Green, but not black, tea may reduce risk for CAD

Laurie Barclay

January 28, 2011

Nanjing, China - Available data do not support a protective role of black tea against coronary artery disease (CAD), but limited data support a tentative association of green-tea intake with a lower risk for CAD, according to the results of a meta-analysis reported online January 19, 2011 in the American Journal of Clinical Nutrition[1].

"Epidemiologic studies are inconsistent regarding the association between tea consumption and the risk of . . . CAD," write Dr Ze-Mu Wang (First Affiliated Hospital of Nanjing Medical University, China) and colleagues. "The objective was to perform a meta-analysis to determine whether an association exists between tea consumption and total CAD end points in observational studies."

Of 18 studies identified from a search of PubMed and EMBASE databases from 1966 through November 2009 and included in the meta-analysis, 13 studies were on black tea and five studies were on green tea. A random-effects model allowed combination of study-specific risk estimates.

The meta-analysis showed no significant association between black-tea intake and the risk for CAD (highest vs lowest consumption, summary relative risk [RR] 0.92; for an increase of one cup/day, summary RR 0.98). The summary RR for green tea showed a significant association between the highest intake of green tea and a lower risk for CAD (summary RR 0.72). Increased green-tea intake by one cup per day was associated with a 10% reduction in the risk for the development of CAD (summary RR 0.90).

"Our data do not support a protective role of black tea against CAD," the study authors write. "The limited data available on green tea support a tentative association of green-tea consumption with a reduced risk of CAD. However, additional studies are needed to make a convincing case for this association."

Limitations of this meta-analysis include significant heterogeneity among studies; the small number of included studies; search for English-language articles only; observational study design with possible uncontrolled confounders; differing methods for measuring tea consumption; and studies mainly conducted in the US, Europe, China, and Japan, limiting generalizability to other populations.

"It is biologically plausible that black tea and green tea might have an inconsistent effect on CAD risk," the study authors conclude. "Green tea catechins have been shown to inhibit oxidation, vascular inflammation, atherogenesis, and thrombogenesis and to favorably modulate the plasma lipid profile and vascular reactivity, which suggests a wide spectrum of beneficial effects on CAD. Because of the different degrees of fermentation, the content and composition of catechins vary substantially between green and black tea."

The National Natural Science Foundation of Chi na supported this study. The study authors have disclosed no relevant financial relationships.

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