Green tea (Camellia sinensis) and cancer prevention: a systematic review of randomized trials and epidemiological studies.

BACKGROUND: Green tea is one of the most popular beverages worldwide. This review summarizes the beneficial effects of green tea on cancer prevention. METHODS: Electronic databases, including PubMed (1966-2008), the Cochrane Library (Issue 1, 2008) and Chinese Biomedical Database (1978-2008) with supplement of relevant websites, were searched. There was no language restriction. The searches ended at March 2008. We included randomized and non-randomized clinical trials, epidemiological studies (cohort and case-control) and a meta-analysis. We excluded case series, case reports, in vitro and animal studies. Outcomes were measured with estimation of relative risk, hazard or odd ratios, with 95% confidence interval. RESULTS: Forty-three epidemiological studies, four randomized trials and one meta-analysis were identified. The overall quality of these studies was evaluated as good or moderate. While some evidence suggests that green tea has beneficial effects on gastrointestinal cancers, the findings are not consistent. CONCLUSION: Green tea may have beneficial effects on cancer prevention. Further studies such as large and long term cohort studies and clinical trials are warranted.