Green but not black tea may lower the risk of coronary artery disease (CAD), the limited evidence suggests.

The researchers performed a meta-analysis of 18 observational studies identified from a search of PubMed and EMBASE databases from 1966 through November 2009. Thirteen studies were on black tea and five on green.

For green tea there was a significant association between the highest intake of green tea and a lower risk for CAD, summary relative risk (RR) = 0.72 (95% CI: 0.58-0.89). Furthermore, an increase in green tea consumption of 1 cup/day was associated with a 10% decrease in the risk of developing CAD, RR = 0.90 (95% CI: 0.82-0.99).

However, for black tea there was no significant association with the incidence of CAD. When highest was compared with lowest consumption, summary RR = 0.92 (95% CI: 0.82-1.04); with an increment of 1 cup/day, summary RR = 0.98 (95% CI: 0.94-1.02).

There is a potential biological explanation for this difference.

Green tea catechins have been shown to inhibit oxidation, vascular inflammation, atherogenesis, and thrombogenesis and favourably effect the plasma lipid profile and vascular reactivity. These effects could clearly translate into a clinical reduction in the manifestation of CAD.

Furthermore, due to different degrees of fermentation, the content and composition of catechins can vary substantially between green and black tea.

The authors do recognise several limitations to this analysis such as a small number of included studies, observational study designs prone to confounding and differing methods for measuring tea consumption. They therefore conclude that additional studies are needed before firm conclusions can be drawn. So no need to switch teas just yet then.

Dr Peter Savill

• Wang Z-M, Zhou B, Wang Y-S et al. Black and green tea consumption and the risk of coronary artery disease: a meta-analysis. Am J Clin Nutr 2011; DOI: 10.3945/ajcn.110.005363