Green and Black Tea May Fight Cataracts Caused by Diabetes

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Green and Black Tea May Fight Cataracts Caused by Diabetes

Herbalists and doctors of Oriental medicine have long recommended tea to help treat myriad conditions, including indigestion, high cholesterol and weight gain.

Now, study results in the March 10, 2005, issue of the Journal of Agricultural and Food Chemistry (JAFC)¹ show that green and black tea may inhibit the development of diabetes-related cataracts.

Researchers monitored the effects of green and black tea in four groups of rats: a normal (non-diabetic) group; a diabetic control group; a group of diabetic rats given green tea; and a diabetic group given black tea. Over a three-month period, the teas were included in the rats' drinking water at a solution of 1.25%, equivalent to a human drinking 4.6 eight-ounce cups of tea per day. The chemical composition of the rats' blood and eye lenses was then analyzed to determine whether the teas lowered blood glucose levels and reduced the incidence of cataracts, a common side-effect associated with diabetes.

Results: The teas "significantly decreased glucose, and ... also inhibited the pathological pathways of diabetes in lens, plasma, and red blood cells," the researchers noted. On average, plasma glucose levels in the diabetic rats drinking tea were reduced between 28% and 32%, which corresponded favorably to a 2003 study of oolong tea in people with type 2 diabetes.² In addition, tea consumption appeared to reduce the severity of cataracts. Rats in the diabetic control group had an average cataract rating of 3.02 (0-4 scale; 0 = normal vision; 4 = nuclear opacity beginning). In diabetic rats given green tea, the average cataract rating was only 2.61; in diabetic rats taking black tea, the average rating was only 2.24.

"This paper is the first study to examine three mechanisms of diabetic pathology and show a relationship to a diabetic complication, cataracts," the authors wrote in their conclusion. "Both green and black teas appear to be of equal efficacy for improving the diabetic state by means of a hypoglycemic effect, which in turn inhibits the biochemical indicators of diabetic pathology."

"Black and green tea represent a potential inexpensive, nontoxic, and, in fact, pleasurable hypoglycemic agent," they added. The authors also suggested the need for further studies to determine the role of teas in the prevention or treatment of diabetes in humans.

1. Vinson JA, Zhang J. Black and green teas equally inhibit diabetic cataracts in a streptozotocin-induced rat model of diabetes. Journal of Agricultural and Food Chemistry 2005;53:3710-3713.

2. Hosoda K, Wang MF, Liao ML, et al. Antihyperglycemic effect of oolong tea in type 2 diabetes. Diabetes Care 2003;26:1714-1718.