The Japanese custom of drinking green tea came from China about 800 AD. The use of tea started when Buddhist monks, who had gone to China for study, returned to Japan bringing tea with them as a medicinal beverage. In the Kamakura era (1191-1333),the monk Eisai stressed the beneficial effects of tea in his book Maintaining Health by Drinking Tea (1211) : "Tea is a miraculous medicine for the maintenance of health. Tea has an extraordinary power to prolong life. Anywhere a person cultivates tea, long life will follow. In ancient and modern times, tea is the elixir that creates the mountain-dwelling immortal." From this passage we can see that green tea has from early times been highly valued as a powerful medication. But in recent years research into the effects of green tea has progressed so far it can now provide scientific confirmation for the legendary saying that "Tea is a miraculous medicine for the maintenance of health." It is becoming increasingly clear that green tea has a broad efficacy in preventing disease. 

(#1) I.Oguni and Y. Hara,"Green tea has many medicinal activities for preventing disease such as cancer, cardio-vascular diseases and diabetes". (published by The Chunichi-shinbun, Nagoya, Japan),PP.1-289 (1990).