Medically Reviewed By: Tom Iarocci, MD

Many times when people hear the word diabetes, they associate it with sweet foods and high blood sugar levels. This is an accurate association, since all forms of diabetes mellitus involve a lack of control of blood sugar levels. High blood sugar, also known as hyperglycemia, can affect people with both type 1 and type 2 diabetes. High blood sugar can occur in response to food or at baseline, in the fasting state, before you have eaten anything. In fasting hyperglycemia, blood sugar is elevated despite not having eaten or consumed anything but water for at least 8 hours. The other kind of high blood sugar is called postprandial or after-meal hyperglycemia. Postprandial levels are taken 2 hours after beginning a meal. In someone who does not have diabetes, it is normal for postprandial blood sugar to be less than 140 mg/dl. However, in those with diabetes the official target from the American Diabetes Association (ADA) is less than 180 mg/dl.

In terms of the long-term complications of diabetes, it is the cumulative or additive exposure to all those overages—over weeks, months, years and decades—that does damage to nerves, blood vessels, and many vital organs. But shorter-term complications along with high blood sugar are also possible.

When your body can’t use blood sugar properly, it goes to its backup energy sources, breaking down fats and proteins instead. When this occurs, the use of your body’s alternative fuel reserves produces acids known as ketones. Excess ketones build up in the blood and eventually make it into the urine. This condition is called diabetic ketoacidosis, or DKA, and it’s a serious complication of diabetes. People with type 1 diabetes are more prone to DKA. DKA usually develops slowly, starting with thirst, frequent urination and ketones in the urine; however, when vomiting occurs, the life-threatening course of DKA can develop in a few hours, with abdominal pain, difficulty breathing, confusion and lethargy.

Hyperglycemic hyperosmolar nonketotic syndrome or HHNS can happen in type 1 or type 2 diabetes, but it occurs most often in people with type 2 diabetes. HHNS only occurs when diabetes is uncontrolled. The body at first tries to get rid of the excess glucose by passing it into the urine. This can work up to a point, but the body starts running out of options when blood sugar continues to be elevated above the threshold for passing it in the urine. If HHNS continues, it can result in severe dehydration with seizures, coma and even death. Warning signs include blood sugar level over 600 mg/dl, symptoms of dehydration such as dry, parched mouth and extreme thirst, fever, confusion and symptoms involving the nervous system. The best way to avoid HHNS is to check blood sugar often.

Many different things can cause a person with diabetes to have high blood sugar, and there are different levels of hyperglycemia. For instance, fasting blood sugar levels of 160 – 240 mg/dL are too high and need to be addressed; however fasting levels 300 mg/dL and higher should prompt an immediate call to your doctor. These are general thresholds, only; warning levels that your doctor wants to know about, as well as blood glucose target in general, may differ from one individual to the next, so always be on the same page with your doctor about what constitutes an emergency.

Things that can bring about hyperglycemia include the more common things like your intake of carbohydrates and the amount of insulin you took; however, a long list of other items can also factor in to your blood glucose levels such as stress, skipped or forgotten insulin doses, new medications, infection, and differing levels of exercise or physical activity.

To prevent high blood sugar, know your diet, test your blood sugar regularly, and tell your doctor about any odd blood sugar levels you may be experiencing.