Green tea (Camellia sinensis) is the second-most consumed beverage in the world (water is the first) and has been used medicinally for centuries in India and China. A number of beneficial health effects are attributed to regular consumption of green tea and dried/powdered extracts of green tea are available as dietary supplements. Green tea is prepared by picking, lightly steaming the leaves, and allowing them to dry. Black tea, the most popular type of tea in the U.S., is made by allowing the leaves to ferment before drying. Due to differences in the fermentation process, a portion of the active compounds are destroyed in black tea, but remain active in green tea. The active constituents in green tea are a family of polyphenols (catechins) with potent antioxidant activity. Tannins, large polyphenol molecules, form the bulk of the active compounds in green tea, with catechins comprising nearly 90%. Several catechins are present in significant quantities; epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG) and epigallocatechin gallate (EGCG). EGCG makes up about 10-50% of the total catechin content and appears to be the most powerful of the catechins – with antioxidant activity about 25-100 times more potent than vitamins C and E. A cup of green tea may provide 10-40mg of polyphenols and has antioxidant activity greater than a serving of broccoli, spinach, carrots or strawberries. A number of commercial green tea extracts are standardized to total polyphenol content and/or EGCG content and many are marketed with claims for preventing cancer, enhancing immune function, boosting antioxidant protection, reducing cholesterol, and stimulating weight loss.
Green tea consumed either as a beverage or as a daily dietary supplement is especially beneficial for individuals at high risk for cancer (e.g. family history) or those undergoing or recovering from chemotherapy or radiation treatment. Green tea is also beneficial as a general protective measure and dietary “insurance” of adequate polyphenol intake (which would otherwise be obtained from a diet high in fruits and vegetables). Recent data provides strong evidence that green tea may be effective in stimulating thermogenesis, increasing caloric expenditure, promoting fat oxidation and controlling body weight.
Because the active compounds, the catechins, found in green tea are known to possess potent antioxidant activity, they may provide beneficial health effects by protecting the body from the damaging effects of oxidative damage from free radicals. A number of chronic disease states have been associated with free radical induced oxidative damage, including cancer, heart disease, suppressed immune function and accelerated aging.
Although numerous laboratory investigations have shown the powerful antioxidant activity of green tea and green tea extracts (August et al. 1999, Benzie et al. 1999), prospective clinical studies in humans are few (Hakim et al. 2003, Hakim et al. 2004). From the laboratory findings, it is clear that green tea is an effective antioxidant, that it provides clear protection from experimentally induced DNA damage and that it can slow or halt the initiation and progression of cancerous tumor growth (Ahn et al. 2003). There is also evidence from some studies that green tea provides significant immunoprotective qualities, particularly in the case of cancer patients undergoing radiation or chemotherapy (Elmets et al. 2001, Pisters et al. 2001). White blood cell count appears to be maintained more effectively in cancer patients consuming green tea compared to non-supplemented patients.
Several epidemiological studies show an association between consumption of total flavonoids in the diet and the risk for cancer and heart disease. Men with the highest consumption of flavonoids (from fruits and vegetables) have approximately half the risk of heart disease and cancer compared to those with the lowest intake. The primary catechin in green tea, EGCG, appears to inhibit the growth of cancer cells as well as play a role in stimulating apoptosis (programmed cell death), both of which are crucial aspects for cancer prevention (Pisters et al. 2001, Weisburger et al. 1998).
In terms of heart disease protection, the potent antioxidant properties of polyphenols would be expected to reduce free radical damage to cells and prevent the oxidation of LDL cholesterol – both of which would be expected to inhibit the formation of atherosclerotic plaques (Hodgson et al. 2000).
Aside from the clear benefits of green tea as an antioxidant, recent studies have suggested a role of catechins in promoting weight loss. Animal studies have shown green tea (and oolong tea) to suppress food intake, body weight gain, and fat tissue accumulation, while human studies have shown increases in metabolic rate and better weight maintenance following weight loss (Komatsu et al. 2003, Kovacs et al. 2004).
In some studies, green tea is associated with a mild increase in thermogenesis (increased caloric expenditure) – which is generally attributed to its caffeine content. However, a handful of studies have shown that green tea extract stimulates thermogenesis to an extent much greater than can be attributed directly to its caffeine content alone – meaning that the thermogenic properties of green tea may be due to an interaction between caffeine and its high content of catechin-polyphenols (Chantre and Lairon et al. 2002, Dulloo et al. 1999). A probable theory for the thermogenic effect of green tea is an increase in levels of norepinephrine – because catechin-polyphenols are known to inhibit catechol-O-methyl-transferase (the enzyme that degrades norepinephrine). One study examined this theory, and the effect of green tea extract on 24-hour energy expenditure, in 10 healthy men – who each consumed 3 treatments of green tea extract (50mg caffeine and 90mg epigallocatechin gallate), caffeine (50 mg), and placebo at breakfast, lunch, and dinner (Dulloo et al. 1999). The results of the study showed that, relative to placebo, the green tea extract resulted in a significant (4%) increase in 24-hour energy expenditure (approximately 80 calories per day) and a significant increase in the body’s use of fat as an energy source (24-h respiratory quotient). In addition, the 24-hour urinary norepinephrine excretion was 40% higher during treatment with the green tea extract than with the placebo. It is interesting to note that treatment with caffeine in amounts equivalent to those found in the green tea extract (50mg) had no effect on energy expenditure or fat oxidation – suggesting that the thermogenic properties of green tea are due to compounds other than its caffeine content alone (Komatsu et al. 2003, Kovacs et al. 2004).
Because norepinephrine levels in humans are also associated with alertness, mental focus, attention, and overall mood – maintaining normal levels of this important neurotransmitter may have benefits for improving mood (reducing depression) and maintaining mental function (reducing attention deficit symptoms) through a mechanism of action similar to a number of pharmaceutical agents (Concerta, Strattera, Effexor).
Green tea consumption of as much as 20 cups per day has not been associated with any significant side effects. In high doses, however, teas that contain caffeine may lead to restlessness, insomnia, heart palpitations and tachycardia (rapid heartbeat). Decaffeinated versions of green tea and green tea extracts are available – but due to differences in caffeine extraction methods, the amounts of phenolic/catechin compounds can vary between extracts. In addition, individuals taking aspirin or other anticoagulant agents (including vitamin E and ginkgo biloba) on a daily basis should be aware of the possible inhibition of platelet aggregation (blood clotting) associated with green tea (in some cases, green tea may prolong bleeding times). Typical dosage recommendations are for 100-500mg/day – preferably of an extract standardized to at least 40% polyphenols and/or EGCG as a marker compound (roughly equivalent to 4-10 cups of brewed green tea).
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EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.