Diabetes results in increased rates of atherosclerosis, neuropathies, blindness, and gangrene? why?

Simple answer is: High sugars increase activity of various biochemical pathways that damage tissues eye, kidney, nerve, blood vessels.

Good question: It is thought that the high levels of glucose cause damage to the nerves and the lining of the blood vessels. Nerve damage causes peripheral numbness and vascular damage causes poor circulation. Both contribute to problems with healing, open wounds, infections and gangrene. Blindness has to do with vessel injury, and can be slowed by an ophthalmologist.