From the National Kidney Foundation news article:
If your diabetes is not well controlled, the sugar level in your blood goes up. This is called “hyperglycemia” (high blood sugar). High blood sugar can cause damage to very small blood vessels in your body. Imagine what happens to sugar when it is left unwrapped overnight. It gets sticky. Now imagine how sugar “sticks” to your small blood vessels and makes it hard for blood to get to your organs. Damage to blood vessels occurs most often in the eyes, heart, nerves, feet, and kidneys.
Read more at the link: