system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does diabetes cause damage to the point that your eyes go blind and they have to amputate your limbs?
gpt
Accumulation of excess glucose in your blood eventually collects on the walls of your blood vessels in a process called glycosylation. This buildup of glucose leads to a series of changes in the blood vessel walls which causing the blood flow to certain organs to be blocked off.

Another place where glucose tends to accumulate is inside nerve cells. Eventually this buildup starts interfering with cell traffic and is toxic to these nerve cells. Diabetes results in many complications for the body, mainly through its effects on the blood vessels and nerves discussed above.

In the retina of your eye, this leads either your cells starving and dying off (retinopathy) or an abnormal response where the eye attempts to grow new blood vessels to supply itself. These new blood vessels are not very structurally sound and tend to leak, causing bleeds and the retinal cells to swell up thus impairing vision. High glucose also predisposes you to cataracts and other causes of blindness, but retinopathy is largely irreversible.

In your legs the nerves are often the first to be affected in chronic diabetes because they are the longest in your body. Because you start to lose sensation in your feet, your body does not automatically correct itself when it is putting too much pressure in a particular area e.g on your heels when walking or your toes in a tight fitting shoe. Compound this with the poor blood supply in diabetes, the skin gets damaged and heals poorly (an ulcer) and becomes easily infected. Your immune cells can't reach the wound site through the diabetic vessels to fight the infection so it spreads into your bone and up your leg. By this stage the best option is to amputate the leg before the bacteria reach the rest of your body.