Summary

  • Tea is an infusion of the leaves of the Camellia sinensis plant, which is not to be confused with herbal teas. (More information)
  • All the tea types, including white, green, oolong, black, and Pu-erh tea, are produced from the leaves of the Camellia sinensis plant. Different processing methods yield the various types of tea. (More information)
  • Biologically active chemicals in tea include flavonoids, caffeine, fluoride, and theanine. Green teas are especially rich in a group of flavonoids called flavan-3-ol monomers or catechins. Black teas contain more complex chemicals — theaflavins and thearubigins — derived from catechins. (More information)
  • Observational studies in humans suggest that daily consumption of tea may be associated with a reduced risk of cardiovascular disease (CVD). Intervention studies showed that tea exhibited cholesterol-lowering, anti-inflammatory, antioxidant, and anti-hypertensive properties, which might be beneficial in the prevention of CVD. (More information)
  • Tea consumption has been associated with reduced risk of developing type 2 diabetes mellitus in large prospective cohort studies. Yet, the mechanisms behind this association are complex, possibly involving a role for tea bioactive compounds in the regulation of energy balance, lipid and glucose metabolism, insulin sensitivity, body composition, and/or body temperature. (More information)
  • Despite promising results from animal studies, current evidence does not support a role for tea consumption in the prevention of most cancers in humans. (More information)
  • It is still unclear whether tea consumption is associated with increased bone mineral density and/or reduced risk of osteoporotic fractures. (More information)
  • Limited research suggests an inverse association between tea consumption and risk of tooth cavities. The incorporation of green tea extract in mouthwashes and/or toothpastes may also help reduce dental plaque and gum inflammation in patients. (More information)
  • The pooled analysis of three large US prospective cohort studies found an 11% lower risk of kidney stones with the highest versus lowest level of tea intake. More research is needed to assess whether the oxalate content in tea may affect subjects with a history of kidney stones. (More information)
  • The mounting evidence for a role of tea consumption in the prevention of cognitive decline comes from observational studies. Clinical studies would be needed to establish whether tea or its bioactive constituents could limit cognitive decline and/or improve cognitive dysfunction in older individuals. (More information)
  • The use of green tea extracts in clinical trials was found to cause gastrointestinal disorders and liver toxicity. Tea consumption may also potentially interfere with certain medications, including the anticoagulant, warfarin, and some cardiovascular drugs. (More information)

Introduction

Tea is an infusion of the leaves of the Camellia sinensis plant and, aside from water, is the most widely consumed beverage in the world (1). Different processing methods of tea leaves involve variable degrees of oxidation yielding different types of tea (green, oolong, or black tea). In 2014, Americans consumed 3.6 billion gallons of tea, of which 84% was black tea, 15% was green tea, and the remaining was white, oolong, and dark tea (2). Herbal teas are infusions of herbs or plants other than Camellia sinensis and will not be discussed in this article. Although tea contains a number of bioactive chemicals, including caffeine and fluoride, most research has focused on the potential health benefits of a class of compounds in tea known as flavonoids. In many cultures, tea is an important source of dietary flavonoids.

Definitions

Types of tea

All teas are derived from the leaves of the tea plant Camellia sinensis, but different processing methods produce different types of tea. Fresh tea leaves are rich in polyphenolic compounds known as flavonoids (see the article on Flavonoids). Flavonoids are divided in six subclasses: flavan-3-ols, anthocyanidins, flavanones, flavonols, flavones, and isoflavones (Figure 1). Tea leaves contain a polyphenol oxidase (PPO) enzyme in separate compartments from flavan-3-ol monomers or catechins (Figure 2) (3). When tea leaves are intentionally broken or rolled during processing, cell compartmentalization is disrupted and PPO comes into contact with catechins. This causes catechins to condense (join together) forming dimers and polymers known as theaflavins (Figure 3) and thearubigins, respectively (4). This oxidation process is often described as "fermentation" in the tea industry. Steaming, firing, or baking tea leaves inactivates PPO and stops the oxidation process (5).

The two prominent varieties of Camellia sinensis used in tea cultivation are Camellia sinensis var. sinensis and Camellia sinensis var. assamica. The former is native of China and usually used to make white and green tea. The latter originates from the Assam region of India, as well as regions of Southeast Asia, and is often used to make black teas, including pu-erh tea in the Yunnan province of China.

Although there are thousands of tea cultivars derived from the principal Camellia sinensis tea varieties, teas are usually divided into five types based on the extent of oxidation they undergo during processing. The withering method (the process of allowing the fresh leaves to dry) and the process of deactivating PPO may also differ among tea preparations (1).

Figure 1. Basic Structures of Flavonoid Subclasses

 

Figure 2. Chemical Structures of Principal Flavan-3-ols (Catechins)

Figure 3. Chemical Structures of Some Theaflavins in Tea. 

White tea

White tea is made from unopened buds and immature leaves, which are steamed or fired to inactivate polyphenol oxidase, and then dried. Thus, due to minimal oxidation, white tea retains the high concentrations of catechins present in fresh tea leaves (see Flavonoids below).

Green tea

Green tea is made from more mature tea leaves than white tea, and tea leaves may be withered prior to steaming or firing, and then rolled and dried. Like white teas, green teas are high in catechins, but the total content and composition of catechins may vary depending on the cultivar and the commercial source (6). Of note, green teas and white teas may sometimes contain similar amounts of catechins but still exhibit different antioxidant capacities; this is due to the presence of other non-catechin antioxidants in teas (6).

Oolong (Wulong) tea

Tea leaves destined to become oolong teas are "bruised" to allow the release of some of the polyphenol oxidase present in the leaves. Oolong teas are allowed to oxidize to a greater extent than for white or green teas, but for less time than black teas, before they are heated and dried. Consequently, the catechin, theaflavin, and thearubigin levels in oolong teas are generally between those of unfermented green and white teas and completely oxidized black teas (1).

Black tea

Tea leaves destined to become black tea are fully rolled or broken to maximize the interaction between catechins and polyphenol oxidase. Because they are allowed to oxidize completely before drying, most black teas are relatively low in monomeric flavan-3-ols, like (-)-epigallocatechin gallate (EGCG), and rich in theaflavins (2%-6% of extracted solids) and thearubigins (>20% of extracted solids) (see Table 1 below). Some theaflavins have shown greater antioxidant activities than EGCG (7).

Pu-erh tea (also pu’erh tea, pu’er tea, or Chinese black tea)

Most pu-erh tea is produced in the Yunnan province of China from the larger leaves of the assamica variety of Camellia sinensis. The making process may include both enzymatic oxidation and fungus-led fermentation. In the case of "raw (aged) pu-erh tea," the initial preparation resembles that used to make green tea. The leaves are heated, dried, and then dampened before being pan-fired and compressed; the preparation is then carefully stored in a controlled environment and left to age for decades. A faster aging process, combining oxidation and fermentation by the fungus Aspergillus niger for several months, can also be used to produce "ripened pu-erh tea."

Cup sizes

The definition of a cup of tea varies in different countries or regions. In Japan, a typical cup of green tea may contain only 100 mL (3.5 ounces). A traditional European teacup holds approximately 125 to 150 mL (5 ounces), while a mug of tea may contain about 240 mL (8 ounces).

Bioactive Compounds in Tea

Tea contains over 2,000 components, including polyphenols (flavonoids), pigments (carotenoids and chlorophyll), alkaloids (caffeine, theophylline, theobromine), lignans, carbohydrates, lipids, proteins, amino acids (including L-theanine), vitamins (vitamin C, vitamin E, riboflavin), and various minerals and trace elements (8). Only some of them are described below.

Flavonoids

Dietary flavonoids are divided in six subclasses: flavan-3-ols, anthocyanidins, flavanones, flavonols, flavones, and isoflavones (see the article on Flavonoids). Total flavonoid content in green tea and black tea is of about 138 mg and 118 mg per 100 mL, respectively (9). A major subclass of flavonoids in tea is that of flavan-3-ols. Flavan-3-ol monomers, also known as catechins, constitute 30%-42% of the solid weight of brewed green tea. The principal catechins found in tea are (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG), and (-)-epigallocatechin gallate (EGCG) (see Figure 2). When catechins are enzymatically oxidized by polyphenol oxidase during the oxidation process that yields black tea, they form low molecular weight dimers known as theaflavins (see Figure 3) and complex polymers (of mostly unknown structures) called thearubigins. Non-oxidized teas are rich in catechins, while fully oxidized teas are rich in theaflavins and thearubigins (Table 1) (5).

Table 1. Flavan-3-ol Monomers and Thearubigins Content of Tea (mg/100 mL) (10)
Type of Tea1 EC ECG EGC EGCG Thearubigins
Tea, white, brewed -2 8.3 18.6 42.4 -
Tea, green, brewed 8.3 17.9 29.2 70.2 1.1
Tea, oolong, brewed 2.5 6.3 6.1 34.5 -
Tea, black, brewed 2.1 5.9 8.0 9.4 81.3
11 g of tea leaves infused in 100 mL of boiling water (1% weight/volume)
2The lack of a value for a particular flavonoid in a food in the database does not imply a zero value, but only that data were unavailable.

Tea is also a good source of another class of flavonoids called flavonols. Flavonols found in tea include kaempferol, quercetin, and myricitin. The flavonol content of tea is minimally affected by processing, and flavonols are present in comparable quantities in oxidized and non-oxidized teas. Unlike flavan-3-ols, flavonols are usually present in tea as glycosides, i.e., bound to a sugar molecule (Figure 4). Despite their poor bioavailability, flavonoids are thought to contribute substantially to the health benefits associated with daily tea consumption (10). For more detailed information, see the article on Flavonoids.

Figure 4. Chemical Structures of Some Flavonols in Tea.

Caffeine

All teas contain caffeine, unless they are deliberately decaffeinated during processing. The caffeine content of different varieties of tea may vary considerably and is influenced by factors like brewing time, the amount of tea and water used for brewing, and whether the tea is loose or in teabags. In general, a mug of tea contains about half as much caffeine as a mug of coffee (11). The caffeine contents of more than 20 green and black teas prepared according to package directions are presented in Table 2 (12). The caffeine content of oolong teas is comparable to green teas (13). There is little information on the caffeine content of white teas, since they are often grouped together with green teas (14). Buds and young tea leaves have been found to contain higher levels of caffeine than older leaves (15), suggesting that the caffeine content of some white teas may be slightly higher than that of green teas (16).

Table 2. Caffeine Content of Teas and Coffee (12, 14, 17)
Type of Tea Caffeine (mg/L) Caffeine (mg/8 ounces)
Green 40-234 9-63
Black 177-333 42-79
Coffee, brewed 306-553 72-130

Caffeine is a known stimulant of the central nervous system, thought to protect dopaminergic neurons by antagonizing adenosine A2A receptors (Figure 5) (18). Because adenosine has mostly inhibitory effects in the central nervous system, the effects of adenosine antagonism by caffeine are generally stimulatory.

Figure 5. Caffeine (and Adenosine)

Fluoride

Tea plants accumulate fluoride in their leaves. In general, the oldest tea leaves contain the most fluoride (19). Most high-quality teas are made from the bud or the first two to four leaves — the youngest leaves on the plant. Fluoride levels in green, oolong, and black teas are generally comparable to those recommended for the prevention of dental caries (cavities). Thus, daily consumption of up to one liter of green, oolong, black, or pu-erh tea would be unlikely to result in fluoride intakes higher than those recommended for dental health (20, 21). The fluoride content of white tea is likely to be less than other teas, since white teas are made from the buds and youngest leaves of the tea plant. A comparative study of green, oolong, and black teas from six provinces of China found that fluoride content was inversely correlated with the quality level of tea sensory attributes (i.e., appearance, taste, flavor) (22). The fluoride content of 17 brands of green, oolong, and black teas is presented in Table 3 (20). These values do not include the fluoride content of the water used to make the tea. For more information, see the article on Fluoride.

Table 3. Fluoride Content of Teas (21)
Type of Tea Fluoride (mg/liter)1 Fluoride (mg/8 ounces)
Green 1.2-1.7 0.3-0.4
Oolong 0.6-1.0 0.1-0.2
Black 1.0-1.9 0.2-0.5
Pu-erh tea 0.9-1.6 0.2-0.4
1Fluoride in 1% weight/volume tea prepared by continuous infusion from 5 minutes (number before hyphen) to 360 minutes (number after hyphen).

L-theanine

L-theanine (also L-g-glutamylethylamide) is a non-protein amino acid that constitutes about 1%-2% (w/w) of Camellia sinensis dry leaves (23). L-theanine is rapidly absorbed in the small intestine and has a bioavailability close to 100% (24). L-theanine can cross the blood-brain barrier and exert neuroprotective effects (25). Because its chemical structure resembles that of glutamate, a neurotransmitter critically involved in memory, theanine may compete with glutamate for binding to glutamate receptors (Figure 6). This glutamate antagonism has been associated with the prevention of neuronal death by theanine after brain ischemia (reviewed in 25).

Figure 6. Theanine (and Glutamate)

Disease Prevention

Cardiovascular disease

Many epidemiological studies have considered the relationship between tea consumption and manifestations of cardiovascular disease (CVD), including coronary heart disease (CHD) and stroke (reviewed in 26). A recent meta-analysis by Zhang et al. included the results of prospective observational studies (cohort or nested case-control studies) that examined the association between tea consumption and cardiovascular morbidity and mortality (27). The results showed that a three-cup (125 mL/cup) increase in daily tea intake was associated with a 27% lower risk of CHD (seven studies), an 18% lower risk of total stroke (eight studies), a 16% lower risk of ischemic stroke (four studies), a 21% lower risk of intracerebral hemorrhage (a type of hemorrhagic stroke), a 26% lower risk of cardiac deaths (12 studies), and a 24% lower risk of total deaths (seven studies). No associations were found between tea consumption and stroke mortality (five studies) or risk of subarachnoid hemorrhage (a subtype of stroke; two studies). Further subgroup analyses indicated that green tea consumption was specifically linked to a reduced risk of stroke, cardiac mortality, and all-cause mortality, while a lower CHD risk was associated with black tea consumption (27). Another recent meta-analysis of prospective cohort studies found that the highest versus lowest level of green tea consumption was associated with a 33% lower risk of cardiovascular mortality (five studies) and a 20% lower risk of all-cause mortality (five studies) (28). Black tea consumption was linked to a 10% reduced risk of all-cause mortality, but not specifically to cardiovascular-related mortality (28).

Tea is a major source of flavonoids in US and European diets (29, 30). The results of several epidemiological studies suggest that dietary flavonoids might influence cardiovascular health. A recent meta-analysis of 14 prospective cohort studies reported that highest versus lowest quantiles of flavonol, flavan-3-ol, flavanone, flavone, anthocyanidin, and proanthocyanidin intake were associated with modest reductions (~10%) in cardiovascular risk (31). A dose-response analysis based on the results of 13 studies in nearly 350,000 individuals and 12,445 CVD cases found a 5% risk reduction with an average 10 mg-incremental increase in daily flavonol intakes (31). It is not clear whether the antioxidant, anti-inflammatory, and/or vasodilatory properties of flavonoids are responsible for some of the cardiovascular benefits associated with tea consumption (see also the article on Flavonoids).

Intervention studies

A number of intervention trials have investigated the effects of tea consumption on markers of cardiovascular health, including biological parameters related to lipid and glucose metabolism, inflammation, blood coagulation, endothelial health, and body composition. 

Metabolic markers of cardiovascular disease: Clinical trials examining the effects of green and/or black tea beverages or extracts have been relatively heterogeneous, especially regarding concentrations of active substances, duration of interventions, and included populations. Pooled analyses of mostly short-term interventions (<3 months) have suggested a reduction in total and LDL cholesterol concentrations with green tea catechin consumption, but the data regarding a potential lipid-lowering effect of black tea are inconsistent (32-34).The studies mentioned below investigated the effect of tea/tea extracts given for at least three months.

Black tea: The Tea’s Effect on Atherosclerosis (TEA) pilot study in 28 older adults at increased risk for cardiovascular disease (CVD) did not find any effect of a six-month black tea consumption intervention (three glasses per day equivalent to 318 mg/day of black tea catechins) on specific biomarkers, including circulating lipoproteins, inflammation markers, homocysteine concentration, adhesion molecules, and hemostatic factors (35). Yet, in a randomized, placebo-controlled study in 47 individuals with borderline-to-moderate hypercholesterolemia, the daily consumption of 1 g of pu-erh tea (Chinese black tea) extract for three months reduced the blood concentrations of total cholesterol, LDL-cholesterol, and triglycerides (36). A three-month, placebo-controlled trial in 77 healthy subjects who drank 9 g of black tea infused in 600 mL of boiling water per day (about 740 mg/day of black tea polyphenols) had improvements in lipoprotein and triglyceride profiles, fasting serum glucose concentrations, and measures of antioxidant activities in the plasma (37). The same protocol was found to also reduce markers of oxidative stress and inflammation in the plasma of individuals at risk for CVD (38). In contrast, a recent randomized, double-blind, placebo-controlled trial in 77 regular tea drinkers (35 to 75 years old) found that the daily consumption of 429 mg of black tea polyphenols for six months had no effect on fasting blood glucose and serum lipids (39).

Green tea: Daily supplementation with a capsule containing 150 mg of green tea catechins, 75 mg of black tea theaflavins, and 150 mg of other polyphenols, for six months significantly lowered plasma LDL-cholesterol concentrations and the ratio total cholesterol:HDL in a randomized, double-blind, placebo-controlled study in 220 individuals with mild-to-moderate hypercholesterolemia (40). In a pilot study in 74 overweight/obese breast cancer survivors, the daily consumption of green tea (containing 26.7 mg of caffeine and 235.6 mg of catechins with 128.8 mg as EGCG) for six months increased HDL concentration compared to a citrus-based herbal placebo but had no effect on LDL concentration and markers of glycemic control (41). Another placebo-controlled study conducted in obese subjects with controlled hypertension found that the daily ingestion of one capsule of green tea extract (379 mg/capsule containing 208 mg of EGCG) for three months significantly lowered systolic and diastolic blood pressure and improved blood lipid profile, antioxidant status, and measures of glycemic control and inflammation (42).

Endothelial dysfunction: The vascular endothelial cells that line the inner surface of all blood vessels synthesize an enzyme, endothelial nitric oxide synthase (eNOS), which plays a critical role in maintaining cardiovascular health. Specifically, eNOS utilizes L-arginine to produce nitric oxide (NO), a compound that regulates vascular tone and blood flow by promoting the relaxation (vasodilation) of all types of blood vessels, including arteries (26). NO also regulates vascular homeostasis and protects the integrity of the endothelium by inhibiting vascular inflammation, leukocyte adhesion, platelet adhesion and aggregation, and proliferation of vascular smooth muscle cells (43). In the presence of cardiovascular risk factors (e.g., hypertension, hypercholesterolemia, hyperglycemia), early alterations in the structure and function of the vascular endothelium are associated with the loss of normal NO-mediated endothelium-dependent vasodilation. Endothelial dysfunction results in widespread vasoconstriction and coagulation abnormalities and is considered to be an early step in the development of atherosclerosis. The measurement of brachial flow-mediated dilation (FMD) is often used as a surrogate marker of endothelial function; FMD values are inversely correlated with the risk of future cardiovascular events (44).

Black tea: Two small controlled clinical trials found that daily consumption of 900 to 1,250 mL of black tea for four weeks significantly improved endothelium-dependent FMD in patients with coronary heart disease (45) and in patients with mildly elevated serum cholesterol concentrations (46). Improvements were noted in comparison to an equivalent amount of hot water. Incremental doses of black tea flavonoids (0, 100, 200, 400, and 800 mg/day; each dose being given for one week) have been associated with dose-dependent increases in brachial FMD in 19 healthy volunteers. Specifically, FMD values went from 7.8% at baseline (no flavonoids) to 10.3% with 800 mg/day of flavonoids (47). Of note, in this study, the ingestion of black tea flavonoids significantly lowered systolic and diastolic blood pressure in a non dose-dependent manner, while other variables, including markers of arterial stiffness, glucose metabolism, inflammation, endothelial activation, and lipid profile, remain largely unchanged (47). In a recent randomized trial, seven days of black tea consumption (450 mL/day) followed by the ingestion of two cups (300 mL) 20 minutes before an experimental ischemia-reperfusion (IR) injury procedure on healthy participants failed to prevent IR injury-associated FMD reduction. Even if tea flavonoids were able to limit the impact of IR injury on FMD — for example by counteracting the production of reactive oxygen species — the presence of caffeine in black tea (and the lack of control for it) could have confounded this effect (48).

Green tea: In a small study conducted in 14 healthy young adults (50% smokers), a significant increase in brachial FMD was reported 30 to 120 minutes after the consumption of 450 mL of green tea (6 g of green tea, including 125 mg of caffeine) compared to caffeine alone or hot water (49). Another study that compared the acute effect of black and green tea on brachial FMD in 21 postmenopausal women found a similar increase in FMD two hours after the ingestion of either tea preparation (50). Both black and green teas have been found to be equally able to increase eNOS activity and NO production in cultured endothelial cells (50, 51). Specifically, the prominent green tea catechin, EGCG, and black tea polyphenols, theaflavins and thearubigins, are thought to contribute to the protective effect of drinking tea by promoting antioxidant activity and endothelium-dependent vasodilation (51). Other flavonoids, like (-)-epicatechin and quercetin glucoside have recently failed to show an effect on FMD (and blood pressure) in hypertensive adults (52). For more information, see the article on Flavonoids.

A meta-analysis of nine human intervention studies estimated that the acute and/or short-term (up to four weeks), daily ingestion of 500 mL of tea — containing about 248 mg of flavonoids in green tea and 415 mg in black tea — significantly increased brachial FMD (53). Yet, the clinical relevance of these FMD improvements is unclear. It is also not clear whether the chronic consumption of tea might benefit vascular endothelial function and eventually lower the risk of cardiovascular disease.

Hypertension: Hypertension (high blood pressure) is a risk factor for CVD morbidity and mortality.

Black tea: A recent meta-analysis of 11 small randomized controlled trials in either healthy or at-risk individuals found a significant reduction of 2 mm Hg in systolic and 1 mm Hg in diastolic blood pressure with the daily consumption of at least 400 mL (13 oz) of black tea for one week to six months, providing a minimum of 240 mg/day of flavonoids (54). Two recent trials also reported that black tea lowered the rate of circadian variations in blood pressure at nighttime and variations after a dietary fat challenge. A six-month intervention in 76 participants from the general population (most with moderate hypertension) showed that the consumption of three cups per day of black tea, supplying a daily total of 1.29 g of polyphenols and 288 mg of caffeine, lowered the rate of nighttime blood pressure variations compared to a polyphenol-free caffeine-matched placebo (55). Two cups of black tea per day, equivalent to 300 mg of polyphenols, also limited blood pressure variations after a fat-rich meal in 19 patients with primary (idiopathic) hypertension (56). Mechanisms underlying the blood pressure-lowering properties of black tea may involve the inhibition of angiotensin-II synthesis by flavonoids (see below).

Green tea: Several recent meta-analyses of randomized controlled trials indicated that the consumption of green tea or green tea extracts could significantly lower blood pressure (57-60). In one of them, the pooled analysis of 13 trials in 1,367 subjects found a 2.0 mm Hg reduction in systolic blood pressure and a 1.9 mm Hg reduction in diastolic blood pressure with green tea polyphenols (208 mg/day-1,207 mg/day) for a median period of 12 weeks (60). Subgroup analyses suggested greater blood pressure lowering effect with polyphenol intake levels lower than 582.8 mg/day and with adjustment for the confounding effect of caffeine. Another meta-analysis included randomized controlled trials that specifically explored the effect of green tea or green tea extracts on blood pressure in overweight or obese subjects. The pooled analysis of 14 trials showed significant reductions of 1.4 mm Hg in systolic blood pressure and 1.3 mm Hg in diastolic blood pressure (58). The anti-hypertensive effect of green tea may be mediated by a number of mechanisms. For example, pharmacological concentrations of several catechins have been shown to inhibit the activity of a key regulator of arterial blood pressure, angiotensin-converting enzyme (ACE), in vitro (61). ACE catalyzes the conversion of angiotensin-I into angiotensin-II, a potent inducer of vasoconstriction. In addition, studies in rats showed that chronic treatment with epicatechin prevented salt-induced hypertension, partly through inhibition of endothelin-1 expression and NADPH oxidase (NOX) activity (62). Other potential benefits of green tea consumption, including improvements in blood lipid profile, insulin sensitivity, and endothelial function, may also contribute to its blood pressure-lowering effects.

Type 2 diabetes mellitus

Impaired glucose tolerance in patients with prediabetes is often associated with loss of insulin sensitivity, impaired lipid metabolism, low-grade inflammation, and endothelial dysfunction (63). Without changes in lifestyle behavior (especially regarding dietary habits and physical activity), individuals with prediabetes will eventually progress to develop overt type 2 diabetes mellitus (64).

In this context, the association between tea consumption and risk of type 2 diabetes has been examined in a recent European, multicenter, nested case-control study — the “EPIC-InterAct” project — that included 16,835 diabetes-free participants and 12,043 individuals with diabetes (65). The results showed that tea consumption was inversely associated with diabetes incidence. The consumption of four cups per day rather than none was found to be associated with a 16% lower risk of diabetes (65). Of note, in a meta-analysis of 15 prospective cohort studies, including the EPIC-InterAct study, an incremental increase of two cups per day in tea consumption was found to be associated with an estimated 4.6% risk reduction (66). The EPIC-InterAct study also found that participants in the highest quintile (>608.1 mg/day) of total flavonoid intake had a 10% lower risk of diabetes than those in the lowest quintile (<178.2 mg/day) (67). Specifically, the risk of diabetes was inversely correlated with the consumption of flavan-3-ols (catechins, proanthocyanidins, and theaflavins) and flavonols (67, 68). The intakes of other flavonoid subclasses that are less abundant in tea, namely anthocyanidins, flavanones, flavones, and isoflavones, were not associated with a reduced risk of diabetes (67).

Recent meta-analyses of randomized controlled trials that examined the possible health benefits of green tea catechins on glucose metabolism have provided conflicting results. A meta-analysis of seven trials in prediabetic and diabetic patients found no effect of green tea or green tea extracts on fasting plasma glucose, fasting serum insulin, or measures of glycemic control (HbA1c) and insulin sensitivity (HOMA-IR) (69). Conversely, another meta-analysis of 17 trials in prediabetic, diabetic, or overweight/obese subjects found that administration of green tea extracts for 4 to 16 weeks improved fasting plasma glucose and HbA1c levels (70). The effect on fasting glucose was observed only with high doses of catechins (≥457 mg/day) and when the confounding effect of caffeine was removed. However, a third meta-analysis of 25 trials found that ingestion of green tea extracts for at least two weeks could lower fasting blood glucose in both the presence or absence of caffeine (71).

Overweight and obesity

A recent meta-analysis of five small, randomized controlled trials (<100 participants per study) found that regular consumption of green or pu-erh tea extracts reduced body weight and body mass index (BMI) in overweight/obese participants with metabolic syndrome (72). The influence of green tea on body composition may be attributed to the regulation of appetite, fat absorption, fatty acid oxidation, and thermogenesis by catechins and caffeine (73). Yet, studies in overweight/obese people who are otherwise healthy have provided mixed results (reviewed in 74). Intervention studies in Caucasian populations have shown a less favorable effect of green tea catechins on body weight and energy expenditure compared to those conducted in Asian subjects. These discrepancies suggested that differences in genetic background, body composition, and dietary habits (including caffeine consumption) might interfere with the possible anti-obesity effect of green tea consumption. Large-scale, intervention trials that control for energy intake and physical activity are needed to determine if tea or tea extracts promote weight loss or improve weight maintenance in different populations with obesity and/or metabolic syndrome (74).

Cancer

Tea and tea constituents have been found to have cancer preventive activities in a variety of animal models of cancer, such as cancer of lung, mouth, esophagus, stomach, colon, and prostate (75). However, the results of epidemiological studies in humans have been mostly inconclusive.

Breast cancer

An early meta-analysis of prospective cohort studies had reported that black tea intake (five cohorts) — but not green tea (three cohorts) — was associated with a 15% higher risk of breast cancer (76). The relationship between tea consumption and breast cancer has been recently examined in the Swedish Women’s Lifestyle and Health prospective cohort Study (WLHS), which followed 42,099 women for 20 years and documented 1,395 breast cancer cases (77). The results indicated a 14% higher risk of breast cancer with each cup (200 mL) of tea consumed daily. The risk was specifically increased in postmenopausal women rather than in premenopausal women. Similarly, in a recent case-control study in Chinese women in Hong Kong, regular tea consumption was inversely correlated with breast cancer risk in premenopausal women but associated with an increased risk in postmenopausal women (78). The risk associated with consuming tea was also significantly higher for estrogen- and progesterone-receptor-positive (ER+/PR+) breast cancer type in the Swedish cohort, while the highest risk was found in women with ER-negative tumors in the Chinese study (77, 78). Yet, in the European Prospective Investigation into Nutrition and Cancer Study (EPIC) in 335,060 women followed for 11 years (10,198 incidental breast cancer cases), tea intake was not associated with breast cancer overall or when the data were analyzed for menopausal status or breast cancer type (79). The most recent meta-analysis of prospective cohort studies found no association between consumption of green or black tea and breast cancer (80). Thus, current epidemiological evidence does not suggest a benefit of tea in breast cancer prevention despite promising data from cultured cells and rodent models (81).

However, some observational studies found that consumption of certain subclasses of flavonoids might have the potential to reduce the incidence of breast cancer in postmenopausal women (82). Recently, the effects of decaffeinated green tea extracts on biomarkers of breast cancer risk were examined in the Minnesota Green Tea Trial (MGTT) in 1,075 high-risk postmenopausal women randomized to receive the equivalent of four 8-ounce mugs/day (960 mL/day) in green tea extracts (1,315±116 mg/day of catechins) or a placebo for one year (83). The results have yet to be published.

Mouth, throat, and esophageal cancers

In the large, prospective NIH-AARP Diet and Health study (1995-2006) in 481,563 US adults, 1,305 cases of oral (392), pharyngeal (178), laryngeal (307), and esophageal (428) cancers have been identified during the follow-up period (84). The highest versus lowest level of tea intake (≥1 cup/day vs. non-consumption) was correlated with a 63% lower risk of pharyngeal cancer but with no other above-cited cancer types (84). Observational studies do not currently provide clear evidence for an association between tea consumption and laryngeal (85) or esophageal (86) cancers. In the case of esophageal cancer, the consumption of high-temperature beverages (including very hot tea) might even damage the epithelium and increase the risk of cancer (87). High temperature may act as a confounding factor that complicates the interaction between tea consumption and esophageal cancer (88). In 2009, a phase II, randomized, double-blind trial was conducted in 41 patients with high-risk oral premalignant lesions (OPLs). Participants were randomly assigned to orally receive 0.5, 0.75, or 1 g of green tea extract per m2 (body surface area) or a placebo, thrice a day for three months. While the results suggested that OPLs might be clinically responsive to green tea extract treatment, larger trial populations are needed to confirm these preliminary data (89).

Gastric cancer

Several prospective cohort studies reported no association between tea consumption and risk of gastric cancer (90-92), including the US NIH-AARP Diet and Health study (84). Tea consumption also failed to predict gastric cancer cases in the EPIC study, which followed 477,312 participants and identified 683 cases during a median 11.6 years of follow-up (93). Yet, a decreased risk of intestinal type gastric cancer was observed in women in the highest versus lowest quartile of tea consumption (≥475 mL/day vs. ≤21 mL/day). Interestingly, women (but not men) in the highest versus lowest quartile of flavonol, flavanol, theaflavin, or total flavonoid intakes had a significantly reduced risk of developing gastric cancer (94). A pooled analysis of six Japanese cohort studies, including 219,080 total participants and 3,577 cases, found an inverse association between green tea consumption and gastric cancer in women but not in men: daily consumption of at least five cups of tea was associated with a 21% lower risk of gastric cancer in women compared to low intakes (<1 cup/day) (95).

Gynecologic cancers

Meta-analyses of case-control studies have found a significant 34% reduction in risk of ovarian cancer for highest versus lowest intake of green tea (four studies) (96), but no association was observed for black tea (six studies) (97). Yet, a meta-analysis of six prospective studies showed an inverse relationship between black tea consumption and ovarian cancer (98). A recent analysis of the Nurses’ Health Studies (NHS I and II) showed a 31% lower risk of ovarian cancer in women consuming at least 1 cup/day compared to rare/non-black tea drinkers (99). In a prospective study in 244 women diagnosed with ovarian cancer and followed for over three years, green tea consumption was associated with a mean survival time greater for consumers (5.39 years) than for non-consumers (4.19 years) (100). However, in a single-arm, phase II trial in 16 women in complete remission from advanced stage ovarian cancer, the daily intake of 500 mL of green tea (containing 319.8 mg of EGCG) failed to effectively prevent cancer recurrence within the 18-month follow-up period (101). Further, a recent systematic review and meta-analysis of observational studies suggested a possible benefit of green tea — but not black tea — for endometrial cancer (102).

Additional studies have examined the association between tea consumption and risk of lung, prostate, liver, or colorectal cancer in humans, providing mixed results (reviewed in 103).

Bone health and osteoporosis

The etiology of osteoporosis is complex, involving factors such as aging, decreased sex hormones, inadequate nutrition, physical inactivity, genetic predisposition, as well as socioeconomic determinants. Tea bioactive components, including flavonoids, caffeine, and fluoride, have the potential to influence health and the risk of osteoporosis and fracture (104, 105). A small prospective study in 164 elderly women found that consumption of tea limited the age-related loss in total hip bone mineral density (BMD) over a four-year follow-up period (106). Also, in a six-month randomized, placebo-controlled trial in 171 postmenopausal women with osteopenia, the daily consumption of green tea catechins (500 mg) alone or combined with Tai Chi exercise (3 hours/week) improved bone turnover by stimulating bone formation (107).

Hip fracture is one of the most serious consequences of osteoporosis. A recent prospective study in 1,188 elderly women (mean age, 80 years) followed for 10 years found that participants in the highest versus lowest tertile of tea consumption (≥3 cups/day vs. ≤1 cup/week) had a 30% lower risk of any osteoporotic fracture. However, no interaction was found when the analysis was conducted on major fractures (hip, spine, humerus, and wrist) or hip fractures only (108). Yet, a meta-analysis based on 147,488 individuals from 11 observational studies published between 1990 and 2010 suggested that the consumption of 1-4 cups/day was associated with a significantly lower risk of hip fracture (109). The results of another recent meta-analysis of mostly case-control studies did not suggest any interaction between tea consumption and risk of any fracture or hip fracture (110). Additional studies are required to determine whether tea consumption affects the development of osteoporosis or the risk of osteoporotic fracture in a meaningful way.

Dental health

The cross-sectional analysis of the Ohsaki prospective cohort study that included data from 25,078 Japanese participants found an inverse association between the daily consumption of at least one cup of tea and the risk of tooth loss (111). Specifically, the risk of tooth loss was 11% lower in women and 23% lower in men who consumed at least five cups per day of tea compared to those drinking less than one cup per day. An earlier cross-sectional study of more than 6,000 14-year old children in the UK found that those who drank tea had significantly fewer dental caries than coffee drinkers; results were independent of the amount of beverage consumed or whether sugar was added (112). Although tea is a good source of fluoride — a recognized anticaries agent — both flavonoids and tannins in tea have been shown to have antimicrobial properties (reviewed in 113). Oral bacteria like Streptococcus mutans and Porphyromonas gingivalis have been associated with plaque formation, dental caries, and periodontal (gum) diseases. Untreated caries and gum inflammation can lead to severe pain, local infection, tooth loss or extraction, nutritional problems, and serious systemic infections in susceptible individuals. A pilot study in 25 adults suggested that mouthwash with a 2% green tea solution could lower acidity level and Streptococcus mutans count in saliva and plaque and improve measures of gum bleeding after exposure to sugar (114). A small, randomized study in 66 young volunteers (12-18 years old) also reported a significant antibacterial effect of a mouth rinse made with pulverized tea leaves compared to a placebo solution (115). Recent randomized, double-blind, controlled studies demonstrated further that tea extract-containing mouthwashes could benefit dental health and offer a possible alternative to current chlorhexidine- and fluoride-containing rinsing solutions (116-118). Finally, the incorporation of tea extract in toothpastes was found to be as effective — if not better — than regular pastes (containing fluoride and triclosan) to reduce dental plaque and gum inflammation in patients with mild to moderate periodontitis (119).

For more information on dental caries, see the article on Fluoride.

Kidney stones

The formation of kidney stones, usually composed of calcium oxalate or calcium phosphate, is a common condition that affects 7% of US women and 11% of US men during their lifetime (120). A pooled analysis of three ongoing prospective cohort studies — the Health Professionals Follow-up Study and the Nurses’ Health Studies I and II, including a total of 194,095 participants — found that the risk of developing symptomatic kidney stones was 11% lower in individuals consuming at least one 8-ounce mug of tea per day compared to those consuming less than one cup per week (121). High fluid intake, including tea intake, is generally considered the most effective and economical means of preventing kidney stones (122). However, the finding that black tea may contain high amounts of oxalate (48 to 92 mg/100 mL) suggested that black tea consumption may increase urinary oxalate concentrations, a risk factor for calcium oxalate stone formation (123). The Academy of Nutrition and Dietetics recommends that kidney stone patients restrict oxalate intake to 40 mg/day-50 mg/day, and some experts advise those with a history of calcium oxalate stones to limit the consumption of oxalate-rich food, including black (but not green) tea (123, 124). Yet, recent studies have reported amounts of oxalate in different samples of green teas (0.8 to 14 mg/100 mL) (125) and black teas (1 to 2.6 mg/100 mL) (126, 127) much lower than previously published, suggesting that tea consumption would not increase kidney stone incidence or recurrence.

Mood

The term "mood" refers to an emotional state of mind that includes aspects like contentedness, relaxation, alertness, energy, and relief from depression, anxiety, and feelings of guilt and failure (128). Clinical depression is described as a mood disorder. An analysis of the NIH-AARP Diet and Health Study (1995-2006) in 263,923 participants — of which 11,311 self-reported depression — found that consumption of decaffeinated (but not caffeinated) hot tea was associated with an increased risk for depression (129). However, smaller cohort studies had previously recorded significantly less depressive symptoms in participants with higher versus lower intakes of tea (130, 131).

Tea consumption may have short-term effects on mood. In a recent cross-sectional study in 95 university staff members, consumption of tea recorded during 10 working days was associated with self reports of feeling less tired and performing better at work (132). Tea was also found to increase the positive valence of mood immediately after consumption in a small randomized controlled study in 150 participants (133).

Cognitive function

Cognitive function includes the domains of attention, memory, processing speed, and executive function, which decline gradually with increasing age.

Cognitive performance

A few studies investigated whether tea consumption was associated with cognitive benefits, especially in the domain of attention (reviewed in 134). Two cross-over, randomized, double-blind, placebo-controlled studies evaluated the effects of two servings of black tea over the course of 60 min (study 1; 26 volunteers) or three servings of black tea over the course of 90 min (study 2; 32 volunteers) on measures of attention and alertness (135). Both studies reported improved performance on objective attention tests and self-reported alertness with black tea compared to placebo. In a small open-label study, 19 participants were asked to consume either black tea (with/without caffeine), coffee (with caffeine), or water (with/without caffeine) before undergoing a battery of psychometric tests (136). Most of the improvements in cognitive function (measured with the Critical Flicker Fusion Threshold [CFFT] task) and subjective alertness were attributed to caffeine in the beverages. In addition, CFFT task scores were greater after consumption of caffeinated tea compared to caffeinated water (136). In a follow-up study, caffeinated tea outperformed caffeinated coffee in the CFFT test, suggesting that tea ingredients other than caffeine might have acute effects on cognitive function (137). A recent meta-analysis of small, randomized controlled trials (<50 participants/trial) that measured the acute effect of L-theanine (36 mg-250 mg) with or without caffeine (40 mg-250 mg) suggested an increase in alertness and attention-switching accuracy but no change regarding other parameters, such as calmness, contentedness, or anxiety (138).

Cognitive decline

The cross-sectional data analysis of 2,501 participants (≥55 years old) in the Singapore Longitudinal Ageing Study (SLAS) indicated that higher intakes of tea correlated with better global cognitive function, as assessed by Mini-Mental State Examination (MMSE) scores (139). Conversely, lower levels of tea consumption were associated with a higher prevalence of cognitive impairments, defined as MMSE scores ≤23. Similar observations have been reported in several other cross-sectional studies (140-143). In the SLAS study, the follow-up of 1,438 cognitively healthy people for one to two years showed that the risk of cognitive decline (defined as a drop of ≥1 point in the MMSE score) was up to 43% lower in tea drinkers compared to non-drinkers (139). Further research in 716 SLAS participants (mean age, 64.5 years) with normal cognitive function confirmed that those consuming tea scored higher in the MMSE global cognition test than non-consumers. Tea consumption was also correlated with higher cognitive test scores regarding memory, executive function, and information-processing speed (144). Tea consumption was also modestly associated with a reduced risk of cognitive decline in 2,722 women (but not in men) followed for a median 7.9 years in the US population-based Cardiovascular Health Study, despite a much lower frequency of tea consumption (up to 5 cups/week) than that observed in the SLAS (over 10 cups/day) (145). In a prospective cohort study in Japanese older people (>60 years old) followed for nearly five years, daily green tea consumers were shown to have lower risks of mild cognitive impairments (MCI) and dementia compared to non-consumers (146). In a recent pilot trial, the consumption of green tea extracts (2 g/day, of which 227 mg of catechins and 42 mg of theanine) for three months resulted in higher MMSE scores (compared to baseline) due to improved short-term memory scores in 12 elderly nursing home residents (ages, 70 to 98 years) with symptoms from MCI to severe dementia (147).

Long-term, large randomized controlled trials are needed to establish whether tea or its bioactive components could limit cognitive decline and/or improve cognitive dysfunction in older individuals.

Parkinson’s disease

Parkinson’s disease (PD) is a neurodegenerative disease characterized by the selective death of dopaminergic brain cells in the substantia nigra. PD is estimated to affect 0.5 to 4% of older people (≥65 years old) worldwide (148). A retrospective study in 279 subjects with PD suggested that the onset of motor symptoms in those drinking more than 3 cups/day of tea was delayed by several years compared to nondrinkers (149); yet, after disease onset, a similar rate of disease progression was observed among tea drinkers and non-drinkers (150). A meta-analysis of eight case-control studies, including 4,250 controls and 1,418 PD cases, found a 15% reduced risk of PD with higher versus lower intake of tea (151). Another meta-analysis of four case-control studies and four prospective cohort studies published between 1999 and 2012 indicated that individuals in the highest category of tea consumption had a 37% lower risk of PD compared to those in the lowest category. The authors estimated that each 2 cups/day-increase in tea consumption was associated with a 26% lower risk of developing PD (152). If a protective effect of tea consumption can be further demonstrated, several bioactive compounds, especially caffeine (153) and flavonoids (154), could be responsible for the tea benefits in PD prevention.

Safety

Adverse effects

Tea

Tea is generally considered to be safe, even in large amounts. However, two cases of hypokalemia (abnormally low serum potassium concentrations) in the elderly have been attributed to excessive consumption of black and oolong tea (3 L/day-14 L/day) (155, 156). Hypokalemia is a potentially life-threatening condition that has been associated with caffeine toxicity (157, 158). Case reports of stomach cramps (159), kidney stones (160), and skeletal fluorosis (161-163) due to excessive tea consumption have also been published.

Tea extracts

In clinical trials employing caffeinated green tea extracts, cancer patients who took 6 g/day, in three to six divided doses, experienced mild-to-moderate gastrointestinal side effects, including nausea, vomiting, abdominal pain, and diarrhea (164, 165). Central nervous system symptoms, including agitation, restlessness, insomnia, tremors, dizziness, and confusion, have also been reported. In one case, confusion was severe enough to require hospitalization (164). In a systematic review published in 2008, the US Pharmacopeia (USP) Dietary Supplement Information Expert Committee identified 34 adverse event reports implicating the use of green tea extract products (containing 25%-97% of polyphenols) as the likely cause of liver damage (hepatotoxicity) in humans (166). Nineteen additional cases of hepatotoxicity associated with the consumption of herbal products containing green tea have been reported for the period 2008-2015 (167). In a four-week clinical trial that assessed the safety of decaffeinated green tea extracts (800 mg/day of EGCG) in healthy individuals, a few of the participants reported mild nausea, stomach upset, dizziness, or muscle pain (168). In the Minnesota Green Tea Trial (MGTT), 1,075 postmenopausal women were randomized to receive green tea extracts (1,315±116 mg/day of catechins; the equivalent of four 8-ounce mugs of brewed decaffeinated green tea) or a placebo for one year. The total number of adverse events and the number of serious adverse events were not different between the treatment and placebo groups (169). However, the use of green tea extracts was directly associated with abnormally high liver enzyme levels in 7 out of the 12 women who experienced serious adverse events. Also, the incidence of nausea was twice as high in the green tea arm as in the placebo group (169).

Pregnancy and lactation

The safety of tea extracts or supplements for pregnant or breast-feeding women has not been established. Some organizations, like the American College of Obstetricians and Gynecologists, suggest to limit caffeine consumption during pregnancy to less than 200 mg/day (170), because higher caffeine intakes have been associated with increased risk of miscarriage and low birth weight in some epidemiological studies (171, 172).

Drug interactions

Green tea

Excessive green tea consumption may decrease the therapeutic effects of the anticoagulant, warfarin (Coumadin, Jantoven). Such an effect was documented in only one patient who began drinking one-half gallon to one gallon of green tea daily (173). It is probably not necessary for people on warfarin therapy to avoid green tea entirely; however, large quantities of green tea may increase the risk of bleeding in warfarin-treated patients (174). Green tea extracts may also reduce the efficacy or increase the toxicity of at least two other cardiovascular drugs, namely simvastatin (Zocor) and nadolol (Corgard) (175). Preclinical studies suggested that green tea extracts may interfere with drug metabolism by affecting the activity of cytochrome P450 3A4 (CYP3A4), which catalyzes the metabolism of about one-half of all marketed drugs in the US and Canada (176). Additional information on drug interactions is available in the article on Flavonoids.

Caffeine

A number of drugs can impair the metabolism of caffeine, increasing the potential for adverse effects from caffeine (177). Such drugs include cimetidine (Tagamet), disulfiram (Antabuse), estrogens, fluoroquinolone antibiotics (e.g., ciprofloxacin, enoxacin, norfloxacin), fluconazole (Diflucan), fluvoxamine (Luvox), mexiletine (Mexitil), riluzole (Rilutek), terbinafine (Lamisil), and verapamil (Calan). High caffeine intakes may increase the risk of toxicity of some drugs, including albuterol (Ventolin), metaproterenol (Alupent), clozapine (Clozaril), ephedrine, stimulant drugs (e.g., epinephrine), monoamine oxidase inhibitors, phenylpropanolamine, and theophylline. High caffeine intakes may also reduce the bioavailability and/or the efficacy of drugs like carbamazepine, valproate, dipyridamole (Persantine), pentobarbital (Nembutal), and phenobarbital (Luminal). Abrupt caffeine withdrawal has been found to increase serum lithium levels in people taking lithium, potentially increasing the risk of lithium toxicity.

Nutrient interactions

Iron

Flavonoids in tea can bind nonheme iron, inhibiting its intestinal absorption (178, 179). Nonheme iron is the principal form of iron in plant foods, dairy products, and iron supplements. The consumption of one cup of tea with a meal has been found to decrease the absorption of nonheme iron in that meal by about 70% (180, 181). Flavonoids can also inhibit intestinal heme iron absorption (182). Interestingly, ascorbic acid (vitamin C) greatly enhances the absorption of iron and is able to counteract the inhibitory effect of flavonoids on nonheme and heme iron absorption (179, 182, 183). To maximize iron absorption from a meal or iron supplements, subjects with poor iron status should not consumed tea at the same time (184). In addition, healthy individuals at no risk of iron deficiency do not need to restrict their consumption of tea (184, 185).


Authors and Reviewers

Written in January 2005 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in January 2008 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in October 2015 by:
Barbara Delage, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in January 2016 by:
Richard Draijer, Ph.D.
Lead Scientist, Unilever R&D
Vlaardingen, The Netherlands

Reviewed in January 2016 by:
Guus Duchateau, Ph.D.
Science Leader, Unilever R&D
Vlaardingen, The Netherlands

Reviewed in January 2016 by:
Suzanne Einöther
Scientist, Unilever R&D
Vlaardingen, The Netherlands

Copyright 2002-2016  Linus Pauling Institute


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