Since ancient times, tea has been used as both beverage and medicine. Both black and green teas contain numerous active ingredients, including polyphenols and flavonoids, which are potent antioxidants.
One class of flavonoids called catechins has recently become the focus of widespread study for their anti-cancer potential. Tea is the best source of catechins in the human diet, and green tea contains about three times the quantity of catechins found in black tea.
In laboratory studies, green tea has been shown to slow or completely prevent cancer developent in colon, liver, breast and prostate cells. Other studies involving green tea have shown similar protective effects in tissues of the lung, skin and digestive tract.
Studies that track the diets of human subjects over several years (particularly studies conducted in Asia, where green tea consumption is common) have also associated regular usage of green tea with lower risk for bladder, colon, stomach, pancreatic and esophageal cancers.
AICR's second expert report, Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective, was unable to make a recommendation regarding the relationship between tea consumption and cancer risk due to insufficient evidence.
Note: Very high amounts of green tea components (usually associated with overdosage of green tea supplements) have been shown to interact with drugs that affect blood clotting such as aspirin and change the way the body metabolizes certain medications.