Tea and Cancer Prevention: Strengths and Limits of the Evidence
Tea and Cancer Prevention: Strengths and Limits of the Evidence
What is tea?
Tea is one of the most ancient and popular beverages consumed around the world. Black tea accounts for about 75 percent of the world’s tea consumption (1). In the United States, United Kingdom (UK), and Europe, black tea is the most common tea beverage consumed; green tea is the most popular tea in Japan and China (2). Oolong and white tea are consumed in much lesser amounts around the world (2).
Tea is made from the leaf of the plant Camellia sinensis. Shortly after harvesting, tea leaves begin to wilt and oxidize. During oxidation, chemicals in the leaves are broken down by enzymes, resulting in darkening of the leaves and the well-recognized aroma of tea. This oxidation process can be stopped by heating, which inactivates the enzymes. The amount of oxidation and other aspects of processing determine a tea’s type. Black tea is produced when tea leaves are wilted, bruised, rolled, and fully oxidized. In contrast, green tea is made from unwilted leaves that are not oxidized. Oolong tea is made from wilted, bruised, and partially oxidized leaves, creating an intermediate kind of tea. White tea is made from young leaves or growth buds that have undergone minimal oxidation. Dry heat or steam can be used to stop the oxidation process, and then the leaves are dried to prepare them for sale.
Tea is brewed from dried leaves and buds (either in tea bags or loose), prepared from dry instant tea mixes, or sold as ready-to-drink iced teas. So-called herbal teas are not really teas but infusions of boiled water with dried fruits, herbs, and/or flowers.
What are the ingredients of tea?
Tea is composed of polyphenols, alkaloids (caffeine, theophylline, and theobromine), amino acids, carbohydrates, proteins, chlorophyll, volatile organic compounds (chemicals that readily produce vapors and contribute to the odor of tea), fluoride, aluminum, minerals, and trace elements (3). The polyphenols, a large group of plant chemicals that includes the catechins (4), are thought to be responsible for the health benefits that have traditionally been attributed to tea, especially green tea. The most active and abundant catechin in green tea is epigallocatechin-3-gallate (EGCG). The active catechins and their respective concentrations in green tea infusions are listed in the table below.
Catechin Concentrations of Green Tea Infusions
Catechin in Green Tea Infusion Catechin Concentration
(mg/8 fl oz)*
Epigallocatechin-3-gallate (EGCG) 117–442 25–106 Epigallocatechin (EGC) 203–471 49–113 Epicatechin-3-gallate (ECG) 17–150 4–36 Epicatechin (EC) 25–81 6–19
*mg = milligram; L = liter; fl oz = fluid ounce. See reference 5.
Black tea contains much lower concentrations of these catechins than green tea (6). The extended oxidation of black tea increases the concentrations of thearubigins and theaflavins, two types of complex polyphenols (2). Oolong tea contains a mixture of simple polyphenols, such as catechins, and complex polyphenols (2). White and green tea contain similar amounts of EGCG but different amounts of other polyphenols (7).
Although iced and ready-to-drink teas are becoming popular worldwide, they may not have the same polyphenol content as an equal volume of brewed tea (8). The polyphenol concentration of any particular tea beverage depends on the type of tea, the amount used, the brew time, and the temperature (3). The highest polyphenol concentration is found in brewed hot tea, less in instant preparations, and lower amounts in iced and ready-to-drink teas (3). As the percentage of tea solids (i.e., dried tea leaves and buds) decreases, so does the polyphenol content (9). Ready-to-drink teas frequently have lower levels of tea solids and lower polyphenol contents because their base ingredient may not be brewed tea (10). The addition of other liquids, such as juice, will further dilute the tea solids (9). Decaffeination reduces the catechin content of teas (11).
Dietary supplements containing green tea extracts are also available (1). In a U.S. study that evaluated 19 different green tea supplements for tea catechin and caffeine content, the product labels varied in their presentation of catechin and caffeine information, and some values reported on product labels were inconsistent with analyzed values (1).
How might tea help prevent cancer?
Among their many biological activities, the predominant polyphenols in green tea―EGCG, EGC, ECG, and EC―and the theaflavins and thearubigins in black teas have antioxidant activity (12). These chemicals, especially EGCG and ECG, have substantial free radical scavenging activity and may protect cells from DNA damage caused by reactive oxygen species (12). Tea polyphenols have also been shown to inhibit tumor cell proliferation and induce apoptosis in laboratory and animal studies (1, 13). In other laboratory and animal studies, tea catechins have been shown to inhibit angiogenesis and tumor cell invasiveness (14). In addition, tea polyphenols may protect against damage caused by ultraviolet (UV) B radiation (13, 15), and they may modulate immune system function (16). Furthermore, green teas have been shown to activate detoxification enzymes, such as glutathione S-transferase and quinone reductase, that may help protect against tumor development (16). Although many of the potential beneficial effects of tea have been attributed to the strong antioxidant activity of tea polyphenols, the precise mechanism by which tea might help prevent cancer has not been established (13).
Are there safety considerations regarding tea consumption?
Tea as a food item is generally recognized as safe by the U.S. Food and Drug Administration. Safety studies have looked at the consumption of up to 1200 mg of EGCG in supplement form in healthy adults over 1- to 4-week time periods (17, 18). The adverse effects reported in these studies included excess intestinal gas, nausea, heartburn, stomach ache, abdominal pain, dizziness, headache, and muscle pain (17, 18). In a Japanese study, children aged 6 to 16 years consumed a green tea beverage containing 576 mg catechins (experimental group) or 75 mg catechins (control group) for 24 weeks with no adverse effects (19). The safety of higher doses of catechins in children is not known.
As with other caffeinated beverages, such as coffee and colas, the caffeine contained in many tea products could potentially cause adverse effects, including tachycardia, palpitations,insomnia, restlessness, nervousness, tremors, headache, abdominal pain, nausea, vomiting, diarrhea, and diuresis (20). However, there is little evidence of health risks for adults consuming moderate amounts of caffeine (about 300 to 400 mg per day). A review by Health Canada concluded that moderate caffeine intakes of up to 400 mg per day (equivalent to 6 mg perkilogram [kg] body weight) were not associated with adverse effects in healthy adults (21). The amount of caffeine present in tea varies by the type of tea; the caffeine content is higher in black teas, ranging from 64 to 112 mg per 8 fl oz serving, followed by oolong tea, which contains about 29 to 53 mg per 8 fl oz serving (4). Green and white teas contain slightly less caffeine, ranging from 24 to 39 mg per 8 fl oz serving and 32 to 37 mg per 8 fl oz serving, respectively (22). Decaffeinated teas contain less than 12 mg caffeine per 8 fl oz serving (22). Research on the effects of caffeine in children is limited (20). In general, caffeine doses of less than 3.0 mg per kg body weight have not resulted in adverse effects in children (20). Higher doses have resulted in some behavioral effects, such as increased nervousness or anxiety and sleep disturbances (21).
Aluminum, a neurotoxic element, is found in varying quantities in tea plants. Studies have found concentrations of aluminum (which is naturally taken up from soil) in infusions of green and black teas that range from 14 to 27 micrograms per liter (μg/L) to 431 to 2239 μg/L (4). The variations in aluminum content may be due to different soil conditions, different harvesting periods, and water quality (4). Aluminum can accumulate in the body and cause osteomalaciaand neurodegenerative disorders, especially in individuals with renal failure (4). However, it is not clear how much of the aluminum in tea is bioavailable, and there is no evidence of any aluminum toxicity associated with drinking tea (4).
Black and green tea may inhibit iron bioavailability from the diet (4). This effect may be important for individuals who suffer from iron-deficiency anemia (4). The authors of a systematic review of 35 studies on the effect of black tea drinking on iron status in the UK concluded that, although tea drinking limited the absorption of non-heme iron from the diet, there was insufficient evidence to conclude that this would have an effect on blood measures (i.e., hemoglobin and ferritin concentrations) of overall iron status in adults (23). However, among preschool children, statistically significant relationships were observed between tea drinking and poor iron status (23). The interaction between tea and iron can be mitigated by consuming, at the same meal, foods that enhance iron absorption, such as those that contain vitamin C (e.g., lemons), and animal foods that are sources of heme iron (e.g., red meat) (4). Consuming tea between meals appears to have a minimal effect on iron absorption (4).
What evidence from human studies links tea to cancer prevention?
Tea has long been regarded as an aid to good health, and many believe it can help reduce the risk of cancer. Most studies of tea and cancer prevention have focused on green tea (13). Although tea and/or tea polyphenols have been found in animal studies to inhibit tumorigenesis at different organ sites, including the skin, lung, oral cavity, esophagus, stomach, small intestine, colon, liver, pancreas, and mammary gland (24), the results of human studies—both epidemiologic and clinical studies—have been inconclusive.
More than 50 epidemiologic studies of the association between tea consumption and cancer risk have been published since 2006. The results of these studies have often been inconsistent, but some have linked tea consumption to reduced risks of cancers of the colon, breast, ovary, prostate, and lung (6, 25–57). The inconsistent results may be due to variables such as differences in tea preparation and consumption, the types of tea studied (green, black, or both), the methods of tea production, the bioavailability of tea compounds, genetic variation in how people respond to tea consumption, the concomitant use of tobacco and alcohol, and other lifestyle factors that may influence a person’s risk of developing cancer, such as physical activity or weight status.
Several clinical trials have investigated the role of tea and tea polyphenols in cancer prevention (58–66). However, few trials have examined the effects of tea or tea polyphenols on cancerincidence or mortality.
Two randomized trials evaluated the effects of tea extracts on premalignant oral lesions (58, 59). One of the trials was a double-blind interventional trial involving 59 people with leukoplakia, which is a putative precursor lesion for oral cancer (58). The trial’s participants were randomly assigned to receive either 3 grams of a mixed tea product, given both orally and topically, or aplacebo. After 6 months, 38 percent of the participants in the treatment group had partialregression of their oral lesions compared with 10 percent of the participants in the placebo group. In addition, fewer participants in the treatment group than in the placebo group had an increase in lesion size (3 percent in the treatment group versus 7 percent in the placebo group). Furthermore, mucosal cell proliferation decreased in the treatment group, suggesting a possible protective effect of tea on the development of oral cancer. In contrast, in the second trial, 39 people with high-risk premalignant oral lesions were randomly assigned to receive one of three doses of a green tea extract—500 mg per square meter of body surface area (mg/m2), 750 mg/m2, or 1000 mg/m2—or a placebo three times daily for 12 weeks (59). At the end of the trial, no differences in lesion responses or histology were found between the groups.
Two other randomized trials examined the effects of tea on urine levels of 8-hydroxydeoxyguanosine (8-OHdG), a biomarker of oxidative DNA damage that may be a predictor of increased cancer risk. Urinary 8-OHdG levels are higher in individuals with lung cancer than in control subjects, and human breast, lung, liver, kidney, brain, stomach, and ovarian tumor tissue has a higher content of 8-OHdG than adjacent nontumor tissue (60). In one trial, 133 adult heavy smokers were randomly assigned to drink 4 cups of one of the following beverages each day for 4 months: decaffeinated green tea, decaffeinated black tea, or water (60). Among those who drank green tea, there was a statistically significant 31 percent decrease in urinary levels of 8-OHdG; in the black tea group, there was no change in urinary 8-OHdG levels (60). In the second trial, 124 individuals at increased risk of liver cancer due to hepatitis B virus infection and aflatoxin exposure took a placebo or 500 mg or 1000 mg of a green tea polyphenol supplement daily (61). The two supplement doses were reported to be equivalent to 2 or 4 cups, respectively, of green tea infusions. No other tea or tea products were consumed. Compared with those in the placebo group, individuals who took the green tea supplement at either dose for 3 months had substantially lower urinary 8-OHdG levels (61). Although these trials indicate that green tea polyphenols from tea or supplements can reduce urinary 8-OHdG levels, it is unclear if reduced 8-OHdG levels are associated with reduced cancer risk.
Additional trials have investigated whether green tea catechins or green tea extracts alter prostate cancer risk. In a double-blind, placebo-controlled study, 60 men took 200 mg of green tea catechin or a placebo three times daily for 1 year (62). These men had high-grade prostatic intraepithelial neoplasia, which is thought to be a precursor of prostate cancer. After 1 year, fewer prostate cancers were detected in the green tea catechin group (1 cancer in 30 men) compared with the placebo group (9 cancers in 30 men) (62). Two other clinical trials, bothuncontrolled studies, investigated the use of green tea extracts to reduce prostate-specific antigen levels in men with prostate cancer and found no evidence of such a reduction (63, 64).
Another trial examined the effect of tea polyphenols on serum pepsinogen levels in 163 individuals with high serum pepsinogen levels (65). Serum pepsinogen is a biomarker of gastric atrophy and an indicator of increased risk for stomach cancer. The participants in this trial were given either one or six 100-mg capsules of tea polyphenols daily for 1 year. Each capsule was the equivalent of about 1.7 cups of tea. After 1 year, no decrease in serum pepsinogen levels was observed in either treatment group (65).
In yet another trial, a possible role for green tea supplements in treating precancerous lesions of the esophagus was investigated (66). In the trial, 200 Chinese participants with such lesions were treated with 5 mg of a decaffeinated green tea extract daily or a placebo. After 12 months, lesion histopathology was scored as improved, unchanged, or deteriorated. The trial found no difference between the treatment and placebo groups with regard to changes in the esophageal lesions or in abnormal cell proliferation (66).
Does the National Cancer Institute (NCI) recommend the use of tea to prevent cancer?
NCI is a research institution. It develops evidence-based research results for others to interpret. In general, therefore, NCI does not make recommendations about specific medical or dietary interventions.
Moreover, as noted above (see Question 5), the evidence regarding the potential benefits of tea consumption in relation to cancer is inconclusive at present.
Source : http://www.cancer.gov/cancertopics/causes-prevention/risk-factors/diet/tea-fact-sheet