It appears that adding milk to tea negates its benefits in preventing cardiovascular disease. The caseins (proteins) in the milk interact with the catechins in the tea which are responsible for its protective effects. Sixteen postmenopausal women were asked to drink half a litre of tea (either freshly brewed black tea or black tea with 10% skimmed milk) or boiled water at three different sessions.

Ultrasonography was used to measure brachial artery cell function in the forearm before and after consumption. Black tea significantly improved the ability of the arteries to soften and relax, an effect which disappeared when the tea was taken with milk.

The milk also destroyed the antioxidant effects of the tea. (European Heart Journal online, January 9th, 2007).