Too much glucose acts as a toxin to the lining of your blood vessels and injures both large vessels (like arteries) and small vessels (capillaries). And one thing is clear: damage to blood vessels is something that all the main complications of diabetes have in common. Damage to the large vessels of your body is called macroangiopathy, and it can lead to heart disease and stroke.

Many cells in the body, such as those of the skeletal muscles and liver, require insulin to admit glucose. This isn’t true of the cells that line blood vessels, called endothelial cells. They can’t reduce their glucose intake. When levels of blood glucose are very high, these cells develop high glucose levels as well. The high glucose levels start a whole chain of events inside the cells that damages them in numerous ways.

One of the ways high glucose injures vessels is by increasing the formation of molecules known as advanced glycation end products, or AGEs. In people without diabetes, AGEs are created and simply excreted in the urine as waste products. But in someone with diabetes, AGE levels may be 10 times higher than normal. As the body becomes unable to excrete all of them, AGEs build up in the blood vessel tissue. There they form bonds with many of the proteins present in the blood vessel walls, a process called glycosylation, which ultimately damages arteries.

In addition to AGEs, high blood glucose increases the activity of certain enzymes that stimulate the formation of other sugars. These abnormal sugars can’t be used by the cell. Instead, they accumulate within the cell and ultimately injure it.