In someone with diabetes, sugar is unable to enter the cells and starts to build up in the person's blood. Over time, this extra sugar can cause problems in several places throughout the body.
For example, high levels of sugar in the blood, as well as high blood pressure, can make the small blood vessels in your eyes begin to swell and bleed. When the bleeding goes into the back part of the eye -- called the retina -- it can cause vision problems and may even lead to blindness in some cases.
Having high levels of sugar in the blood can also damage the blood vessels that bring oxygen and nutrients to the kidneys and harm the kidney's cells. This makes your kidneys have to work harder. When this happens, the waste products that the kidneys usually filter out start to build up in the blood. Sometimes, the kidneys will even stop working if the sugar level has been too high for a long time. This is called kidney failure.
Over time, high blood sugar can harm the small blood vessels that supply your nerves with blood, oxygen, and nutrients. As a result, less blood reaches the nerves, and they start becoming damaged as well. This is known as a "neuropathy."
Some problems in the nerves can make your feet feel like they are burning or tingling, or even numb. You might notice pain in your arms, legs, or hands as well. Nerve problems can also make it difficult for you to go to the bathroom, eat, or have sex
In addition to these types of problems, many people with diabetes have both high blood pressure and high cholesterol levels. This puts them at an increased risk for heart disease. Heart disease occurs when blood vessels in the heart become blocked by a build-up of fat and cholesterol called plaque. Eventually, this blocked blood vessel can lead to a heart attack.
These problems take time to appear, so diabetes can be damaging your body without you even being aware of it.