The prospective cohort study followed 17 413 persons in 25 different communities across Japan for five years. The researchers, from Osaka University, reported that those people who had high intakes of coffee and green tea were associated with lower rates of diabetes.
In the US, there are over 20 million people with diabetes, equal to seven per cent of the population. The total costs are thought to be as much as $132bn, with $92bn being direct costs from medication, according to 2002 American Diabetes Association figures.
The study, published in the Annals of Internal Medicine (Vol. 144, pp. 554-562), gathered data on beverage consumption of the Japanese subjects, aged between 40 and 65, using food frequency questionnaires. The researchers focussed on consumption of coffee, and black, green, and oolong teas.
While no relationship was found between black or oolong tea intake and the risk of type-2 diabetes, the researchers found that, after adjusting for risk factors such as age, sex and body mass index, people who drank more than six cups of green tea per day had a 33 percent lower risk of diabetes, compared to people who drank less than one cup per week.
Drinking three or more cups of coffee per day was associated with a 42 percent lower risk of diabetes. The average American consumption is 3.2 cups of coffee per day.
"Total caffeine intake from these beverages was associated with a 33 percent reduced risk for diabetes," reported lead author Hiroyasu Iso.
The Japanese researchers also found that the benefits of caffeine consumption were more pronounced in women and in overweight men.
The study has several limitations, including using self-reporting of diabetes, and no data collection on caffeine-containing soda drinks.
Last year, a Canadian study (Diabetes Care, Vol 28, pp 566-572), reported that coffee could reduce insulin sensitivity. However, coffee contains several other substances that may affect sugar metabolism, such as antioxidants, potassium and magnesium, suggested the researchers.
Coffee is one of the world's largest traded commodities produced in more than 60 countries and generating more than $70bn in retail sales a year.
The global tea market is worth about $941m. Green tea accounts for about 20 per cent of total global production, while black tea (green tea that has been oxidized by fermentation) accounts for about 78 per cent.