Table of Contents
Green tea is made of the lightly steamed and then dried leaf of the shrub Camilla sinensis. When processed in this way, the leaves retain many of the chemical properties that are thought to provide health benefits. 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 packaged as a liquid.
Green tea has been drunk for thousands of years, especially in Asia. Traditionally it has been used to treat colds, cough, asthma, nausea, vomiting, and diarrhea, as an astringent, and as a diuretic (“water pill”). Green tea or green tea extract has also been proposed as a treatment for reducing cholesterol, 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. Few of these health claims have been studied in rigorous ways that satisfy the standards of conventional medicine, although many health care professionals feel that there is no harm and likely some health benefit in drinking green tea.
Green tea, oolong tea, and black tea all are made from leaves of the same shrub, Camilla sinensis. This plant 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. However, the difference between green, oolong, and black tea is in the processing of the leaves. Green tea is the least processed. The leaves are picked, steamed lightly, and then dried. Oolong tea is made by allowing the
leaves to ferment slightly before drying. With black tea, the leaves are more heavily fermented for longer periods. Because green tea is not fermented, it retains more of its nutrients than either oolong or black tea. Most green tea comes from India or Sri Lanka.
Green tea has become increasingly popular in Europe and the United States. As more information about 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 sale of green tea and green tea extract is regulated by the Food and Drug Administration (FDA). 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 stores. Manufacturers of green tea extract do not have to prove that their products are either safe or effective before they can be sold to the public. This differs from conventional pharmaceutical drugs, which must undergo extensive human testing to prove their safety and effectiveness before they can be marketed. Also unlike conventional drugs, the label for a dietary supplement such as green tea extract does not have to contain any statements about possible side effects.
- health claims. These statements indicate a relationship between an ingredient in the supplement and the reduction in the risk of developing a disease or condition. (e.g. Increased intake of folic acid by pregnant women helps reduce the risk of neural tube defects in their offspring.)
- nutrient content claims. These statements describe the amount of supplement in the product and may contain words such as “high in,” “good source of,” “fortified,” “enriched,” or “high potency.”
- structure or function claims. These claims describe how the supplement may affect organs or systems in the body without mentioning a specific disease or condition. (e.g. Calcium builds strong bones.). Labels with structure or function claims must also contain the words “This statement has not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.”
Active ingredients in green tea
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 (genetic material) and certain fats (lipids) in cell membranes. Antioxidants react with free radicals to neutralize them. The damage that free radicals cause to cells is believed to play a role in the development of certain diseases, especially cancer. 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 that are capable of neutralizing free radicals and, by extension, help prevent disease. In comparison, black tea contains only 3-10% polyphenols.
The six major polyphenols in green tea belong to a group called catechin compounds. The most active of these catechins is epigallocatechin gallate (EGCG). Some manufacturers of green tea extract standardize the amount of polyphenols and EGCG in each capsule. Standardization ranges from 50-90% polyphenols or 100-750 mg of polyphenols. By comparison, one brewed cup of green tea contains about 50-150 mg polyphenols. U. S. law does not require the standardization of dietary supplements, so consumers should
read all labels carefully. Green tea also contains caffeine and caffeine-like compounds. Caffeine is a central nervous system stimulant. The average cup of green tea contains about 50 mg of caffeine. Decaffeinated green tea is 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.
CANCER. Many claims have been made that regularly drinking green tea can help prevent skin, esophagus, stomach, colon, pancreas, lung, bladder, prostate, and breast cancer. The basis for these claims lies in laboratory experiments with EGCG. In cell cultures and in some animal studies, EGCG has been found to kill cancer cells and to shrink tumors, possibly by preventing blood vessels from growing into the tumor (a process called angiogenesis) and thus cutting off the tumor’s supply of food and oxygen. Human studies have shown mixed results. For example, one study found that green tea protected against the development of esophageal cancer, while another found it had no effect. In a large study of more than 40,500 Japanese, researchers found that participants who drank green tea regularly were less likely to die of cardiovascular disease (e.g. heart attack, stroke) but just as likely to die of cancer as non-tea drinkers.
There are many difficulties associated with studying the role of green tea in the development of cancer in human populations. These include:
- The amount and strength of green tea and green tea extract are not standardized and a wide range of doses are used in different studies
- Many studies done on green tea are poorly designed so that it is impossible to show a direct link between cause and effect, or they poorly reported, making analysis of the results difficult
- Many human studies have a small sample size
- Cancer takes a long time to develop, making it difficult to follow study participants and determine outcomes
- Many studies are sponsored by tea growers, manufacturers, or importers who have a financial interest in obtaining positive results
Despite these drawbacks, the possibility that EGCG and other antioxidants in green tea can slow or prevent cancer is strong enough that many research studies are being supported by government health agencies around the world. The official position of the American Cancer Society states, “While the results of lab studies have been promising, at this time there is no conclusive evidence that green tea can help prevent or treat any specific type of cancer in humans.”
WEIGHT LOSS. Some studies in mice have shown that the polyphenols in green tea lower the level of blood glucose (sugar), lipids (fats), and cholesterol, and reduce the amount of body fat deposited under the skin. Other studies have shown that green tea increases body metabolism. However, these results have not been rigorously duplicated in humans. Although green tea may be good food for dieters when used in conjunction with a calorie-reduced diet and increased exercise, it is not a magic bullet that will cause weight loss by itself.
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.
CARDIOVASCULAR DISEASE. Claims have been made green tea decreases cholesterol and fats in the blood. This is thought to reduce the risk of clogged arteries and thus help prevent heart attack and stoke. There is not enough reliable evidence to determine if these claims are true. One large study done in Japan did show improved cardiovascular health in individuals that used green tea, but these individuals tended to be thinner than the average American and have other dietary differences. It is not clear how the Japanese results might apply to other populations.
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.
Green tea has been safely used for thousands of years. Most negative effects are attributable to the caffeine it contains. Children and pregnant and breast feeding women may want to avoid the effects of caffeine by choosing decaffeinated green tea.
There are no known drug or herbal interactions with green tea when used in moderate quantities.
Parents should be aware that the safe dose of many herbal supplements has not been establsihed for children. Accidental overdose may occur if children are give adult herbal supplements.
Fragakis, Allison. The Health Professional’s Guide to Popular Dietary Supplements Chicago: American Dietetic Association, 2003.
Heiss, Mary Lou. Green tea: 50 Hot Drinks, Cool Quenchers, and Sweet and Savory Treats. Boston: Harvard Common Press, 2006.
PDR for Herbal Medicines, 3rd ed. Montvale, NJ: Thompson Healthcare, 2004.
Pierce, Andrea. The American Pharmaceutical Association Practical Guide to Natural Medicines. New York: William Morrow, 1999.
Tracy, Timothy S. and Richard L. Kingston, eds. Herbal Products: Toxicology and Clinical Pharmacology. Totowa, NJ, Humana Press, 2007.
Wildman, Robert E. C., ed. Handbook of Nutraceuticals and Functional Foods, 2nd ed. Boca Raton, FL: CRC/Tay-lor&Francis, 2007.
Hoshiyama, Y. et al. “Green Tea and Stomach Cancer—A Short Review of Prospective Studies.” Journal of Epidemiology 2005, suppl. 2 (June 15, 2005):S109-12.
“Common Dietary Supplements for Weight Loss.” American Family Physician 70, no. 9 (November 1, 2004):1731-8. <http://www.aafp.org/afp/20041101/1731.html>
Alternative Medicine Foundation. P. O. Box 60016, Potomac, MD 20859. Telephone: (301) 340-1960. Fax: (301) 340-1936. Website: <http://www.amfoundation.org>
National Center for Complementary and Alternative Medicine Clearinghouse. P. O. Box 7923, Gathersburg, MD 20898. Telephone: (888) 644-6226. TTY: (866) 464-3615. Fax: (866) 464-3616. Website: <http://nccam.nih.gov>
American Cancer Society. “Green Tea.” American Cancer Society, October 3, 2005. <http://www.cancer.org/docroot/ETO/content/ETO_5_3x_green_tea.asp?sitearea=ETO>
Medline Plus. “Green Tea (Camellia sinensis).” U. S.
National Library of Medicine, August 1, 2006. <http://www.nlm.nih/gov/medlineplus/druginfo/natural/patient-green_tea.html>
National Center for Complementary and Alternative Medicine. “Green Tea.” National Center for Complementary and Alternative Medicine, May 2006. <http://nccam.nih.gov/health/greentea/index.htm>
Maryland Medical Center Programs Center for Integrative Medicine. “Green Tea.” University of Maryland Medical Center, April 2002. <http://www.umm.edu/altmed/ConsHerbs/GreenTeach.html>
Tish Davidson, A.M.