Tea has been cultivated and consumed by humans throughout much of recorded history. Flavonoids are present in abundance in tea and are considered to be powerful antioxidants. It has been proposed that these substances might prevent the oxidation of LDL ("bad") cholesterol in the body. Oxidation of LDL cholesterol is one step in the development of coronary artery plaque, or coronary heart disease (CHD).
Black tea may also serve to reverse some of the changes in the blood vessels associated with CHD. When the lining of the blood vessels (the endothelium) is altered, it becomes easier for plaque to form. In a study involving 66 patients with proven CHD, the consumption of black tea appeared to improve this endothelial dysfunction (Circulation 2001; 104, 151). A study that examined the eating habits of 1,900 people reported that those individuals who had previously had a heart attack and who drank large quantities of green tea were reported to have a reduction in the mortality rate compared with those who consumed little or no green tea (Circulation 2002; 105, 476).
Tea consumption alone is not associated with any significant side effects or toxicity other than the typical caffeine effects of nervousness and irritability in sensitive persons. Green tea may interfere with calcium absorption and increase the stimulatory effects of some asthma medications.