The findings appear in the May 4 issue of the Journal of Agricultural and Food Chemistry.
"Black and green tea represent a potentially inexpensive, nontoxic, and, in fact, pleasurable [blood-sugar-lowering] agent," the researchers write. "Tea may be a simple, inexpensive means of preventing or retarding human diabetes and the ensuing complications."
In the study, the researchers gave green and black teas to diabetic rats for three months.
They found both kinds of tea inhibited diabetic cataracts. The teas also had a blood-sugar-lowering effect.
To get the same dose of tea given to the rats, a 143-pound person would have to drink 4.5 8-ounce cups of tea every day.
The researchers recommend that tea -- black and green -- should be studied for an antidiabetes effect in humans.