High blood sugar levels injure the retina, the film in the camera. Since the blood vessels are critical to the normal function of the retina their resistance to injury is very important to vision.
Some people have greater resistance to high blood sugar than others.
Chronically elevated blood sugar levels eventually cause some damage in most people. In young people it may take longer than in older patients. There is good evidence that well controlled diabetes can prevent not only the eye, but also the heart, kidney, nerve and other blood vessel difficulties that can occur in the disease - if that control starts early and continues throughout life. These tests measure the sugar levels in the red blood cells of the blood stream. The sugar level in these cells does not change as quickly as the usual serum blood sugar and this gives an indication of control over a 6-8 week period.