The Cancer Research
Inconsistent results in studies of tea and cancer risk may reflect differences we don’t yet fully understand related to the type of tea and how it’s prepared. Moreover, individual differences in microbes of the gut microbiota that convert compounds in tea to other compounds that can be absorbed, and genetic differences in handling these compounds add complexity. And interaction with other foods and with lifestyle choices (such as smoking and drinking alcohol) could also affect whether a relationship between tea and cancer risk is identified in a study.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how tea and its nutrients affect the risk of developing cancer.
- Evidence categorized as “convincing” or “probable” means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
- A convincing or probable judgment is strong enough to justify recommendations.
- Evidence categorized as “limited suggestive” means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce the risk of cancer.
- Limited evidence suggests that tea may DECREASE the risk of:
- Bladder cancer
- Tea, and green tea, in particular, was discussed by the AICR/WCRF Continuous Update Project (CUP) Expert Panel for multiple other cancers, but the evidence was too limited to draw a conclusion.
Ongoing Areas of Investigation
- Laboratory Research
Most lab research has focused on green tea and EGCG, its most active compound. Black tea also contains EGCG. In cell and animal studies, EGCG is a powerful antioxidant. Thearubigins and theaflavins (other flavan-3-ols that are higher in black tea) also act as antioxidants in lab studies. However, since the body breaks down these compounds and forms others, such tests don’t demonstrate the effects when you consume them in tea.
- Tea polyphenols also inhibit the development of various types of cancer in cell and rodent studies through other mechanisms, including changing the expression of tumor suppressor and other genes. EGCG can act throughout the cancer process: stimulating enzymes that deactivate carcinogens, decreasing tumor growth, increasing self-destruction of cancer cells, and restraining the spread of cancer cells.
- As in many lab studies that focus on individual compounds, studies often use amounts of EGCG that are far higher than people typically get from drinking tea. This practice increases researchers’ ability to detect a potential effect.
- Animal models have shown that polyphenols found in green tea inhibited the growth of bladder cancer tumors. EGCG or blends of green tea polyphenols also decrease the development and spread of lung and prostate cancer in studies with rodents.
Caffeine speeds the passage of any carcinogens through the digestive tract, reducing the time the colon is exposed to these substances. Cell studies show caffeine influences cell signaling to decrease colorectal cancer development.
- Human Studies
Some population studies look at green and black tea separately, but others look at total tea consumption.
The AICR Third Expert Report identifies limited evidence that tea may reduce risk of bladder cancer. The Expert Panel for the AICR Third Expert Report categorized evidence for other cancers — including mouth, pharynx and larynx; nasopharynx; lung; stomach; pancreas; gallbladder; liver; colorectum; breast; ovary; endometrium; prostate; and kidney — as too limited to draw a conclusion.
- Some observational studies of large groups of people show reduced cancer risk with higher consumption of tea or green tea. However, results tend to be inconsistent. Often, study types that are more prone to bias show an association with tea, but stronger study designs (prospective cohort studies) do not.
- Some small controlled intervention trials of green tea preparations have shown promise in raising the ability of cells to have antioxidant activity among people exposed to oxidative stress, and in decreasing the development of prostate cancer in men at greater risk, but studies as a whole have not been successful.
- Inconsistent results in population studies may reflect differences we don’t yet understand related to the type of tea and how it’s prepared. Genetic differences in handling these compounds and interaction with other lifestyle choices (such as smoking and drinking alcohol) could also affect whether a relationship between tea and cancer risk is identified in a study.
When reduced cancer risk is seen in human observational studies and intervention trials, it often involves consumption of three to six cups per day. In Japanese population studies, reduced risk may relate to consumption of 10 cups per day; but here standard cup size is only 4 fluid ounces,11 so this is quite different than 10 cups or mugs in the U.S.
- Tips for Selection, Storage and Preparation
You can choose from four categories of tea that each have a distinct appearance and flavor:
- Green tea has a delicate flavor and its color resembles the fresh tea leaf. In processing, the leaves are not fermented (oxidized).
- Oolong tea’s flavor is halfway between green and black tea. The leaves ferment for about 2 hours creating a darker color, but still slightly delicate flavor.
- Black tea has a darker color and stronger flavor that come from the oxidized phytochemicals such as thearubigins formed as tea leaves ferment. Leaves are oxidized for 3-4 hours.
- White tea is a more expensive tea that is least processed of all. Its flavor is very delicate and slightly sweet. EGCG content (though not necessarily total antioxidant content) is even higher than green tea’s.
Ready-to-drink bottled tea is convenient, but it often contains added sugar, so read the label to find the unsweetened versions. Although it is much lower in polyphenols and total antioxidants than brewed tea, unsweetened bottled tea is a refreshing way to replace sugary drinks.
Black tea has one-third to one-half the caffeine of coffee and green tea’s content is even lower. Decaffeinated teas have only a trace and although they lose at least half of their polyphenols, decaf teas still contain good amounts.
To retain fresh flavor, store tea in an airtight container in a dark, dry cabinet away from heat.
- Cassidy A, Minihane A-M. The role of metabolism (and the microbiome) in defining the clinical efficacy of dietary flavonoids. Am J Clin Nutr. 2016;105(1):10-22.
- Yang CS, Chen JX, Wang H, Lim J. Lessons Learned from Cancer Prevention Studies with Nutrients and Non-Nutritive Dietary Constituents. Mol Nutr Food Res. 2016;60(6):1239-1250.
- de Melo FHM, Oliveira JS, Sartorelli VOB, Montor WR. Cancer Chemoprevention: Classic and Epigenetic Mechanisms Inhibiting Tumorigenesis. What Have We Learned So Far? Frontiers in oncology. 2018;8:644-644.
- Yang CS, Wang H. Cancer Preventive Activities of Tea Catechins. Molecules. 2016;21(12):1679.
- World Cancer Research Fund/American Institute of Cancer Research. Continuous Update Project Expert Report 2018. Non-alcoholic drinks and the risk of cancer. Available at dietandcancerreport.org.
- Najaf Najafi M, Salehi M, Ghazanfarpour M, Hoseini ZS, Khadem-Rezaiyan M. The association between green tea consumption and breast cancer risk: A systematic review and meta-analysis. Phytotherapy Research. 2018;32(10):1855-1864.
- Livingstone TL, Beasy G, Mills RD, et al. Plant Bioactives and the Prevention of Prostate Cancer: Evidence from Human Studies. Nutrients. 2019;11(9):2245.
- Ellinger S, Muller N, Stehle P, Ulrich-Merzenich G. Consumption of green tea or green tea products: Is there an evidence for antioxidant effects from controlled interventional studies? Phytomedicine. 2011.
- Boehm K, Borrelli F, Ernst E, et al. Green tea (Camellia sinensis) for the prevention of cancer. Cochrane Database Syst Rev. 2009(3):CD005004.
- Henning SM, Wang P, Heber D. Chemopreventive effects of tea in prostate cancer: Green tea versus black tea. Mol Nutr Food Res. 2011;55(6):905-920.
- Fujiki H, Watanabe T, Sueoka E, Rawangkan A, Suganuma M. Cancer Prevention with Green Tea and Its Principal Constituent, EGCG: from Early Investigations to Current Focus on Human Cancer Stem Cells. Mol Cells. 2018;41(2):73-82.