Green tea consumption and risk of cancer: a systematic review and meta-analysis of observational studies

Introduction Tea is one of the most highly consumed drink in the world after water. Between 2007 and 2016, world tea production grew by an average annual rate of 4.4%. Global tea consumption was 5.53 million tonnes in 2016 with an annual growth rate of 4.5 percent between 2007 and 2016 1. Brewed tea is obtained from the infu- sion of leaves and buds of Camellia sinensis. The most common- ly consumed type of tea varies cross-culturally, but green and black tea are the main ones. It has been described that Camellia sinensis contains polyphenols, particularly green tea that contain a high amount of catechins, powerful antioxidants. Laboratory studies have suggested that these compounds may inhibit cancer cell proliferation and 2 and some experimental and nonexperi- mental epidemiologic studies have suggested that green tea may have cancer-preventative effects 3 4. We aimed to assess associa- tions between green tea consumption and the risk of cancer incidence and mortality. Materials and methods We searched eligible studies up to January 2019 online databases as well as from reference lists of previous reviews and included studies. We included all observational epidemiological stud- ies (both cohort and case-control studies), that investigated the association of green tea consumption with cancer risk. Two or more authors independently applied the study criteria, extracted data and assessed methodological quality of studies. We also per- formed a meta-analysis using a random effects model comparing the highest category of green tea intake with the lowest, and us- ing the most adjusted multivariable model. We summarized the results according to cancer type diagnosis Results In this review update, we included 131 studies, including 46 co- hort and 85 case-control studies with over 1,100,000 participants., we found a lower overall cancer incidence. Regarding overall cancer mortality we found no difference in risk For most of the site-specific cancers we observed a decreased sRR. However, af- ter stratifying the analysis according to study design, we found strongly conflicting results for some cancer sites: esophageal, prostate and urinary tract cancer showed an increased sRR in co- hort studies and a decreased/null sRR in case-control studies. Conclusions Overall, findings from observational epidemiological studies yielded inconsistent and even contrasting results for the effect of green tea consumption on cancer risk. In addition, since observa- tional studies generally suffered from the inherent limitation of this study design, potential unmeasured confounding and expo- sure misclassification, well conducted and adequately powered experimental studies are clearly needed to elucidate the possible beneficial effects of green tea consumption on cancer risk in hu- mans.