Insulin is a hormone produced in the pancreas in response to the level of blood sugar in the bloodstream. Insulin serves 2 purposes: 1) lowers blood sugar levels, and 2) increases the availability of sugar levels for normal cell functions.

In diabetics, either the pancreas secretes little or no insulin in response to rising blood sugar leveles, or the pancreas secretes normal levels, but the cells do not respond properly to the insulin. In either case, blood sugar levels become and remain abnormally high, can spill into urine, and result in many bodily complications. 

Green tea may provide benefits to diabetics. The polyphenol Epicatechin has been shown to bring down high levels of blood sugar.  In addition, Epicatechin also has insulin-like properties in the the body. 

Once again, Resveratrol and Red Wine grape extracts have similar effects on diabetics as they indeed possess many of the same polyphenols.