Drinking tea can reduce the risk of heart disease and stroke but only if milk is not added to the brew, German scientists said Tuesday.

Research has shown that tea improves blood flow and the ability of the arteries to relax but researchers at the Charite Hospital at the University of Berlin in Mitte found milk eliminates the protective effect against cardiovascular disease.

The beneficial effects of drinking black tea are completely prevented by the addition of milk, said Verena Stangl, a cardiologist at the hospital.

"If you want to drink tea to have the beneficial health effects you have to drink it without milk. That is clearly shown by our experiments," she said.

Tea is second only to water in worldwide consumption so any benefits could have important public health implications. But until now it was not known whether adding milk had an impact.

Stangl and her team discovered that proteins called caseins in milk decrease the amount of compounds in tea known as catechins which increase its protection against heart disease.

They believe their findings, which are reported in the European Heart Journal, could explain why countries such as Britain, where tea is regularly consumed with milk, have not shown a decreased risk of heart disease and stroke from drinking tea.

The researchers compared the health effects of drinking boiled water and tea with and without milk on 16 healthy women.

Using ultrasound, they measured the function of an artery in the forearm before and two hours after drinking tea.

Black tea significantly improved blood flow compared to drinking water but adding milk blunted the effect of the tea.

"We found that, whereas drinking tea significantly increased the ability of the artery to relax and expand to accommodate increased blood flow compared with drinking water, the addition of milk completely prevents the biological effect," said Dr Mario Lorenz, a molecular biologist and co-author of the study.

Tests on rats produced similar results. When rodents were exposed to black tea, they produced more nitric oxide which promotes dilation of blood vessels. But adding milk blocked the effect.

Tea has also been shown to have a protective effect against cancer so the findings could have further implications.

"Since milk appears to modify the biological activities of tea ingredients, it is likely that the anti-tumor effects of tea could be affected as well," said Stangl.

Editor: Donald