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American Heart Association journal report: Black tea tames artery disease
DALLAS, July 10 -- A new study finds a strong link between drinking black tea and healthy arteries in individuals who have heart disease, according to a report in today's Circulation: Journal of the American Heart Association.
Preliminary results from the study were presented at the American Heart Association's Scientific Sessions 2000 last November.
Black tea offers some of the same benefits as other foods rich in the antioxidants known as flavonoids -- such as purple grape juice, onions and red wine. Flavonoids, the major antioxidants found in tea, have been shown to prevent the oxidation of low-density lipoproteins (LDL) the so-called "bad" cholesterol that leads to the formation of plaque in artery walls. However, very high concentrations of tea flavonoids are needed to produce this effect.
This study provides an alternative explanation for the heart benefits of tea. The new study shows that flavonoids improve the function of the vascular endothelium. The vascular endothelium, which forms the inner lining of cells in all blood vessels, produces substances that regulate the diameter of the blood vessel. It responds to minute to minute changes in the body's oxygen and blood flow needs, by causing blood vessels to expand (dilate) or contract. The vessels expand when the need for blood flow is higher, as occurs during exercise, and the vessels return to original size when the individual is at rest. The healthy endothelium also inhibits the formation of blood clots and the development of inflammation in the vessel wall. All of these functions are often impaired in individuals with atherosclerosis, a condition called endothelial dysfunction. Endothelial dysfunction is believed to be a contributing cause of heart attack and stroke.
"No previous study has examined the effects of tea on endothelium. The present study provides a plausible biological mechanism in humans to explain the inverse relationship between black tea and cardiovascular disease," says Joseph Vita, M.D., senior author of the study, and a professor of medicine at Boston University School of Medicine. "Our findings fit well with the growing appreciation that diet and lifestyle modifications are important approaches to the prevention and treatment of atherosclerosis."
Researchers measured the immediate and longer-term effects of black tea vs. water consumption on the arteries of 50 individuals who had coronary artery disease. They found that tea improved endothelial-dependent dilation in their arteries, while water had no effect.
Study participants were Boston Medical Center patients with either a history of revascularization (a procedure to re-open blocked arteries) or at least one coronary artery with greater than 70 percent blockage. Participants maintained their usual diet, but excluded red wine and other tea consumption during the eight-week study. Half the group drank tea for four weeks and half drank water, then they switched to the other beverage for another four weeks.
Endothelial function was assessed at six time points: at baseline; two hours after drinking 450 milliliters (mL) of black tea (acute tea); after four weeks of drinking 900 mL of black tea daily (chronic tea). In the chronic tea drinkers, function was measured two hours after drinking 450 mL of tea (acute-on-chronic tea). Measurements were also taken after four weeks of drinking 900 mL of water daily (chronic water); and two hours after drinking 450 mL of water (acute water).
To find out if the tea effects were related to caffeine, the researchers assessed endothelial function and blood pressure in 11 patients before and two hours after a 200-milligram dose of caffeine, the caffeine content of 450 mL of black tea.
Chronic water or tea consumption had no effect on fasting lipid, glucose, or vitamin C levels; however, acute and chronic tea consumption increased total catechins, a type of flavonoid found in tea. Total plasma catechin concentrations were 25.5 nanograms per milliliter (ng/mL) at baseline, 21.5 ng/mL after acute water, 25.1 ng/mL after chronic water, 32.8 ng/mL after acute tea, 34.6 ng/mL after chronic tea, and 33.7 ng/mL after acute-on-chronic tea.
The function of the endothelium was examined non-invasively by ultrasound of an artery in the arm (brachial artery). The technique uses a blood pressure cuff to stimulate an increase in blood flow in the arm. In healthy individuals, the endothelium releases nitric oxide when stimulated in this fashion, causing the artery to dilate (enlarge). However, in patients with coronary artery disease, the release of nitric oxide is diminished and the artery dilates less well.
Thus, this simple, non-invasive test provides information about the functioning of the endothelium, and previous studies have shown that the results in the arm also predict the function of the endothelium in the coronary arteries. The response of the blood vessel is termed flow-mediated dilation (FMD).
FMD was 6.0 percent at baseline, which is approximately half of the normal response. FMD was not significantly changed after acute water (5.7 percent) and chronic water (6.1 percent). However, FMD improved to 9.4 percent after acute tea, 9.5 percent after chronic tea, and 10.8 percent after acute-on-chronic tea.
"The results suggest that black tea acted to reverse endothelial dysfunction. Endothelial dysfunction is a key part of the development of cardiovascular disease," says Vita. "We've shown that black tea may help reverse this dysfunction and may reduce the risk of cardiac events."
Vita notes that acute tea consumption increased systolic blood pressure (the top number in a blood pressure reading) by about 5 millimeters of mercury (mmHg), however, he says this effect was not evident after chronic tea consumption. "The blood pressure response was likely due to the caffeine in tea. A caffeine pill had a similar systolic blood pressure response," he says.
This research was partially funded by grants from the National Institutes of Health and the AHA.
Co-authors include Stephen J. Duffy, Ph.D.; John F. Keaney, Jr., M.D.; Monika Holbrook, M.A.; Noyan Gokce, M.D.; Peter L. Swerdloff, B.A.; and Balz Frei, Ph.D.
NR01-1311 (Circ/Vita)Media Alert: Dr. Vita can be reached by phone at (617) 638-4885 and by e-mail at [email protected]. (Please do not publish contact information.)
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Circulation, 3-Jul-2001 (3-Jul-2001)