Numerous studies have suggested that tea drinking may do a heart good, with effects on cholesterol, blood clotting and blood vessel function being among the proposed mechanisms.

The new study, reported in the American Journal of Cardiology, suggests it also has a more immediate beneficial effect. The authors suspect that black tea improved the dilation of the men's blood vessel, allowing better blood flow.

Tea is rich in antioxidant compounds called flavonoids, and these may be the key to the beverage's potential heart benefits, study co-author Dr. Kenei Shimada of Osaka City University told Reuters Health.

For the study, the researchers used a special ultrasound method to gauge "coronary flow velocity reserve" or CFVR. This reflects how much blood-flow can speed up when demands are put on the heart, and paints a picture of the healthiness of the coronary circulation.

Shimada's team measured the CFVR of each of the men after they drank either black tea or a caffeinated beverage, and found that it increased significantly after the black tea.

"The results of this study suggest that black tea consumption has a beneficial effect on coronary circulation," the researchers report.

They speculate that the flavonoids in black tea improve the functioning of the lining of the blood vessels, increasing how much the vessels dilate in response to blood flow. Dysfunction in this lining, called the endothelium, is one of the things that goes wrong as heart disease develops.

It's not clear what the long-term implications of the findings on CFVR might be, but Shimada said research has shown coronary flow reserve to be related to heart disease risk.

Larger studies, particularly in people with coronary artery disease, are needed to establish how tea affects the coronary circulation, the researchers conclude.

SOURCE: American Journal of Cardiology, June 1, 2004.