Tests on volunteers showed that black tea significantly improves the ability of the arteries to relax and expand, but adding milk completely blunts the effect. Supporting tests on rat aortas (aortic rings) and endothelial (lining) cells showed that tea relaxed the aortic rings by producing nitric oxide, which promotes dilation of blood vessels. But, again, adding milk blocked the effect.
The results have led the researchers from the Charité Hospital, Universitätsmedizin-Berlin, Campus Mitte, Germany to suggest that tea drinkers who customarily add milk should consider omitting it some of the time. Their study showed that the culprit in milk is a group of proteins called caseins, which they found interacted with the tea to decrease the concentration of catechins in the beverage. Catechins are the flavonoids in tea that mainly contribute to its protection against cardiovascular disease.
Senior researcher Dr Verena Stangl, Professor of Cardiology (Molecular Atherosclerosis) at the hospital, said: "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."
Dr Mario Lorenz, a molecular biologist and first author said: "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."
MEDICA.de; Source: European Society of Cardiology