Green tea is processed from the leaves of the tea plant, Camellia sinensis. While all forms of traditional tea come from this plant, green tea is processed differently from black and oolong teas. For green tea, immediately after the tea leaves are picked, they are steamed to stop the fermentation process and then dried. By minimizing the fermentation process in green tea, more of the naturally present compounds are preserved. Green tea is rich in active compounds called catechins, as well as vitamins, minerals and amino acids (such as L-theanine). It also has caffeine, but only about two-thirds the amount in coffee. 

Of the catechins in green tea, epigallocatechin gallate (EGCG) is the most well-studied. Another important constituent in green tea is L-theanine, which has been shown to help with relaxation and concentration. Green tea has been studied in humans for prevention of heart disease, obesity and cancer. This profile will focus on green tea’s ability to help reduce the risk of cancer.

Cancer prevention effects

The evidence supporting green tea’s cancer prevention effects comes from large and small population studies. While these types of studies are not as definitive as double-blinded clinical trials, they are very helpful in assessing the long-term impact of green tea as a part of day-to-day life—which is exactly how cancer prevention works. Research has been done on drinking green tea and taking green tea supplements. Keep in mind that when research mentions one cup, it means a traditional Japanese cup—about 100 ml or 3.5 oz. A typical American mug of tea is about 8 oz. 

Prostate cancer is perfectly suited to the prevention benefits of green tea because it typically takes decades to grow, giving ample opportunity to use targeted cancer prevention strategies like green tea over a long period of time. In a 2006 study in Cancer Research, 60 volunteers with high-grade prostate intraepithelial neoplasia, a precancerous condition, participated in a double-blind, placebo-controlled study. The treatment group took 600 mg of green tea daily. After one year, only one tumor was diagnosed among the 30 men taking the green tea, whereas nine cancers were found among the 30 placebo-treated men.

Cancers of the digestive tract also seem to respond to the preventive effects of green tea. In a study that looked at the health histories of 2,658 non-smoking and non-drinking adults, half of whom eventually developed esophageal cancer, those who drank green tea had almost 50 percent less likelihood of developing esophageal cancer than non–green tea drinkers. A similar relationship between green tea and stomach cancer exists, showing that regular long-term green tea drinkers can experience a 50 percent reduction in cancer risk. A study published in 2001 in the International Journal of Cancer found that green tea reduced the incidence of chronic gastritis, or inflammation and ulceration of the stomach, which is a risk factor for stomach cancer. 

A study published in a 2008 issue of Cancer Epidemiology, Biomarkers and Prevention featured 136 adults with a history of colon polyps (a cancer risk factor). The treatment group took 1,500 mg of green tea extract tablets that were standardized to 80 percent catechins and also drank six 3.5-ounce cups of green tea daily for one year. The control group only drank the green tea. After a year, all participants underwent colonoscopy. Thirty-one percent of those who did not take the tablets developed at least one colon polyp, whereas only 15 percent of the treatment group developed one or more polyp. The addition of the green tea supplement lowered the risk of colon polyps by 50 percent. By adding the capsules, the daily consumption of green tea was raised to more than 10 cups daily, a quantity that is most consistently associated with cancer prevention across all cancer types. This study also indicates the role supplements can play in boosting the level of green tea—great news for people who can’t drink 10 cups of green tea every day. Plus, green tea capsules are more than 99 percent caffeine-free.

Population studies have also shown breast cancer to respond positively to green tea. A 2001 issue of Cancer Letters featuring 1,160 post-surgical women with breast cancer demonstrated that those who drank three or more cups of green tea daily (average consumption was five cups daily) had a 31 percent decreased risk of breast cancer recurrence over their non-tea drinking counterparts. The protective effects from green tea regarding recurrence in the women with early (stages I and II) breast cancer equaled a 51 percent reduction. A similar effect was found in a 1998 study in the Japanese Journal of Cancer Research, which studied 472 Japanese women diagnosed with breast cancer. Women with early stage breast cancer who drank an average of seven cups daily had an 8 percent reduction in recurrence rate, and the recurrences were delayed by almost a full year compared to women who drank an average of less than two cups of green tea daily. 

Several large population studies have also demonstrated protective benefits against the development of cancers of the cervix, lung, bladder and head and neck. 

Dosage, quality and cautions

A 300 mg capsule of green tea extract that is standardized to contain at least 50 percent catechins (80 percent catechins is preferred for optimal anticancer effects), of which at least 45 percent is EGCG, is equivalent to two Japanese cups of tea. To achieve the equivalent of 10 cups daily, you would need to take four to five capsules daily. For L-theanine, look for a standardized extract containing 100 mg per capsule. The typical dose of L-theanine is 200 to 300 mg daily. 

Green tea extracts should be chosen with care. Commercially grown green tea may contain pesticide residue, which can remain in the green tea extract. Green tea extract may also contain carcinogenic solvent residues. It is important to learn from the manufacturer how they ensure that each batch of their green tea extracts is solvent- and pesticide-free. Generally, the best quality green tea will be made from organically grown plants, and each batch will be screened for solvent residues. 

High-quality green tea extract is generally considered safe. However, it can alter the effectiveness of other medications such as blood thinners (such as Coumadin), oral contraceptives and beta-blockers. In some people, high doses of green tea can cause anxiety, tremors, palpitations and insomnia. Green tea can also cause weight loss, which some people may count as an added benefit.