From the Harvard Heart Letter:
“Tea is a good source of compounds known as catechins and epicatechins, which are thought to be responsible for tea’s beneficial health effects,” says Dr. Howard Sesso, associate professor of medicine at Harvard Medical School and associate epidemiologist at Brigham and Women’s Hospital. These compounds belong to a group of plant chemicals called flavonoids. Research suggests that flavonoids help quell inflammation, and that in turn may reduce plaque buildup inside arteries. Green tea has slightly higher amounts of these chemicals than black tea. Both black and green teas also contain modest amounts of caffeine, ranging from about 20 to 45 milligrams per 8-ounce cup. That’s roughly half the amount of caffeine in the same amount of coffee.
Short-term studies have shown that drinking tea may improve vascular reactivity—a measure of how well your blood vessels respond to physical or emotional stress. There’s also evidence that drinking either black or green tea may lower harmful LDL cholesterol levels. Blood pressure may also dip slightly in people who drink tea, but results from these studies have been mixed.
Several large, population-based studies show that people who regularly drink black or green tea may be less likely to have heart attacks and strokes. However, people who drink tea tend to be different from people who don’t drink tea. “We can’t quite disentangle whether it’s their tea drinking or something else those people are doing that lowers their risk of cardiovascular disease,” explains Dr. Sesso. “Some experts believe that tea may have cardiovascular benefits, but it’s not considered a slam-dunk proposition.”