A good fasting blood sugar is below 100 mg/dL. That’s the threshold the American Diabetes Association uses to define a normal result. Between 100 and 125 mg/dL falls into the prediabetes range, and 126 mg/dL or higher on two separate tests indicates diabetes. But “normal” and “optimal” aren’t always the same thing, and several factors can shift your number on any given morning.
The Three Categories
Fasting blood sugar is measured after you haven’t eaten for at least 8 hours, with only sips of water allowed. The result places you in one of three categories:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
A single reading above 126 mg/dL doesn’t confirm diabetes on its own. Your doctor will typically repeat the test or order an A1C blood test, which reflects your average blood sugar over the previous two to three months. An A1C below 5.7% is normal, 5.7% to 6.4% signals prediabetes, and 6.5% or above indicates diabetes.
Why Lower Within Normal May Be Better
A fasting glucose of 98 mg/dL and one of 78 mg/dL both count as “normal,” but they may not carry the same long-term risk. A study published in the Journal of the American Heart Association tracked over 20,000 adults whose fasting glucose stayed within the normal range throughout the study. Even among these people, those whose levels trended toward the higher end of normal (around 86 to 98 mg/dL) had a 40% greater risk of developing coronary heart disease compared to the group that stayed at the lower end.
This doesn’t mean a reading of 95 is dangerous. It means that within the “normal” bucket, lower numbers are generally associated with better cardiovascular outcomes over time. Many clinicians informally consider a fasting glucose in the 70 to 90 mg/dL range to be the sweet spot for long-term metabolic health.
Fasting Glucose During Pregnancy
If you’re pregnant, the targets are tighter. The American Diabetes Association recommends a fasting glucose between 70 and 95 mg/dL for managing gestational diabetes, along with levels under 140 mg/dL one hour after meals and under 120 mg/dL two hours after meals. These stricter thresholds exist because even mildly elevated blood sugar during pregnancy can affect both the mother and baby, increasing the risk of complications like high birth weight and preeclampsia.
Targets for Older Adults
For adults over 65 who are otherwise healthy and independent, blood sugar targets are the same as for younger adults. But for older people managing multiple health conditions or living with frailty, guidelines from Diabetes Canada recommend relaxing those targets to reduce the risk of blood sugar dropping too low. A fasting glucose up to about 144 mg/dL (8 mmol/L) may be acceptable for someone who is functionally dependent, and up to 162 mg/dL (9 mmol/L) for someone who is frail or living with dementia. The priority shifts from hitting perfect numbers to avoiding dangerous lows, which can cause falls, confusion, and hospitalization.
Why Your Morning Number Can Be Misleading
You might fast properly, sleep well, and still wake up with a fasting glucose that seems higher than expected. One common reason is the dawn phenomenon. Between roughly 3 a.m. and 8 a.m., your body releases a surge of cortisol and growth hormone. These hormones signal your liver to push extra glucose into your bloodstream, giving you the energy to wake up. In people with insulin resistance or diabetes, this early-morning spike isn’t counterbalanced effectively, so the fasting number reads higher than it would at, say, midnight.
Certain medications also raise fasting glucose. Corticosteroids like prednisone are among the most common culprits. They block insulin’s ability to move sugar from the blood into cells and simultaneously trigger the liver to release extra glucose. The effect is dose-dependent, becoming more pronounced at higher doses. Certain blood pressure medications, antipsychotics, and statins can also nudge fasting numbers upward.
Stress, poor sleep, and even a very high-fat dinner the night before can temporarily elevate your morning reading. If your number lands higher than expected on a single test, the context matters as much as the number itself.
How Fasting Glucose Relates to A1C
Fasting glucose gives you a snapshot of one moment. A1C gives you the trend over two to three months. The two don’t always line up neatly. Data from the National Glycohemoglobin Standardization Program shows that an A1C of 5% corresponds to an average blood sugar of about 97 mg/dL, while an A1C of 6% corresponds to roughly 126 mg/dL. But fasting glucose alone tends to underestimate what your A1C will show, especially as blood sugar control worsens. That’s because fasting glucose misses the spikes that happen after meals, which A1C captures.
For this reason, a normal fasting glucose doesn’t guarantee a normal A1C. If you have risk factors for diabetes (family history, excess weight around the midsection, a history of gestational diabetes, or a sedentary lifestyle), getting an A1C test alongside a fasting glucose gives a more complete picture.
What Pushes Fasting Glucose Up Over Time
Fasting blood sugar rises when your body becomes less efficient at using insulin. This process, called insulin resistance, develops gradually. Your pancreas produces insulin to help cells absorb glucose, but over time, cells stop responding as well. The pancreas compensates by producing more insulin, which works for a while. Eventually it can’t keep up, and glucose starts accumulating in the blood.
The factors that accelerate this process are well established: carrying excess weight (particularly visceral fat around the organs), physical inactivity, chronically poor sleep, and diets high in refined carbohydrates and added sugars. Genetics play a role too. Some people develop insulin resistance at a lower body weight than others, and certain ethnic groups, including South Asian, Hispanic, Black, and Native American populations, face higher risk at the same BMI.
The encouraging part is that prediabetes is reversible for many people. Losing 5% to 7% of body weight and getting about 150 minutes of moderate physical activity per week has been shown to cut the risk of progressing to type 2 diabetes by more than half. Even small improvements in fasting glucose, dropping from 110 to 100 mg/dL for instance, reflect meaningful metabolic changes happening under the surface.