Drinking caffeinated green tea and coffee may protect women and overweight men against developing type 2 (non-insulin dependent) diabetes, according to a Japanese study.

Among 17,413 men and women aged 40 to 65 who were free of diabetes at baseline, new cases of diabetes occurred in 231 men and 213 women over a 5-year follow up period.

Compared to subjects who did not consume these beverages, those who reported drinking six or more cups of green day per day were 33 percent less likely to develop type 2 diabetes. Those who reported drinking three or more cups of coffee per day were 42 percent less likely to develop the condition than non-coffee drinkers.

Drinking black or oolong teas did not appear to protect against type 2 diabetes.

The apparent protection was most pronounced in women and overweight men - persons at higher risk for insulin resistance, a precursor to diabetes, Dr. Hiroyasu Iso from Osaka University, Japan, and colleagues note in the Annals of Internal Medicine.

These findings from a Japanese population mirror results obtained in European and US populations in which people who drank seven or more cups of coffee per day had a 29 to 52 percent reduced risk of diabetes compared with those who drank fewer cups of coffee per day or none at all.

The researchers think these associations are mostly due to caffeine, because green tea and coffee are both major sources of caffeine in Japan.

Clinical studies are needed to confirm the protective effect of green tea and coffee for type 2 diabetes, the authors emphasize in their report.

SOURCE: Annals of Internal Medicine April 18, 2006.