Although caffeine has received bad press in the West, it might have been this very quality in tea, which instantly invigorates the body, that attracted early religious leaders and physicians. Scientists have identified caffeine as one of a potent group of drugs called methylxanthines, which are found in over sixty plant species. Caffeine blocks adenosine, a natural tranquilizer, so the brain is overstimulated, thus heightening intellectual activity. The highest quality Japanese green teas, which are picked from tender spring leaves high in caffeine, may have more caffeine than coffee. But their high tannin and vitamin C content is believed to moderate the stimulating effect. This synergistic quality of vitamin C and tannin with caffeine may explain why Zen monks use green tea during long meditations to stay alert but calm.
Other side effects of caffeine, such as widening of the arteries and pulmonary vessels, increasing blood flow to the heart, and stimulating kidney and bladder functions, were viewed as medicinal effects by ancient healers, who used green tea with moderation.