Researchers in London recently found that drinking a glass of water with one gram of instant black tea produced more robust blood-insulin levels than drinking a similar beverage with just caffeine. The study, published in the Journal of the American College of Nutrition, suggested that the polyphenols in black tea may have an insulin-stimulating effect. Participants drank water with sugar, and then a tea or caffeine beverage. After two hours the tea group had higher insulin levels, suggesting that black tea may help lower blood sugar and possibly maintain more consistent blood-insulin levels in healthy adults.