After drinking black tea instead of other beverages for four weeks, blood vessel functioning improved by nearly 50 percent, leading Dr. Vita to conclude that drinking black tea can prevent or reverse some types of artery diseases and reduce the risk of heart attack and stroke. Dr. Vita and other researchers believe that the tea's concentration of flavonoids, plant compounds that behave as antioxidants, prevent the formation of plaque in artery walls and may be responsible for these results.