It should be said that much of the green tea that is available in stores has a lot of sugar, usually in the form of high fructose corn syrup. If you have diabetes or diabetic symptoms, then you would want to monitor your intake of green tea that is sweetened. Tea that you brew yourself would be the most beneficial in that case.
There have been various studies on the health benefits of green tea. Clearly there are health benefits. However, research with regard to green tea's affects on diabetes is not as widely proven, but the following is a link to a scientific article that studied green tea's affects in both a mice and a human study.
Research tells us that green tea has many health benefits. However this research has been limited to mostly in the lab. There are not a lot of studies from the "real world" and the benefits of green tea.
There is no real proof that this is true for human beings. It has clearly worked in lab animals, though. In human beings, it seems that it may do something to help people who are just slightly diabetic. But there is no proof that you can help yourself by drinking tea if you are seriously diabetic.
Here's a link:
Since green tea contains the antioxidants polyphenols and polysaccharides that are known for lowering the blood sugar, we can say that green tea is helpful in type-two diabetes.
It is no need to say that you are not allowed to ignore your doctor's recommendations concerning diabetes.
Anyway, drinking a cup of green tea is good for your health, in general.