Now, a study appearing in the medical journal Circulation: Journal of the American Heart Association provides a possible answer. The study indicates that drinking black tea may improve functioning of cells lining blood vessels in the heart and elsewhere-- known as endothelial cells -- which help regulate flow of blood. Abnormal functioning of these endothelial cells has been shown to be a key step on the path to the artery hardening disease atherosclerosis, which causes coronary artery disease.
Study author Joseph A. Vita, MD, says black tea appears to have beneficial effects on endothelial cells in both the short term and the longer term. "There is benefit two hours after drinking black tea, and if people continue to drink tea for a month the benefit is sustained," Vita tells WebMD. "Our study provides a plausible explanation for why tea may be beneficial for coronary artery disease."
He is professor of medicine at Boston University School of Medicine.
Black and green teas contain antioxidants, which are believed to neutralize the harmful effects of the body's own natural chemical processes when cells are oxidized. Antioxidants are found in a host of foods, in addition to tea, and have been linked to reduced risk for heart disease, osteoporosis, and some cancers.
But Vita cautions that the study does not prove drinking black tea will prevent heart attacks and stroke. "We can't conclude that you should drink tea to prevent heart attacks, but the study gives us some insight into how tea might be working," Vita tells WebMD.
In the study, 50 patients with coronary artery disease were randomly assigned to drink either tea or water. Two hours after drinking about two cups of either beverage, ultrasound imaging was used to measure the ability of the brachial artery -- an artery that runs down the arm -- to dilate and allow blood to flow. Longer-term results were determined using the same method after drinking about four cups of tea or water daily for four weeks, according to the report.
Results indicate that both short- and longer-term consumption of tea improved function of the artery, while consumption of water had no effect.
The study was supported by the Tea Trade Health Research Association, which is funded by tea manufacturers. Vita is a paid advisor to tea maker Lipton Inc.
"It's promising, but not conclusive," says nutrition and heart disease expert Barbara Howard, PhD, who reviewed the report for WebMD. "Endothelial function is one piece of coronary artery disease, and it is an area where antioxidants have been said to act."
She is president of MedStar Research Institute, which is the research arm of the MedStar Health System, with hospitals in Washington and Baltimore, Md.
Howard stressed that drinking tea is only one part of a balanced diet of fruits, vegetables, and grains. "Just drinking tea is a not a substitute for a balanced diet," she tells WebMD. "People want a magic bullet. They want to drink 10 cups of tea then go to McDonalds all day. That won't cut it."