What is green tea?
Tea has been consumed in Asia since ancient times. Sailors first brought tea to England in the 17th century. Other than water, tea is the most widely consumed beverage in the world. Tea comes from the Camellia sinensis plant. The way the leaves of this plant are processed determines the type of tea produced.
Many of the possible health benefits studied in green tea are thought to be from compounds called polyphenols. Polyphenols are a large group of plant chemicals that include catechins (antioxidants that help protect cells from damage caused by free radicals).
Catechins make up most of the polyphenols in green tea. The most active catechin in green tea is epigallocatechin-3-gallate (EGCG).
To make green tea, the tea leaves are roasted in a wok (or, historically, steamed) to preserve the catechins and retain freshness. Black tea is made using a process that causes the catechins and other compounds in the leaves to oxidize, producing darker colored tea. Oolong tea is made from partially oxidized leaves.
Some studies suggest that green tea may protect against cardiovascular disease and some types of cancer, including prostate cancer. Clinical trials designed to study whether green tea is useful in treating prostate cancer are in the early stages. There is not enough evidence to show whether green tea is effective in treating prostate cancer.
How is green tea administered or consumed?
Green tea may be consumed as a beverage or taken in dietary supplements.
Have any preclinical (laboratory or animal) studies been conducted using green tea?
Laboratory and animal research has been done to study the effects of green tea in prostate cancer.
Studies of green tea in the laboratory have shown the following:
- EGCG was shown to block the stimulating effect of androgen (a male sex hormone) on human prostate tumor cells, slow their spread, and increase cell death.
- Human prostate cancer cells were treated with EGCG for 30 minutes and then with radiation. Cells treated with EGCG were less likely to die when exposed to radiation than cells not treated with EGCG before radiation.
- Prostate cancer cells were treated with either EGCG or EGCG-loaded nanoparticles. While both treatments decreased cell spread and increased cell death, the nanoparticle treatment was more effective at lower levels, suggesting this type of delivery system for EGCG may make it easier for the body to use and improve EGCG's anticancer activity.
- Green tea polyphenols may cause anticancer effects by blocking histone deacetylases (HDAC) which are found in large amounts in cancer cells, including those in prostate cancer. Treating prostate cancer cells with green tea polyphenols lowered HDAC activity and caused cell death.
Studies of green tea in animal models of prostate cancer have shown the following:
- Strains of mice created to develop prostate cancer that acts like human cancer were given either plain water or water treated with green tea catechins (comparable to a human drinking 6 cups of green tea/ day). After 24 weeks, the mice given plain water had developed prostate cancer while the mice given water with green tea catechins showed only prostatic intraepithelial neoplasia (PIN) lesions. The findings suggested that green tea catechins may help delay the development of prostate cancer by blocking a protein involved in cancer growth.
- In a study of EGCG, mice were implanted with prostate cancer cells and injected with EGCG or placebo 3 times/ week. The mice that received the EGCG treatment had lower levels of proteins needed for androgen activity than those treated with placebo. The findings suggested that EGCG blocks the stimulating effect of androgen on tumor cells in a way that may be useful in prostate cancer that can be treated with hormone therapy and also in prostate cancer that does not respond to hormone therapy.
- In another study of EGCG, strains of mice created to develop prostate cancer that acts like human cancer were given EGCG in drinking water (comparable to a human drinking 6 cups of green tea/ day) starting at either 12 weeks of age or 28 weeks of age. EGCG treatment prevented high-grade PIN lesions in mice that began treatment at 12 weeks but not in those that began treatment at 28 weeks of age.
- In a study of green tea polyphenols, these strains of mice were given polyphenols in drinking water starting at different ages (to match different stages of prostate cancer). All the green tea-fed mice were tumor-free longer than water-fed control mice, and the mice that were fed with green tea the earliest benefitted the most.
- In another study of green tea polyphenols, these strains of mice were fed polyphenols by mouth (comparable to a human drinking 6 cups of green tea/ day). As measured by MRIs over time, tumor development was delayed and tumor growth was slowed in the polyphenol-fed mice compared to water-fed mice. In addition, the polyphenols caused high levels of cell death, possibly limiting the spread of cancer to distant parts of the body.
- Safety studies of Polyphenon E (a green tea extract with a mixture of catechins) have been done in dogs given various doses by mouth. Mixed findings of safety and harms in fasting dogs compared to fed dogs using different types of Polyphenon E are being reviewed.
Have any clinical trials (research studies with people) of green tea been conducted?
Population studies and clinical trials have been done to find out if green tea may be useful in preventing or treating prostate cancer.
Population studies look for risk factors and ways to control disease in large groups of people.
A review of many population studies combined, mainly from Asia, showed mixed findings about whether green tea had a protective effect or no effect on prostate cancer risk. Many factors may be involved in these mixed results, including study location, tobacco and alcohol use, and other dietary differences. Black tea was not found to affect prostate cancer risk.
Overall, population studies suggest that green tea may help protect against prostate cancer in Asian populations. As more people drink green tea worldwide, including in the United States, further population studies will add to information about whether green tea or green tea catechins may help protect against prostate cancer.
Clinical trials of preventing prostate cancer
A study assigned 60 men with high-grade prostatic intraepithelial neoplasia (HGPIN) to take green tea catechin capsules (600 mg / day) or a placebo. After 1 year, 9 men in the placebo group were diagnosed with prostate cancer compared to 1 man in the green tea catechin group. The findings suggest that green tea catechins may lower the risk of prostate cancer in patients at high risk for the disease. Two year follow-up showed that this effect was long-lasting. A larger, multicenter trial is underway.
Clinical trials of treating prostate cancer
Clinical trials designed to study whether green tea is useful in treating prostate cancer have shown the following:
Patients scheduled to undergo radical prostatectomy were assigned to drink green tea, black tea, or soda five times/ day for 5 days. Bioavailable tea polyphenols were found in prostate tissue samples of patients who drank either green tea or black tea. In addition, prostate cancer cells treated with blood taken from patients after they drank tea grew and divided more slowly than cells treated with blood taken from patients before they drank tea.
Fifty patients scheduled to undergo radical prostatectomy were assigned to take Polyphenon E (800 mg EGCG) or a placebo daily for 3 to 6 weeks. Patients treated with Polyphenon E had lower blood levels of prostate specific antigen (PSA) and insulin-like growth factor -1 (a protein linked with increased risk of prostate cancer) than patients treated with placebo, but these differences were not meaningful. The findings suggest that the possible anticancer effects of green tea polyphenols may need to be studied in longer treatment trials.
A small group of hormone-refractory prostate cancer patients were given capsules of green tea extract (375 mg of polyphenols/ day) for up to 5 months. The study showed that the green tea treatment was well tolerated by most of the patients. However, no patient had a meaningful decrease in PSA levels and all 19 patients had disease progression within 1 to 5 months.
Patients with androgen-independent prostate cancer that had spread to other places in the body consumed powdered green tea extract (6 grams / day for up to 4 months). Of the forty-two participants, one had a meaningful decrease in blood PSA levels which did not last longer than 2 months. Green tea extract was well tolerated by most of the study patients. However, there were 6 reports of serious side effects, including insomnia, confusion, and fatigue. The findings suggest that green tea extract may have limited benefits in patients with advanced prostate cancer.
Have any side effects or risks been reported from green tea?
Four phase I studies of Polyphenon E in single doses or multidoses were done in healthy volunteers. Polyphenon E was given in a range of doses and found to be well tolerated. Side effects were generally mild, with no serious side effects reported. The most frequently reported side effects thought to be related to the drug include headache, nausea, abdominal pain, diarrhea, upset stomach, dizziness, and weakness. Gastrointestinal side effects were usually mild, occurring most often in patients taking the drug on an empty stomach and at the highest doses.
The FDA Division of Drug Oncology Products recommends that Polyphenon E should be taken with food by patients in clinical trials and that liver function tests should be done during treatment.
Various types and doses of green tea extracts taken by mouth have been linked with several cases of liver damage in recent years. Most of those affected were women and many were taking green tea extract for weight loss. Most patients recovered within 4 months after stopping the green tea extract. However, there is one case report of acute liver failure in a woman who then needed a liver transplant. Her doctors concluded that her condition was likely caused by over-the-counter green tea extract capsules for weight loss.
Green tea has been well tolerated in clinical studies of patients with prostate cancer. One study found that the most commonly reported side effects of green tea were gastrointestinal symptoms. These were mild except for two reports of severe anorexia and moderate breathing problems. There is evidence that consuming 10 or more cups of green tea/ day for long periods of time may cause headaches, which may be due to caffeine content in the tea.
Is green tea approved by the U.S. Food and Drug Administration (FDA) for use as a cancer treatment in the United States?
The U.S. Food and Drug Administration has not approved the use of green tea as a treatment for cancer or any other medical condition.
Green tea is available in the United States in food products and dietary supplements. Because dietary supplements are regulated as foods, not as drugs, FDA approval is not required unless specific claims about disease prevention or treatment are made.