Issue: 37 Page: 38
The Health Benefits of Tea.
HerbalGram. 1996; 37:38 American Botanical Council
Most of the scientific evidence for the health benefits of tea consumption has accumulated over the past 15 years. Many of these studies focus on the polyphenols as the principal and most active health-promoting ingredients in green tea. These substances protect lipids from oxidative degradation, have antibacterial and antiviral action, protect skin from UV damage, are anticarcinogenic and antimutagenic, and may have other beneficial physiological effects as well. While the evidence of these effects, mainly shown in animal and cell culture models, warrants further study in humans, few epidemiologic studies have been done.
The most convincing evidence for protection has been summarized by Mukhtar, Katiyar, and Agarwal (1994). In studies of the anticarcinogenic effects of GTE on mouse skin, each step in the process of conversion to malignancy (DiGiavanni, 1992) was inhibited by the green tea polyphenols, especially by EGCG. Tea protects against chemically induced tumor initiation (Katiyar, et al., 1992a; Huang et al., 1992), promotion (Katiyar, et al., 1992b) both at stages I and II (Katiyar, 1993a), and against the progression of benign tumors to malignancy (Wang, et al., 1992a; Katiyar, et al., 1993b). It is radioprotective (Agarwal, et al., 1993; Wang, 1992b; Uchida, et al., 1992), especially against UVB-induced carcinogenesis. (UVB, normally in sunlight, is a major skin cancer risk factor in humans (Elmets, 1991)). It also reduces, if not prevents, UVB-induced inflammatory responses and depletion of epidermal anti-oxidant defenses (Agarwal, et al., 1993). Black tea, even when decaffeinated, giv es a similar level of radioprotection to murine skin (Wang, et al., 1994).
With the use of mouse models of chemically (usually nitrosamine) induced lung and stomach cancer, oral administration of green tea polyphenols reduces both tumor incidence and multiplicity (Xu, et al., 1992; Wang, 1992c; Katiyar, 1993c). Similar results are found using tumor-bearing rats (Yamane, et al., 1995). EGCG, when given perorally to mice, is shown to inhibit lung metastasis of experimental and spontaneous tumors (Taniguchi, et al., 1992).
Oral administration of EGCG or GTE (0.002-1.25 percent) to mice or rats in which tumors of the esophagus (Chen, 1992), duodenum (Fujita, et al., 1989), small intestine (Hirose, et al., 1993), large intestine (Yin, et al., 1994), colon (Narisawa, et al., 1993), mammary gland (Hirose, et al., 1994), or liver (Hirose, et al., 1993) can be chemically induced, proves to have a potent inhibitory effect on carcinogenesis.
The results of human studies are less clear. The Working Group of the International Agency for Research on Cancer studied the available epidemiologic evidence for an association between tea consumption and cancer and concluded it was inconsistent and inconclusive (IARC, 1991). Recent studies have shown that tea drinking reduces the risk of esophageal cancer among Shanghai women (Gao, et al., 1994), of cancer of the mouth in northern Italians (Franceschi, et al., 1992), of gastric cancer among Swedish adolescents (Hansson, et al., 1993), of pancreatic cancer among the elderly of Opole, Poland (Zatonski, et al., 1993) and residents of a retirement community in the U.S. (Shibata, et al., 1994), and of colon cancer among retired male self-defense officials in Japan (Kono, et al., 1991). No association is found with gastric cancer in Spain (Agudo, et al., 1992) or with esophageal cancer in the U.S. (Brown, et al., 1995). On the other hand, a positive risk of esophageal cancer is foun d in Kashmir where it is the practice to drink boiling hot and salty tea (Kumar, et al., 1992; Dhar, et al., 1993). (Of course, harm cannot be ruled out from drinking tea that is too strong, hot, or salty.) These recent studies may still be said to be inconsistent and inconclusive; but most of them now conclude that tea is a chemopreventative in humans. Perhaps this conclusion might be more convincing if the studies on which it is based were not confounded by variables associated with the tea-drinking habit. (Schwarz, et al., 1994).
Chemical carcinogens alter the cell's genetic material, i. e., are mutagens. It is widely believed that carcinogenesis begins with exposure to dietary, occupational, or environmental substances that are mutagenic. Green tea polyphenols are antimutagenic (Wang, et al., 1989; Bu Abbas, et al., 1994b); they lower the formation of heterocyclic amines (carcinogens formed during the cooking of meats and fish) (Weisburger, et al., 1994); and they reduce the occurrence of chromosome aberrations during mutagen exposure (Sasaki, et al., 1990; Sasaki, et al., 1993).
Atherosclerosis and Hypertension
The second important target of preventive medicine is cardiovascular disease. It has been suggested that lipid peroxidation, especially oxidation of low density lipoprotein (LDL), is critical to the development of atherosclerosis in humans (Steinberg, et al., 1989). The tea polyphenols are known to block the test tube oxidation of LDL by cupric (copper) ions (Miura, et al., 1995) and to inhibit lipid peroxidation in a mouse model of sunlight-induced skin cancer (Katiyar, et al., 1994).
GTE prevents the increase of serum cholesterol in mice fed an atherogenic diet but has no effect on their serum triglyceride or high density lipoprotein (HDL) levels (Yamaguchi, et al., 1991). This pattern is seen also in the serum lipid profiles of tea-drinking humans (Kono, et al., 1992). However, a recent cross-sectional study (Imai, 1995) shows that increased green tea consumption, especially more than 10 cups per day, is not only associated with decreased total serum cholesterol but also decreased LDL, very low density lipoproteins and triglycerides, increased HDL, and reduced atherogenic index. This study is also the first one to report that green tea consumption causes a decrease in serum markers of liver damage.
The Zutphen Elderly Study in the Netherlands investigated the risk of coronary heart disease (CHD) in elderly men who consumed flavonoids from all dietary sources. These sources included tea, although probably not green tea. The study concludes that flavonoids in regularly consumed foods may reduce the risk of death from CHD (Hertog, et al., 1993).
In addition, the polyphenols of green tea appear to reduce chronic psychosocial hypertension in mice (Henry, et al., 1984), possibly through the sedative active of the brain neurotransmitter, gamma-aminobutyric acid (GABA). (GABA-rich tea is made by fermenting green tea leaves under nitrogen.) The drinking of GABA-rich green tea by salt-sensitive, hypertensive rats significantly lowers their blood pressure (Abe, et al., 1995). In a study of Japanese women in the city of Sendai, no relation between hypertension and green tea drinking is seen. Nevertheless, the same study concludes that medical history of stroke in this group varies inversely with increasing green tea intake (Sato, et al., 1989).
The antibacterial activity of tea catechins has a broad range and is particularly useful for treatment of the diarrheal disease, e. g., cholera and typhus (Shetty, et al., 1994). Results show that the bactericidal catechins (EGCG and EC) primarily act on and damage bacterial membranes (Ikigai, 1993).
Green tea can also be protozoacidal (Ryu, 1982) and virucidal. Many viruses are adversely affected by the presence of the tea catechins, including influenza (Nakayama, et al., 1993) and even HIV (Nakane & Ono, 1990); but tea's important in the treatment of human viral infection has yet to be proven.
Immune Response Stimulator
There are a few studies that show how green tea polyphenols might affect the immune system. EGCG appears to stimulate mouse splenic B-cell proliferation and the galloyl group appears to be responsible for this effect (Hu, et al., 1992).
The polyphenolic compounds may protect teeth from decay by killing the causative bacteria (Horiba, et al., 1991, Otake, et al., 1991). However, they could protect against caries in two other ways: 1) by inhibiting the collagenase activity of bacteria that lodge in fluid below the gum line (Makimura, et al., 1993), and 2) by increasing the acid resistance of human tooth enamel (Yu, et al., 1995).
The effect of green tea drinking on life extension is not well studied. The recent medical literature contains but one example. This study looks at mortality among female practitioners of Chanyou (Japanese "tea ceremony") and concludes that green tea may be a protective factor for several fatal diseases (Sadakata, et al., 1992).
Article copyright American Botanical Council.