A new study of coffee and diabetes (Jan 2004) has shown that men who drank 6 cups of coffee a day reduced their chances of developing type-2 diabetes by half, and women who drank the same amount cut their risk by 30 percent. 126,000 people filled out questionnaires over the past 12-18 years with information about their coffee intake and other health questions.
A number of older studies have shown that caffeine may increase your risk of developing diabetes. The theory is that the beneficial chemicals are able to offset the damage done by the caffeine. So drinking decaffeinated coffee would be the best bet if you are thinking of drinking coffee to prevent diabetes.
Tea also has an effect on diabetes. Drinking tea can improve insulin activity up to 15 times, and it can be black, green or oolong. Herbal teas don't have any effect. The active compounds don't last long in the body, so you would have to drink a cup or more of tea every few hours to maintain the benefit. The catch is that you should drink it without milk (even soy milk), because milk seems to interact with the necessary chemicals and render them unavailable to your body. Enjoy it but don't abuse it !