Green tea (Camellia sinensis) for the prevention of cancer

Filippini T, Malavolti M, Borrelli F, Izzo AA, Fairweather-Tait SJ, Horneber M, Vinceti M · Cochrane Database of Systematic Reviews · 2020 March 02

DOI 10.1002/14651858.CD005004.pub3

Objective

To assess associations between green tea consumption and the risk of cancer incidence and mortality, as well as safety data and quality of life.

Conclusions

Green tea consumption was associated with a lower overall cancer incidence (summary RR 0.83, 95% CI 0.65 to 1.07) but not with cancer-related mortality (summary RR 0.99, 95% CI 0.91 to 1.07); experimental studies showed a potential preventive effect on incident prostate cancer (RR 0.50, 95% CI 0.18 to 1.36), uncertain effects on gynaecological cancer (RR 1.50, 95% CI 0.41 to 5.48), and no evidence for non-melanoma skin cancer (RR 1.00, 95% CI 0.06 to 15.92); adverse effects included gastrointestinal disorders, elevated liver enzymes, insomnia, raised blood pressure, and skin reactions; green tea extracts slightly improved quality of life in experimental studies.

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Insights

Green tea, containing antioxidative catechins, may possess some beneficial effects in reducing the risk of specific types of cancer.

The study team performed an extensive review of 142 completed and 2 ongoing studies up to January 2019. These studies encompassed all types of epidemiological studies: experimental (randomised controlled trials) and nonexperimental (observational studies with both cohort and case-control design) that looked into the association of green tea consumption with cancer risk or quality of life, or both. Data extraction and methodological quality assessment of these studies were performed independently by two or more review authors. These efforts were organized and the results summarized according to the type of cancer diagnosis. The data was sourced from CENTRAL, MEDLINE, Embase, ClinicalTrials.gov, and referenced lists of earlier reviews and included studies. The studies focused on investigating the links between cancer incidence and mortality rates, safety data, quality of life, and green tea consumption. Numerous studies came with a high degree of methodological quality, based on 'Risk of bias' assessment. The placebos in 11 studies involving a total of 1795 participants were green tea extracts. On another note, the studies indicated some side effects linked to high intakes of green tea with more than a million participants undertaking the non-experimental studies. Despite inconclusive evidence, experimental and nonexperimental epidemiological studies suggested potential beneficial effects of green tea consumption on the overall risk of cancer or on certain types of cancer. Variations of the beneficial effects of green tea were observable across different cancer types, indicating a decrease in rate ratios (RR) in the highest category of green tea consumption versus the lowest. A detailed analysis based on study design exhibited conflicting results for some types of cancers, showing an increased RR in cohort studies and a decreased or no difference in RR in case-control studies. However, the overall interpretability of these RR estimates was limited by methodological issues, such as low study numbers and inconsistencies with cohort study results. Lastly, it's important to note that the majority of included studies were conducted within populations with a high intake of green tea, mainly Asian, thus limiting the general applicability of the findings to other populations.