A strong body of research has connected black tea to heart health benefits thanks to its antioxidant, anti-inflammatory and vasodilating effects.
But researchers from the Charite Hospital in Berlin compared the endothelial function in the brachial artery of 16 postmenopausal women, after drinking black tea with no milk, black tea with milk, or hot water. They found that while tea increased the artery's ability to relax and expand to accommodate increased blood flow compared to water, this effect was blocked when milk was added to the beverage.
One level this research, widely reported in the consumer press, could lead some to reconsider a drop of milk in their tea. "As worldwide tea consumption is second only to that of water, its beneficial effects represent an important public health issue," the authors wrote in the European Heart Journal.
But it also flags up the need to consider consumption context. Olive oil and red wine, for instance, are also lauded for their antioxidant content and health potential, but it is hard to separate the observed effects from the surrounding matrix, which may bias the results.
"Interactions of food-derived flavonoids with milk proteins may impede their physiological effects," said the researchers, who also cited research indicating that consumption of chocolate containing epicatechin, but not milk or dark chocolate, increases the antioxidant capacity of human blood plasma.
They do note that not all studies have come to the same conclusions over tea-drinking and milk. While the reasons for this discrepancy are largely unknown, it could be due to different physiological/experimental endpoints.
The Charité team extended its findings to a functional model, exposing the aortic rings of rats to tea alone and to tea with individual milk proteins to measure vasodilation. The results were the same, and the researchers were able to identify three caseins that appeared to be responsible for the blocking the action.
They said this was most likely down to formation of complexes with tea catechins.
Senior researcher Dr Verena Stangl connected the findings to cancer risk reduction:
"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 we re-examine the association between tea consumption and cancer protection, to see if that is the case," she said.
The research team is also involved in comparing the effects of black tea with those of green tea (almost always consumed without milk) on vascular function.
In the meantime, however, the message the researchers are sending out to those who like a dash of milk in their tea is to drink it black every now and then.
According to the UK's Tea Council, tea with milk provides 16 per cent of a person's daily calcium requirement, based on four cups consumed a day. There are no statistics on how much of a person's average 1.6 litres of milk per week is consumed in tea.
The Milk Development Council, however, drew attention to the small size of the study.
The recent study looking at the health benefits of tea and the inclusion of milk in tea was conducted on just 16 people, which almost anyone would recognise is a very small sample size.
Liz Broadbent, director of Marketing at the Milk Development Council, said: "The majority of adults get most of their calcium intake from dairy. We advocate tea drinking as a good way of enjoying and maintaining your intake of milk." Source: European Heart Journal Doi: 10.1093/eurheartj/ehl442 "Addition of milk prevents vascular protective effects of tea". Authors: Mario Lorenz, Verena Stangl et al.