Milk Blunts Benefits of Tea on Endothelium
from Heartwire — a professional news service of WebMD
Lisa Nainggolan
January 9, 2007 (Berlin, Germany) – German researchers have shown that adding milk to tea--as is customary in the UK and some other countries--appears to blunt any protective effect that tea may have on endothelial cells [1]. Dr Mario Lorenz (Charité Hospital, Universitätsmedizin-Berlin, Campus Mitte, Germany) and colleagues report their findings online January 9, 2007 in the European Heart Journal.
Tests on 16 volunteers showed that black tea significantly improves dilation in the brachial arteries, but that adding milk completely attenuated the effect. Senior author Dr Verena Stangl (Charité Hospital, Universitätsmedizin-Berlin) told heartwire that further work in rats showed that "the very pronounced effect of tea on endothelial nitric oxide synthase (eNOS) activity was completely blocked by adding milk to the tea."
But Stangl cautions that "it is too early--on the basis of this one study in just 16 people--to advocate that people not add milk to tea." A spokesperson for the British Heart Foundation agreed. "It is difficult to say from this small study the impact of adding a drop of milk to your tea," says June Davison (British Heart Foundation). "However, the study highlights the importance of not just thinking about one food in isolation but the effect of the actual interaction between different foods. Leaving milk out of your tea is far less likely to help protect your heart health than other measures, such as taking regular exercise, avoiding smoking, and eating a healthy balanced diet," she adds.
Caseins in the milk inactivate catechins in the tea
In the current study, a crossover design was employed, in which 16 healthy postmenopausal women drank either half a liter of freshly brewed black tea, black tea with 10% skim milk, or boiled water (as a control) on three separate occasions under the same conditions. The endothelial function of the brachial artery was measured by high-resolution ultrasound before and two hours after drinking, with measurements being taken every 15 seconds for up to two minutes a time. Black tea significantly improved flow-mediated dilation compared with water, whereas the addition of milk completely blunted the effects.
"Milk may counteract the favorable health effects of tea on vascular function. Our results thus provide a possible explanation for the lack of beneficial effects of tea on the risk of heart disease in the UK, where milk is usually added to tea," say the researchers.
This is not the first time that it has been suggested that milk may blunt the protective effects of tea. A meta-analysis of tea consumption [2] found an 11% reduction in MI for every additional three cups per day but noted heterogeneous results among the studies--eg, an increased risk of coronary heart disease in a UK trial and an increased risk of stroke in a study done in Australia, but reductions in risk in continental Europe. In the UK, 98% of tea consumed is taken with milk, and there are many people of British descent living in Australia. In contrast, in continental Europe, tea is almost exclusively consumed without milk.
Stangl stressed to heartwire that this study looked only at the effects of tea on endothelial function and that there have been conflicting reports on whether milk may block the antioxidant properties of tea. It is also important to note that the milk is not in itself deleterious, she says; rather, it blocks the benefits of the tea, which her team believe results from caseins in the milk binding to catechins in the tea. The latter are thought to mediate the effects on endothelial cells.
But much more research is needed to fully determine the impact of adding milk to tea, Stangl concludes. She hopes to do further research on the subject, with a four-week study, but admits that nutritional studies like this are notoriously difficult to conduct.
Results do not apply to green tea
"It is important to bear in mind that green tea is almost exclusively drunk without milk. So we are talking only about those countries and regions where black tea is consumed and where milk is added," lead author Lorenz notes. "We certainly don't want to dismiss the consumption of black tea: the results of our study merely attempt to encourage people to consider that, while the addition of milk may improve its taste, it may also lower its health-protective properties."
Stangl says her team is now in the process of comparing the effects of green and black tea on vascular function. "It's an ongoing question whether green tea, with its higher catechin content, is superior to black tea in regard to endothelial function. In addition, because of the antiatherogenic potential of tea ingredients, we want to investigate the effects of the ingredients on chronic cardiovascular processes such as the development of restenosis after catheter procedures."
Lorenz M, Jochmann N, von Krosigk A, et al. Addition of milk prevents vascular protective effects of tea. Eur Heart J 2007; DOI:10.1093/eurheartj/ehl442. Available at:
http://eurheartj.oxfordjournals.org.
Peters U, Poole C, and Arab L. Does tea affect cardiovascular disease? A meta-analysis. Am J Epidemiol 2001: 154:495-503.
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