D are among the factors that affect bone health. Knowing the risk factors is important for preventing and treating bone health problems throughout life. Nutrition is one of the modifiable risk factors. Recently, green tea has been highlighted in relation to bone health in terms of nutrition. It is believed that green tea positively affects bone health by reducing oxidative stress and inflammation [81, 82]. In women with decreased bone density during menopause, green tea polyphenol supplementation and Tai Chi practice have been shown to increase bone formation biomarkers and improve bone turnover rate [83]. Another study in mice found that green tea polyphenol supplementation had positive effects on bone health. It is believed that green tea achieves this effect by increasing antioxidant capacity and suppressing inflammation [84]. In another study, it was found that increased consumption of coffee and tea raised circulating vitamin D levels independently of physical activity, sun exposure, age, gender, and BMI [85]. A study on oolong tea suggested that drinking oolong tea could help prevent bone loss in menopausal Chinese women [86].

The Effect of Tea on Neurological Disorders

The prevalence of neurodegenerative disorders is increasing. EGCG found in green tea can affect various neurological functions [87]. Therefore, it is predicted that green tea polyphenols may provide protection against Parkinson’s and Alzheimer’s diseases and other neurodegenerative diseases [88, 89]. In a cohort study, tea consumption was associated with a lower prevalence of cognitive impairment in Chinese adults, with black and oolong teas showing the most significant effects [90]. In another cohort study, it was found that green tea consumption was associated with a lower frequency of depressive symptoms in the elderly [91]. In addition, it is believed that theanine may have anti-stress effects by preventing cortical neuron excitation and reducing psychological and physiological stress responses [92].

Table 2. Animal studies related to tea

Subject of Study	How it works	Subjects	The work plan	Conclusion	Source
	Experimental study	New Zealand black mice (male)	-3 groups were created: group control group (0% green tea extract) group (0.5% w/w green tea extract) was given.group (1% w/w green tea extract) was given. -This application was carried out for 4 weeks.	Depending on the dose, with green tea extract; -Decrease in body fat storage -Decrease in food digestibility and long-term energy absorption -There was a decrease in leptin and stearoyl-CoA desaturase-(1SCD1) gene expression in white adipocytes, but there was no change in brown adipocytes. – A decrease in lipogenesis and an increase in fat oxidation were observed.
					[39]
Tea – Obesity	Experimental study	Male Sprague-Dawley rats fed high-fat diet	3 groups were created: the group was fed with a normal fat diet. The group was fed with a high fat diet. The group was fed with a high fat diet and received 20 g/kg of green tea extract. -This application was carried out for 14 days.	With green tea extract, – Decrease in body fat gain – Increase in thermogenesis due to brown adipose tissue has been observed.
					[40]
	Experimental study	Male Wistar Rats	-Two groups were created: The first group was given water (control group) and the second group was given green tea instead of drinking water for 3 weeks.	It has been observed that green tea reduces adipose tissue weight and significantly reduces cholesterol and free fatty acids in plasma.
					[42]
	Experimental study	Male adult Sprague-Dawley rats (Mice with spinal cord injury)	-4 groups were created: group sham surgery groupgroup trauma groupgroup EGCG treatment group (50 mg/kg EGCG immediately after injury)group EGCG treatment group (50 mg/kg EGCG 1 hour after injury)	In EGCG treatment groups, -Inflammatory cytokines were observed to be suppressed.
					[52]
Tea- inflammation	Experimental study	Male BALB/c mice	-4 groups were created: group control groupgroup toxin control group (10 mg / kg oral NaAsO2) group given (10 mg / kg oral NaAsO2 followed by 10 mg / kg EGCG) group given group (10 mg / kg EGCG) -for 30 days It was implemented in this way.	With EGCG, it has been observed that – Inflammatory cytokines are suppressed – Oxidative stress is reduced.
					[53]

Tea-bone health

Experimental study

Virgin Sprague Dawley obese female mice

-one. group fed with high fat (45% of the energy is taken from fat) and given green tea polyphenols (free access to water containing 0.5% green tea polyphenols) 2nd group, the group fed only with high fat – This application was carried out for 4 months.

-Consumption of water containing 0.5% green tea polyphenols has been shown to have positive effects on bone health.

[84]

Table 3. Microbiological studies on tea

Subject of Study	How it works	How it works	The work plan	Conclusion	Source
	İn vitro	Escheria coli isolated from urinary tract infections	-Different concentrations of green tea extracts (0, 2.5, 3.0, 3.5, and 4.0 mg/ml) were applied.	Green tea extract effect on strains; It was found to be 99% effective at ≤4.0 mg/mL, 94% effective at ≤3.5 mg/mL, 76% effective at ≤3.0 mg/mL, 40% effective at ≤2.5 mg/mL. Green tea is effective against Escherichia coli. It has been shown to have a potential antimicrobial effect.
					[68]
Tea- antimicrobial effect
	İn vitro	6 different types of bacteria	-Different concentrations of green tea extracts 10 mL, 20 mL, 30 mL were applied.	-Significant antibacterial activity was observed at all concentrations.
					[69]
	İn vitro	H1N1, A/H3N2 and B virus in Madin–Darby canine kidney (MDCK) cells	-120 mM EGCG, 1200 mM ECG and 120 mM EGC were applied separately. – 8-24-36. The reproduction amount of viruses in the presence of catechins was compared over time.	EGCG has been shown to be the most effective antiviral catechin.
					[71]

Table 4. Human studies on tea

Randomized, double-blind, placebo-controlled study

Cross-sectional study

48 healthy adults

(aged 18-28)

1210 adults (≥20 years)

• Participants were divided into 4 groups, and their blood pressures were compared:

group received 250 mg of caffeine.

group received 200 mg of theanine.

Group received both caffeine and theanine.

group received a placebo.

• Tea consumption and other lifestyle characteristics were evaluated through surveys.
• It was observed that theanine acted as an antagonist to caffeine, lowering blood pressure.
• Individuals who had been consuming tea habitually for over 10 years experienced a 19.6% reduction in body fat percentage.

[31]

• Body fat percentage was evaluated using bioelectrical impedance analysis.
• In a meta-analysis involving 1243 participants, it was observed that the combined application of green tea catechins with caffeine led to statistically significant reductions in BMI, body weight, and waist circumference.
• It was found that the application of green tea catechins alone did not positively alter anthropometric measurements.

Prospective study involving 36,908 participants (aged 45-74):

• The frequency and amount of coffee, black tea, and green tea consumption were queried. It was found that consuming black tea (≥1 cup of black tea/day) resulted in a 14% reduction in the risk of diabetes.
• Such an effect was not observed with green tea.

[32]

Tea-diabetes	Retrospective study	17413 participants (40-65 years old)	-Black, green and oolong tea consumption was determined through a survey.	-Caffeine taken from these drinks has been shown to reduce the risk of diabetes by 33%, -Green tea reduces the risk of diabetes by 70%.
					[48]
	Prospective study	Men (n 4055) women (n = 1768) (35-55 years old)	-Coffee and tea consumption was questioned through a survey.	-It has been determined that coffee and tea consumption is not associated with the incidence of diabetes.	[49]
	Case control study	130 cancer patients, 274 non-patient participants (>45 years old)	-Tea consumption was questioned.	Green tea has been shown to be protective against prostate cancer.
					[59]
Tea- cancer	Case control study	160 cancer patients, 320 control group	-Tea consumption habits were questioned. -Information about stomach cancer risks was collected.	It has been observed that habits such as -drinking tea regularly, -consuming too much, -drinking green tea at low temperatures are associated with a lower risk of stomach cancer.
					[61]
	İn vitro	PC-3 cell	0-50 mM EGCG was applied.	-EGCG was found to have an antiproliferative effect.	[62]
	İn vitro	MDA-MB-231 human breast cancer cell	-50-80 mg/mL EGCG was administered	It has been observed that cell apoptosis is induced.
					[63]
	İn vitro	Saliva collected from healthy volunteers	-Green tea, black tea infusions (50 mL/mL) and EGCG (50 μM) were added to saliva.	-Consumption of green tea at these doses has high Oral peroxidase (OPO) activity, providing extra protection against oxidative stress in the oral cavity.
					[76]
	Double-blind randomized controlled trial	60 children (8-12 years old)	-Participants were divided into 2 groups. Participants were asked to rinse their mouths with 0.05% sodium fluoride and 0.5% green tea twice a day for 2 weeks. Before and 2 weeks after the intervention, salivary bacteria levels were measured.	It was observed that there was no significant difference in the average bacterial colonies between the groups before and after the intervention. It was observed that there was a significant difference between the average number of bacterial colonies before and after the intervention within the group. Green tea was found to be more suitable for children because it had fewer side effects.
					[77]
Tea-oral health	Cross-sectional study	25 people (21-46 years old)	Streptococcus mutans, pH values, and Gingival Bleeding Index (GBI) were examined in saliva and plaque. Evaluation was made 5 minutes before and after rinsing with 2% green tea solution.	It was observed that there was a statistically significant difference in saliva and plaque pH values ​​and GBI values ​​regarding S. mutans.
					[79]
	Prospective study	25,078 people (40-64 years old)	-Participants’ green tea consumption and tooth loss were questioned.	Consumption of ≥1 cup/day of green tea has been found to be associated with reduced tooth loss.	[80]
	Randomized placebo-controlled study	-171 people Women who have been in menopause for at least 2 yearsOsteopenia (+)	-1. Daily Tai exercise + 500 mg starch was applied to the group. Daily Tai exercise + 500 mg green tea polyphenol was applied to the 2nd group. – All participants were given 500 mg calcium (Ca) and 200IU vitamin D daily. -This practice was carried out for 6 months.	Green tea polyphenol supplementation and tai exercise have been shown to increase bone formation markers and improve bone ratio. Green tea polyphenol supplementation and tai exercise have been observed to increase muscle strength in menopausal women.
					[83]
Tea- bone health	Cross-sectional study	-330 adolescents (11- 14 years old)	-Coffee and tea consumption status was questioned through a survey. The group was formed as the group consuming 0-4 cups/week of coffee/tea, the group consuming 5-8 cups/week of coffee/tea, and the group consuming 9-12 cups/week of coffee/tea.	Vitamin D levels were found to be highest in those who consumed tea 9–12 times/week. -Individuals who consumed tea 0-4 times a week were found to have lower 25 (OH) D levels than those who consumed tea 8-12 times a week.
					[85]

Cross-sectional study

Women in the menopausal period, the group consuming at least 250 ml of oolong tea per day in their routine (n=124) and the group not consuming tea  (n=556)

-Information was collected through a survey.

-It has been found that bone density in tea drinkers is higher than in those who do not drink tea. -It has been stated that oolong tea helps prevent bone loss after menopause.

[86]

CONCLUSION

Due to differences in processing, the amounts of bioactive compounds in tea can vary. In black tea, theaflavins and thearubigins are more abundant due to the fermentation process, while in green tea, EGCG, ECG, EGC, and EC are found in greater quantities as fermentation is not applied. Although various results have been found in many studies in recent years, tea can be effective in the prevention and treatment of many diseases. It can be said that the most important mechanism of action on health is the inhibition of oxidative enzymes by the polyphenols present in its composition. Thus, it can be used in the prevention and treatment of diseases such as cardiovascular diseases and different types of cancer. Additionally, it can have positive effects on diabetes and obesity due to its effects on carbohydrate and lipid metabolism. While some studies mention the anti-inflammatory, antimicrobial, and antiviral effects of tea, there are also many studies showing its significant role in neurological and psychological diseases. To eliminate contradictory results, there is a need for more experimental human studies that take into account individual differences, compare tea quantities, and methods of preparation.

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