Intermittent fasting can temporarily raise blood sugar during the fasting window itself, even though it tends to improve blood sugar control over weeks and months. This surprises many people who check their glucose while fasting and see higher numbers than expected. The explanation lies in how your body fuels itself when food isn’t coming in.
Why Blood Sugar Rises When You’re Not Eating
When you stop eating for several hours, your body doesn’t just sit quietly waiting for the next meal. It actively works to keep your blood sugar stable, because your brain depends on a steady glucose supply. The main tool it uses is your liver, which stores glucose in a compact form called glycogen and releases it into your bloodstream as needed.
As a fast stretches past 12 to 16 hours, those glycogen stores start running low, and your liver shifts to manufacturing new glucose from scratch, using raw materials like amino acids and the byproducts of fat breakdown. In controlled studies, this glucose-manufacturing process accounted for about 50% of the liver’s glucose output at the 16-hour mark and rose to roughly 62% by 24 hours. At the same time, total glucose output from the liver actually decreases as the fast lengthens, but the proportion coming from new production climbs steadily.
Cortisol plays a key role here. Fasting activates your body’s stress-response system, and cortisol levels rise to help maintain blood sugar. Cortisol signals the liver to ramp up glucose production and shifts your muscles toward burning fat instead, preserving glucose for the brain. A systematic review confirmed that severe caloric restriction and fasting acutely elevate cortisol in humans. This is a normal, protective response, but it can push your blood sugar readings higher than you’d expect on an empty stomach.
The Dawn Phenomenon Makes Mornings Worse
If you’re checking your blood sugar first thing in the morning after an overnight fast, you may be running into a well-known pattern called the dawn phenomenon. Between roughly 3 and 8 a.m., your body releases a surge of cortisol and growth hormone to prepare you for waking up. These hormones tell your liver to dump glucose into your bloodstream. In people without diabetes, insulin rises to match, keeping things balanced. In people with diabetes or insulin resistance, that compensating insulin response falls short, and morning readings come in high.
About half of people with type 1 or type 2 diabetes experience the dawn phenomenon, according to the American Diabetes Association. If you’re practicing intermittent fasting and skipping breakfast, you’re extending the window during which this hormonal surge goes unopposed by incoming food. That can make the morning spike more noticeable on a glucose monitor, even though nothing has gone wrong metabolically.
Physiological Insulin Resistance During Fasting
There’s another layer to this that’s worth understanding. During a fast, your muscles deliberately become less responsive to insulin. This isn’t the same harmful insulin resistance that develops with type 2 diabetes. It’s a temporary, protective adaptation. When fatty acids flood the bloodstream during fasting (your body breaking down fat for fuel), muscles reduce their glucose uptake so that the limited glucose supply gets reserved for the brain.
Researchers describe this as “glucose sparing.” Your heart and skeletal muscles switch to burning fat, and they become transiently insulin resistant to enforce that switch. If you were to take a glucose tolerance test during this state, your results might look worse than usual, not because your metabolic health has declined, but because your body is in conservation mode. This effect reverses once you eat again and insulin levels normalize.
The important distinction: in pathological insulin resistance, both glucose and fatty acids flood tissues simultaneously, creating cellular damage. In fasting-related insulin resistance, the body is deliberately routing different fuels to different tissues. One is protective, the other is harmful.
Long-Term Effects on Blood Sugar Control
Despite the short-term glucose bumps during fasting windows, the overall trajectory for most people points downward. A meta-analysis of four clinical trials involving 280 participants with type 2 diabetes found that intermittent fasting reduced HbA1c (a measure of average blood sugar over two to three months) by 1.85 percentage points compared to control groups. That’s a clinically meaningful improvement.
Across a broader set of 12 studies, people taking oral diabetes medications saw an average HbA1c reduction of 0.54%, while those on insulin saw reductions averaging 2.8%. Even in longer trials lasting a full year, the benefits held up. One 52-week study showed a 0.2% HbA1c decrease with a fasting-mimicking diet, while another 52-week trial found reductions of 0.3% to 0.5% depending on the fasting pattern used.
The American Diabetes Association’s 2025 Standards of Care acknowledges intermittent fasting and time-restricted eating as generally safe for adults with type 1 or type 2 diabetes, noting that these approaches “may be useful strategies for people with diabetes who are looking for practical eating management tools.” The key caveat: people taking insulin or medications that stimulate insulin release should be medically monitored during fasting periods because of the risk of blood sugar dropping too low.
Dehydration Can Skew Your Readings
If you’re not drinking enough water during your fasting window, that can also nudge blood sugar numbers upward. Dehydration triggers the release of a hormone called vasopressin, which among other things stimulates cortisol production and increases the liver’s glucose output. A systematic review found that while dehydration doesn’t significantly change insulin levels, it may increase glucagon (a hormone that raises blood sugar), with one study showing a 32% jump in glucagon after just 1% body mass loss from fluid deficit.
The practical effect on glucose readings appears modest in otherwise healthy people, but for someone already dealing with insulin resistance, even a small nudge matters. Staying well hydrated with water, black coffee, or unsweetened tea during your fasting window can help keep readings more accurate and stable.
What Higher Readings Actually Mean for You
If you’re monitoring your blood sugar during intermittent fasting and seeing numbers that seem too high, context matters. A reading of 95 to 110 mg/dL during a 16-hour fast doesn’t carry the same meaning as that same reading after a meal. Your body is in a fundamentally different metabolic state, actively producing and rationing glucose rather than processing incoming food.
True hyperglycemia during fasting, the kind worth paying attention to, comes with noticeable symptoms: extreme thirst, frequent urination, unusual tiredness, nausea, or dizziness. These signal that blood sugar has climbed well above normal ranges and your body can’t bring it back down. By contrast, if your fasting glucose is mildly elevated but you feel fine, you’re likely seeing the normal hormonal and metabolic adjustments described above.
It’s also worth distinguishing this from the opposite problem. Low blood sugar during fasting produces a distinctly different set of symptoms: a racing pulse, cold sweats, shakiness, sudden intense hunger, and difficulty concentrating. If you experience these, breaking your fast with a small amount of food is the right move. People with diabetes who experience overnight lows may also see a rebound high in the morning, known as the Somogyi effect, where the body overcompensates for the nighttime drop by flooding the bloodstream with glucose before you wake.
Making Sense of Your Numbers
The pattern most people experience with intermittent fasting is straightforward: blood sugar may run slightly higher during the fasting window (especially in the morning), drop after the first meal, and trend lower over weeks to months as insulin sensitivity improves and body weight decreases. The temporary spikes are a feature of normal physiology, not a sign that fasting is backfiring.
If you’re using a continuous glucose monitor, keep in mind that these devices can be less accurate in the low glucose range, with one pivotal study reporting a mean error rate of about 9%. During prolonged fasts, CGMs may overreport low readings, which can be confusing when paired with the occasional high readings from cortisol-driven glucose production. Fingerstick checks can help verify unusual numbers.
For people without diabetes, the short-term glucose fluctuations during fasting are self-correcting and rarely cause problems. For people with diabetes, the same fluctuations are generally manageable but require closer attention, particularly around medication timing and the risk of overnight lows that trigger morning rebound highs.