Can Tea Protect Against Heart Disease?

Source: Tufts University
December 6, 2001 (Reviewed: December 4, 2003)

Some convincing science suggests that tealeaves - like many other plant components - contain phytochemicals that may provide some protection against disease. But a group of U.S. scientists are concerned that publication bias may be affecting what health professionals read on the proposed protective properties of tea. They conducted a meta-analysis on studies of tea and cardiovascular risk and reported their findings in a recent issue of the American Journal of Epidemiology.

A closer look at the science

Researchers from the University of North Carolina looked at 10 cohort studies and 7 case control studies conducted in the U.S., Australia, and Europe. As expected, it was difficult to combine the results of these studies into a single finding because they differed greatly as to population group, study duration, and definitions of exposure and outcome. In an attempt to standardize the findings, data were recalculated to estimate risk based on a total tea consumption of 3 cups per day (1 cup=237 mL, or about 8 oz). Data collected in U.S. studies were further adjusted on the assumption that tea consumed in the United States is generally half as strong as tea consumed in other parts of the world.

Potential health benefits

Most of the studies suggested an inverse association between tea consumption and cardiovascular risk, although several studies conducted in England showed an apparent positive effect. Using this pooled data, the researchers estimated that people who consume at least 3 cups of tea per day are 11% less likely than those who do not drink tea to suffer from cardiovascular disease, an effect of borderline significance (RR=0.89, 95% CI 0.79-1.01).

There is a plausible biological pathway to explain the apparent protective effect of the phytochemicals in tealeaves. Tea is a rich source of flavonoids, antioxidants thought to reduce the risk of cardiovascular disease by limiting the oxidation of LDL cholesterol within artery walls.

More research needed

But the researchers believe that small studies that have produced results inconsistent with the current hypothesis of the protective effect of tea may be subject to publication bias - that is, studies of no effect may be less likely to be published than those that solidly support or refute the hypothesis. Graphs using the Begg-Mazumdar test of publication bias appear to support the claim of bias; the small number of studies included in the meta-analysis, however, limits the test's predictive power.

In spite of their concerns, though, the authors do not refute the potential health benefits that may come from drinking tea. They argue only that the proposed protective effect may not be as large as some believe. Given the evidence currently available, tea - as part of an overall healthy lifestyle - appears to contribute to the health benefits gained from a diet that includes plenty of phytochemical-rich plant foods.

Source

• Does tea affect cardiovascular disease? A meta-analysis U. Peters, C. Poole, L. Arab, Am J Epidemiol., 2001, vol. 154, pp. 495--503

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