Green tea (Camellia sinensis)

Preventive efficacy

A Cochrane systematic review1 of green tea for the prevention of cancer published in 2009 included 51 studies with over 1.6 mio participants. Overall it concluded that there is insufficient and conflicting evidence to establish an association between green tea consumption and the incidence of cancer. Most studies tested whether there is an association between green tea consumption and cancer of the digestive tract (n=27), mainly of the upper gastrointestinal tract. Results from studies assessing associations between green tea and risk of digestive tract cancer incidence were highly contradictory. There was limited evidence that green tea could reduce the incidence of liver cancer. The evidence for oesophageal, gastric, colon, rectum, and pancreatic cancer was conflicting. In prostate cancer (n=4), observational studies with higher methodological quality and the only included RCT suggested a decreased risk in men consuming higher quantities of green tea or green tea extracts. However, there was limited to moderate evidence that the consumption of green tea reduced the risk of lung cancer (n=3), and urinary bladder cancer (n=3) or that it could even increase the risk of the latter. There was moderate to strong evidence that green tea consumption does not decrease the risk of dying from gastric cancer. There was limited moderate to strong evidence for benefit in lung, pancreatic and colorectal cancer. The review authors concluded that the results of this review, including its trends of associations, need to be interpreted with caution. They also concluded that desirable green tea intake is 3 to 5 cups per day (up to 1200 ml/day), providing a minimum of 250 mg/day catechins and recommend that, if not exceeding the daily recommended allowance, those who enjoy a cup of green tea should continue its consumption.

A 2008 systematic review2 of forty-three epidemiological studies, four randomized clinical trials and one systematic review concluded that while some evidence suggests that green tea has beneficial effects on gastrointestinal cancers, the findings are not consistent.

One meta-analysis3 investigating the impact of green tea and black tea consumption on lung cancer risk with 22 studies. The summary relative risk showed a borderline significant association between highest green tea consumption and reduced risk of lung cancer. An increase in green tea consumption of two cups/day was associated with an 18% decreased risk of developing lung cancer. A relationship between black tea and reduction of lung cancer risk was, however, not found.

One meta-analysis4 involving 5,617 cases found that increased green tea consumption (more than three cups a day) was inversely associated with breast cancer recurrence. This inverse association was found in the analysis of case-control studies but not in the cohort studies. Combining all studies of breast cancer incidence resulted in significant heterogeneity.

One meta-analysis5 of epidemiologic studies including 25 articles was conducted to assess the association between green tea and black tea consumption and colorectal cancer risk. The combined results from eight studies indicated a reduced risk of colorectal cancer with intake; the protective effect was mainly found among the three case-control studies of colon. Formal tests for homogeneity across studies revealed statistically significant differences in findings across all studies and the study concluded that available epidemiologic data were insufficient to conclude that either tea type may protect against colorectal cancer in humans.

One meta-analysis6 of 13 epidemiologic studies investigated the quantitative association between the consumption of green tea and the risk of stomach cancer in epidemiologic studies. There were discrepancies in the effects of green tea consumption on stomach cancer risk between case–control studies and cohort studies as well as between the crude data and adjusted data in the meta-analysis of published epidemiologic studies. When using the crude data, the highest green tea consumption was seen to significantly increase the stomach cancer risk in the cohort studies. However, when using the adjusted data, green tea consumption at the highest level, when compared with that at the lowest level, was seen to significantly decrease the risk of stomach cancer in a meta-analysis of the case–control studies and in a combined meta-analysis of the case–control and cohort studies. In addition, no significant association was seen in a meta-analysis of the cohort studies.

Further details of the studies reviewed above are presented in table 1. [PDF of table linked to here].

Pre-clinical studies

A large number of in vitro and in vivo studies have demonstrated the anti-carcinogenic properties of green tea polyphenols by inducing apoptosis, cell-growth inhibition, cyclin kinase inhibition, and inhibition of urokinase (an enzyme crucial for cancer growth).6 The mechanism of action may include antioxidant and free-radical scavenging activity, and stimulation of detoxification systems through selective induction or modification of phase I and phase II metabolic enzymes.