Addition of Milk Prevents Vascular Protective Effects of Tea

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Endothelial Function, Milk and Tea Consumption

http://www.vitasearch.com/CP/experts/VStangAT04-11-07.htm

Prof. Dr. med. Verena Stangl

Charité der Humboldt-Universität

Universitätsmedizin Berlin

Medizinische Klinik mit Schwerpunkt

Kardiologie und Angiologie

Chariteplatz 1, 10117 Berlin

+49 (0) 30-4 50 51 31 42 / +49 (0) 30-4 50 51 31 53

+49 (0) 30-4 50 51 39 32 (FAX)

verena.stangl@...

" Addition of Milk Prevents Vascular Protective Effects of Tea, "

Eur Heart J, 007; 28(2): 219-23. 45109 (6/2007)

Kirk Hamilton: Can you please share with us your educational

background and current position?

Verena Stangl: I received my high school diploma in Germany,

basic medical training in France and specialist training in Germany

(Munich and Berlin). Since 2004 I have been a Professor of Cardiology

(molecular atherosclerosis) at the Charité Hospital, Universitätsmedizin

Berlin, Campus Mitte, Germany.

KH: What got you interested in studying the role of tea, specifically

black tea, and endothelial

function? How does black tea’s effect improve endothelial function? What

are the components

of black tea that provide this effect?

VS: Our research group has been studying the impact of nutritional

components on endothelial function for many years. We are especially

interested on the effects of tea and single tea compounds on endothelial

function. We have studied the ability of tea and tea substances to exert

vasodilation in vitro in isolated rat aortic rings and in vivo by

measuring flow-mediated dilation (FMD) in human volunteers. In addition,

we have measured the tea-mediated increase in the production of nitric

oxide. This gaseous molecule is responsible for mediating the

vasodilation in blood vessels.

Black tea is able to stimulate the production of nitric oxide in

endothelial cells, the inner cell lining of the blood vessels. An

increase in the amount of nitric oxide in the blood will lead to

improved vasodilation of the vessels. In addition, adding black tea to

isolated rat aortic rings leads to vasodilation of these rings. This

effect is prevented by blocking the enzyme that is producing nitric

oxide. Finally, consumption of black tea (as compared to water) by human

subjects leads to improved dilation of the brachial arteries after cuff

occlusion, measured as flow-mediated dilation. These findings confirm

the beneficial effects of tea in vitro are also valid in vivo.

Tea, especially black tea, contain numerous individual substances. The

group of compounds thought to be responsible for the beneficial effects

of tea are called polyphenols. Within the polyphenols, a number of

subgroups called flavonols, flavonoids, flavonols etc. exist. Generally,

the catechins (and in black tea also probably the theaflavins) are

thought to provide the beneficial biological effects of black (and also

green) tea. Although a number of additional compounds may be involved.

KH: Why did you think that milk might have an inhibitory effect on

black tea’s positive effect

on endothelial function? What is the physiology of this proposed effect?

VS: It has been known for many years that consumption of tea has a

beneficial impact on the cardiovascular systems and can lower the

incidence of heart diseases. However, whereas these beneficial effects

could be clearly demonstrated in East Asian countries, studies in Great

Britain often failed to show these beneficial effects of tea in terms of

cardiovascular health. In East Asian countries tea is preferentially

consumed as green tea, almost exclusively without milk. Whereas in Great

Britain and other regions in the world black tea is consumed, mostly

with the custom of adding milk to tea to improve its taste. In the

scientific literature there has been a debate for a while whether the

addition of milk can blunt or prevent its biological activities. The

results appeared rather conflicting and were mainly focused on measuring

antioxidant activities. So, we decided to look at the impact of milk on

the well-known beneficial effects of tea and on endothelial function.

Impaired endothelial function is an early and sensitive marker for

future cardiovascular events.

The physiological mechanisms by which tea has protective effects on

endothelial function are do to its strong antioxidative properties, the

ability to prevent the activation of platelets and immune cells in the

blood, as well its ability to produce nitric oxide in endothelial cells.

KH: Can you tell us about your study and the basic results?

VS: We measured flow-mediated dilation (FMD) in 16 healthy

postmenopausal women, and found that, whereas drinking black tea

significantly increased the ability of the artery to relax and expand to

accommodate increased blood flow compared with drinking water, the

addition of 10% milk to tea completely prevented these biological

effects of tea. To extend our findings to a functional model, we

measured vasodilation in isolated rat aortic rings by exposing them to

tea on its own and tea with milk, and got the same results. The

production of nitric oxide, the molecule mediating vasodilation, was

stimulated by tea alone in endothelial cells, whereas the addition of

milk prevented this effect. To identify the substances in milk

responsible for its inhibiting effects, we added individual milk

proteins to tea and measured the production of nitric oxide in

endothelial cells and vasodilation in rat aortic rings. We found that

among the various milk proteins, the group of milk caseins blocked the

stimulatory effect of tea on vasodilation and nitric oxide production.

By centrifugation of the beverage we could show that after addition of

milk to tea preferentially a group of tea substances, the catechins,

were complexed whereas a number of other individual tea substances were

not affected. Hence, we conclude that addition of milk may lead to

formation of complexes between milk caseins and tea catechins, and in

this way could lower the biological activity of tea.

KH: If you were strictly commenting on the medicinal use of black tea

would you then not

recommend the addition of milk to one’s tea?

VS: The aim of our study was to investigate the effects of tea with

or without milk on vascular function. Although we could clearly show

that the addition of milk to tea lowers its vascular protective effects,

our study was not designed to give nutritional recommendations. To this

intention larger endpoint studies are of course required. From our point

of view an individual should decide by him/herself whether or not to add

milk to black tea. Or maybe to decide to omit the milk from time to time.

KH: How significant could this be as inhibiting a cardio-preventive

strategy with the addition

of milk to black teas worldwide?

VS: At this point it should be noted that consumption of tea is only

one of numerous factors affecting vascular health. Thinking about

cardio-preventive strategies – physical exercise, not smoking, avoiding

being overweight and high blood pressure, to name a few, will have a

much greater impact on cardiovascular health. Without any doubt, a

healthy life style including a balanced diet will certainly improve

cardiovascular health. Whether the addition of milk to black tea will

have an impact on cardiovascular mortality needs still to be confirmed.

KH: Do you have any further comments on this interesting observation?

VS: Our findings that addition of milk to black tea suppresses the

protective effects of tea alone in terms of vascular function, point to

the general problem of conducting nutritional studies. It is necessary

to exclude confounding factors as much as possible. In particular, the

effects of a single nutritional compound or beverage such as grape

juice, chocolate, red wine, olive oil and so on, have to be separated

from the surrounding food matrix. It is therefore important to include

potentially confounding factors in the analysis.

--

ne Holden, MS, RD < fivestar@... >

" Ask the Parkinson Dietitian " http://www.parkinson.org/

" Eat well, stay well with Parkinson's disease "

" Parkinson's disease: Guidelines for Medical Nutrition Therapy "

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