— Those looking for an antidote to holiday overindulgence may have added drink more tea to their lists of New Years resolutions, given past studies linking the drink to health benefits. But new research indicates this could all be in vain if you add a splash of milk.
German researchers have found that the relaxing effect of a few cups of ordinary black tea on the arteries is completely wiped out by milk.
After water, tea is the most widely drunk beverage in the world, and is increasingly popular in countries such as the US after reports that people who drink more tea have less cardiovascular disease and cancer. But these effects have been seen most clearly in east Asia, not in tea-loving countries such as the UK.
That could be because there they almost always drink tea with milk, says Verena Stangl of the Charité Hospital in Berlin. She and her colleagues found that when middle-aged women drank half a litre of black Darjeeling, their arteries relaxed much more in response to artificially increased blood flow than without the tea. This ability prevents cardiovascular problems, and depends on eNOS, an enzyme that synthesises the chemical messenger nitrous oxide.
They found that tea also relaxed rings of rat aorta in the lab, and increased the activity of eNOS in cultured arterial cells fourfold. But when the tea had 10% of skimmed milk in it, it had none of these effects in women, rat aortas or cells.
Dissecting this further, the team found that casein proteins from milk blocked the effect of tea all by themselves. These proteins bind specifically to tea chemicals which cause rat aorta to relax, and especially a catechin called EGCG. Catechins are a kind of polyphenol, a group of chemicals long thought to underlie teas healthful effects.
Researchers in the past have claimed that milk had no effect on tea because it does not change overall concentration of polyphenols in the blood. But, Stangl told New Scientist, what is important are specific polyphenols, such as EGCG and milk does block those. It probably also blocks teas effect on other things, such as cancer, she says.
The group is now looking at whether green or black tea is better for you. Previous studies may have been confounded by the fact that black tea is often drunk with milk, while green tea is not.
Journal reference: European Heart Journal (DOI: 10.1093/eurheartj/ehl442)