Fasting and Blood Sugar: What the Research Really Shows

Fasting lowers blood sugar by reducing insulin demand. Within 12-24 hours, insulin levels fall and insulin sensitivity improves. A 2019 Cell Metabolism trial by Sutton et al. found early time-restricted feeding improved insulin sensitivity after five weeks. Prediabetes intervention studies report HbA1c reductions of 1-2%. Blood sugar management requires medical supervision.

Of all the metabolic benefits attributed to fasting, the impact on blood sugar is the most studied and the most directly relevant to the 96 million Americans with prediabetes and the 38 million with type 2 diabetes. But fasting is also one of the most double-edged interventions in metabolic health — profoundly helpful for many, potentially dangerous for some. Here's what the research actually shows.

How fasting affects glucose and insulin

Every time you eat, carbohydrates are broken down into glucose, which enters the bloodstream. In response, the pancreas releases insulin — the hormone that shuttles glucose into cells. Frequent eating means frequent insulin spikes; over time, cells can become less responsive, a state called insulin resistance that underlies prediabetes and type 2 diabetes.

Fasting interrupts this cycle. Without incoming food, insulin levels fall. The body draws on liver glycogen for glucose, then shifts to fat oxidation. Lower average insulin means cells can recover sensitivity to the signal. The de Cabo and Mattson NEJM review describes intermittent fasting as a metabolic switch that improves glucose regulation and insulin sensitivity across multiple mechanisms.

During fasting, the depletion of hepatic glycogen stores triggers a metabolic switch toward fatty acid and ketone body oxidation, which improves glucose regulation and insulin sensitivity.— de Cabo & Mattson, New England Journal of Medicine, 2019

This is the central mechanism by which fasting helps blood sugar: by lowering the average insulin exposure across the day, cells recover responsiveness. The beta cells of the pancreas — which produce insulin — also get a rest. In people with insulin resistance, beta cells are often overworked trying to produce enough insulin to overcome the resistance. Periods of low insulin demand give them recovery time, which is one reason fasting improves markers of beta-cell function (like the homeostasis model assessment of beta-cell function, HOMA-%B) in studies like Sutton et al.

What happens to blood sugar during a fast

A common misconception is that fasting causes blood sugar to crash. In metabolically healthy people, it doesn't. The body maintains blood glucose within a narrow range through several mechanisms:

• Hours 0-12: Blood glucose is maintained from recently absorbed food, then from liver glycogen (glycogenolysis).
• Hours 12-24: Glycogen is depleting; the liver begins producing glucose from non-carbohydrate sources (gluconeogenesis) using amino acids, lactate, and glycerol.
• Hours 24+: Gluconeogenesis continues, but the brain increasingly runs on ketones produced from fat. Blood glucose stays in a healthy range.

In metabolically healthy people, fasting glucose typically stays between 70-90 mg/dL during a fast. The body is remarkably good at maintaining this — your liver is essentially a glucose thermostat.

Fasting and insulin sensitivity

One of the strongest findings in the fasting literature is the improvement in insulin sensitivity. A 2019 study by Sutton et al. in Cell Metabolism tested early time-restricted feeding (eTRF) — eating between 8am and 2pm — in men with prediabetes. After five weeks, eTRF improved insulin sensitivity, reduced fasting insulin, and lowered blood pressure, without changing weight or total caloric intake.

Early time-restricted feeding improved insulin sensitivity, beta cell responsiveness, and blood pressure in men with prediabetes, independent of food intake and weight loss.— Sutton et al., Cell Metabolism, 2019

A 2021 review in the journal Nutrients surveyed the broader evidence and concluded that intermittent fasting — particularly time-restricted eating — shows consistent improvements in fasting insulin, HOMA-IR (a measure of insulin resistance), and HbA1c, especially in people with metabolic dysfunction.

Fasting for prediabetes and type 2 diabetes

For people with prediabetes or type 2 diabetes not on insulin or sulfonylureas, fasting can be a powerful adjunct — but only under medical supervision. Several small trials and case studies report meaningful improvements:

• Reductions in HbA1c of approximately 1-2% in some studies, often comparable to first-line medications.
• Reductions in fasting insulin and improvements in HOMA-IR.
• Weight loss, which independently improves insulin sensitivity.
• Reduced need for glucose-lowering medication in some patients (under medical supervision).

The Diabetes Prevention Program — the landmark NIH study — established that lifestyle intervention (diet, exercise, modest weight loss) reduces progression from prediabetes to type 2 diabetes by 58%. Fasting is a tool within that broader lifestyle approach, not a replacement for it.

Dawn phenomenon and the morning glucose rise

Many people who start fasting are confused to see higher morning glucose readings — even after fasting all night. This is the dawn phenomenon: a normal physiological surge in cortisol, growth hormone, and glucagon between roughly 4am and 8am that prompts the liver to release glucose. In people with insulin resistance, the dawn phenomenon is exaggerated.

A higher morning fasting glucose after starting fasting isn't necessarily a regression. It may reflect the dawn phenomenon, late-evening eating, poor sleep, or stress. Continuous glucose monitors (CGMs) are often the best way to understand what's actually happening across the full 24-hour cycle, not just at the morning snapshot.

CGM insights and practical monitoring

Continuous glucose monitors — small sensors that track interstitial glucose every few minutes — have transformed how people understand fasting. CGM data typically shows:

• Lower and more stable glucose during the fasting window (after adaptation).
• Lower postprandial (after-meal) spikes when meals are eaten earlier in the day — supporting the early time-restricted feeding findings.
• Highly individual responses to specific foods — the same carb can spike one person and barely move another.
• The dawn phenomenon clearly visible as a morning rise independent of eating.

If you use a CGM, focus on trends over days and weeks, not single readings. And remember that glucose is one metric — insulin, HbA1c, triglycerides, and inflammation markers all matter.

Fasting glucose vs. postprandial glucose — what's the difference?

Two glucose numbers tell different stories. Fasting glucose is measured after at least 8 hours without food — typically first thing in the morning. It reflects the body's baseline glucose regulation and the dawn phenomenon. Postprandial glucose is measured 1-2 hours after eating and reflects how well your body handles the carbohydrate load of a meal. Both matter for metabolic health, and both are independently predictive of long-term diabetes risk.

• Normal fasting glucose: under 100 mg/dL (5.0 mmol/L). Prediabetes: 100-125 mg/dL. Diabetes: 126 mg/dL or higher.
• Normal postprandial glucose (2 hours after eating): under 140 mg/dL. Prediabetes: 140-199 mg/dL. Diabetes: 200 mg/dL or higher.
• HbA1c: an average of glucose over the prior ~90 days. Normal: under 5.7%. Prediabetes: 5.7-6.4%. Diabetes: 6.5% or higher.

Fasting primarily improves fasting glucose, fasting insulin, and HbA1c — but postprandial glucose also improves when meals are spaced out and eaten earlier in the day (the early time-restricted feeding effect Sutton et al. demonstrated). The combination of lower average insulin across the day and fewer post-meal spikes is what drives the metabolic improvements seen in the research.

Who should be cautious about fasting

Fasting is not appropriate for everyone. The following groups should not fast, or only under close medical supervision:

• Type 1 diabetes — high risk of hypoglycemia and diabetic ketoacidosis without careful management.
• Anyone on insulin or sulfonylureas — hypoglycemia risk requires medical supervision and dose adjustment.
• Pregnant or breastfeeding women — glucose needs are different.
• People with a history of hypoglycemia — fasting can trigger episodes.
• People with a history of eating disorders — fasting can trigger relapse.
• Underweight individuals (BMI under 18.5).
• Children and adolescents still growing.

Even for people without these contraindications, fasting can interact with many common medications (blood pressure drugs, thyroid hormone, steroids). Always talk to your healthcare provider before starting — and use fasting as a complement to, not a replacement for, evidence-based medical care.

The bottom line

Fasting is one of the most evidence-backed tools for improving blood sugar regulation — but it is not benign. The 2019 Sutton et al. Cell Metabolism trial, the 2021 Nutrients review, and the de Cabo and Mattson NEJM review all converge on the same conclusion: intermittent fasting can meaningfully improve insulin sensitivity, fasting insulin, and HbA1c, especially in people with metabolic dysfunction. If you have diabetes, take glucose-lowering medication, or have a history of low blood sugar, do not fast without medical supervision. Stay hydrated, use electrolytes, and consider black coffee, plain tea, or yerba mate during the fasting window — none of which significantly raise insulin. The science is promising; the personal application requires care.