Why Are My Fasting Glucose Levels High?

If you have high fasting blood glucose numbers, it may be because of how your body is using the hormones involved with glucose and diabetes—not the nighttime snack before bed.

a glucose monitor on a plate shaped like a clock
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If you have diabetes, you're probably tuned in to your body's blood glucose (AKA blood sugar) levels, and how they change throughout the day. But have you found yourself feeling stumped by high fasting blood glucose results? Join the club. Sometimes people find that their fasting blood glucose is even lower when they eat something before bed, further confusing the matter.

"It's logical for people to point the finger for high fasting blood sugar numbers at what they eat between dinner and bed, but surprisingly food isn't the lead villain," says Robert Chilton, M.D., a cardiologist and professor of medicine at the University of Texas Health Science Center at San Antonio. The true reason for why blood sugar goes up when fasting is related to compromised hormonal control of blood glucose levels.

Essential hormones for blood glucose control

During the years (up to a decade) that type 2 diabetes develops, the hormonal control of blood glucose breaks down. Four hormones are involved in glucose control:

  • Insulin, made in the beta cells of the pancreas, helps the body use glucose from food by enabling glucose to move into the body's cells for energy. People with type 2 diabetes have slowly dwindling insulin reserves.
  • Amylin, secreted from the beta cells, slows the release of glucose into the bloodstream after eating by slowing stomach-emptying and increasing the feeling of fullness. People with type 1 and type 2 diabetes are amylin-deficient.
  • Incretins, a group of hormones secreted from the intestines that includes glucagon-like peptide 1 (GLP-1), enhance the body's release of insulin after eating. This in turn slows stomach-emptying, promotes fullness, delays the release of glucose into the bloodstream and prevents the pancreas from releasing glucagon, putting less glucose into the blood.
  • Glucagon, made in the alpha cells of the pancreas, breaks down glucose stored in the liver and muscles and releases it to provide energy when glucose from food isn't available.

How the essential hormones work in the body

When diabetes is not present, the body handles the changing supply of and demand for glucose (the energy from food) 24 hours a day. This system involves the four hormones—what we'll call the messengers—and a continuous feedback loop that moves messages between the brain, gut, pancreas and liver. Here is how the system works in someone without diabetes:

When fasting

As blood glucose falls after peaking from the last food eaten, the pancreas puts out less insulin hormone. At the same time, two other hormones wane: amylin and glucagon-like peptide 1 (GLP-1), which help store and use glucose. A fourth hormone, glucagon, kicks into gear to offer a constant flow of glucose. Glucagon sends messages to the liver and muscles to make glucose from stored energy.

After eating

Food raises blood glucose and sends a message to the intestines to release GLP-1, which primes the insulin and amylin spigots. These hormones help cells use the glucose from food to fuel the body. The glucagon spigot turns off because there's little need for glucose from the liver or muscles when food is available. The impact of food on blood glucose, even for a large, high-fat meal, lasts less than six hours.

The Dawn Phenomenon and Somogyi Effect

Here are two other situations that may cause high fasting blood glucose levels:

Dawn phenomenon happens as part of the body's normal circadian rhythm to wake up and get going. Hormones, such as growth hormone and cortisol, are released and raise glucose. Without diabetes, the body simply responds to this early morning effect by putting out more of the hormones that keep blood glucose in control. That doesn't happen when you have type 1 or type 2 diabetes, which is why those with diabetes can sometimes wake up with high blood sugar levels.

Somogyi effect is very high fasting blood glucose thought to be caused by the liver making a lot of excess glucose in response to hypoglycemia (low blood glucose) during the night. Somogyi effect is uncommon in those with type 2 diabetes. There's controversy as to whether it even exists with the rapid- and long-acting insulin products available today.

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