Experimental and clinical studies indicate that tea exerts protection against cardiovascular diseases. However, a group of German researchers (abstract, European Heart Journal 2007, ASAP contents) have found that the beneficial effects of drinking tea may be reduced if milk is added to the tea.
By measuring the blood vessel’s ability to expand (and thereby reduce the blood pressure) the researchers found that this ability was improved by tea, but the effect was completely blunted if milk was added to the tea. It was found that the caseins were responsible for the observed inhibition, probably by formation of complexes with catechins. It is believed that catechins (polyphenolic compounds, belong to the group of flavonoids, structure of epicatechin shown below) trigger the release of other active substances that are responsible for the expansion of blood vessels (also known as vasodilation).
The results of this study are not limited to tea, because catechins are found in many other foods, including citrus fruits, wine and chocolate.