In pre-diabetes, blood sugar levels are slightly higher than normal, but still not as high as in diabetes. If diabetes is "runaway blood sugar" think of pre-diabetes as blood sugar that is "halfway out the door."
People almost always develop pre-diabetes before they get type 2 diabetes. The rise in blood sugar levels that is seen in pre-diabetes starts when the body begins to develop a problem called "insulin resistance." Insulin is an important hormone that helps you to process glucose (blood sugar). If usual amounts of insulin can't trigger the body to move glucose out of the bloodstream and into your cells, then you have insulin resistance.
Once insulin resistance begins, it can worsen over time. When you have pre-diabetes, you make extra insulin to keep your sugar levels near to normal. Insulin resistance can worsen as you age, and it worsens with weight gain. If your insulin resistance progresses, eventually you can't compensate well enough by making extra insulin. When this occurs, your sugar levels will increase, and you will have diabetes.
Depending on what a blood sugar test finds, pre-diabetes can be more specifically called "impaired glucose (sugar) tolerance" or "impaired fasting glucose." Impaired fasting glucose means that blood sugar increase after you haven't eaten for a while – for example, in the morning, before breakfast.
Impaired glucose tolerance means that blood sugar levels reach a surprisingly high level after you eat sugar. To diagnose impaired glucose tolerance, doctors usually use what is called a "glucose tolerance test." For this test you drink a sugary solution, and then you have blood drawn after a short time.
Having pre-diabetes does not automatically mean you will get diabetes, but it does put you at an increased risk. Pre-diabetes is also a risk factor for heart disease. Like people with type 2 diabetes, those with pre-diabetes tend to be overweight, have high blood pressure and have unhealthy cholesterol levels.