Reviewed: Gianfredi V, Nucci D, Abalsamo A, et al. Green tea consumption and risk of breast cancer and recurrence — A systematic review and meta-analysis of observational studies. Nutrients. December 2018;10(12):E1886. doi: 10.3390/nu10121886.
Tea is obtained from leaves and buds of the tea plant (Camellia sinensis, Theaceae). Green, oolong, white, pu-erh, and black teas are all produced from this species. Their distinction comes from how the plant is processed, with green tea being minimally processed. Fresh tea leaves are exposed to heat or hot steam immediately after picking, resulting in minimal oxidation of polyphenols, which are bioactive compounds believed to be responsible for some of the health benefits of green tea. Recent in vitro studies have found that phenolic compounds in green tea may inhibit angiogenesis (formation of new blood vessels) through various pathways and mechanisms.
Breast cancer (BC) is the most frequently diagnosed cancer in women and has the highest mortality rate worldwide. Previous studies are limited or provide inconclusive evidence to support an association between green tea consumption and BC risk. The purpose of this systematic review and meta-analysis was to evaluate green tea consumption and BC risk, recurrence, and risk in relation to menopausal status.
Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines were followed. A literature search was performed using PubMed, Scopus, and the Web of Science. The researchers used an extensive search strategy including search terms and filters. Observational studies (case-control studies, cohort studies, and cross-sectional studies) evaluating BC risk in adult populations were included. Selected studies evaluated green tea consumption using a questionnaire or interview. A total of 194 studies were retrieved. Of those, 39 were duplicates, 115 did not meet the inclusion criteria, an additional 25 were excluded because there was no differentiation between green tea and other teas consumed, and two were excluded because data were not extrapolated. In the end, 13 studies were included in the meta-analysis.
Of the 13 studies included, seven were conducted in Japan, five in China, and one in the United States. Participants varied between 20 and 87 years of age. Seven of the studies analyzed recurrence in women with a history of BC, and six studies followed healthy individuals to establish BC risk. The pooled sample size was 163,810 participants. There was an overall odds ratio (OR; a measure of the effect size) of 0.85 (95% confidence interval [CI] = 0.80-0.92, P = 0.000) on BC risk when comparing the highest versus the lowest category of green tea consumption.
Three of the seven studies with a focus on BC recurrence showed a possible protective effect of green tea consumption, while the analysis of the remaining four studies did not find a statistically significant correlation. The pooled effect size was 0.81 (95% CI = 0.74-0.88, P = 0.000). The pooled sample size was 13,956, and individual studies ranged from 472 to 6,928 participants. Statistically significant heterogeneity was observed (P = 0.001). The remaining six studies analyzed focused on the risk of a BC diagnosis. None of the studies showed statistical significance and the combined effect size was 0.97 (95% CI = 0.86-1.11, P = 0.684).
To determine a potential beneficial amount of green tea, a meta-analysis was performed with those studies that reported consumption of five cups of tea per day. The effect size was 0.97 (95% CI = 0.81-1.18, P = 0.783). The pooled sample size included 148,511 participants, and individual studies ranged from 427 to 67,422 participants.
In addition, a meta-analysis comparing the risk of BC in women before and after menopause was completed. A statistically significant protective role of green tea in pre-menopausal women was found with an effect size of 0.88 (95% CI = 0.79-0.99, P = 0.035). The pooled sample size was 1,729 participants, and individual studies ranged from 79 to 1,302 participants. No statistically significant heterogeneity was observed (P = 0.385). No protective effects of green tea consumption were observed in post-menopausal women.
A potential protective effect of green tea consumption was found with a 15% reduction in BC risk in the overall meta-analysis. A significant reduction in BC recurrence was also observed in the majority of cohort studies included in the meta-analysis; however, the same was not confirmed in the case-control studies. Results indicate that green tea consumption was not associated with the risk of a new BC diagnosis. Conversely, green tea consumption appears to significantly reduce BC recurrence by 19%. The authors concluded that “[t]he presented results highlight the important role of green tea in tertiary prevention rather than in primary prevention.”
The authors note several limitations of the meta-analysis. The amount of green tea consumed varied considerably among the studies. Some studies reported the amount in grams, others in cups. Serving size ranged between 100 mL and 350 mL. The food frequency questionnaire (FFQ) was used by participants, but only five studies used validated tools. FFQs, which are not without potential bias, were measured at baseline only. Ongoing green tea consumption was not measured.
Other limitations of the analysis included demographics. The majority of the studies (12) were completed in Asia. Only one study from the United States was included, and no studies conducted in Europe were used in the analyses. Other factors, such as diet and cultural differences, were not considered.
In conclusion, the results of this study suggest a potential beneficial link between green tea consumption and BC risk, particularly a reduction in BC recurrence. The need for further studies is warranted. Future studies should include BC staging, accurate consumption amounts before, during, and at the conclusion of the studies, and inclusion of diverse demographical pool.