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Why Is My Fasting Blood Glucose Level High?

author image Martin Booe
Martin Booe writes about health, wellness and the blues. His byline has appeared in the Washington Post, the Los Angeles Times and Bon Appetit. He lives in Los Angeles.
Why Is My Fasting Blood Glucose Level High?
A person's hands testing blood glucose levels on their finger. Photo Credit Wavebreakmedia Ltd/Wavebreak Media/Getty Images

The body uses complex mechanisms to maintain blood sugar within a healthy, narrow range. Your fasting blood glucose, or FBG, is the amount of sugar in your blood after not eating or drinking fluids other than water for at least 8 hours. Elevated FBG can mean the body's ability to regulate glucose is faltering, which most commonly indicates either prediabetes or diabetes. Less frequently, a high FBG may be due to another medical condition or a medication side effect. For those already living with diabetes or prediabetes, high FBG levels often indicate the condition is progressing or a medication adjustment is needed. Whatever your current status, it's important to determine the cause of an elevated FBG so you can receive appropriate treatment.


Prediabetes occurs prior to the onset of type 2 diabetes (T2DM), as the body loses its ability to metabolize glucose normally. The condition causes no symptoms, so the first detectable sign is usually an elevated FBG. With prediabetes, FBG is higher than normal but not high enough to signal T2DM. People with prediabetes have a metabolic abnormality called insulin resistance, in which the body doesn't respond normally to the hormone insulin. This pancreatic hormone enables body tissues to absorb blood glucose and use it to generate energy.

With insulin resistance, the pancreas initially produces more insulin to compensate. Prediabetes develops when the extra insulin isn't sufficient to keep blood glucose levels normal. Insulin resistance also causes the liver to overproduce glucose, further contributing to high blood sugar. Without intervention, prediabetes usually progresses to T2DM. But with modest weight loss, regular exercise and possibly a medicine called metformin (Fortamet, Glucophage, Glumetza), the development of T2DM can often be delayed or prevented entirely.


Type 1 diabetes occurs when the insulin-producing cells are mistakenly destroyed by the immune system, leaving the body with little to no insulin. T2DM develops due to a combination of insulin resistance and failing insulin production due to degeneration of the pancreatic cells that make the hormone. High blood glucose levels characterize all forms of diabetes.

According to the American Diabetes Association, a normal FBG reading is less than 100 milligrams per deciliter (mg/dL). An FBG of 100 to 125 mg/dL is considered prediabetes, while 126 mg/dL or higher indicates diabetes. To confirm the diagnosis, the FBG may be repeated and other tests may be done. An oral glucose tolerance test evaluates how the body handles a specific amount of glucose over several hours. An A1C test reflects average blood sugar over the past 3 months. Both tests are commonly used to diagnose diabetes. Diabetes can lead to serious complications but doesn't have to interfere with a full life if well-managed.

Poor Blood Sugar Control

For people living with diabetes, several situations can lead to a sudden or gradual rise in FBG. T2DM may progress over time due to increasing insulin resistance, diminishing insulin production, or both -- causing higher blood sugar levels. High FBG can also be due to a problem called the dawn phenomenon. This scenario occurs because the liver naturally produces glucose in the early morning, but the insulin that would normally keep it in check isn't abundant enough to do the job.

Rebound hyperglycemia, also called the Somogyi effect, occurs when the liver floods the body with glucose in the early morning in response to a low blood sugar level during the night. Sorting out whether high morning FBG is due to the dawn phenomenon or the Somogyi effect usually requires checking blood glucose in the middle of the night. High morning FBG might also be due to a change in diet or not following your nutrition plan. Blood sugar levels commonly increase with infections as well. A rise in FBG may require a change in medications, dosages or timing.

Medications and Other Conditions

Some diseases and illnesses can cause a high FBG in people with or without existing diabetes. An overactive or underactive thyroid can cause high blood sugar levels. Notably, people with diabetes have an increased risk for thyroid disease. Certain adrenal gland tumors and disorders, such as Cushing disease, are other possible causes of high blood sugar. Inflammation of the pancreas, or pancreatitis, can damage the insulin-producing cells and lead to elevated blood glucose levels. Tumors that produce substances that interfere with blood sugar regulation are another possible cause of high FBG.

High blood sugar is also a possible side effect of certain medications in both people with diabetes and those without. As reported in the November 2011 issue of "Diabetes Spectrum," some common examples include:
-- Corticosteroids, such as prednisone and hydrocortisone, for inflammatory diseases like rheumatoid arthritis and ulcerative colitis.
-- Heart and blood pressure medications called beta-blockers, such as atenolol (Tenormin), metoprolol (Lopressor) and propranolol (Inderal).
-- Certain water pills for high blood pressure, such as hydrochlorothiazide (Microzide) and metolazone (Zaroxolyn).
-- Some types of antiviral medicines used to treat HIV/AIDS, including didanosine (Videx), stavudine (Zerit), zidovudine (Retrovir), indinavir (Crixivan) and lopinavir-ritonavir (Kaletra).

Next Steps and Precautions

High FBG is a warning sign that should not be ignored. Over time, high blood sugar damages blood vessels, nerves and organs. Possible complications of diabetes include vision problems, blindness, kidney failure, stroke, heart disease, loss of sensation and even amputations. The earlier prediabetes or diabetes is diagnosed and treated, the better the chances are of avoiding complications.

If you have diabetes and experience high morning blood sugars but are otherwise feeling well, contact your doctor as soon as possible. Seek emergency medical care if you have high blood sugar accompanied by nausea, vomiting, abdominal pain, weakness, sleepiness or confusion. These symptoms could indicate a diabetic crisis, which can be life-threatening.

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