American Diabetes Association Complete Guide to Diabetes: The Ultimate Home Reference from the Diabetes Experts


Glucose Facts

• What Is Glucose?

• How Glucose Works

• Glucose Tests

It sounds like a technical term, but glucose is just a fancy name for sugar. Your health care provider tested the glucose in your blood when he or she diagnosed you with diabetes. Perhaps it was the first time you heard the term. In any case, you’ll probably hear it a lot more.

Glucose is probably the most important term you’ll need to understand for managing your diabetes. In this chapter, we’ll discuss what it is and how it affects your diabetes. We’ll also cover the tests that measure glucose in your blood.

What Is Glucose?

Glucose is a simple sugar in your blood and your body’s main source of energy. It is called blood glucose or sometimes blood sugar. It comes primarily from carbohydrates in many of the foods that you eat, including sugary treats like cupcakes, starchy things like vegetables, and grains like breakfast cereal.

How Glucose Works

The purpose of glucose is to provide energy. The key to providing energy is eating food. Yes, food!

When people eat certain foods, their bodies break them down into simple sugars called glucose. Glucose goes into their bloodstream, where it travels to all of the cells in their body. Cells use this glucose for energy to do all the big and small jobs that keep the body humming. To operate at peak performance, your body needs to keep blood glucose levels within a normal range. If you have too little glucose, you run out of energy; too much, and the extra glucose will be stored and cause weight gain. In people with diabetes, excess glucose stays in the blood and may also damage different parts of the body, such as blood vessels and nerves.


The optimal, “normal” range of blood glucose is measured using a plasma glucose test. A normal fasting glucose level is 70–99 milligrams per 1 deciliter of blood, which is abbreviated as 70–99 mg/dl. Fasting means that you haven’t eaten for at least eight hours before the test.

But how does the human body regulate glucose levels? This feat requires a delicate balance of hormones and stored glucose.


The pancreas is an organ that does not get much attention—unless it stops doing its job. It is an important player in your digestive system, and it sits right behind your stomach.

The pancreas secretes many hormones, including two very important hormones for regulating glucose: insulin and glucagon. These hormones are made by cells in the pancreas known as the islets of Langerhans. Within the islets of Langerhans, alpha cells produce glucagon and beta cells produce insulin.

Islets of Langerhans

The islets of Langerhans are named for the German physician Paul Langerhans, who first described them in 1869 while still a medical student. He presented a thesis that described these cells as looking different than other cells in the pancreas. However, Langerhans could not determine what these cells did.

Insulin and Glucagon

Insulin and glucagon are two important hormones that help keep your blood glucose on target. Insulin helps move glucose (energy) to your cells and glucagon helps raise blood glucose.

Insulin is the “special key” to make sure glucose effectively gets to cells. Insulin also prevents the liver from making too much glucose when you are not eating. The pancreas needs to produce the right amount of insulin to move glucose from the bloodstream to cells. It releases insulin in response to rising blood glucose levels during snacks and meals. It also releases a small, steady stream of insulin throughout the day. This keeps your liver from making too much glucose between meals or overnight.

Conversely, the pancreas produces glucagon to raise blood glucose levels between meals or during exercise when your body uses a lot of energy. Glucagon raises blood glucose by stimulating the liver to release stored glucose.

We’ve only just recently begun to understand the delicate and complicated process of insulin and glucagon secretion. Together, these processes work to maintain a steady level of glucose in the blood all the time. This process is central to understanding how diabetes develops and how to treat it.

People have known about diabetes since antiquity. Unfortunately, for thousands of years, they didn’t know how it worked or how to treat it. Beginning in the Enlightenment (17th and 18th centuries), this began to change.

Early Glucose Discoveries

• In 1776, scientists discovered that glucose was in the blood of both people with and people without diabetes. That led them to suspect that people with diabetes pass glucose from blood into urine. But they didn’t know how.

• Over one hundred years later, in 1889, two German physiologists, Oskar Minkowski and Joseph von Mering, accidently discovered that the pancreas is involved in diabetes. As part of their experiments on how the body uses fat, they removed the pancreas of a laboratory dog. Much to their astonishment, the dog urinated again and again. Luckily, the scientists tested the dog’s urine for glucose. Sure enough, the dog had developed diabetes when its pancreas was removed. This led the scientists to suspect that some substance in the pancreas somehow prevented diabetes.

• It would take another 30 years for scientists to find this magic pancreatic substance—insulin. This discovery, one of the greatest of modern medical history, is discussed in chapter 13.

Too Much Glucose

People with diabetes don’t produce enough or don’t produce any insulin. Or they don’t use insulin effectively. This results in a buildup of too much glucose in their blood.

In all types of diabetes, glucose does not get into the cells that need it and instead builds up in the bloodstream. In addition, cells don’t have the energy they need to do their work.

The buildup of glucose in blood can have various effects, depending on its severity. For one thing, the body may try to flush out excess glucose by filtering it through the kidneys and expelling it from the body in urine. Therefore, people with high levels of glucose in their blood may urinate a lot or feel thirsty because of dehydration.

In other cases, the body may try to “grab” energy from muscle and stored fat cells because it can’t get energy from glucose in the blood. This can cause muscle deterioration and weight loss.

Symptoms of High Blood Glucose

People with very high blood glucose levels share many similar symptoms. You may have had some of these symptoms before you were diagnosed with diabetes.

Some Early Symptoms of Diabetes

• Extreme thirst

• A frequent need to urinate

• Blurred vision

• A feeling of being tired most of the time for no apparent reason

However, some people do not have any symptoms of high blood glucose. Because the signs of diabetes can be so mild, many people walk around for years with dangerously high glucose levels, which can lead to long-term damage. In fact, many people don’t realize they have diabetes until they begin to suffer from complications of the disease.

Of course, only a health care provider can make an accurate diagnosis of diabetes. A trip to the doctor is the next logical step if you or someone you love has symptoms of diabetes or you run a risk of developing diabetes.

Glucose Tests

Although you or your health care provider may suspect that you have diabetes because of your symptoms, the only sure way to tell is with glucose tests.

Diabetes causes your blood glucose levels to be above normal some or all of the time. Your blood glucose levels may be high even though you haven’t eaten recently. So, checking the amount of glucose in your blood can determine whether you have diabetes or not. There are four types of tests used to diagnose diabetes: A1C test, fasting plasma glucose test, random plasma glucose test, and two-hour oral glucose tolerance test.

Plasma glucose tests measure the amount of glucose in the plasma of your blood to determine whether the level is higher than normal. Plasma glucose is different from whole-blood glucose, which contains blood cells. In the laboratory, your blood sample is spun in a machine to remove blood cells, platelets, and cell debris. Only the plasma is left. Scientists measure the amount of glucose in the plasma, and these numbers can be 15% higher than whole-blood readings.

A1C Test

• The A1C test can be used to diagnose diabetes.

• Blood is collected from a fingerstick or vein.

• A1C values represent average blood glucose levels over the past 2–3 months.

• The test measures the concentration of hemoglobin molecules that have glucose attached to them. The measure is given as a percentage. An 8% level means that 8% of your molecules are glycated (sugar coated).

• An A1C of 6.5% or higher is used to diagnose diabetes.

Fasting Plasma Glucose Test

• In diabetes, extra glucose remains in the blood, even after fasting.

• For this test, you will be asked not to eat or drink anything but water for at least 8–10 hours. Then, a sample of your blood is taken and the amount of glucose in the blood is measured.

• For those without diabetes, the amount of glucose after fasting is usually less than 100 mg/dl.

• However, when the amount of fasting plasma glucose is 126 mg/dl or higher, diabetes is suspected. A firm diagnosis of diabetes is made when two fasting plasma glucose tests, done on different days, are at least 126 mg/dl.

Random Plasma Glucose Test

• The simplest way to detect diabetes.

• This test measures the amount of plasma glucose at any given time and is done without fasting.

• You may be diagnosed with diabetes if your plasma glucose is 200 mg/dl or higher and you have obvious symptoms, such as frequent urination, intense thirst, blurred vision, unexplained weight loss, and extreme tiredness.

Oral Glucose Tolerance Test

• This test can also be used to diagnose diabetes.

• For this test, you will be asked not to eat or drink anything overnight. Then, in the morning, a sample of your blood is taken before and two hours after you have a drink that contains glucose.

• If your fasting plasma glucose is 126 mg/dl or higher and/or your post-drink plasma glucose is 200 mg/dl or higher, then you will be diagnosed with diabetes regardless of your symptoms.

Categories of Increased Risk for Diabetes

Sometimes increased risk for diabetes is apparent before diabetes develops. If your fasting plasma glucose test is greater than 100 mg/dl but less than 126 mg/dl, you may have impaired fasting glucose. Some people also have impaired glucose tolerance, a condition in which blood plasma glucose levels are higher than normal (140 mg/dl to 199 mg/dl) 2 hours after the start of an oral glucose tolerance test. If your A1C is 5.7–6.4%, you may be at similar risk.

If you have impaired fasting glucose and/or impaired glucose tolerance, you may be diagnosed with prediabetes. This is not the same as having diabetes, but it sometimes occurs before diabetes develops.

If you have been diagnosed with prediabetes, you will want to have your blood glucose tested routinely and watch for symptoms of diabetes. Also, you need to talk with your health care provider about reducing your risk of heart disease. Keeping your weight in the healthy range and exercising regularly will lower your chances of developing diabetes.


Some people with prediabetes never get diabetes, especially if they make lifestyle changes that help improve their health, such as exercising more, making healthier food choices, and losing some unwanted weight. However, some of the same problems that result from having diabetes also occur in people with prediabetes.

Which Type of Diabetes?

If tests reveal that you have diabetes (and you’re not pregnant), the next question is whether you have type 1, type 2, or another type of diabetes. Although the symptoms and blood test results can be similar for both type 1 and type 2 diabetes, the causes are very different.

If you find an error or have any questions, please email us at Thank you!