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Researchers warn milk eliminates
cardiovascular health benefits of tea.
Research
published online today in the European Heart Journal has found that the
protective effect that tea has on the
cardiovascular system is totally
wiped out by adding milk.
Tests on volunteers showed that black tea significantly improves the
ability of the arteries to relax and expand, but adding milk completely
blunts the effect. Supporting tests on rat aortas (aortic rings) and
endothelial (lining) cells showed that tea relaxed the aortic rings by
producing nitric oxide, which promotes dilation of blood vessels. But,
again, adding milk blocked the effect.
The findings, by cardiologists and scientists from the Charité
Hospital, Universitätsmedizin-Berlin, Germany, are bad news for
tea-drinking nations like the British, who normally add milk to their
beverage. The results have led the
researchers to suggest that tea
drinkers who customarily add milk should consider omitting it some of
the time.
Their study showed that the culprit in milk is a group of proteins
called caseins, which they found interacted with the tea to decrease
the concentration of catechins in the beverage. Catechins are the
flavonoids in tea that mainly contribute to its protection against
cardiovascular disease.
Senior researcher Dr Verena Stangl, Professor of Cardiology
(Molecular Atherosclerosis) at the hospital, said: “There is a broad
body of evidence from experimental and clinical studies indicating that
tea exerts antioxidative, anti-inflammatory and vasodilating effects,
thereby protecting against cardiovascular diseases. As worldwide tea
consumption is second only to that of water, its beneficial effects
represent an important public health issue. But, up to now, it’s not
been known whether adding milk to tea, as widely practised in the UK
and some other countries, influences these protective properties. So,
we decided to investigate the effects of tea, with and without milk, on
endothelial function, because that is a sensitive indicator of what is
happening to blood vessels.”
Sixteen healthy postmenopausal women drank either half a litre of
freshly brewed black tea, black tea with 10% skimmed milk, or boiled
water (as a control) on three separate occasions under the same
conditions. The endothelial function of the brachial artery in the
forearm 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.
Said first author Dr Mario Lorenz, a molecular biologist: “We found
that, whereas drinking tea significantly increased the ability of the
artery to relax and expand to accommodate increased blood flow compared
with drinking water, the addition of milk completely prevents the
biological effect. To extend our findings to a functional model, we
determined vasodilation in rat aortic rings by exposing them to tea on
its own and tea with individual milk proteins added, and got the same
result.”
Milk contains a number of different proteins: by testing each one
separately, the researchers found that it was the three caseins that
accounted for the inhibiting effect, probably by forming complexes with
tea catechins.
Said Dr Stangl: “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.”
She said their findings could also have implications for cancer,
against which tea has also been shown to be protective. “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 that we re-examine the association between tea
consumption and cancer protection, to see if that is the case.”
Said Dr Lorenz: “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. 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.”
Dr Stangl said that another important lesson from their research was
that it was vital in nutritional studies to exclude confounding factors
as far as possible. Often, the effects of a single nutritional compound
or beverage such as red wine, olive oil and so on, are analysed. But,
it is difficult to assign clearly the observed effects and separate
them from the surrounding food matrix (such as adding milk) that may
bias results. It was therefore important to collect all data accurately
and include potentially confounding factors in the analysis.
She said that the team was 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 (re-narrowing of
arteries) after catheter procedures.”
Source: European Society of Cardiology