Green Tea and Cancer

Since 1990, approximately 10 million new cancer cases have been diagnosed in the United States alone, with approximately 4 million cancer deaths. Six million people worldwide and 550,000 Americans will likely die from cancer this year. The economic impact of cancer in terms of lost productivity to society has been estimated in the trillions to quadrillions of dollars as we enter the next century.

Cancer results from a gene-environment interaction, and elements in the diet have been implicated in cancer causation and the progression of established tumors. First, there is an excess of dietary fat and calories combined with inadequate physical activity causing an epidemic of obesity both in the U.S. and internationally. Obesity has been shown to contribute significantly to cancer risk, and may produce its effects through various hormones and other factors as well as by promoting oxidant stress. Second, there is a deficiency in the intake of fruits, vegetables, and high fiber cereals and grains in industrialized nations which also impact multiple mechanisms including oxidant defense. Finally, there are potent phytochemicals in edible plants which may have preventive benefits through antioxidation and via gene-nutrient interactions. A number of chemoprevention agents including resveratrol from grapes and red wine, sulforaphane from broccoli and other cruciferous vegetables, organosulfides from garlic and other allium species, limonene and perillyl alcohol from the lipid fraction of citrus peels, isoflavones and enterodiols from soy and flax proteins, and lycopene from tomatoes have been proposed as potential chemoprevention agents based on animal and laboratory evidence of anti-tumor effects. Flavonoids found in green tea and available as botanical dietary supplements are among the dietary factors that may play a role in cancer protection, and have recently been shown to have potent antioxidant and anti-tumor effects.

Green tea contains potent antioxidants called polyphenols which have been shown to reduce tumor growth, metastatic capacity and angiogenesis in several studies. The four major catechins in green tea are (-)-epicatechin, (-)-epicatechin-3-gallate, (-)-epigallocatechin, and (-)-epigallocatechin-3-gallate (EGCG). EGCG is the primary component, accounting for 40% of the total polyphenolic mixture, but it is not clear that this is the only component important in the action of green tea. One cup of green tea usually contains about 300-400 mg of polyphenols, and capsules containing polyphenols in amounts equivalent to four to six cups of green tea per day with reduced caffeine are available as dietary supplements. Preliminary work suggests that these polyphenols protect DNA to a much greater extent than either vitamin E or beta-carotene.

Green Tea-Overview

The long history of the safe ingestion of green tea by humans makes it likely that its ingestion in reasonable doses is safe. In fact, tea is accepted as a flavoring agent and is listed on the Generally Recognized as Safe (GRAS) list of the FDA. The human use of the plant camellia sinensis is believed to have originated 4,000 to 6,000 years ago and 2.5 million tons of tea are produced each year in about 30 countries. It is the most widely consumed beverage in the world. Tea exists as one species with many hybrids and cultivars. The original sources of tea were China, Burma (Myanmar), Thailand (Siam), Laos, and Vietnam. Tea reached Europe in the 1600's through the Dutch and Portuguese. A broader leaf variety was discovered in India in 1823 and large plantations were established in the 1870's in Sri Lanka and Indonesia. The plants can grow up to 30 feet tall, but are usually trimmed to remain short so that the upper leaves, called the flush, can be harvested by hand. Tea is manufactured as black (78%), green (20%), or oolong (2%). Whereas black tea is more popular in Western countries, green tea is primarily consumed in Asian countries, such as Japan, China, and India, and a few countries in North Africa and the Middle East, although India also produces and consumes large amounts of black tea. Leaves meant for green tea are picked by the same method as those picked for black tea. Black tea is prepared from fresh leaves by allowing enzymatic digestion to occur shortly after harvest. This process gives black tea its distinctive flavor. Oolong tea is only partially oxidized. Green tea is heated by steaming or in a pan over a flame to prevent the enzymatic action of polyphenol oxidases in the leaf preserving the polyphenols which are believed to be the bioactive ingredient.

Green Tea Composition and Polyphenol-Enriched Extracts

Tea is a complex mixture of many substances including caffeine (2-4%), amino acids (4%), lignin (6.5%),organic acids (1.5%), protein (15%), chlorophyll (0.5%) and polyphenols (8-12%). Green tea contains 35-52% (measured in weight percent of extract of solids) catechins and flavonols combined. Catechins belong to the general natural product class of flavonoids. The four major catechins in green tea are (-)-epicatechin, (-)-epicatechin-3-gallate, (-)-epigallocathechin, and (-)-epigallocatechin-3-gallate (EGCG). EGCG is the primary component, accounting for 40% of the total polyphenolic mixture. One cup of green tea usually contains about 300-400 mg of polyphenols, which are thought to be nontoxic. Commercial preparations are now available which have reduced caffeine content and are enriched to contain 60 to 80% or more dry weight polyphenols. Numerous in vitro, human, and animal studies have identified these antioxidant polyphenols, in particular EGCG, as cancer-chemopreventive agents. Many of the animal and in vitro studies report the effects of crude green tea extracts, the green tea phenolic fraction, and its major and most active constituent EGCG.

Oxidant Stress, DNA Damage, and Carcinogenesis

When oxygen oxidizes xenobiotic substances, it undergoes a series of reactions that result in the formation of reactive oxygen species with unpaired electrons. These substances are often free radicals and can react with a variety of cell constituents including carbohydrates, lipids, proteins, and DNA. This damage is minimized by compartmentalization (e.g. restriction by lipid membranes) or the actions of protective enzymes (superoxide dismutase, peroxidases, glutathione peroxidase, glutathione transferase, and catalase) or intercepting species (glutathione, ascorbic acid, urea, carotenoids, tocopherols, etc.). It is generally accepted that oxidation is an important pathogenetic event in atherosclerosis, cancer, dementia, and aging. This conclusion is based on the presence of oxidized species or byproducts. While a strong association of oxidation with chronic diseases including cancer exists, the detailed causative mechanisms in each case are incompletely understood. Cancer is the second most common disease-related cause of death in the United States and is the result of gene-environment interactions. A combination of epidemiologic and basic science evidence strongly suggests that diet and plant-derived phytochemicals may play an important role in cancer prevention. Advances in early detection, chemotherapy, radiation, and surgery have improved the outlook for many common forms of cancer.

Nutrition 101 - Natural Remedies - Weight Management - Physician Education
Vitamins & Minerals - Nutrition & Aging - Nutrition & Disease - Research