Questions and Answers About Green Tea
- 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. All tea comes from the Camellia sinensis plant. The way the leaves of this plant are processed determines the type of tea produced.
To make green tea, the tea leaves are steamed and dried; this causes very little oxidation (a chemical reaction that takes place when a substance comes into contact with oxygen or another oxidizing substance). Black tea is made by crushing tea leaves to cause full oxidation. Oolong tea is made from partially oxidized leaves.
Some studies suggest that green tea may protect against cardiovascular disease and some types of 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.
Many of the possible health benefits studied in 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).
The most active catechin in green tea is epigallocatechin-3-gallate (EGCG).
- 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?
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.
- 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.
- Strains of mice created to develop prostate cancer that acts like human cancer were given green tea 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, but the mice that were fed with green tea the earliest benefitted the most. The findings suggested that green tea might be most beneficial either in men diagnosed with early PIN lesions, men at high risk for developing prostate cancer, or men undergoing watchful waiting.
- In another study, these strains of mice 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.
- 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, suggested that green tea may help protect against prostate cancer in Asian populations. Black tea was not found to affect prostate cancer risk.
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 green tea (6 grams / day). Of the forty-two participants, one had a meaningful decrease in blood PSA levels which did not last longer than 2 months. Green tea 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 may have limited benefits in patients with advanced prostate cancer.
- Have any side effects or risks been reported from green tea?
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 does not cause ill effects and that any side effects that do occur are 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?
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.
General information about clinical trials is also available from the NCI Web site.