Previous studies have shown that drinking tea is a boon for cardiovascular health, but now researchers have found that the beneficial effects of the drink are "totally wiped out" by putting milk.
German researchers publishing their findings in the online version of the European Heart Journal show that a group of proteins called caseins found in milk prevent the dilation of blood vessels which occurs when pure black tea is consumed.
As a result they argue that those who drink milk with their tea should seriously consider going without on occasion, in order to boost the healthiness of their heart.
"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," Senior researcher Dr Verena Stangl, Professor of Cardiology (Molecular Atherosclerosis) at the hospital, said
In the study, reported in the European Heart Journal, 16 women drank half a litre of freshly brewed tea, tea with skimmed milk, or plain boiled water. Tests were taken on an artery in the forearm for two hours afterwards.
They showed that drinking black tea significantly improves the ability of the artery to relax and expand – but adding milk completely blunts the effect.
"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," Dr Mario Lopez, who also worked on the study, said.
Stangl also said their findings could also have implications for cancer, against which tea has also been shown to be protective.
"Since milk appears to modify the biological activities of tea ingredients, it is likely that the anti-tumour 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," she said.