Dose-response relationship between tea consumption and risk of cardiovascular disease and all-cause mortality: a systematic review and meta-analysis of population-based studies

Authors: CHUNG, MEI - Tufts University; ZHAO, NAISI - Tufts University; WANG, DEENA - D&v Systematic Evidence Review Consulting; SHAMS-WHITE, MARISSA - University Of New England; KARLSEN, MICAELA - University Of New England; CASSIDY, AEDIN - University Of East Anglia; FERRUZZI, MARIO - North Carolina State University; JACQUES, PAUL - Jean Mayer Human Nutrition Research Center On Aging At Tufts University; JOHNSON, ELIZABETH - Jean Mayer Human Nutrition Research Center On Aging At Tufts University; WALLACE, TAYLOR - George Mason University

Submitted to: Advances in Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2020
Publication Date: 2/14/2020
Citation: Chung, M., Zhao, N., Wang, D., Shams-White, M., Karlsen, M., Cassidy, A., Ferruzzi, M., Jacques, P.F., Johnson, E.J., Wallace, T.C. 2020. Dose-response relationship between tea consumption and risk of cardiovascular disease and all-cause mortality: a systematic review and meta-analysis of population-based studies. Advances in Nutrition. https://doi.org/10.1093/advances/nmaa010.
DOI: https://doi.org/10.1093/advances/nmaa010

Interpretive Summary: Flavonoids are a large group of naturally occurring plant metabolites (phytonutrients) found in almost all fruits and vegetables. Accumulating evidence suggests that consumption of flavonoids can be beneficial for metabolic and cardiovascular health. Tea is one of the top dietary sources of flavonoids in the U.S. diet and the second most widely consumed beverage worldwide. Differences in post-harvest processing result in several categories of tea including: black (fermented), oolong (partially fermented), and green (unfermented), each with a different array of flavonoids including flavan-3-ol monomers (in green tea) to oxidized flavan-3-ols including theaflavins and thearubigens (oolong and black tea). Black tea is the primary type of tea consumed in the North America and Europe, whereas green tea is consumed principally in Asia, and oolong tea in southeast China. About 21% of Americans report consuming tea on a daily basis, and tea consumers were previously shown to have approximately 20 times higher flavonoid intake compared to non-consumers. Tea has been investigated (via in vitro, animal, observational, preclinical, and clinical studies) for its potential to reduce the risk and/or progression of various chronic disease outcomes, particularly cardiovascular disease (CVD) and cerebrovascular disease. Tea flavonoids can act as modulators of oxidative and inflammatory stress, which may in part contribute to the benefits on cardiovascular health. However, due to differences in tea profiles (i.e., concentrations of flavonoids) and variable doses, it has been difficult to compare results across studies. To provide comprehensive and updated evidence on the relationship between tea consumption and risks of CVD and all-cause mortality, we conducted a systematic review and dose-response meta-analysis of prospective cohort studies and randomized controlled trials (RCTs). Evidence from human studies of tea and risk of cardiovascular events, stroke events, CVD-specific mortality, or all-cause mortality was collected from a large number of sources (PubMed, EMBASE, Web of Science, Cochrane Central Register of Controlled Trials, Food Science and Technology Abstracts, and Ovid CAB Abstract databases) from English-language scientific publications through June 20, 2019. Two reviewers independently performed data extractions from these sources and assessed study quality for the identified publications. Data from 37 publications from prospective cohort studies were included in our analyses. Our analyses showed that each cup increase in daily tea consumption (estimated 280 mg and 338 mg total flavonoids/day for black and green tea, respectively) was associated with an average of 4% lower risk of CVD mortality, 2% lower risk of CVD events, 4% lower risk of stroke, and 2% lower risk of all-cause mortality. Although these publications included in the analysis presented many flaws such as limited data for higher tea consumption and potential for publication bias, our findings provide support for the use of daily tea consumption as part of a usual dietary pattern to lower risks of CVD and all-cause mortality among generally healthy adults.

Technical Abstract: Background: Tea flavonoids can modulate oxidative stress and improve endothelial function, which may in part contribute to the potential benefits on cardiovascular health. Purpose: To synthesize the evidence on the dose-response relationship between tea consumption and risks of cardiovascular disease (CVD) and all-cause mortality among generally healthy adults. Data Sources: PubMed, EMBASE, Web of Science, Cochrane Central Register of Controlled Trials, Food Science and Technology Abstracts, and Ovid CAB Abstract databases from English-language publications through June 20, 2019. Study Selection: Randomized trials, prospective cohort, and nested case-control (or case-cohort) studies with data on tea consumptions and risk of incident cardiovascular events (cardiac or peripheral vascular events), stroke events (including mortality), CVD-specific mortality, or all-cause mortality. Data Extraction: Two reviewers independently performed data extractions and assessed risk of bias. Data Synthesis: Data from 37 prospective cohort publications were synthesized. The linear meta-regression analyses showed that each cup increase in daily tea consumption (estimated 280 mg and 338 mg total flavonoids/day for black and green tea, respectively) was associated with an average of 4% lower risk of CVD mortality, 2% lower risk of CVD events, 4% lower risk of stroke, and 2% lower risk of all-cause mortality. CVD events and all-cause mortality outcomes also showed non-linear associations with total tea flavonoids intake levels. Subgroup meta-analysis results showed that the magnitude of association was larger in elderly (pooled adjusted relative risk = 0.90 [95% CI, 0.85, 0.95] for CVD mortality; pooled adjusted relative risk = 0.92 [95% CI, 0.90, 0.94] for all-cause mortality) than that in adults. Generally, studies with higher risk for biases appeared to show larger magnitudes of associations than studies with lower risk for biases. Strength of evidence was rated as low and moderate (depending on the age groups of the study populations) for CVD-specific and all-cause mortality outcomes and was rated as low for CVD events and stroke outcomes. Limitations: Dose-response meta-regression was limited by heterogeneity, ecological bias, and measurement errors for tea intake. Data were scarce regarding high tea intake levels (>7 cups/day or >1600 mg total tea flavonoids/day). Publication bias cannot be ruled out. Conclusions: Daily tea intake as part of a usual dietary pattern may be associated with lower risks of CVD and all-cause mortality among generally healthy adults.