Fasting Blood Glucose Test Results Explained

Plain-English guide · Last updated October 20, 2026 · Based on NIH and U.S. government sources

The short answer: A fasting glucose of 99 mg/dL or below is considered normal, 100 to 125 mg/dL suggests prediabetes, and 126 mg/dL or higher may mean diabetes. Doctors usually confirm a high result with a repeat test or an A1C before making any diagnosis, and healthy habits can often help bring an elevated number down.

What the fasting glucose test measures

A fasting plasma glucose test measures the sugar (glucose) in your blood at a single point in time. For the most reliable result, it is usually done in the morning after you have had nothing to eat or drink except sips of water for at least 8 hours (NIDDK). It is different from an A1C, which shows your average over about 3 months. See our A1C guide.

Fasting glucose ranges

The NIDDK and MedlinePlus list the same cutoffs for adults who are not pregnant (NIDDK, MedlinePlus):

Sources: NIDDK and MedlinePlus. mmol/L values from MedlinePlus. Different cutoffs apply during pregnancy.
Fasting glucose	What it generally means
70 to 99 mg/dL (3.9 to 5.5 mmol/L)	Normal
100 to 125 mg/dL (5.6 to 6.9 mmol/L)	Prediabetes (impaired fasting glucose)
126 mg/dL (7 mmol/L) or higher	May mean diabetes

A single result is not a diagnosis. Your provider may repeat the test or order an A1C or glucose tolerance test to confirm (MedlinePlus).

What a high or low result may mean

These are possible causes, not a diagnosis.

High fasting glucose

Prediabetes and diabetes are the most common causes. Other possible causes MedlinePlus lists include an overactive thyroid, pancreas problems such as pancreatitis, stress from surgery, serious illness or trauma, adrenal gland problems, and certain medicines (MedlinePlus).

Low fasting glucose

If you don't have diabetes, low blood glucose is not common. When it happens, possible causes include liver or kidney disease, underactive adrenal or thyroid glands, certain pituitary disorders, malnutrition and certain medicines (MedlinePlus). For some people, mainly those who are thin and young, a fasting level below 70 mg/dL may be normal (MedlinePlus).

If you have diabetes, low glucose can come from diabetes medicines, not eating enough, or more activity than usual. The NIDDK advises people with diabetes to treat a level below their target or less than 70 mg/dL (NIDDK).

Root causes and risk factors for high glucose

The NIDDK lists these risk factors for prediabetes (NIDDK):

Having overweight or obesity, and not being physically active
Older age and a family history of diabetes
Being African American, American Indian, Asian American, Hispanic, Latino or Pacific Islander
Smoking or being exposed to secondhand smoke
A history of gestational diabetes, or conditions such as polycystic ovary syndrome

The NIDDK notes that insulin resistance, type 2 diabetes and high triglycerides make fatty liver more likely (NIDDK). See our ALT guide and cholesterol guide.

Plant-based steps to discuss with your doctor

According to the NIDDK, healthy living, including healthy eating, physical activity, weight management and enough sleep, may help prevent or reverse insulin resistance and prediabetes (NIDDK). Plant-based ideas that fit that advice:

Build meals around whole plant foods: beans, lentils, whole grains, vegetables, fruit, nuts and seeds.
Choose whole grains such as oats, brown rice and whole wheat over refined white flour and white rice.
Swap sugary drinks for water or unsweetened tea.
Move more in ways your doctor agrees are safe, such as a short walk after meals.
Protect your sleep and don't smoke.

Supplements: what NIH says

NCCIH says that for a few supplements there is weak evidence of possible benefit for blood sugar, and that most studies have limitations (NCCIH):

Cinnamon: a 2019 review of 16 studies found cinnamon supplements helped reduce fasting glucose in people with prediabetes and type 2 diabetes, but the studies varied and more research is needed. Doses below 6 grams a day don't appear to pose a health risk. Cassia cinnamon contains coumarin, which might worsen liver disease in sensitive people (NCCIH, NCCIH cinnamon).
Chromium: a 2022 review suggested it may help fasting glucose and A1C in type 2 diabetes. Large doses have been linked to rare reports of kidney, liver and muscle problems (NCCIH).
Berberine: may help, but can cause digestive side effects, may interact with medicines, and should not be used during pregnancy or breastfeeding (NCCIH).

Supplements are not a replacement for diabetes care. Combining herbal supplements with diabetes medicines can cause unwanted effects, so tell your providers about anything you take (NCCIH, NCCIH).

Medicines and safety notes

Don't stop or change any diabetes medicine on your own. If you take glucose-lowering medicine, big diet changes or new supplements can lower your blood sugar, so talk with your prescriber first.
If you have diabetes and your glucose drops below 70 mg/dL, the NIDDK advises eating or drinking 15 to 20 grams of glucose or carbohydrates right away, such as four glucose tablets or half a cup of regular fruit juice, then waiting 15 minutes and checking again (NIDDK). Follow the plan your care team gave you.

When to talk to your doctor

Your fasting glucose is 100 mg/dL or higher, or your lab flags it.
You have frequent low readings, or symptoms of low blood glucose such as feeling shaky, hungry, dizzy, confused or irritable (NIDDK).
You have symptoms of high blood glucose such as feeling thirsty, tired or weak, headaches, urinating often or blurred vision (MedlinePlus).
You feel very unwell, confused or faint, or someone with diabetes passes out. Call 911 or seek emergency care.

Questions to ask at your next appointment

Was I truly fasting for this test, and should we repeat it?
Should I also have an A1C or glucose tolerance test?
Could a medicine or another condition be affecting my glucose?
Which food and activity changes would help me most?
When should I retest?

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Frequently asked questions

What is a normal fasting glucose level?

The NIDDK lists a fasting plasma glucose of 99 mg/dL or below as normal, 100 to 125 mg/dL as prediabetes, and 126 mg/dL or above as diabetes. MedlinePlus describes 70 to 99 mg/dL as normal.

How long do I need to fast before a glucose test?

The NIDDK says to fast for at least 8 hours, with nothing to eat or drink except sips of water. The test is usually done in the morning.

Does one high fasting glucose mean I have diabetes?

Not necessarily. Providers usually repeat the test or order an A1C or glucose tolerance test before making a diagnosis. Stress, illness and some medicines can also raise glucose.

Can prediabetes be reversed?

According to the NIDDK, healthy living, including healthy eating, physical activity, weight management and enough sleep, may help prevent or reverse insulin resistance and prediabetes.

Does cinnamon lower blood sugar?

NCCIH says some studies suggest cinnamon supplements may modestly lower fasting glucose, but the evidence is weak and more research is needed. Cassia cinnamon contains coumarin, which might worsen liver disease. Talk with your doctor, especially if you take diabetes medicine.

Sources

Medical disclaimer: This article is general education, not medical advice. It does not diagnose, treat, cure or prevent any disease. Always talk with your doctor about your lab results, and talk with your prescriber before starting, stopping or changing any medicine or supplement. If you think you have a medical emergency, call 911. Last updated October 20, 2026.