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Beneficial Effects of Green Tea—A Review

Departamento de Nutrición y Bromatología, Facultad de Farmacia, Universidad de Granada, Granada, SPAIN

Address reprint requests to: Carmen Cabrera, PhD, Dpto. Nutrición y Bromatología, Facultad de Farmacia, Campus Universitario de Cartuja, 18012-Granada, SPAIN. E-mail: carmenc{at}ugr.es

	ABSTRACT

TOP ABSTRACT INTRODUCTION Green Tea Processing Green Tea Composition Bioavailability of Green Tea... Green Tea and Human... Green Tea Nutritional Value Harmful Effects of Tea... Conclusions ACKNOWLEDGMENTS REFERENCES

 
Tea is the most consumed drink in the world after water. Green tea is a ‘non-fermented’ tea, and contains more catechins, than black tea or oolong tea. Catechins are in vitro and in vivo strong antioxidants. In addition, its content of certain minerals and vitamins increases the antioxidant potential of this type of tea. Since ancient times, green tea has been considered by the traditional Chinese medicine as a healthful beverage. Recent human studies suggest that green tea may contribute to a reduction in the risk of cardiovascular disease and some forms of cancer, as well as to the promotion of oral health and other physiological functions such as anti-hypertensive effect, body weight control, antibacterial and antivirasic activity, solar ultraviolet protection, bone mineral density increase, anti-fibrotic properties, and neuroprotective power. Increasing interest in its health benefits has led to the inclusion of green tea in the group of beverages with functional properties. However, although all the evidence from research on green tea is very promising, future studies are necessary to fully understand its contributions to human health, and advise its regular consumption in Western diets, in which green tea consumption is nowadays limited and sporadic.

Key words: green tea, polyphenols, catechins, antioxidant activity, human health

Abbreviations: CI = confidence interval • DNA = deoxyribonucleic acid • DPPH = 2,2-diphenyl-l-picrylhydrazyl assay • DMPD = N,N-dimethyl-p-phenylendiamine assay • EC = (–)-epicatechin • ECG = (–)-epicatechin-3-gallate • EGC = (–)-epigallocatechin • EGCG = (–)-epigallocatechin-3-gallate • FRAP = ferric reducing ability of plasma assay • GA = gallic acid • GTP = green tea polyphenols • HDL = high density lipoproteins • HR = hazard ratio • LDL = low density lipoproteins • OR = odd ratio • ORAC = oxygen radical absorbance capacity assay • PCL = photochemiluminescence assay • RR = relative risk • TEAC = Trolox equivalent antioxidant capacity assay • TRAP = total radical-trapping antioxidant parameter assay • UV = ultraviolet

Key teaching points:

• Green tea contains numerous components with antioxidant activity: polyphenols (especially catechins), minerals, vitamins.

• Green tea contains more catechins than black or oolong teas.

• The strong antioxidant potential of catechins, and especially EGCG, are widely demonstrated in vitro and in animal studies. In addition, catechins possess antimutagenic, antidiabetic, anti-inflammatory, antibacterial and antiviral properties.

• Recent human studies suggest that green tea may contribute to reduce the risk of cardiovascular disease and cancer, and has another beneficial effect on health.

• Although research of green tea is very promising, future studies considering dietetic, environmental and life style factors, are necessary to fully understand its contribution to human health.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Green Tea Processing Green Tea Composition Bioavailability of Green Tea... Green Tea and Human... Green Tea Nutritional Value Harmful Effects of Tea... Conclusions ACKNOWLEDGMENTS REFERENCES

 
Tea, a product made up from leaf and bud of the plant Camellia sinensis, is the second most consumed beverage in the world, well ahead of coffee, beer, wine and carbonated soft drinks [1–2]. Originating from China, tea has gained the world’s taste in the past 2000 years. The economic and social interest of tea is clear and its consumption is part of many people daily routine, as an everyday drink and as a therapeutic aid in many illnesses.

Depending on the manufacturing process, teas are classified into three major types: ‘non-fermented’ green tea (produced by drying and steaming the fresh leaves to inactivate the polyphenol oxidase and thus, non oxidation occurs); ‘semi-fermented’ oolong tea (produced when the fresh leaves are subjected to a partial fermentation stage before drying); and ‘fermented’ black and red (Pu-Erh) teas which undergo a post-harvest fermentation stage before drying and steaming, although the fermentation of black tea is due to an oxidation catalyzed by polyphenol oxidase, and that of Pu-Erh tea is attained by using microorganisms [3–4]. McKay and Blumberg [4] reported a per capita mean consumption of tea in the world of 120 mL/day. Approximately 76–78% of the tea produced and consumed is black tea, 20–22% is green tea and less than 2% is oolong tea [5]. Black tea is consumed principally in Europe, North America and North Africa (except Morocco) while green tea is widely drunk in China, Japan, Korea and Morocco; oolong tea is popular in China and Taiwan [5–6]. In USA, the 80% of tea consumed is black ice tea [7].

Although health benefits have been attributed to green tea consumption since the beginning of its history, scientific investigations on this beverage and its constituents have been underway for less than three decades [4]. In vitro and animal studies, and clinical trials employing putative intermediary indicators of disease, particularly biomarkers of oxidative stress status, provide strong evidence that green tea polyphenols (GTP) may play a role in the risk and pathogenesis of several chronic diseases, especially cardiovascular disease and cancer, and related pathologies. In addition, several studies suggest a beneficial impact of green tea intake on bone density, cognitive function, dental caries and kidney stones, among other effects [4–5]. Over the last years, numerous epidemiological and clinical studies have revealed several physiological responses to green tea which may be relevant to the promotion of health and the prevention or treatment of some chronic diseases. However, the results from epidemiological and clinical studies of the relationship between green tea consumption and human health are mixed. For example, conflicting results between human studies may arise in part, from ignoring socioeconomic and lifestyle factors as well as by inadequate methodology to define tea preparation and intake [2,4–7].

Foodstuff can be regarded as functional if it is satisfactorily demonstrated to affect beneficially one or more target functions in the body, beyond adequate nutritional effects in a way which is relevant to either the state of well-being and health or the reduction of the risk of a disease [5,8–9], so green tea has been proved to have functional properties and at present, its consumption is widely recommended.

The aim of this article is to revise the most recent studies on green tea beneficial effects and to evaluate its potential interest in Western diets.

	Green Tea Processing

TOP ABSTRACT INTRODUCTION Green Tea Processing Green Tea Composition Bioavailability of Green Tea... Green Tea and Human... Green Tea Nutritional Value Harmful Effects of Tea... Conclusions ACKNOWLEDGMENTS REFERENCES

 
Green tea is mainly produced from Camellia sinensis var. sinensis. The Assan type (Camellia sinensis var. assamica) has a too high content of polyphenols, which would make green tea taste excessively bitter [3]. The production of green tea is characterized by an initial heating process, which kills the enzyme polyphenol oxidase, which is responsible for the conversion of the flavanols in the leaf into the dark polyphenolic compounds that colour black tea. The other important process is rolling, in which leaves are cut and twisted. The final form of green tea depends on the particular variant being produced. The rolling stage is very similar to the operation with the same name in black tea production. Green tea production is restricted mainly to China and Japan [3,6]. Fig. 1 shows the principal differences between green and black tea processing.

View larger version (22K): [in this window] [in a new window]   Fig. 1. Principal differences between green and black tea processing and its influence on the final polyphenols content.

	Green Tea Composition

TOP ABSTRACT INTRODUCTION Green Tea Processing Green Tea Composition Bioavailability of Green Tea... Green Tea and Human... Green Tea Nutritional Value Harmful Effects of Tea... Conclusions ACKNOWLEDGMENTS REFERENCES

View larger version (7K): [in this window] [in a new window]   Fig. 2. Chemical structure of caffeine and theophylline.

View larger version (10K): [in this window] [in a new window]   Fig. 3. Chemical structure of gallic acid and the four major catechins in green tea. GA, gallic acid; EGCG, (–)-epigallocatechin-3-gallate; EGC, (–)-epigallocatechin; ECG, (–)-epicatechin-3-gallate; EC, (–)-epicatechin.

Wu and Wei [5] indicated that a cup of green tea (2.5 g of green tea leaves/200 mL of water) may contain 90 mg of EGCG. Lin et al. [19] analyzed 31 commercial teas, and detected that the levels of EGCG and total catechins were in the following order: green tea (old leaves) > green tea (young leaves) and oolong tea > black tea and Pu-Erh tea. Fernández et al. [20] determined the contents of GA, EGCG, EGC, EC and ECG, in a set of 45 tea samples, including black and green teas from different geographical origins (i.e. China, Japan, Kenya, Sri Lanka, and India); the GA levels were always higher in black tea because the amount of GA increases during the fermentation process due to its liberation from catechin gallates [21]. The amount of catechins were always higher in green tea; EGCG and EGC were the major catechins present with average contents of 7.358% and 3.955%, respectively; ECG presented values ranging between 0.910 and 3.556%. For black tea, EGCG and ECG were the catechins present in higher percentages, with average contents of 1.583 and 0.706%, respectively. Cabrera et al. [22] reported the mean content of the four major catechins (EGCG, EGC, ECG and EC) and gallic acid in 45 samples of different types of tea including black, red, oolong and green teas; the higher levels of EGCG appeared in green tea samples. Results are summarized in Fig. 4. However, Henning et al. [23] suggested that the large variation of the catechin content in tea is not taken into consideration in most of the epidemiological studies.

View larger version (30K): [in this window] [in a new window]   Fig. 4. Mean content of gallic acid and catechins in different types of tea (n = 45). Data refereed to dry weight of commercial samples (Source: Cabrera et al. [22]).

	Bioavailability of Green Tea Catechins

TOP ABSTRACT INTRODUCTION Green Tea Processing Green Tea Composition Bioavailability of Green Tea... Green Tea and Human... Green Tea Nutritional Value Harmful Effects of Tea... Conclusions ACKNOWLEDGMENTS REFERENCES

Lu et al. [30] reported that GTP have biological activities including modulation of key signal transduction pathways; however, the possible significance of these activities in inhibition of carcinogenesis in vivo depends on the polyphenol bioavailability. These authors observed that after oral administration of green tea to rats, about 14% of EGC, 31% of EC, and <1% of EGCG appeared in the blood; in mice, the bioavailability of EGCG was higher, but the biological activities of the catechin metabolites still need to be investigated. Inter-individual variations in the bioavailability of GTP can be substantial and may be due, in part, to differences in colonic microflora and genetic polymorphisms among the enzymes involved in polyphenol metabolism [31]. The effect of green tea drinking may also differ by genotype [32]. To sum up, there appear to be species differences in the bioavailability of EGCG compared to other tea catechins. Further research results are largely consistent in demonstrating that the addition of milk to tea does not interfere with catechin absorption [33–35], but milk may affect the antioxidant potential of tea, depending upon milk fat content, milk volume added, and the method used to assess this parameter [33–36]. Xu et al. [37] observed that the epimerisation reaction occurring in manufacturing canned and bottled tea drinks would not significantly affect antioxidant activity and bioavailability of total tea polyphenols.

	Green Tea and Human Health

TOP ABSTRACT INTRODUCTION Green Tea Processing Green Tea Composition Bioavailability of Green Tea... Green Tea and Human... Green Tea Nutritional Value Harmful Effects of Tea... Conclusions ACKNOWLEDGMENTS REFERENCES

 
Green tea has been considered a medicine and a healthful beverage since ancient times. The traditional Chinese medicine has recommended this plant for headaches, body aches and pains, digestion, depression, detoxification, as an energizer and, in general, to prolong life. Green tea leaves contain three main components which act upon human health: xanthic bases (caffeine and theophylline), essential oils and especially, polyphenolic compounds. Caffeine acts mainly upon the central nervous system, stimulating wakefulness, facilitating ideas association and decreasing the sensation of fatigue [38]. Some of the effects caused by caffeine are influenced by theophylline tea content. Theophylline induces psychoactive activity, it also has a slightly inotrope and vasodilator effect, and a much higher diuretic effect than caffeine. However, its most interesting effects can be seen at the bronchopulmonar and respiratory level. Theophylline causes a non-specific relaxation on the bronchial smooth muscle, and respiratory stimulation is also observed. Essential oils are in a great extent volatile and they evaporate from the beverage after some time, thus it is not very convenient to overextend the brewing time. Among their properties, the one of facilitating digestion must be highlighted [5,39]. Green tea is the type of tea with the higher percentage of essential oils [38–39]. However, green tea has received a great deal of attention especially due to its content of polyphenols, which are strong antioxidants and present important biological properties. Numerous studies have also demonstrated that the aqueous extract of GTP possesses antimutagenic, antidiabetic, antibacterial, anti-inflammatory, and hypocholesterolemic properties [21,40–44]. Beneficial effects in oral diseases such as protection against dental caries, periodontal disease, and tooth loss (which may significantly affect a person’s overall health) have been also described [5]. Among all GTP, catechins and gallic acid have been especially considered to be the main players in the beneficial effects on human health next detailed.

Antioxidant Activity.
Green tea is considered a dietary source of antioxidant nutrients: green tea is rich in polyphenols (catechins and gallic acid, particularly), but it also contains carotenoids, tocopherols, ascorbic acid (vitamin C), minerals such as Cr, Mn, Se or Zn, and certain phytochemical compounds. These compounds could increase the GTP antioxidant potential. GTP present antioxidant activity in vitro by scavenging reactive oxygen and nitrogen species and chelating redox-active transition metal ions; GTP can chelate metal ions like iron and copper to prevent their participation in Fenton and Haber-Weiss reactions [4,45–46]. They may also function indirectly as antioxidants through 1) inhibition of the redox-sensitive transcription factors; 2) inhibition of ‘pro-oxidant’ enzymes, such as inducible nitric oxide synthase, lipoxygenases, cyclooxygenases and xanthine oxidase; and 3) induction of antioxidant enzymes, such as glutathione-S-transferases and superoxide dismutases.

The antioxidant capacity of GTP has been assessed by several methods. For example, Cao et al. [47] using the oxygen radical absorbance capacity (ORAC) assay found that green tea has a much higher antioxidant activity against peroxyl radicals than vegetables such as garlic, kale, spinach and Brussels sprouts. Using the ferric reducing ability of plasma (FRAP) assay. Langley-Evans [48] found that the total antioxidant capacity of green tea is more potent than that of black tea. Saffari and Sadrzadeh [49] investigated the antioxidant capacity of EGCG using erythrocyte membrane-bound. ATPases as a model, and the results indicated that EGCG is a powerful antioxidant that is capable of protecting erythrocyte membrane-bound ATPases against oxidative stress. Several studies have shown that EGCG can act in vitro as an antioxidant by trapping proxyl radicals and inhibiting lipid peroxidation [49–50]. However, the antioxidant capacity of catechins determined in vitro is dependent upon the type of assay employed and it does not reflect factors such as bioavailability and metabolism. The fact that catechins are rapidly and extensively metabolized emphasizes the importance of demonstrating their antioxidant activity in vivo to better represent the physiological impact of green tea consumption. Frei and Higdon [51] reported that in order to determine whether or not GTP act as effective antioxidants in vivo, future studies in animals and humans should employ sensitive and specific biomarkers of oxidative damage of lipids, proteins and DNA.

Nevertheless, a substantial number of human intervention studies with green tea demonstrate a significant increase in plasma antioxidant capacity in humans after consumption of moderate amounts (1–6 cups/day); there are also initial indications which show that the enhanced blood antioxidant potential leads to a reduced oxidative damage in macromolecules such as DNA and lipids [2,4,23,28,37]. However, these authors indicated that the measurement of oxidative damage through biomarkers needs to be further established. McKay and Blumberg [4] reported that the repeated consumption of green tea and encapsulated green tea extracts for one to four weeks has been demonstrated to decrease biomarkers of oxidative status. Furthermore, Klaunig et al. [52] observed in a study with 40 male smokers in China and 27 men and women (smokers and non-smokers) in the United States, that oxidative DNA damage, lipid peroxidation, and free radical generation were reduced after consuming 6 cups/day of green tea for seven days. Therefore, GTP may contribute to defenses against oxidative damages [5]. Erba et al. [53] suggest the ability of green tea, consumed within a balanced controlled diet, to improve overall the antioxidative status and to protect against oxidative damage in humans.

Antimutagenic and Anticarcinogenic Potential.
Lifestyle-related diseases, including cancer, are also characterized as aging-related diseases, where aging may be the most potent causal factor. Therefore, prevention of lifestyle-related diseases will depend on slowing the aging process and avoiding the clinical appearance the disease. Dietary components that are capable of retarding cellular aging and inhibiting the growth of cancer cells without affecting the growth of normal cells are receiving considerable attention for the development of novel cancer-preventive approaches [54–56]. The role of green tea in protection against cancer has been supported by ample evidence from studies in cell culture and animal models [54]. Animal studies have shown that green tea inhibit carcinogenesis of the skin, lung, oral cavity, esophagus, stomach, liver, kidney, prostate and other organs [55,57–60]. In some studies, the inhibition correlated with an increase in tumor cell apoptosis and a decrease in cell proliferation [56]. Today, green tea is accepted as a cancer preventive on the basis of numerous in vitro, in vivo and epidemiological studies. The Chemoprevention Branch of the National Cancer Institute has initiated a plan for developing tea compounds as cancer-chemopreventive agents in human trials [61]. The chemopreventive effects of green tea depend on: (1) its antioxidant action; (2) the specific induction of detoxifying enzymes; (3) its molecular regulatory functions on cellular growth, development and apoptosis; and (4) a selective improvement in the function of the intestinal bacteria flora. D’Alessandro et al. [62] also indicated that an important aspect of cancer risk is related to inflammatory response; currently, anti-inflammatory agents are used in chemopreventive strategies. The inflammatory response involves the production of cytokines and proinflammatory oxidants such as hypochlorous acid and peroxynitrite produced by neutrophils and macrophages, respectively. These oxidants react with phenolic tyrosine residues on proteins to form chloro- and nitrotyrosine. Green tea catechins and soy isoflavones have also been shown to be chemopreventive; the aromatic nature of polyphenols makes them potential targets of hypochlorous acid and peroxynitrite, and these reactions may create novel pharmacophores at the site of inflammation. In addition, a major mechanism of the anticarcinogenic activity of green tea in animals is the impairment of the interaction of carcinogens with DNA leading to mutations. Nevertheless, the antimutagenic activity of green tea as well as its underlying mechanisms must be reviewed, and the role of GTP, the postulated bioactive components, and caffeine must be critically evaluated. EGCG from green tea especially imparts a growth inhibitory effect on cancer cells [56,63–64]. EGCG possesses promising anticancer potential due to its antioxidant, antimutagenic and chemopreventive properties [65–66].

Rosengren [67] indicated that the green tea catechins reduce the proliferation of breast cancer cells in vitro and decrease breast tumor growth in rodents. Furthermore, in vitro studies have demonstrated that the combination of EGCG and tamoxifen is synergistically cytotoxic to breast cancer cells; these results suggest that the catechins have significant potential in the treatment of breast cancer. Mittal et al. [56] reported that the treatment with EGCG decreased cell viability at different stages studied (approx. 80% inhibition) in human breast carcinoma MCF-7 cells, but had no adverse effect on the growth of normal mammary cells. These authors found that this treatment inhibited telomerase activity (40–55%); telomerase is elevated in >90% of breast carcinomas and therefore has received much attention as a target for breast cancer therapy and cancer diagnostic research. According to Wu et al. [68], green tea drinkers showed a significantly reduced risk of breast cancer; compared to women who did not drink green tea regularly (i.e., less than once a month). Furthermore, there was a significant trend of decreasing risk with increasing amount of green tea intake. Two studies in Japanese women diagnosed with breast cancer indicate that green tea consumption is inversely associated with the rate of recurrence, especially in the early stages of breast cancer [69–70]. Zhou et al. [71] also reported that breast cancer is significantly less prevalent among Asian women, whose diets contain high intake of soy products and green tea. These authors suggested that dietary soy phytochemical concentrate plus green tea may be used as a potential effective dietary regimen for inhibiting progression of estrogen-dependent breast cancer.

Zhang et al. [72] reported that ovarian cancer risk declined with increasing frequency and duration of green tea consumption. Green tea is also an effective chemopreventive agent to human prostate cancer. In the same line of research, Yu et al. [73] reported that EGCG inhibited the growth of prostate cancer adenoma cells and induced apoptosis. Jian et al. [74] conducted a case-control study in China in order to investigate whether green tea consumption has an etiological association with prostate cancer. Prostate cancer risk declined with increasing frequency, duration and quantity of green tea consumption. The dose-response relationships were also significant, suggesting that green tea is protective against prostate cancer. Yamamoto et al. [59] reported that GTP could be applied to enhance the effectiveness of chemo/radiation therapy to promote cancer cell death while protecting normal cells.

On the one hand, epidemiological studies have suggested that high consumption of green tea protects against the development of chronic active gastritis and decreases the risk of stomach cancer; in addition, the ingestion of green tea before fasting protects the intestinal mucosa against atrophy [75]. On the other hand, Borrelli et al. [76] concluded in a systematic review, that an inverse association does not seem to exist between green tea consumption and the risk of gastric and intestinal cancer, although green tea did show a protective effect on adenomatous polyps and chronic gastritis [76]. In the same way, Hoshiyama et al. [66] and Koizumi et al. [77] found no association between green tea consumption and the risk of stomach cancer; these authors indicated that green tea consumption had no protective effect against stomach cancer, and suggested the implication of other factors such as age, smoking, socioeconomic status, Helicobacter pylori infection, history of pectic ulcer, and family history of stomach cancer along with certain dietary components. Sasazuki et al. [78] reported an inverse association between green tea consumption and distal gastric cancer among women; however, these authors indicated that more prospective studies with detailed information are needed to confirm the role of green tea in the occurrence of gastric cancer. Il’yasova et al. [7] examined the association between black tea consumption and colon cancer in a population-based study in North Carolina, and concluded that, contrary to expectation, black tea drinking did not decrease the risk of colon cancer. However, it is important to remark that major risk factors for colorectal cancer are family history of colorectal cancer, exposure to non-steroid anti-inflammatory drugs, certain meat cooking practices, smoking, low physical activity and an elevated body mass index, and elevated intake of red meat and alcoholic beverages. Data on the effects of green tea in the prevention of this type of cancer are not available.

Several authors have noted that some epidemiological studies have generated inconsistent results [54,79]. Some of these studies related tea consumption with reduced risk of cancer. In a similar way, other authors found that tea lacks protective activity against certain human cancers; these results raise questions about the actual role of green tea components in human cancer that need to be addressed. For example, Arab and Il’yasova [80] provided a brief synopsis of 30 studies aimed at examining tea consumption as a factor in the incidence of colon and rectal cancers; the 30 papers examined populations in 12 countries and provided data on consumption of both black and green tea. These studies do not provide consistent evidence to support the theory from animal studies and basic research of tea being a potent chemopreventive agent. A negative association is stronger in observational epidemiologic studies of rectal cancer than in colon cancer. There is no consistent adjustment for important potential confounders of any tea relationship, such as coffee and alcohol consumption and physical activity levels. Finally, these authors indicated that the assessment of tea in most of these studies was based on a single question and therefore may have significant measurement error compared with more recent studies specifically aimed at assessing green tea consumption. Table 2 summarizes some recent epidemiological studies on the association of green tea consumption and cancer risk.

Epidemiological studies suggest that green tea consumption is associated with a reduced cardiovascular disease risk, but the mechanisms for these observations have remained uncertain. Several studies have demonstrated that green tea may affect the cardiovascular function through mechanisms of action related to LDL-cholesterol oxidation [4,89]. The oxidation of LDL-cholesterol, associated with a risk for atherosclerosis and heart disease, is inhibited by green tea due to EC and EGCG antioxidant activity. The in vitro antioxidant activity of EGCG on LDL oxidation was stronger than that of EC [90]. In accordance with these observations, Trevisanato and Kim [91] indicated that GTP may slow atherogenesis by reducing the oxidative modification of LDL-cholesterol and associated events such as foam cell formation, endothelial cytotoxicity and induction of proinflammatory cytokines. Gomikawa and Ishikawa [90] suggested that catechins suppressed the susceptibilities of human LDL to oxidation by CuSO₄ in vitro and plasma oxidation in vivo after ground green tea ingestion. Recent bioavailability studies indicate that GTP can accumulate in the body at concentrations comparable to those employed in vitro by several investigators [31]. Other data report that catechins have been shown to reduce plasma cholesterol levels and the rate of cholesterol absorption. Raederstorff et al. [92] investigated the dose-response and the mechanism of action of EGCG on these parameters in rats which were fed a diet high in cholesterol and fat; after 4 weeks of treatment, total cholesterol and LDL-cholesterol plasma levels were significantly reduced in the group fed 1% EGCG when compared to the non-treatment group. Plasma triglycerides and HDL-cholesterol did not change significantly. These authors suggested that one of the underlying mechanisms by which EGCG affects lipid metabolism is by interfering with the micellar solubilization of cholesterol in the digestive tract, which then in turn decreases cholesterol absorption. Yokozawa et al. [93] reported that the administration of GTP effectively inhibited LDL-cholesterol oxidation and elevated serum antioxidative activity. Furthermore, GTP increased the levels of HDL-cholesterol, leading to dose-dependent improvement of the atherogenic index. Thus, GTP may exert an antiatherosclerotic action by virtue of its antioxidant properties and by increasing HDL-cholesterol levels. Consistent with these results are the data reported by Hertog et al. [94] that demonstrated an inverse correlation between catechin intake and coronary heart disease mortality after a 25-year follow-up of 12763 men from seven different countries. Similarly, another research showed that men and women from the Boston Area Health study who consumed one or more cups per day of green tea in the previous year had a 44% lower risk of myocardial infarction than those who drank no tea [95]. Recently, Peters et al. [96] have provided a meta-analysis that suggested a decrease in the rate of cardiovascular disease outcomes with increasing green tea consumption. Through seven studies the incidence rate of myocardial infarction was estimated to decrease by 11% with an increase in green tea consumption of three cups per day (RR = 0.89; 95% CI = 0.79–1.019). In addition, an inverse association of green tea intake and myocardial infarction and its genetic variation has been found by Hirano et al. [97] and Ohmori et al. [98].

Impaired endothelium-derived nitric oxide activity contributes to the pathogenesis of atherosclerosis, and in coronary circulation, it has been linked with future cardiovascular disease events. Furthermore, this endothelial dysfunction is associated with increased oxidative stress and may be reversed by antioxidant interventions [4]. Duffy et al. [99] observed that tea consumption improved flow-mediated dilation, in association with an increased plasma catechin concentration (p < 0.001). No effects were observed with an equivalent dose of caffeine (200 mg) or on endothelium-independent nitroglycerin-mediated dilation. As flow-mediated dilation is blunted in coronary heart disease patients compared to healthy subjects, these results suggest that green tea reverses endothelial vasomotor dysfunction. Hertog et al. [100] reported no association of catechin by tea intake with ischemic heart disease incidence in a 14-year follow-up of 334 men, 45 to 59 years of age, conducted in Caerphilly, Wales. According to McAnlis et al. [101], the discrepancy between the effect of green tea in vivo and ex vivo, on the susceptibility of LDL-cholesterol to oxidation may be due to the inability to achieve concentrations in vivo as great as those obtained with the former methods. The possible variations between the different studies may be also due to their ignorance of socioeconomic and lifestyle factors associated with the green tea drinking (i.e., geographical differences, social class, body mass index, healthy lifestyle, higher prevalence of smoking, higher fat intake, alcohol intake, coffee consumption).

Oral Health.
Oral diseases including dental caries, periodontal disease, and tooth loss may significantly impact a person’s overall health. Among these, dental caries is a multifactorial infectious disease in which nutrition, microbiological infection, and host response play important roles. Earlier reports in experimental animals and humans suggested that green tea consumption (without added sugar) reduces dental caries [5,102–103]. Linke and LeGeros [104] indicated that frequent intake of green tea can significantly decrease caries formation, even in the presence of sugars in the diet. In vivo animal studies have shown that specific pathogen-free rats infected with Streptococcus mutans and then fed with a cariogenic diet containing GTP have significantly lower caries scores [105]. Supplementing drinking water of rats with 0.1% GTP along with a cariogenic diet also significantly reduced total fissure caries lesions [5]. Recent findings of Okamoto et al. [106] suggest that green tea catechins may have the potential to reduce periodontal breakdown resulting from the potent proteinase activity of Porphyromonas gingivalis. In addition, green tea decoctions inhibit -amylase in human saliva, reducing maltose release by 70% and effectively lowering the cariogenic potential of starch-containing food [4]. Similarly, Zhang and Kashket [107] reported that green tea extracts inhibits human salivary amylase and may reduce the cariogenic potential of starch-containing food such as crackers and cakes because it may reduce the tendency of this kind of food to serve as slow-release sources of fermentable carbohydrate. It is likely that the cariogenic challenge in a cariogenic diet may be reduced by the simultaneous presence of green tea in the diet.

Apart from their polyphenol content, both green and black tea, are a natural source of fluoride and an effective vehicle for fluoride delivery to the oral cavity. According to Simpson et al. [108], after cleansing the mouth with tea, approximately 34% of the fluoride is retained and shows a strong binding ability to interact with the oral tissues and their surface integuments. This fluoride content may have a beneficial impact on caries and may carry out a wide range of biological activities including prevention of tooth loss and oral cancer [106,109]. Nonetheless, the data have suggested that GTP extract may be responsible for the noted effects on oral health and it has been also demonstrated that GTP rather than fluoride contribute to anticariogenic potential [5,105] by inhibition of oral bacteria growth such as Escherichia coli, Streptococcus salivarius, and Streptococcus mutans. Several studies have indicated that GTP inhibit growth, acid production, metabolism, and glucosyltransferase enzyme activity of S. mutans and dental plaque bacteria [5]. In consequence, green tea has been considered as functional food for oral health and is widely used in toothpaste formulation.

Solar Ultraviolet Protection.
Epidemiological, clinical and biological studies have shown that solar ultraviolet (UV) light is a complete carcinogen and repeated exposure can lead to the development of various skin disorders including melanoma and non-melanoma skin cancers. EGCG is considered to be a topic protector agent against some types of radiation, since it prevents skin disease, photoaging and potential cancer problems due to prolonged exposure [109–111]. It seems that the rest of catechins also favour this action [5,109–110]. Katiyar [111] indicated that topical treatment or oral consumption of GTP inhibits chemical carcinogen or UV radiation-induced skin carcinogenesis in different laboratory animal models. Topical treatment of GTP or ECCG and oral consumption of GTP resulted in prevention of UVB-induced inflammatory responses, immunosuppression and oxidative stress, which are the biomarkers of several skin disease conditions. Topical application of GTP and EGCG prior to exposure of UVB protects against UVB-induced local as well as systemic immune suppression in laboratory animals. This fact was associated with the inhibition of UVB-induced infiltration of inflammatory leukocytes. The in vitro and in vivo animal and human studies have suggested that GTP are photoprotective in nature, and can be used as pharmacological agents for the prevention of solar UVB light-induced skin disorders including photoaging, melanoma and non-melanoma skin cancers [4–5,109,111].

Body Weight Control.
Obesity has increased at an alarming rate in recent years and is now a worldwide health problem. Current interest in the role of functional foods in weight control has focused on plant ingredients capable of interfering with the sympathoadrenal systems [112]. The effects of long-term feeding with tea catechins have been widely studied, and some investigators suggest a potential role of green tea in body weight control. In addition, caffeine and theanine have been found to strengthen polyphenol effects on body weight control and fat accumulation in mice [113]. In vitro studies with green tea extracts containing 25% of catechins have shown its capacity (in conditions similar to physiological ones) to significantly inhibit the gastric lipase, and in a lower extent also the pancreatic lipase. Thus, the lipolysis of long-chain triglycerides is reduced in a 37% [114]. In vitro studies have also shown that green tea extracts interfere in the fat emulsification process, which occurs before enzymes act, and is indispensable for lipid intestinal absorption [114–115]. Green tea also exhibits a fatty acid synthase inhibitor activity [116]. In addition, green tea may have thermogenic properties not only attributable to its caffeine content, but to the joint-effect of caffeine and catechins. EGCG can act upon AMPc levels by increasing the energetic expenditure [114]. Dulloo et al. [112] using a green tea extract rich in catechins and caffeine, concluded that green tea has thermogenic properties and promotes fat oxidation beyond than those explained by its caffeine content per se; the green tea extract may play a role in the control of body composition via sympathetic activation of thermogenesis, fat oxidation, or both. Dulloo et al. [117] indicated that the thermogenic properties of green tea could reside primarily in an interaction between its high content in catechins and the presence of caffeine with sympathetically released noradrenaline; since polyphenols are known to be capable of inhibiting catechol-o-methyl-transferase (the enzyme that degrades noradrenaline), and caffeine of inhibiting trancellular phosphodiesterases (enzymes that break down noradrenaline-induced AMPc). Such a synergistic interaction between polyphenols and caffeine to increase and prolong sympathetic stimulation of thermogenesis could be of value in assisting the management of obesity. Kovacs et al. [118] reported that weight maintenance after 7.5% of body weight loss in overweight and moderately obese subjects was not affected by green tea treatment and that regular caffeine consumption affected weight maintenance in green tea treatment. According to some authors, green tea extracts (with a 25% of catechins content) may be advisable for overweight treatment in patients whose body mass index ranges between 25 and 29.9 kg/m², only if they do not present special sensitiveness to xantic bases [118]. Wu et al. [119] indicated that an inverse relationship may exit among regular green tea consumption, body fat percentage, and body fat distribution, especially for subjects who have maintained the habit of tea consumption for more than 10 years.

Glucose Tolerance and Insulin Sensitivity.
Epidemiological observations and laboratory studies have shown that green tea has an effect on glucose tolerance and insulin sensitivity. Anderson and Polansky [120] reported that green tea increases insulin activity, and that the predominant active compound is EGCG; these same authors indicated that addition of lemon to the tea did not affect the insulin-potentiating activity but the addition of 50 g of milk per cup decreased the insulin-potentiating activity similar to 90%. Wu et al. [121] examined the effect of green tea supplementation on glucose tolerance and insulin sensitivity in rats; rats were divided into two groups: a control group was fed with standard chow and deionized distilled water, while the other was fed with the same chow diet but with green tea instead of water (0.5 g of lyophilized green tea powder dissolved in 100 mL of deionized distilled water); after 12 weeks of green tea supplementation, this group had lower fasting plasma levels of glucose, insulin, triglycerides, and free fatty acid than the control rats. In addition, GTP significantly increased basal and insulin-stimulated glucose uptake of adipocytes [4]. Some investigations have also shown that EGCG does not only regulate the glucose level in blood, but also may rehabilitate damaged beta-cells, which are responsible for producing insulin [4,119].

Other Effects.
Green tea catechins have been reported to have antibacterial and antiviral activity. Green tea effectiveness against any type of diarrhoea and typhoid has been known in Asia since ancient times [4,68,119]. Nowadays it is also known that it inhibits the reproduction and growth of many bacteria, among which some types of Salmonella, Clostridium or Bacillus can be named. Takabayashi et al. [122] and Yee et al. [123] reported an inhibitory effect of green tea catechins on Helicobacter pylori infection. Moreover, it has been shown that green tea has not effect over intestinal flora, which is a great advantage against other bactericide agents. Regarding its antiviral action, green tea is well known for preventing tobacco crops from being invaded by the ‘mosaic virus’ of tobacco. Recent investigations have confirmed that catechins completely inhibit its growth and reproduction [3]. Effects of green tea against the influenza virus, especially in its earliest stage, as well as against the Herpes simplex virus have also been demonstrated [124–126]. Furthermore, Weber et al. [127] observed that adenovirus infection is inhibited in vitro by green tea catechins. Hirasawa and Takada [128] indicated the antifungal activity of green tea catechins against Candida albicans, and the convenience of a combined treatment with catechins and lower doses of antimycotics; this treatment may help to avoid the side effects of antimycotics.

Green tea consumption has also been associated with increased bone mineral density, and it has been identified as an independent factor protecting against the risk of hip fractures; this fact has been considered independent of smoking status, hormone replacement therapy, coffee drinking and the addition of milk to tea [129]. Park et al. [130] observed the positive effects of green tea extracts and GTP on the proliferation and activity of bone cells. Wu and Wei [5] indicated that bone mineral density may be influenced by several chemical compounds that are contained in tea extracts (i.e., caffeine, phytostrogen, fluoride, ...).

Green tea polyphenols are known to have anti-fibrotic properties on the skin and on the arteries. The proliferation of hepatic stellate cells is closely related to the progression of liver fibrosis in chronic liver diseases, and EGCG has a potential inhibitory effect on the proliferation of these cells [131–132]. Green tea strengthens the immune system action since green tea protects it against oxidants and radicals. Bayer et al. [133] suggest that oral intake of green tea could act as an adjunctive therapy for prevention of transplant rejection in humans. The neuroprotective power of complex extracts rich in flavonoids like those of Ginkgo biloba, green tea or lyophilized red wine have been demonstrated in several studies [134–135]. Recent studies suggest that GTP possibly protect against Parkinson’s and Alzheimer’s diseases and other neurodegenerative diseases [44,136]. GTP have demonstrated neuroprotectant activity in cell cultures and animal models, such as the prevention of neurotoxin-induced cell injury; the biological effects of GTP may benefit patients with Parkinson’s disease, but further in-depth studies are needed to investigate the safety and effectiveness of green tea in humans and to determine the different mechanisms of green tea in neuroprotection [44]. In the same way, the neuroprotective effects of the theanine contained in green tea are a focus of considerable attention, and further studies are warranted [134].

Finally, the following health effects of green tea consumption have also been described. Green tea is considered to be useful for insect stings due mainly to its antiinflammatory effects and its capacity to stop bleeding [137–138]. Some studies have suggested an inverse association between green tea consumption and the risk of kidney stone formation [4,139]. In addition, green and black tea extracts led to a retardation of the progression of lens opacity in rats with cataracts induced by selenite [140]. Gupta et al. [141] reported that green tea acts by preserving the antioxidant defense system of the lens. Skrzydlewska et al. [142] indicated a beneficial effect of green tea in alcohol intoxication. Besides all the above mentioned properties, which have helped to the recognition of green tea as functional food by some authors [143], it is not to forget its current use in the preparation of a variety of food, pharmaceutical preparations, dentifrices and cosmetics [144]. This additional use is mainly due to its antioxidant activity, which makes it a natural, efficient and safe preservative.

	Green Tea Nutritional Value

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Green tea consumption contributes to the overall daily fluid intake, and if sugar is not added, the calories intake is insignificant; besides, the caffeine intake is lower than in coffee, black tea or cola soft-drinks. In addition, green tea contribution to the dietary intake of antioxidant compounds (catechins and other phytochemical substances, certain vitamins as vitamin C, and minerals as Mn, Cr, Se, Zn) is very interesting to promote human health and well being, and more relevant than that other non-alcoholic beverages widely consumed. The Mn content is high, and tea is considered a rich source of this essential element [21,145]. Manganese is a constituent of three metalloenzymes (i.e., arginase, pyruvate carboxylase, and Mn-superoxide dismutase) and it activates a large number of enzymes, such as glycosyl transferases, involved in mucopolysaccharide synthesis [146]. Manganese deficiency can cause abnormalities in the metabolism of carbohydrates, glycosaminoglycans, and cholesterol [147]. Chromium, selenium and zinc play also an important role in human metabolism, and interest in these elements is increasing since there are reports relating trace element status and oxidative diseases. Chromium is involved in carbohydrate and lipid metabolism; the most frequent sign of Cr deficiency is altered glucose tolerance; this nutrient has been associated with diabetes and cardiovascular diseases [146]. Beneficial effects of dietary Cr supplementation, particularly in groups in which deficiencies are frequent, have been reported [147]. Garcia et al. [148] measured through duplicate diet sampling the Cr dietary intake in Spain and detected that the most elevated intakes are related to high consumption of infusions, especially tea and coffee. Selenium functions through selenoproteins, several of which are oxidant defense enzymes; Se acts as enzymatic cofactor of glutathione peroxidase in the elimination of peroxide radicals from the organism. Epidemiological studies have shown the possible effects of Se in the prevention and regression of cancer [146–147]. Most Se is ingested in food, but food derived from vegetables has a variable Se content depending on the zone where they have been cultivated [149]. Zinc enzymes participate in a wide variety of metabolic processes including carbohydrate, lipid, and protein synthesis or degradation. This element is required for deoxyribonucleic and ribonucleic acid synthesis; it may also play a role in stabilizing plasma membranes [147]. Zinc has been recognized as a cofactor of the superoxide dismutase enzyme, which is involved in protection against oxidative processes [146]. Recently there has been a development of terminology and change in conceptual approaches towards setting nutrient recommendations from adequate to optimum nutrition [150]. Regarding antioxidant minerals, the US Food and Nutrition Board has set an Adequate Intake for Mn at 2.3 and 1.8 mg/day for adult men and women, respectively, and a Tolerable Upper Intake Level at 11 mg/day for adults. Chromium Adequate Intake values are 35 and 25 µg/day for young men and women, respectively. The Recommended Dietary Allowance for Zn is 8 and 11 mg/day for adult men and women, respectively; the Tolerable Upper Intake Level for adults is 40 mg/day. The selenium Recommended Dietary Allowance and Tolerable Upper Intake Level for adults is 55 and 400 µg/day, respectively [151–152]. Table 3 summarizes data on the content of minerals with antioxidant activity in green tea.

	Harmful Effects of Tea Over Consumption

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Harmful effects of tea over consumption (black or green) are due to three main factors: (1) its caffeine content, (2) aluminum presence, and (3) the effects of tea polyphenols on iron bioavailability. A day-long consumption of green tea improved the cognitive and psychomotor performance of healthy adults in a manner similar to coffee, but green tea (which contains less caffeine) is less likely than coffee to disrupt sleep quality at night [4]. Lin et al. [19] compared the caffeine content in the same type of tea but manufactured by different fermentation processes, and concluded that the caffeine level presented the following order: black tea > oolong tea > green tea > fresh tea leaf. Cabrera et al. [22] determined the caffeine content in a total of 45 tea samples, including ‘fermented’ teas (red and black teas), oolong tea and green tea samples; the results showed that caffeine presence is higher in the case of black teas (41.5–67.4 mg/g), whereas green and oolong teas show a mean caffeine content of 32.5 and 29.2 mg/g, respectively. Fernandez et al. [20] also reported that caffeine content is higher in the case of ‘fermented’ teas, showing values between 2.4 and 4.8%, whereas ‘non-fermented’ teas show caffeine levels ranging between 1.47 and 3.86%. Table 4 includes data on the caffeine content in beverages widely consumed. The caffeine content in green tea may vary according to the type of tea and the form of preparation (i.e., brewing time); generally, bagged tea produces a higher percentage of caffeine than tea leaves [3]. In any case, although green tea caffeine content is low, its consumption is not advisable in cases of special sensitiveness to xanthic bases. The negative effects produced by caffeine are nervousness, sleep disorders, vomits, headaches, epigastric pain, tachycardia [38–39]. Theophylline negative effects are similar to those of caffeine, but they only occur with high quantities intake. Thus, green tea should not be taken by patients suffering from heart conditions or major cardiovascular problems. Pregnant and breast feeding women should drink no more than 1–2 cups/day, since it can cause an increase in heart rhythm. It is also convenient to control the concomitant consumption of green tea and some drugs, due to its diuretic effects [39].

Several studies have demonstrated that black tea appears to inhibit the bioavailability of non-heme iron by 79% to 94% when both are consumed concomitantly; the impact of this interaction depends on the iron intake and iron status of the individual [162–163]. Likewise, green tea catechins may have an affinity for iron, and green tea infusions can cause a significant decrease of the Fe bioavailability from the diet [164]. On the one hand, some authors affirm that tea should not be consumed by patients suffering from anaemia. For example, iron deficiency anaemia among children in Saudi Arabia and the United Kingdom may be exacerbated by the regular consumption of tea with meals [165–166]. On the other hand, this effect may be of benefit to patients with genetic hemochromatosis [4]. It is worth noting that the interaction between tea and iron can be mitigated by the addition of lemon or consuming tea between meals.

	Conclusions

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Green tea has been consumed in China and other Asian countries since ancient times in order to maintain and improve health. Nowadays, green tea is considered one of the most promising dietary agents for the prevention and treatment of many diseases and consequently, it is being studied extensively worldwide. Numerous studies in a variety of experimental animal models have demonstrated that aqueous extract of the mayor GTP designed as catechins (EGCG, EGC, ECG and EC) possess antioxidant, antimutagenic, antidiabetic, anti-inflammatory, antibacterial and antiviral, and above all, cancer-preventive properties. Epidemiological studies suggest that consumption of green tea may have a protective effect against the development of several cancers. Preclinical studies of green tea and its polyphenolic components have demonstrated antimutagenic and anticarcinogenic activity, and inhibition of growth of tumor cell lines and animal tumor models, including cancer. Green tea may also have chemopreventive properties, and enhancement of chemotherapeutic agents has been demonstrated. In addition, several epidemiological studies with humans have demonstrated that regular green tea consumption has beneficial effects and it shows a significant rate of protection against the development of some oral diseases and against solar radiations. It also contributes to body weight control and to the rise of bone density as well as being able to stimulate the immune system. Furthermore, green tea consumption has been recently reported to act positively against neurodegenerative diseases such as Parkinson and Alzheimer disease. Catechin antioxidant power is also strengthened by the presence of other phenolic compounds, vitamin C and minerals such as Cr, Mn, Se, and Zn, although specific data regarding this fact are still scarce.