Health Benefits of Tea
By Yoon Hang Kim, MD, MPH and Jeanne Bowers; Dr. Kim is the Director, Georgia Integrative Medicine, Atlanta, and Ms. Bowers is completing a master's degree in Psychology, University of West Georgia; they both report no financial relationship relevant to this field of study.
Tea is a popular form of beverage made from the dried leaves of the plant Camellia sinensis. Popularity of tea consumption worldwide is second only to water.1 About three billion kilograms of tea are produced each year.2 Tea is cultivated in some 30 countries worldwide, and is consumed globally.3 Black tea is consumed predominantly in Western and some Asian countries, and green tea is consumed predominantly in China, Japan, India, and a number of countries in North Africa and the Middle East.4
Tea originated in China 5000 years ago, and was used as a medicine for various illnesses.5 The tea was stored in the form of powder, leaves, and cakes. Small pieces would be broken off the cakes and steeped in water. Traditional Chinese Medicine recommended drinking tea to healthy people as early as 200 BC.6 While people have been enjoying tea consumption more than 5000 years, the possible beneficial health effects of tea are being investigated only recently.
There are many types of tea, but they can be divided into two major groups: green tea and black tea. It is estimated that about 78% of tea produced worldwide is black tea, while 20% is green tea and 2% is oolong tea.
Tea leaves contain the enzyme polyphenol oxidase which oxidizes the chemical contents of tea. In green tea, polyphenol oxidase is inactivated by steaming the tea leaves, briefly followed by drying the steamed tea leaves. Green tea contains a variety of polyphenolic compounds, including epigallocatechin-3gallate (EGCG), epigallocatechin (EGC), epicatechin-3-gallate (ECG), and epicatechin (EC).7 These compounds are collectively referred to as catechins. EGCG is the most abundant catechin and has received, by far, the most attention. In contrast, black tea is produced by crushing the tea leaves and allowing them to oxidize by polyphenol oxidase, leading to polymerization of catechins, resulting in theaflavins and thearubigens.7 Theaflavins are key to the characteristic color and taste of black tea. The health-beneficial chemical constituents are often collectively referred to as flavonoids.
Initial epidemiological surveys have associated tea drinking with reduced risk of cardiovascular diseases (CVD) and cancer.8
Many epidemiological studies have investigated the effects of tea consumption on CVD.9 The results of several, but not all, of these studies are suggestive of a protective effect of black tea. Peters and colleagues performed a meta-analysis of tea consumption in relation to myocardial infarction.10 The analysis included 10 cohort and seven case-control studies. The incidence rate of myocardial infarction was estimated to decrease by 11%, with an increase in tea consumption of three cups per day.
As stated earlier, tea is a rich source of flavonoids. Epidemiologic studies have also explored the relationships between intake of flavonoids derived from tea, fruits, vegetables, and red wine and CVD. Huxeley and Neil performed a meta-analysis of seven prospective studies of flavonoids in relation to coronary heart disease. The highest consumption of flavonoid intake was associated with a 20% reduction in the risk of fatal coronary heart disease compared with the lowest consumption of intake.11
There is an assumption that tea intake and flavonoid intake play a protective role against cardiovascular disease. However, a causative link has not been established. The potential for confounding effects need to be kept in mind when interpreting this information.
One of the proposed mechanisms for the possible protective effect of tea against cardiovascular diseases is that tea polyphenols inhibit the oxidation of LDL, which is known to be involved in the development of atherosclerosis.12
Another mechanism is the hypocholesterolemic effect of tea, which provides another layer of protection from CVD. In animals fed diets high in fat and cholesterol, green tea, black tea, and tea polyphenols prevented elevations in serum and liver lipids, decreased serum total cholesterol, or atherogenic index, and increased fecal excretion of total lipids and cholesterol.13 When hamsters were fed a high fat diet, those drinking green tea or green tea polyphenols had lower serum total cholesterol and higher fecal fat excretions than the control group.14
Recently, there has been a focus on looking at more complex mechanisms involving endothelial function. Endothelial dysfunction is characterized by the loss of normal endothelium-dependent vasodilation. The development of endothelial dysfunction has been linked to the pathogenesis of cardiovascular disease.15 The results of in vitro studies provide evidence for direct effects of flavonoids present in tea on endothelial function.16
Health benefits of tea in cancer have been observed in epidemiologic studies. Bushman reviewed 31 studies and concluded that a protective effect was associated between tea consumption and cancer of the colon, urinary bladder, stomach, esophagus, lung, and pancreas.17 This protective effect was most prominent in Japanese women drinking more than 10 cups of green tea per day.18 Most of the literature regarding tea consumption for prevention of cancer utilized green tea.
Polyphenols found in green tea have been shown to have potent antioxidant and antitumor effects.19 The most widely recognized properties of tea polyphenols are their antioxidant activities. Polyphenols bind to metal ions, preventing them from participating in peroxidase reactions. Green and black tea, and isolated tea polyphenols, have been shown to scavenge reactive oxygen and nitrogen species, reducing their damage to lipid membranes, proteins, and nucleic acids in cell-free systems.20
It should be noted that despite strong evidence suggesting a link between antioxidant activity and anticancer effects, regulatory organizations have been cautious to allow any health claims linking intake of antioxidants to anticancer benefit.21
The FDA has allowed a qualified health claim for antioxidant vitamins and cancer.22 The FDA concluded that despite the scientific evidence for antioxidant vitamins C and E reducing the risk of certain forms of cancer, this evidence is limited and not conclusive.
Cutter and colleagues proposed that the anticancer protective effects of green tea is not limited to the antioxidants, and may result from not yet discovered chemical properties.23 Biochemical elucidation of the pathway is not clear but below summarize the benefits of green tea.24
- Enhance antioxidant and phase II enzyme activities for liver detoxification;
- Inhibition of chemically induced lipid peroxidation preventing oxidative damages;
- Inhibition of irradiation and induced cyclooxygenase activities for reducing inflammation; and
- Inhibition of protein kinase C and cellular proliferation
- Enhancement of gap junction intercellular communication.
Yang and colleagues demonstrated that green tea reduces the proliferation rate and induces apoptosis of cancer cells.25 In addition, a recent study has shown that green tea decreases the levels of two important angiogenic factors, vascular endothelial growth factor and basic and acidic fibroblast growth factors.26 Suzuki and colleagues reported that increased consumption of green tea was correlated with decreased recurrence of stage I and II breast cancer.27
A study of 1256 women in the United Kingdom reported that tea drinkers had significantly greater mean bone mineral density measurements independent of smoking status, the use of hormone therapy, coffee drinking, and whether milk was added to the tea.28 Authors concluded that drinking tea may help protect against osteoporosis in older women.
There exists an abundance of epidemiological and basic science research of the possible benefits of both black tea & green tea for cardiovascular prevention. The evidence for prevention of cancer with green tea is more speculative.
Given that tea is popular beverage, people should consider incorporating tea to their nutritional regimen. Many Asian cultures have integrated tea into healing ceremony and have recognized its value. For patients who want a milder tasting green tea, Gen Mai (roasted brown rice) tea is a good start. It is also recommended that whenever possible, organic green tea should be considered over non-organic counter part.
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22. FDA Docket No. 91N-0101, April 1, 2003 Enforcement Discretion Letter.
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25. Yang CS, et al. Tea and tea polyphenols inhibit cell hyperproliferation, lung tumorigenesis, and tumor progression. Exp Lung Res. 1998;24:629-639.
26. Sartippour MR, et al. Inhibition of fibroblast growth factors by green tea. Int J Oncol. 2002;21:487-491.
27. Inoue M, et al. Regular consumption of green tea and the risk of breast cancer recurrence: Follow-up study from the Hospital-based Epidemiologic Research Program at Aichi Cancer Center (HERPACC), Japan. Cancer Lett. 2001;167:175-182.
28. Hegarty VM, et al. Tea drinking and bone mineral density in older women. Am J Clin Nutr. 2000;71: 1003-1007.