Green tea is derived from young leaves of the common Chinese tea plant Camilla sinensis. The shrub is native to Asia, and all tea is made from the leaves of the same plant, including green, black, and oolong tea. Differences between the teas are the result of varied locations, soil composition, local weather conditions, and processing of the leaves. Leaves for green tea are picked when they are young. When lightly steamed and then dried, the young leaves exhibit chemical properties that are thought to provide health benefits. Green tea is commonly available in tea bags, loose teas, packaged drinks, and extracts. Green tea extract is a concentrated form of green tea that is sold as a dietary supplement. It usually comes in capsules but sometimes is prepared and packaged as a liquid.
Green tea has been a popular beverage for thousands of years, especially in Asia. Because it contains biochemicals called polyphenols, which are known to have antioxidant and anti-inflammatory properties, green tea is believed to provide multiple health benefits. Traditionally, it has been used to treat colds, cough, asthma, nausea, vomiting, and diarrhea, and has been used both as an astringent and a diuretic (“water pill”). Green tea or green tea extract has also been proposed as a treatment for reducing cholesterol levels, preventing heart attack, preventing cancer, increasing fertility, decreasing symptoms associated with menopause, increasing mental alertness, preventing tooth decay, relieving anxiety, protecting skin from sun damage, and aiding in weight loss. Some of these health claims have been investigated in clinical studies, and although the studies may not satisfy the standards of conventional medical research many healthcare professionals support the notion of health benefits obtained from drinking green tea.
Camilla sinensis is farmed in many temperate areas of China, India, Sri Lanka, Indonesia, Kenya, Malawi, Turkey, and Pakistan. The chemical content of the tea leaves varies slightly from location to location, which accounts in part for the differences between green, black, and oolong tea. The processing of the leaves, however, determines the character, taste, and properties of the different teas. Green tea is the least processed. Young leaves of the shrub are picked, steamed lightly, and then dried. The leaves for oolong tea are allowed to ferment slightly before drying. In making black tea, the leaves are fermented more heavily and for longer periods. Because green tea is not fermented, it retains more of its nutrients, and also more caffeine. Most green tea comes from India or Sri Lanka.
Green tea has become increasingly popular in Europe, the United States, and many other regions outside of Asia. As more evidence of its potential health benefits has become available, capsules of green tea extract have been promoted as a dietary supplement that may help with weight loss, prevent cancer, and rid the body of free radicals. In the United States, the Food and Drug Administration (FDA) regulates the sale of green tea and green tea extract. Green tea is treated as a food and is sold in supermarkets everywhere. Green tea extract is considered a dietary supplement under the 1994 Dietary Supplement Health and Education Act (DSHEA) and is available mainly in health food and whole foods stores. Manufacturers of green tea extract, as with other herbal products, do not have to demonstrate that their products are safe or effective before they can be sold to the public, as with regulated pharmaceutical products. Also, unlike conventional drugs, labeling for dietary supplements such as green tea extract does not have to describe possible side effects. However, under the DSHEA, dietary supplements such as green tea are legally prohibited from claiming that they can “cure,” “treat,” “mitigate,” “prevent,” or “diagnose” a specific disease.
Dietary supplements may make the following claims:
Green tea contains a group of compounds called polyphenols. Polyphenols have strong antioxidant properties. Antioxidants help protect the body against damage caused by free radicals. Free radicals are formed during normal metabolic processes. The quantity of free radicals in the body may also be increased by exposure to environmental toxins, ultraviolet light, and radiation. Free radicals have a strong tendency to react with and damage other compounds, especially those in DNA and certain fats (lipids) in cell membranes. Antioxidants react with free radicals to neutralize them. The damage that free radicals cause to cells, especially inflammation, has been shown to play a role in the development of certain diseases, including cardiovascular diseases and cancer. Accordingly, many of the health claims for green tea and green tea extract are based on the fact that green tea leaves contain 30%–40% polyphenol antioxidants. In comparison, black tea contains only 3%–10% polyphenols. To produce the health benefits attributed to antioxidants, polyphenols obtained from foods must be absorbed, metabolized, and transported to body tissues to neutralize free radicals and, by extension, help to prevent disease.
The six major polyphenols in green tea belong to a group called catechin compounds. The most active of these catechins is epigallocatechin-3-gallate (EGCG). The American Cancer Society states that EGCG may cause cancer cells to die in much the same way that normal cells do, an important distinction because cancer cells continue to divide, multiply, and cause cancer to spread. The amount of polyphenols and EGCG in each capsule of green tea is standardized by some manufacturers. Standardization ranges from 50%– 90% polyphenols or 100–750 milligrams (mg) of polyphenols. By comparison, one brewed cup of green tea contains about 50–150 mg polyphenols.
Green tea also contains caffeine and caffeine-like compounds. Students in Asia are noted for drinking green tea to keep them awake and alert while studying. Caffeine is a central nervous system stimulant. The average cup of green tea contains less than 40 mg of caffeine, compared to about 100–200 mg in a cup of coffee. Decaffeinated green tea is also available; it contains little or no caffeine but still contains polyphenols. All teas, including green tea, contain tannin. Tannin is an astringent that slows secretions and helps control bleeding.
Health claims for green tea are based on the way the active ingredients act in laboratory studies and animal studies. The results of green tea studies in humans have been mixed, and the official position of the FDA is that the evidence for health benefits of green tea is not strong enough to meet the requirements of conventional medicine. Similarly, claims for green tea have not been supported in Europe due to lack of sufficient evidence from randomized clinical trials. Yet results of studies are generally promising. The National Center for Complementary and Integrative Health (NCCIH) at the National Institutes of Health has sponsored clinical trials to determine the safety and effectiveness of green tea as a possible treatment for more than a dozen diseases and disorders.
CANCER. Results of laboratory and animal studies of EGCG in green tea have supported claims that regularly drinking green tea may help prevent cancer of the skin, esophagus, stomach, colon, pancreas, lung, bladder, prostate, and breast. EGCG has been found in animal studies to kill cancer cells and shrink tumors, possibly by preventing blood vessels from growing into the tumor (a process called angiogenesis) and thereby cutting off the tumor's supply of nutrients and oxygen carried by blood. A review of many published studies conducted prior to 2010 indicated that multiple studies of EGCG, the main component of green tea, demonstrated its ability to inhibit tumor invasion and angiogenesis, processes that are essential for tumor growth and metastasis. The American Cancer Society reported that more than 50 epidemiologic studies of associations between tea (green, black, or both) and cancer published since 2006 have linked tea consumption to reduced risk of colon, breast, ovary, prostate, and lung cancers. Clinical trials in human subjects continue to examine the effects of tea polyphenols on cancer incidence and mortality, but a definitive cancer prevention role has neither been confirmed or refuted.
Studying the role of green tea in human cancers is difficult, mainly because the amounts and strengths of green tea and green tea extracts are not standardized, and a wide range of doses used by study participants complicates statistical analysis. Also, many investigations of the benefits of green tea are cross-sectional studies across national populations, which precludes making inferences about cause and effect.
MENTAL PERFORMANCE. Any effects of green tea on mental alertness and performance are most likely due to the effects of caffeine and caffeine-like compounds found in green tea. Caffeine is a drug classified as a stimulant, or a psychoactive drug. It acts on the central nervous system, helping the body to be more alert and less drowsy.
CARDIOVASCULAR DISEASE. Claims have been made that green tea decreases circulating levels of cholesterol and fats, reducing the risk of clogged arteries and helping to prevent heart attack and stroke.
OTHER HEALTH CLAIMS. The tannins in green tea have an astringent or drying effect. One folk remedy to stop the bleeding where a tooth has been extracted is to bite down on a used tea bag. Tannins in green tea may also be responsible for helping to control diarrhea. Caffeine may provide additional benefits, including longer life and improved memory, though most caffeine studies are done on coffee, not tea.
No precise dosages are specified for green tea served as a beverage or for green tea extract, mainly because the amount of green tea needed for beneficial effects has not been determined. The American Cancer Society (ACS) reports that the amount of green tea consumed daily varies widely. Typical amounts of green tea consumed as a beverage in Asian countries is three or more cups daily. It is usually brewed using one to two teaspoons of dried tea in one cup of boiling water. Tea bags contain a similar amount. Three capsules of green tea extract per day is a common suggested dosage for supplements on package labeling.
Green tea has been used safely for thousands of years, with no adverse effects reported. The FDA includes tea on their list of substances considered “generally recognized as safe.” However, even though the caffeine content of green tea is lower than that of coffee and black tea, any negative effects are attributed generally to caffeine. Caffeine in any type of drink or product may make individuals more prone to nervousness or restlessness and cause difficulty sleeping. Some individuals are especially sensitive to caffeine. Children and pregnant and breastfeeding women may want to avoid the effects of caffeine by choosing naturally decaffeinated green tea. People with heart disease, fibrocystic breasts, birth defects, reproductive issues, or stomach ulcers may wish to reduce or eliminate caffeine from their diet.
Interactions between substances in green tea (e.g., caffeine, tannin, and EGCG) and various drugs may occur; the effects vary depending on the interacting substance. In some instances, the expected action of a drug may be altered by one or more substances in green tea, including:
The actions of certain other drugs and supplements may be affected by green tea, including iron supplements, verapamil, irinotecan, cytochrome P450 3A4 substrates, and UGT (uridine 5′-diphosphoglucuronosyltransferase) substrates. Consumers are advised to consult with a trusted medical professional or pharmacist before using green tea combined with any drug or herb.
The safety of green tea extract has not been established. Green tea is reported to be generally safe and, aside from the relatively standard effects of caffeine, complications are few even when large amounts of tea are consumed. Individuals with hypersensitivity to caffeine or who use large amounts of green tea may develop caffeine-related insomnia or upset stomach. The tannin in green tea may contribute to iron deficiency in individuals with low dietary iron intake. For this reason, tea should not be consumed with meals.
Parents should be aware that a safe dose of many herbal supplements has not been established for children. Accidental overdose may occur if children are given adult herbal supplements. Green tea can pass into breast milk and may cause sleep disorders in infants and young children. It can also inhibit the metabolism of iron within the bodies of infants, and may cause microcytic anemia (any type of anemia characterized by smaller than normal red blood cells).
See also Antioxidants ; Arthritis diet ; Asian diet ; Cancer-fighting foods ; Diet drugs ; Metabolism .
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L. Lee Culvert