A meta-analysis evaluated evidence from multiple databases on the relation between tea consumption and risks of cardiovascular disease (CVD) and all-cause mortality among generally healthy adults.
Randomized trials, prospective cohort studies, and nested case-control studies with data on tea consumption and risk of incidence of cardiovascular events were pulled from six databases and 39 total publications.
The data synthesized from all combined studies showed that each cup increase in daily tea consumption was associated with an average 4% lower risk of CVD mortality, a 2% lower risk of CVD events, and a 4% lower risk of stroke, and a 1.5% lower risk of all-cause mortality.
The magnitude of the association was larger in elderly individuals for both CVD mortality and all-cause mortality, compared to other demographic factors.
Strengths of evidence were rated as low or moderate, depending on the study population age group for CVD-specific mortality outcome, and was rated as low for CVD events, stroke, and all-cause mortality outcome, signifying to researchers that a daily tea intake, as part of a healthy dietary pattern, may be associated with lower risks of CVD and all-cause mortality among adults.
Tea stood out to researchers in particular for its richness in flavonoids, relative to other foods and beverages typical in the U.S. diet, along with the fact that it is the second most-consumed beverage worldwide. Tea consumers, on average, have a flavonoid intake 20 times greater than non-tea drinkers. Flavonoids are known to benefit heart health in a few ways—they act as moderators of enzymes involved in oxidative and inflammatory stress, enhance NO status, and improve endothelial function.
Due to the variety of flavonoid profiles in various types of tea, little has been done in the way of green tea and black tea cross-comparisons. However, the combined results in this meta-analysis found only incremental differences in the benefits from both black and green tea, based upon which types of tea were reported in the evaluated studies, or which types are more commonly consumed in the various regions of the world in which relevant research has taken place.
“The most important potential biological mechanism is the ability of tea flavonoids to reduce both systolic and diastolic blood pressure,” researchers wrote. “Tea flavonoids improve endothelium-dependent vasorelaxation in healthy individuals as well as in various human pathogenic conditions. Their ability to activate entothelial NO synthase seems to be the likely mechanism underlying improvement in flow-mediated dilation and blood pressure reduction. Colonic microbes convert flavonoids into smaller, more bioavailable compounds with ultimate conversion to hippuric acid, a major compound detected in human urine after consumption of both green and black tea. Thus, one might expect the bioefficacy of green and black tea to be similar, as seen throughout these analyses.”
The amount of flavonoids, however, may be overestimated in certain ready-to-drink products, researchers said. Conversely, certain models may underestimate the amount of flavonoids in tea that was steeped for longer periods of time.
“The development of novel biomarkers of exposure for future research would assist researchers in overcoming measurement error from assessing tea consumption via FFQs [food frequency questionnaires],” the researchers wrote.
Studies considered by the meta-analysis looked exclusively at black or green tea. Studies on alcohol or solvent extracts of tea leaves, other tea supplements, or herbal teas were excluded from the meta-analysis.
“No adverse effects were shown at normal consumption amounts,” the researchers concluded. “Our systematic review provides evidence to begin developing dietary guidance and public health messaging around the consumption of tea, although future, rigorously designed [randomized controlled trials] would greatly strengthen the evidence base and certainty of our findings. Incorporating tea as part of a healthy diet is a simple dietary modification that may have positive public health implications on chronic disease risk reduction worldwide.”