Green tea has an antidiabetic effect, because the polyphenols in green tea and oolong are very effective in reducing blood glucose levels. Sugars and carbohydrates in food are digested mainly in the duodenum, turned into glucose and then absorbed into the bloodstream. The agent that regulates glucose uptake into tissues is insulin, a substance secreted in the pancreas. Diabetes is a disease characterized by insufficient secretion of insulin which prevents proper absorption of glucose and this leads to an increased level of glucose.
60 years ago, Dr. Minowada of Kyoto University noticed that significantly decreased glucose levels existing in the urine in diabetic patients who participated in chanoyu (tea ceremony). So he published a study which shows that green tea concentrate (as traditional tea ceremony drink) may lower blood glucose levels. But this study was ignored due to outbreak of World War II. But special interest in Japanese cuisine in recent times, made to revert to green tea and its ability to lower blood sugar levels.
According to some new U.S. study involving 40 types of teas, only 3 of these teas have shown a significant improvement in reducing blood glucose. Demonstrated that only true tea, produced from the leaves of Camellia sinensis is native to China, has beneficial effects. The remaining commercial teas had only minimal efficiency in improving insulin activity in the body. The conclusion is that only by eating quality teas can take full advantage of the beneficial effects of green and oolong teas.