In earlier studies, Dutch researchers discovered that there are compounds in coffee that aid the body's metabolism of sugar. Their study involved 17,000 men and women in the Netherlands. The results were published in November 2002, in the journal Lancet.
According to their study, people who drank 7 cups a day (or more) were 50% less likely to develop type 2 diabetes. Drinking less coffee had less of an impact on diabetes onset. Researchers are still looking at the connection between coffee and diabetes, and caution people that 7 cups of coffee per day is enough to create other health problems.
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.