Archeological evidence suggests that tea leaves steeped in boiling water were consumed as many as 500,000 years ago. Botanical evidence indicates that India and China were among the first countries to cultivate tea. Today, hundreds of millions of people drink tea around the world, and studies are now suggesting that one variety of tea in particular -- green tea (Camellia sinensis) -- has many health benefits.

There are three main varieties of tea -- green, black, and oolong. The difference between the teas is in their processing. Green tea is made from unfermented leaves and is reputed to contain the highest concentration of polyphenols, chemicals that act as powerful antioxidants. Antioxidants are substances that scavenge free radicals -- damaging compounds in the body that alter cell membranes, tamper with DNA (genetic material), and even cause cell death. Free radicals occur naturally in the body, but environmental toxins (including ultraviolet light, radiation, cigarette smoke, and air pollution) can also increase the number of these damaging particles. Free radicals are believed to contribute to the aging process as well as the development of a number of health problems including cancer and heart disease. Antioxidants such as polyphenols in green tea can neutralize free radicals and may reduce or even help prevent some of the damage they cause.

Green tea has been consumed throughout the ages in India, China, Japan, and Thailand. In traditional Chinese and Indian medicine, green tea has been used as a stimulant, diuretic (to promote the excretion of urine), astringent (to control bleeding and help heal wounds), and to improve heart health. Other traditional uses of green tea include treating flatulence, regulating body temperature and blood sugar, promoting digestion, and improving mental processes.

Green tea has been extensively studied in people, animals, and laboratory experiments. Results from these studies suggest that green tea may be useful for the following health conditions:

Atherosclerosis
Population-based studies indicate that the antioxidant properties of green tea may help prevent atherosclerosis, particularly coronary artery disease. (Population-based studies refers to studies that follow large groups of people over time and/or studies that are comparing groups of people living in different cultures or with different dietary habits, etc.)

High cholesterol
Green tea has demonstrated an ability to lower total cholesterol and raise HDL ("good") cholesterol in both animals and people. One population-based study found that men who drink green tea are more likely to have lower total cholesterol thank those who do not drink green tea. Results from one animal study suggest that polyphenols in green tea may block the intestinal absorption of cholesterol and promote its excretion from the body.

Cancer
The cancer-protective effects of green tea have been reported in several population-based studies. For example, cancer rates tend to be low in countries such as Japan where green tea is regularly consumed. However, it is not possible to determine from these population-based studies whether green tea actually prevents cancer in people. Emerging animal and clinical studies are beginning to suggest that substances in green tea known as polyphenols may play an important role in the prevention of cancer. These substances act as powerful antioxidants. Researchers also believe that polyphenols help kill cancerous cells and stop its progression.

Bladder cancer
Only a few studies have examined the relationship between bladder cancer and green tea consumption. In one study that compared people with and without bladder cancer, researchers found that women who drank black tea and powdered green tea were less likely to develop bladder cancer. A follow-up study by the same group of researchers revealed that bladder cancer patients (particularly men) who drank green tea had a substantially better 5-year survival rate than those who did not.

Breast cancer
Studies in animals and test tubes suggest that polyphenols in green tea inhibit the growth of breast cancer cells. In one study of 472 women with various stages of breast cancer, researchers found that women who consumed the most green tea experienced the least spread of cancer (particularly premenopausal women with early stages of breast cancer). They also found that women with early stages of the disease who drank at least 5 cups of tea every day before being diagnosed with cancer were less likely to suffer recurrences of the disease after completion of treatment. However, women with late stages of breast cancer experienced little or no improvement from drinking green tea.

Colorectal cancer
Studies on the effects of green tea on colon or rectal cancer have produced conflicting results. Some studies show decreased risk in those who drink the tea, while others show increased risk. Further research is needed before green tea can be recommended for the prevention of colorectal cancer.

Esophageal cancer
Studies in laboratory animals have found that green tea polyphenols inhibit the growth of esophageal cancer cells. However, results of studies in people have been conflicting. For example, one large-scale population-based study found that green tea offered significant protection against the development of esophageal cancer (particularly among women). Another population-based study revealed just the opposite -- green tea consumption was associated with an increased risk of esophageal cancer. In fact, the stronger and hotter the tea, the greater the risk. Given these conflicting findings, further research is needed before green tea can be recommended for the prevention of esophageal cancer.

Lung cancer
While green tea polyphenols have been shown to inhibit the growth of human lung cancer cells in test tubes, few studies have investigated the link between green tea consumption and lung cancer in people and even these studies have been conflicting. One population-based study found that Okinawan tea (similar to green tea but partially fermented) was associated with decreased lung cancer risk, particularly among women. A second study revealed that green tea and black tea significantly increased the risk of lung cancer. As with colon and esopageal cancers, further studies are needed before any conclusions can be drawn about green tea and lung cancer.

Pancreatic cancer
In one large-scale study comparing green tea drinkers with non-drinkers, those who drank the most tea were significantly less likely to develop pancreatic cancer. This was particularly true for women -- those who drank the most green tea were half as likely to develop pancreatic cancer as those who drank less tea. Men who drank the most tea were 37% less likely to develop pancreatic cancer. It is not clear from this population-based study, however, whether green tea is solely responsible for reducing pancreatic cancer risk. Although promising, further studies in animals and people are needed before green tea can be recommended for the prevention of pancreatic cancer.

Prostate cancer
Laboratory studies have found that green tea extracts prevent the growth of prostate cancer cells in test tubes. However, both green and black tea extracts were also found to stimulate genes that cause cells to be less sensitive to chemotherapy drugs. Given this potential interaction, black and green tea (as well as extracts of these teas) should not be taken while receiving chemotherapy.

Skin cancer
The main polyphenol in green tea is epigallocatechin gallate (EGCG). Scientific studies suggest that EGCG and green tea polyphenols have anti-inflammatory and anti-cancer properties that may help prevent the onset and growth of skin tumors.

Stomach cancer
Laboratory studies have found that green tea polyphenols inhibit the growth of stomach cancer cells in test tubes, but studies in people have been less conclusive. In two studies that compared green tea drinkers with non-drinkers, researchers found that people who drank tea were about half as likely to develop stomach cancer and gastritis (inflammation of the stomach) as those who did not drink green tea. However, a recent study including more than 26,000 men and women in Japan found no association between green tea consumption and stomach cancer risk. Some studies even suggest that green tea may increase the risk of stomach cancer.

Further studies are underway to determine whether green tea helps reduce the risk of stomach cancer. Although green tea is considered safe for people at risk for stomach cancer, it is too soon to tell whether green tea reduces the likelihood of developing this disease.

Inflammatory Bowel Disease (IBD)
Green tea may help reduce inflammation associated with Crohn's disease and ulcerative colitis, the two types of IBD. Also, if green tea proves to be helpful for preventing colon cancer, this would be an added benefit for those with IBD because they are at risk for colon cancer.

Diabetes
Green tea has been used traditionally to control blood sugar in the body. Animal studies suggest that green tea may help prevent the development of type 1 diabetes and slow the progression once it has developed. People with type 1 diabetes produce little or no insulin, a hormone that converts glucose (sugar), starches, and other foods into energy needed for daily life. Green tea may help regulate glucose in the body. More research in this area would be helpful.

Liver disease
Population-based studies have shown that men who drink more than 10 cups of green tea per day are less likely to develop disorders of the liver. Green tea also appears to protect the liver from the damaging effects of toxic substances such as alcohol. Animal studies have shown that green tea helps protect against the development of liver tumors in mice.

Results from several animal and human studies suggest that one of the polyphenols present in green tea, known as catechin, may help treat viral hepatitis (inflammation of the liver from a virus). In these studies, catechin was isolated from green tea and used in very high concentrations. It is not clear at this time, whether green tea (which contains a lower concentration of catechins) confers these same benefits to people with hepatitis.

Weight loss
Studies suggest that green tea extract may boost metabolism and help burn fat, but there have been no specific studies of this herb in overweight or obese individuals. Some researchers speculate that substances in green tea known as polyphenols, specifically the catechins, are responsible for the herb's fat-burning effect.

Green, black, and oolong tea are all derived from the leaves of the Camellia sinensis plant. Originally cultivated in East Asia, this plant grows as large as a shrub or tree. Today, Camellia sinensis is grown throughout Asia and parts of the Middle East and Africa.

Green and oolong tea are more commonly consumed in Asian countries, while black tea is most popular in the United States. Green tea is prepared from unfermented leaves, the leaves of oolong tea are partially fermented, and black tea is fully fermented. The more the leaves are fermented, the lower the polyphenol content (see What's It Made Of?), and the higher the caffeine content. Green tea has the highest polyphenol content while black tea has roughly two to three times the caffeine content of green tea.

The healthful properties of green tea are largely attributed to polyphenols, chemicals with potent antioxidant properties. In fact, the antioxidant effects of polyphenols appear to be greater than vitamin C. The polyphenols in green tea also give it its somewhat bitter flavor.

Polyphenols contained in teas are classified as catechins. Green tea contains six primary catechin compounds: catechin, gallaogatechin, epicatechin, epigallocatechin, epicatechin gallate, and apigallocatechin gallate (also known as EGCG). EGCG is considered to be the most active component in green tea and is the best researchered of all the green tea polyphenols. Green tea contains roughly 30% to 40% polyphenols and black tea contains only 3% to 10% polyphenols.

Green tea also contains alkaloids including caffeine, theobromine, and theophylline. These alkaloids provide green tea's stimulant effects.

Most green tea products are sold as dried leaf tea. There are also extracts made from the leaves and leaf buds. The average cup of green tea contains about 50 to 150 mg polyphenols. Decaffeinated green tea products contain concentrated polyphenols (60% to 89% total polyphenols). Capsules and liquid preparations are also available.

Pediatric

There are no known scientific reports on the pediatric use of green tea. Therefore, it is not currently recommended for children.

Adult

Three cups of green tea per day (3 g soluble components, or 240 to 320 g polyphenols) or 300 to 400 mg per day of standardized green tea extract (extracts should contain 80% total polyphenols and 55% epigallocatechin) is the recommended dosage.

The use of herbs is a time-honored approach to strengthening the body and treating disease. Herbs, however, contain active substances that can trigger side effects and interact with other herbs, supplements, or medications. For these reasons, herbs should be taken with care, under the supervision of a practitioner knowledgeable in the field of botanical medicine.

People with heart problems, kidney disorders, stomach ulcers, and psychological disorders (particularly anxiety) should not take green tea. Pregnant and breastfeeding women should also avoid green tea.

People who drink excessive amounts of caffeine (including caffeine from green tea) for prolonged periods of time may experience irritability, insomnia, heart palpitation, and dizziness. Caffeine overdose can cause nausea, vomiting, diarrhea, headaches, and loss of appetite. If you are drinking a lot of tea and start to vomit or have abdominal spasms, you may have caffeine poisoning. Lower your caffeine intake and see your health care provider if your symptoms are severe.

If you are currently being treated with any of the following medications, you should not drink green tea or take green tea extract without first talking to your healthcare provider:

Adenosine
Green tea may inhibit the actions of adenosine, a medication administered in a hospital setting for an irregular (and usually unstable) heart rhythm.

Antibiotics, beta-lactam
Green tea may increase the effectiveness of beta-lactam antibiotics by reducing bacterial resistance to treatment.

Benzodiazepines
Caffeine (including caffeine from green tea) has been shown to reduce the sedative effects of benzodiazepines (medications commonly used to treat anxiety, such as diazepam and lorazepam).

Beta-blockers, propranolol and metoprolol
Caffeine (including caffeine from green tea) may increase blood pressure in people taking propranolol and metoprolol (medications used to treat high blood pressure and heart disease).

Blood Thinning Medications
Green tea should not be taken with warfarin, a blood-thinning medication, because the herb contains vitamin K and, thus, can render warfarin ineffective.

Similarly, green tea and aspirin should not be mixed because they both prevent platelets from clotting. Using the two together, therefore, may increase your risk of bleeding.

Chemotherapy
The combination of green tea and chemotherapy medications, specifically doxorubicin and tamoxifen, increased the effectiveness of these medications in laboratory tests. These results have not yet been demonstrated in studies of people, however.

On the other hand, there have been reports of both green and black tea extracts stimulating a gene in prostate cancer cells that may cause them to be less sensitive to chemotherapy drugs. Given this potential interaction, black and green tea (as well as extracts of these teas) should not be taken while receiving chemotherapy for prostate cancer in particular.

Clozapine
The anti-psychotic effects of the medication clozapine may be reduced if taken less than 40 minutes after drinking green tea.

Ephedrine
When taken together with ephedrine, green tea may cause agitation, tremors, insomnia, and weight loss.

Lithium
Green tea has been shown to reduce blood levels of lithium (a medication used to treat manic/depression).

Monoamine oxidase inhibitors (MAOIs)
Green tea may cause a severe increase in blood pressure (called a "hypertensive crisis") when taken together with MAOIs used to treat depression. Examples of MAOIs include phenelzine and tranylcypromine.

Oral contraceptives
Oral contraceptives can prolong the amount of time caffeine stays in the body and may increase its stimulating effects.

Phenylpropanolamine
A combination of caffeine (including caffeine from green tea) and phenylpropanolamine (an ingredient used in many over-the-counter and prescription cough and cold medications and weight loss products)can cause mania and a severe increase in blood pressure. The FDA issued a public health advisory in November 2000 to warn people of the risk of bleeding in the brain from use of this medication and has strongly urged all manufacturers of this drug to remove it from the market.

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