Yes, fasting lowers blood glucose levels. When you stop eating, your body burns through its available sugar within hours, and blood glucose drops as a direct result. How much it drops, how quickly, and whether that’s beneficial depends on your metabolic health, how long you fast, and whether you take diabetes medications.
What Happens to Blood Sugar When You Stop Eating
After your last meal, your body first uses the glucose circulating in your bloodstream. Once that supply runs low, your liver steps in by breaking down its stored form of sugar, called glycogen. A liver can hold enough glycogen to keep your blood sugar stable for a while, but after roughly 12 to 18 hours of fasting, those stores are nearly depleted.
At that point, your body shifts to a backup process: manufacturing small amounts of new glucose from non-sugar sources like amino acids and glycerol. This keeps blood sugar from crashing to dangerous levels, but the overall trend during a fast is downward. Insulin levels fall, and your cells increasingly burn fat for fuel instead of sugar. This is the basic metabolic sequence that makes fasting effective at reducing glucose.
How Much Glucose Drops in Practice
For context, normal fasting blood glucose is below 100 mg/dL. Prediabetes falls between 100 and 125 mg/dL, and diabetes is diagnosed at 126 mg/dL or higher.
If you’re healthy, an overnight fast of 8 to 12 hours typically brings your blood sugar into that normal range without any effort. The more interesting question is what happens for people with elevated blood sugar or type 2 diabetes who use fasting as a deliberate strategy.
A 2021 review of intermittent fasting studies found an average reduction in fasting glucose of about 4 mg/dL across participants. That sounds modest, but the results vary widely depending on the fasting protocol. In a randomized controlled trial by Che and colleagues, 60 participants with diabetes or prediabetes followed a 10-hour eating window (eating only between 8 a.m. and 6 p.m.) for 12 weeks. Their fasting glucose dropped by 15%, and their HbA1c, a measure of average blood sugar over three months, fell by 18%. That HbA1c reduction was nearly double what typical diabetes medication achieves.
Fasting Improves Insulin Sensitivity Too
Lower glucose readings during a fast are partly just the absence of incoming food. But fasting also appears to make your cells more responsive to insulin over time, which means your body handles sugar better even after you start eating again.
One clinical trial measured insulin resistance using a standard index called HOMA-IR. After 26 weeks of intermittent fasting, participants saw their insulin resistance score drop by about 1.0 point on average, while the control group’s score actually worsened by 0.6 points. That’s a meaningful swing. A broader systematic review found similar improvements: fasting reduced insulin levels and insulin resistance scores alongside the glucose reductions.
This is what separates fasting from simply skipping a meal. The repeated cycles of low insulin appear to recalibrate how your body processes sugar, not just during the fast itself but in the hours and days afterward.
Long-Term Effects on Blood Sugar Control
HbA1c reflects your average blood sugar over two to three months, so improvements there signal a lasting change rather than a temporary dip. A meta-analysis pooling four trials with 280 participants found that intermittent fasting produced an average HbA1c reduction of 1.85 percentage points compared to controls. People on insulin saw even larger drops, averaging 2.8 percentage points, while those on oral diabetes medications saw a 0.54 percentage point reduction.
To put that in perspective, most diabetes medications aim for an HbA1c reduction of 0.5 to 1.0 percentage points. Fasting, at least in these trials, often matched or exceeded that benchmark, particularly for people with more severe blood sugar dysregulation.
When Fasting Can Raise Blood Sugar Instead
There’s an important exception that catches many people off guard. Between roughly 3 a.m. and 8 a.m., your body releases cortisol and growth hormone to prepare you for waking up. These hormones tell your liver to push out more glucose. In a healthy person, the pancreas responds by releasing enough insulin to keep things balanced.
If you have diabetes or significant insulin resistance, your body may not produce enough insulin (or respond to it well enough) to counteract that early-morning glucose surge. This is called the dawn phenomenon, and it’s the reason some people with diabetes see their highest blood sugar readings first thing in the morning, even after fasting all night. If you’re checking your fasting glucose and finding it stubbornly high in the morning despite not eating, this hormonal pattern is the likely explanation.
Fasting Risks With Diabetes Medications
If you take medications that actively lower blood sugar, fasting introduces a real risk of hypoglycemia, where glucose drops too low. Two categories of medications are especially concerning. Sulfonylureas, a common class of oral diabetes drugs, stimulate insulin release regardless of whether you’ve eaten, so combining them with an extended fast can push blood sugar dangerously low. Insulin carries the same risk, particularly premixed formulations that combine long-acting and short-acting types.
People with type 2 diabetes who want to try intermittent fasting often need their medication doses adjusted. During Ramadan fasting studies, for instance, patients on premixed insulin were switched to different formulations to reduce hypoglycemic episodes. The timing and dose of sulfonylureas were also modified. This isn’t something to experiment with on your own, since a severe low blood sugar episode can cause confusion, seizures, or loss of consciousness.
What This Means for You
For most people, fasting reliably lowers blood glucose. An overnight fast of 12 or more hours depletes liver glycogen, drops insulin levels, and brings blood sugar down. Practiced regularly as intermittent fasting or time-restricted eating, this pattern can improve insulin sensitivity and lead to sustained reductions in both fasting glucose and long-term blood sugar markers like HbA1c.
The people most likely to benefit are those with prediabetes or type 2 diabetes who have room to improve their blood sugar control. The people who need the most caution are those on insulin or sulfonylureas, where the combination of medication and no food intake can overcorrect blood sugar in a dangerous direction. And if your morning fasting readings seem paradoxically high, the dawn phenomenon is a well-known explanation that has nothing to do with what you ate the night before.