Green Tea

Other common name(s): green tea extract, Chinese tea

Scientific/medical name(s): Camellia sinesis

Description

Green tea is a drink made from the steamed and dried leaves of the Camellia sinesis plant, a shrub native to Asia. Black tea is also made from this plant, but unlike green tea, black tea is made from leaves that have been fermented. Fermentation may reduce the levels of some compounds, such as antioxidants, in the tea.

Overview

Some researchers believe green tea may protect against certain types of cancer because it contains antioxidants. However, results from human studies have been mixed. More research is needed to determine the role of green tea in cancer prevention.

How is it promoted for use?

Green tea is widely consumed in Japan, China, and other Asian nations and is becoming more popular in Western nations. Some reports indicate green tea may have the ability to help prevent cancers of the skin, esophagus, stomach, colon, pancreas, lung, bladder, prostate, and breast.

Green tea contains chemicals known as polyphenols, which have antioxidant properties. The major group of polyphenols in green tea are called catechins, and the most important catechin seems to be epigallocatechin-3-gallate (EGCG). EGCG may cause cancer cells to die in much the same way that normal cells do. This effect is important because cancer cells are different from normal cells in that they do not die when they should—they continue to grow and spread.

Herbalists use green tea and extracts of its leaves for stomach problems, vomiting, and diarrhea and to reduce tooth decay, blood pressure, cholesterol levels, and blockages of the blood vessels in the heart that can lead to heart attacks. Green tea is also promoted as preventing certain bacterial infections. In recent years, some researchers have suggested that black tea may also be effective in cancer prevention. These claims are currently being studied.

What does it involve?

The typical amount of green tea consumed varies widely, and it is not clear how much might be needed for beneficial effects. Three cups a day or more is the amount typically taken in Asian countries. Green tea is usually brewed using 1 to 2 teaspoons of the dried tea in a cup of boiling water or is steeped for 3 to 15 minutes. Bottles and cans of prepared green tea are sold under a variety of brand names, often with additional ingredients.

Green tea extract is also available in capsule form. Three capsules of green tea extract a day is a common recommended dosage, but this dosage and its effects remain uncertain. There is wide variation in the contents of these extracts.

What is the history behind it?

The Chinese have been drinking green tea for at least 3,000 years, and this beverage has been popular in some other Asian countries for at least 1,000 years. In recent years, scientists have begun to study its health effects more closely in laboratory and animal studies and in observational human studies.

What is the evidence?

Many laboratory studies in cell cultures and animals have shown green tea acts against cancer cells. Test tube studies have suggested that compounds in the tea may help stop new blood vessels from forming, thereby cutting off the supply of blood to cancer cells. It is tempting to assume that it may therefore help prevent some cancers, but studies in humans have been mixed. Most human studies have been epidemiologic studies in East Asia, in which researchers compared tea drinkers with non–tea drinkers while trying to account for other lifestyle differences. These types of studies are complex, and it is often hard to draw firm conclusions from them.

Large population-based studies in East Asia generally have not found that green tea drinkers have a lower risk of breast, stomach, or colon cancer than non–tea drinkers. One study found that Asian-American women who drink green tea regularly have a lower risk of breast cancer than those who do not. A Chinese study found that green tea drinking was linked to fewer cancers of the esophagus for people who did not smoke. On the other hand, a 2006 Japanese study showed that those with cancer of the esophagus were more likely to be green tea drinkers than those who did not have the cancer. Other studies of green tea’s ability to prevent or treat lung, prostate, bladder, or other types of cancer have yielded similarly mixed results.

While the results of laboratory studies have been promising, at this time the available scientific evidence does not support claims that green tea can help prevent or treat any specific type of cancer in humans. Controlled, randomized clinical trials are needed to determine its effectiveness. Several studies are currently under way.

Are there any possible problems or complications?

This product is sold as a dietary supplement in the United States. Unlike companies that produce drugs (which must provide the FDA with results of detailed testing showing their product is safe and effective before the drug is approved for sale), the companies that make supplements do not have to show evidence of safety or health benefits to the FDA before selling their products. Supplement products without any reliable scientific evidence of health benefits may still be sold as long as the companies selling them do not claim the supplements can prevent, treat, or cure any specific disease. Some such products may not contain the amount of the herb or substance that is written on the label, and some may include other substances (contaminants). Though the FDA has written new rules to improve the quality of manufacturing processes for dietary supplements and the accurate listing of supplement ingredients, these rules do not take full effect until 2010. And, the new rules do not address the safety of supplement ingredients or their effects on health when proper manufacturing techniques are used.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

Moderate intake of green tea is generally considered safe. Asians have consumed this tea for thousands of years with few dangerous side effects. However, some people may have allergic reactions and should stop drinking it should reactions occur. Drinking large amounts of tea may cause nutritional and other problems because of the caffeine content and the strong binding activities of the polyphenols, which can make it harder for the body to absorb certain medicines and iron supplements. Always tell your doctor and pharmacist about any herbs and supplements you are taking.

Possible effects from too much caffeine are the major concern with green tea. Too much caffeine can lead to nausea, trouble sleeping, and frequent urination. Because caffeine acts as a stimulant, people with irregular heartbeats or who have anxiety attacks should be cautious in its use.

Women who are pregnant or breast-feeding should not drink green tea in large amounts. Caffeine can cross the placenta and affect the fetus and can also be passed along in breast milk. Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

Additional resources

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-ACS-2345).

Guidelines for Using Complementary and Alternative Therapies

Dietary Supplements: How to Know What Is Safe

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management

Complementary and Alternative Methods for Cancer Management

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer

References

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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.