Green tea (Camellia sinensis) for the prevention of cancer

Filippini, Tommaso, Malavolti, Marcella, Borreli, Francesca, Izzo, Angelo, Fairweather-Tait, Susan, Horneber, Markus and Vinceti, Marco (2020) Green tea (Camellia sinensis) for the prevention of cancer. Cochrane Database of Systematic Reviews.

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Abstract

Background Tea is one of the most commonly consumed beverages worldwide. Teas from the plant Camellia sinensis can be grouped into green, black and oolong tea and drinking habits vary cross-culturally. Camellia sinensis contains polyphenols, one subgroup being catechins. Catechins are powerful antioxidants, and laboratory studies have suggested that these compounds may inhibit cancer cell proliferation.Some experimental and nonexperimental epidemiologic studies have suggested that green tea may have cancer-preventative effects. Objectives To assess possible associations between green tea consumption and the risk of cancer incidence and mortality as primary outcomes.Secondary outcomes are safety data and quality of life. Search methods We searched eligible studies up to January 2019 in the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, ClinicalTrials.gov, and reference lists of previous reviews and included studies. Selection criteria We included all epidemiological studies (experimental, i.e. randomised controlled trials (RCTs) and nonexperimental (non-randomised studies - NRS), i.e. observational studies with both cohort and case-control design), that investigated the association of green tea consumption with cancer risk and or quality of life or both. Data collection and analysis Two or more authors independently applied the study criteria, extracted data and assessed methodological quality of studies. We summarized the results according to cancer type diagnosis. Main results In this review update, we included 142 completed studies (11 experimental131 nonexperimental) and two ongoing studies. This was an additional 11 experimental and 85 nonexperimental studies from the previous version of the review. Eleven experimental studies allocated a total of 1,795 participants to either green tea extract or placebo, all demonstrating an overall high methodological quality based on risk of bias assessment. For incident prostate cancer, the summary risk ratio (sRR) in the green tea supplemented participants was 0.50 (95% confidence interval (CI) 0.18 to 1.36), based on 3 studies and involving 201 participants (low certainty-evidence). The sRR for gynaecological cancer was 1.50 (95% CI 0.41 to 5.48; 2 studies, 1157 participants; low certainty-evidence). No effect on risk of non-melanoma skin cancer emerged (sRR 1.00, 95% CI 0.06 to 15.92; 1 study, 1075 subjects; low certainty-evidence). In addition, adverse effects of green tea extract intake were reported, including gastrointestinal disorders, elevation of liver enzymes, and, more rarely, insomnia, raised blood pressure and skin/subcutaneous reactions. Concerning quality of life, consumption of green tea extracts induced a slight improvement compared with placebo, based on three experimental studies. In nonexperimental studies, we included over 1,100,000 participants from 46 cohort studies and 85 case-control studies, which were on average of intermediate to high methodological quality based on Newcastle-Ottawa assessment scale risk of bias assessment. When comparing the highest category of green tea intake with the lowest we found a lower overall cancer incidence (sRR 0.83, 95% CI 0.65 to 1.07) based on 3 studies, involving 52,479 participants (low certainty-evidence). Results for all cancer mortality were null (sRR 0.99, 95% CI 0.91 to 1.07), based on 8 studies and 504,366 participants (low certainty-evidence). For most of the site-specific cancers we observed a decreased sRR.After stratifying the analysis according to study design, we found strongly conflicting results for some cancer sites: oesophageal, prostate and urinary tract cancer, and leukaemia showed an increased sRR in cohort studies and a decreased/no difference sRR in case-control studies. Authors' conclusions Overall, findings from experimental and nonexperimental epidemiologic studies yielded inconsistent results, thus providing limited evidence for the beneficial effect of green tea consumption on the overall risk of cancer or on specific cancers sites. Some evidence of a beneficial effect of green tea at some cancer sites emerged from the RCTs and from case-control studies, but their methodological limitations such as the low number and size of the studies, and the inconsistencies with the results of cohort studies limit the interpretability of the sRR estimates. The trials also indicated the occurrence of several side effects associated with high intakes of green tea. In addition, the majority of included studies were carried out in Asian populations characterized by a high intake of green tea, thus limiting the generalisability of the findings to other populations. Well conducted and adequately powered RCTs would be needed to draw conclusions on the possible beneficial effects of green tea consumption on cancer risk in humans.

Item Type: Article
Uncontrolled Keywords: pharmacology (medical) ,/dk/atira/pure/subjectarea/asjc/2700/2736
Faculty \ School: Faculty of Medicine and Health Sciences > Norwich Medical School
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Depositing User: LivePure Connector
Date Deposited: 07 Feb 2020 05:09
Last Modified: 28 Mar 2020 01:28
URI: https://ueaeprints.uea.ac.uk/id/eprint/73989
DOI: 10.1002/14651858.CD005004.pub3

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