It is known that drinking tea can help to prevent cardiovascular disease. It now seems that this protective effect is negated if milk is added to the tea, so tea without milk is more beneficial.
Over three different sessions, sixteen post-menopausal women, mean age 59, were given either 500ml of Darjeeling tea, or boiled water to drink. The tea was either freshly brewed black tea, or black tea with 10% skimmed milk added. An ultrasound scan was used to measure artery cell function in the forearm, both before and two hours after drinking. Black tea was found to significantly improve the ability of the arteries to soften and relax, but this effect was absent when the tea was taken with milk: the protective catechins in the tea interacted with the milk proteins (caseins) inhibiting their effects. The milk additionally destroyed the antioxidant properties of the tea.
(Addition of milk prevents vascular protective effects of tea. European Heart Journal, on-line 9 January 2007.)