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).

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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 per kilogram [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 osteomalacia and 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.

Epidemiologic Studies

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.