BERLIN -- Black tea's purported cardiovascular benefits disappear when milk, even skimmed milk, is mixed into the brew, found researchers here.
BERLIN, Jan. 9 -- Black tea's purported cardiovascular benefits are blunted when milk, even skimmed milk, is mixed into the brew, found researchers here.
In a study of 16 postmenopausal women, those who drank about two cups of black tea without milk had a greater than four-old increase in flow-mediated vasodilation from baseline in the forearm brachial artery (P
To determine which milk compounds might be responsible for inhibiting the vasodilatory properties of tea, the researchers performed a series of cell culture experiments with the six major single milk proteins.
The experiments were conducted on isolated rat aortic rings and endothelial cell cultures. The researchers measured the amount of nitric oxide, a mediator of flow-mediated dilation, that the cell cultures generated in response to stimulation with each milk protein combined with black tea, compared with black tea alone and black tea with skim milk.
Three of the milk proteins-alpha-casein, beta-casein, and kappa-casein-blunted nitric oxide production about as much as milk did. The three other proteins-alpha-lactalbumin, beta-lactoglobulin, and serum albumin-appeared to have no effect on nitric oxide production.
"Our results thus provide a possible explanation for the lack of beneficial effects of tea on the risk of heart disease in the United Kingdom, where milk is usually added to tea," they said.
The three casein proteins in milk may form complexes with certain flavonoids in tea, called catechins, and thereby block their vasodilatory effect, the researchers speculated.
"In conclusion, milk may counteract the favorable health effects of tea on vascular function," the authors said. "The finding that the tea-induced improvement of vascular function in humans is completely attenuated after addition of milk may have broad implications on the mode of tea preparation and consumption."