Study finds that adding milk reduces healthy benefits of black tea

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The study looked at 16 healthy postmenopausal women with an average age of 59. Each participant's flow-mediated vasodilation from baseline in the forearm brachial artery was measured before and two hours after they were given roughly two cups of freshly brewed black tea without milk, freshly brewed black tea with 10 percent milk, and boiled water as a control.

While drinking the black tea without milk, the subjects' flow-mediated dilation increased more than 400 percent, said lead researcher Veran Stangl and colleagues in the online issue of European Heart Journal. Women who drank 90 percent black tea and 10 percent skimmed milk had the same insignificant vasodilation increase they would get from drinking two cups of hot water.

"The most striking finding of our study is that addition of milk to black tea completely prevents the biological activity of tea in terms of improvement of endothelial function," said the authors.

The next step in the study was to see which milk compounds could be inhibiting the tea's vasodilatory properties. The researchers took six major single milk proteins and conducted a series of cell culture experiments with them on isolated rat aortic rings and endothelial cell cultures. To determine flow-mediated dilation, the researchers measured the amount of nitric oxide produced when the cell cultures were stimulated with each milk protein and black tea, just black tea, and black tea with skim milk.

The milk proteins alpha-casein, beta casein and kappa-casein all stunted the production of nitric oxide to the same degree as skim milk. The alpha-lactalbumin, beta-lactoglobulin and serum albumin did not seem to have an effect on nitric oxide production. The researchers theorized that the three proteins that slowed nitric oxide production might have formed complexes with catechins -- flavonoids in the tea -- and blocked the vasodilatory effects.

"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," the authors noted. "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."

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