Milk Proteins Block Tea Anti-Oxidants

The subjects were asked to drink either 500 ml of freshly brewed black tea, black tea with 10% skimmed milk, or boiled water as the control. Flow-mediated dilation (FMD) of the brachial artery in the forearm was measured by high-resolution vascular ultrasound before and two hours after consumption.

Results published in the January 9, 2007, online edition of the European Heart Journal revealed that although black tea significantly improved FMD compared with water, the addition of milk completely counteracted the effects of the tea.

To refine the study the investigators used cell cultures of isolated rat aortic rings and endothelial cells. They found that tea induced vasorelaxation in rat aortic rings and increased the activity of endothelial nitric oxide synthase by phosphorylation of the enzyme in endothelial cells. These effects were completely inhibited by the addition of milk to the tea. Of the various kinds of milk proteins, the caseins were the active inhibitory molecules, probably forming complexes with tea anti-oxidants.

Senior author Dr. Verena Stangl, professor of cardiology at Charité Hospital said, "The well-established benefits of tea have been described in many studies. Our results thus provide a possible explanation for the lack of beneficial effects of tea on the risk of heart disease in the UK, a country where milk is usually added. Since milk appears to modify the biological activities of tea ingredients, it is likely that the anti-tumor 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.”

The authors wrote that researchers studying the health benefits of various foods must take the surrounding food matrix (such as adding milk to tea) into account.



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