Why does high blood sugar slowly damage eyes, kidneys and nerves?

High blood sugar damages the body slowly, mainly through the blood vessels. The WHO says that over time diabetes can damage blood vessels in the heart, eyes, kidneys and nerves.

One explanation in the sources: the endothelial cells that line blood vessels take in more glucose than normal, because they do not depend on insulin to do so. They then form more surface glycoproteins than normal, and the basement membrane under them grows thicker and weaker. Another source describes advanced glycation end-products (AGEs) as having a central role in the pathophysiology of many complications of diabetes.

Complications are grouped by vessel size. Microvascular ones affect the small vessels: neuropathy (nerves), nephropathy (kidneys) and retinopathy (retina). Macrovascular ones affect the large vessels: cardiovascular disease, stroke and peripheral vascular disease.

Overall, complications are far less common and less severe in people with well-controlled blood sugar levels, according to the source. In the Diabetes Control and Complications Trial (DCCT) in type 1 diabetes, those who maintained good glycaemic control for a mean of 6.5 years were 76% less likely to develop diabetic retinopathy.