Research published on-line (Tuesday 9 January) in European Heart Journal has found that the protective effect that tea has on the cardiovascular system is totally wiped out by adding milk.
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 findings, by cardiologists and scientists from the Charité Hospital, Universitätsmedizin-Berlin, Campus Mitte, Germany, are bad news for tea-drinking nations like the British, who normally add milk to their beverage. The results have led the researchers to suggest that tea drinkers who customarily add milk should consider omitting it some of the time.
Proteins in milk probably bind to the catechins and render them unavailable.
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. 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."
In East Asian countries where green tea is popular the use of milk in tea is relatively rare. So they are deriving a greater benefit from tea drinking.
High resolution ultrasound allowed measure of the effects of tea and milk on an artery.
Sixteen healthy postmenopausal women drank either half a litre of freshly brewed black tea, black tea with 10% skimmed milk, or boiled water (as a control) on three separate occasions under the same conditions. The endothelial function of the brachial artery in the forearm was measured by high resolution ultrasound before and two hours after drinking, with measurements being taken every 15 seconds for up to two minutes a time.
Said first author Dr Mario Lorenz, a molecular biologist: "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."
Milk contains a number of different proteins: by testing each one separately, the researchers found that it was the three caseins that accounted for the inhibiting effect, probably by forming complexes with tea catechins.
Casein proteins make up a substantial portion of cheese. This suggests that red wine (which has a healthy amount of catechins) and cheese is a bad combination. Chocolate milk similarly blunts the effects of the catechins in the chocolate. Drink your wine with nuts. The nuts have their own beneficial compounds such as another type of flavonoids called anthocyanins (tea has some too). As for chocolate milk: Why dilute something as great as chocolate with mere milk?
Casein proteins show up as food additives in a variety of foods and you can watch for the terms caseinate, casein, milk solids, milk protein, and curds as indicators of their presence.
|Share |||Randall Parker, 2007 January 08 06:26 PM Aging Diet Studies|