14 DAY TRIAL // Bad news for the British: the healthy effect of tea on the cardiovascular system is totally annulled by milk.
Scientific research at the Charit? Hospital, Universit?tsmedizin-Berlin, Germany, proved that black tea significantly improves the ability of the arteries to relax and expand, due to nitric oxide, but this effect is aborted by milk.
The culprit in milk was found to be a group of proteins called caseins, which interacted with the tea to decrease the concentration of catechins in the beverage. Catechins are flavonoids (vitamin A like compounds) that mainly contribute to the protection against cardiovascular diseases.
"There is a broad body of evidence from experimental and clinical studies indicating that tea exerts antioxidative, anti-inflammatory and vasodilating effects, thereby protecting against cardiovascular diseases. As worldwide tea consumption is second only to that of water, its beneficial effects represent an important public health issue. But, up to now, it's not been known whether adding milk to tea, as widely practised in the UK and some other countries, influences these protective properties. So, we decided to investigate the effects of tea, with and without milk, on endothelial function, because that is a sensitive indicator of what is happening to blood vessels", said senior researcher Dr Verena Stangl, Professor of Cardiology (Molecular Atherosclerosis).
16 healthy postmenopausal women subjects drank either half a liter of black tea, black tea with 10% skimmed milk, or boiled water (as a control) on three separate sessions.
A high resolution ultrasound was used to measure the endothelial function of the brachial artery, before and after two hours from drinking, the values being taken every 15 seconds for up to two minutes.
"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. To extend our findings to a functional model, we determined vasodilation in rat aortic rings by exposing them to tea on its own and tea with individual milk proteins added, and got the same result", said first author Dr Mario Lorenz, a molecular biologist.
Three caseins were found for inducing the inhibitory effect, probably by interacting with tea catechins. "Our results thus provide a possible explanation for the lack of beneficial effects of tea on the risk of heart disease in the UK, a country where milk is usually added", said Stangl. "Since milk appears to modify the biological activities of tea ingredients, it is likely that the anti-tumour (cancer) effects of tea could be affected as well. I think it is essential that we re-examine the association between tea consumption and cancer protection, to see if that is the case."
"It is important to bear in mind that green tea is almost exclusively drunk without milk. So we are talking only about those countries and regions where black tea is consumed and where milk is added. The results of our study merely attempt to encourage people to consider that, while the addition of milk may improve its taste, it may also lower its health-protective properties", said Lorenz.
Often, studies investigate the effects of a single nutritional compound or beverage, like the red wine, olive oil and others, without assessing their effects with the surrounding food matrix (like milk in the tea case) that could totally change the results.
"It's an ongoing question whether green tea, with its higher catechin content, is superior to black tea in regard to endothelial function. In addition, because of the antiatherogenic potential of tea ingredients, we want to investigate the effects of the ingredients on chronic cardiovascular processes such as the development of restenosis (re-narrowing of arteries) after catheter procedures."