Intermittent Fasting and Insulin Resistance: What the Research Shows

Quick answer: Intermittent fasting consistently improves insulin sensitivity in research studies, primarily by giving your body extended periods of low insulin levels. This allows cells to resensitize to insulin's signal. The effect is particularly significant for people with prediabetes or metabolic syndrome.

Insulin resistance is not just a concern for diabetics. It is arguably the central metabolic dysfunction of our time, underlying type 2 diabetes, obesity, cardiovascular disease, polycystic ovary syndrome, and non-alcoholic fatty liver disease. An estimated 40% of American adults aged 18-44 have some degree of insulin resistance (Li et al., JAMA Internal Medicine, 2024).

And one of the most effective levers for improving it may be something remarkably simple: not eating for a while.

How Insulin Works (And How It Breaks)

Insulin is a hormone produced by beta cells in your pancreas. When you eat — particularly carbohydrates and, to a lesser extent, protein — blood glucose rises and insulin is released. Insulin tells your cells to open up and absorb that glucose for energy or storage.

In a healthy system, this works efficiently. Glucose arrives, insulin signals, cells respond, blood sugar normalizes.

Insulin resistance occurs when cells become less responsive to insulin's signal. The pancreas compensates by producing more insulin. Blood insulin levels rise — a condition called hyperinsulinemia — and for a while, blood sugar stays normal because the extra insulin forces glucose into cells.

But the system is under strain. Eventually, the pancreas cannot keep up. Blood sugar rises. Prediabetes develops. Left unaddressed, type 2 diabetes follows.

The critical insight: insulin resistance is driven not just by what you eat but by how often and how constantly insulin is elevated. Dr. Jason Fung, nephrologist and fasting researcher, has argued extensively that hyperinsulinemia is both the cause and consequence of insulin resistance — a vicious cycle that requires lowering insulin levels to break (Fung, The Obesity Code, 2016).

What Fasting Does to Insulin Levels

When you stop eating, insulin drops. This is not controversial — it is basic physiology.

Within 12-16 hours of fasting, insulin levels fall significantly. With insulin low, several things happen:

Cells get a break. Without constant insulin bombardment, insulin receptors can resensitize. Think of it like turning down loud music — after a while, you can hear normal conversation again.

Fat burning activates. Low insulin allows hormone-sensitive lipase to mobilize stored fat for energy. This is why fasting is particularly effective for reducing visceral belly fat, which is closely tied to insulin resistance.

Hepatic glucose output normalizes. The liver, which becomes insulin resistant early in the disease process, begins to respond more appropriately when insulin levels are allowed to fall.

The Research on Fasting and Insulin Sensitivity

The evidence base here is substantial and growing.

Halberg et al. (Journal of Applied Physiology, 2005) studied healthy young men doing alternate-day fasting for 15 days. Insulin-mediated glucose uptake improved significantly, and insulin levels during a glucose tolerance test dropped — both markers of improved insulin sensitivity.

Sutton et al. (Cell Metabolism, 2018) conducted one of the most well-designed studies to date. Men with prediabetes followed early time-restricted eating (eating only between 8 AM and 2 PM) for five weeks. Even without weight loss, insulin sensitivity improved, beta cell function improved, blood pressure dropped, and oxidative stress decreased. This is significant because it isolated the timing effect from calorie reduction.

Harvie et al. (International Journal of Obesity, 2011) compared a 5:2 fasting approach (two days of very low calorie intake per week) to continuous daily calorie restriction in overweight women. Both groups lost similar weight, but the fasting group showed greater improvements in insulin sensitivity and larger reductions in waist circumference.

Cienfuegos et al. (Cell Metabolism, 2022) found that both 4-hour and 6-hour time-restricted eating windows reduced insulin resistance in obese adults over eight weeks compared to controls, even after adjusting for weight loss.

The pattern across studies is clear: giving the body extended periods without food improves the way it handles insulin and glucose.

The Visceral Fat Connection

Insulin resistance and visceral fat — the deep abdominal fat surrounding organs — are intimately connected. Visceral fat is metabolically active tissue that secretes inflammatory cytokines and free fatty acids, both of which worsen insulin resistance.

Fasting appears particularly effective at targeting visceral fat. Varady et al. (Nutrition Journal, 2013) found that alternate-day fasting reduced visceral fat more effectively than heart-healthy diets with the same calorie deficit. The mechanism likely involves the hormonal environment created by fasting — low insulin, elevated growth hormone, increased norepinephrine — which favors mobilization of visceral stores.

Reducing visceral fat improves insulin sensitivity, which further reduces visceral fat accumulation. This is the virtuous cycle that fasting can initiate.

Fasting and Type 2 Diabetes

For people already diagnosed with type 2 diabetes or prediabetes, the research is particularly relevant — and requires particular caution.

Furmli et al. (BMJ Case Reports, 2018) published case reports of three patients with type 2 diabetes of 10-25 years' duration who used therapeutic fasting (24-42 hour fasts, three times per week) under medical supervision. All three were able to discontinue insulin therapy within one month. Two were able to stop all diabetes medications.

Carter et al. (JAMA Network Open, 2018) conducted a 12-month randomized controlled trial comparing 5:2 fasting to continuous energy restriction in adults with type 2 diabetes. Both approaches produced similar improvements in HbA1c (a measure of long-term blood sugar control), but the fasting group reported greater satisfaction and adherence.

These results are promising, but a critical caveat applies: people with diabetes who take insulin or sulfonylureas must work with their doctor before fasting. These medications lower blood sugar, and combining them with fasting can cause dangerous hypoglycemia. Medication adjustments are typically necessary and should be managed by a healthcare provider.

The Role of Hormones Beyond Insulin

Insulin does not operate in isolation. Fasting affects an entire hormonal cascade that influences glucose metabolism.

Glucagon rises during fasting, promoting glycogen breakdown and gluconeogenesis. This is normal and helps maintain blood sugar during fasts.

Growth hormone increases significantly, which preserves lean mass and promotes fat utilization. This hormonal shift is part of why fasting does not cause the muscle loss people fear.

Cortisol rises modestly during fasting. While chronically elevated cortisol worsens insulin resistance, the acute, transient rise during a fast is a normal physiological response that supports glucose availability without pathological effects.

Adiponectin — a hormone secreted by fat cells that improves insulin sensitivity — tends to increase with intermittent fasting, particularly as visceral fat decreases (Bhutani et al., Obesity, 2010).

Practical Recommendations for Insulin Sensitivity

If improving insulin sensitivity is your primary goal, the research suggests several strategies:

Earlier eating windows are better. The Sutton et al. (2018) study specifically demonstrated benefits from eating earlier in the day. This aligns with circadian biology — insulin sensitivity is naturally higher in the morning. Eating your main meals earlier and stopping food intake by mid-afternoon or early evening may maximize insulin-sensitizing effects.

Consistency matters more than duration. A daily 16:8 protocol maintained over months is likely more beneficial for insulin sensitivity than occasional 24-hour fasts. The regular cycling of insulin levels appears to be what drives resensitization.

What you eat still matters. Fasting improves insulin sensitivity, but breaking your fast with a large bolus of refined carbohydrates will spike insulin dramatically. Prioritizing protein, fiber, and healthy fats when you eat supports the metabolic improvements fasting provides.

Reduce refined carbohydrates in your eating window. Combining intermittent fasting with a lower glycemic load diet creates synergistic improvements in insulin sensitivity.

Incorporate movement. Even moderate walking after meals significantly improves glucose clearance. Exercise during the fasted state further enhances insulin sensitivity through AMPK activation.

How Fasted Helps

Tracking your fasting consistency is critical when the goal is improving insulin sensitivity — it is the regular cycling of insulin levels that drives results. Fasted lets you set and track your preferred fasting schedule, monitor your streaks, and log your meals so you can identify patterns between what you eat, when you eat, and how you feel. The weight tracking feature helps you monitor the visceral fat reduction that goes hand-in-hand with improved metabolic health.

Frequently Asked Questions

How quickly does fasting improve insulin sensitivity?

Improvements can occur rapidly. The Sutton et al. (2018) study found measurable improvements in insulin sensitivity within five weeks, and some studies show changes in insulin levels within days. However, sustained improvement — particularly for people with significant insulin resistance — requires consistent practice over months.

Can fasting cause low blood sugar?

In healthy individuals, the body tightly regulates blood sugar during fasting through gluconeogenesis and glycogen breakdown. Symptomatic hypoglycemia during intermittent fasting is uncommon in people who do not take diabetes medications. If you experience shakiness, confusion, or dizziness during fasting, consult a healthcare provider.

Is intermittent fasting better than medication for insulin resistance?

This is a conversation to have with your doctor. For prediabetes, lifestyle interventions — including intermittent fasting — may be sufficient. For established type 2 diabetes, fasting can be a powerful complement to medical treatment but should not replace medication without medical supervision. Some patients have been able to reduce or eliminate medications under their doctor's guidance.

Does the type of fasting matter for insulin sensitivity?

Both time-restricted eating (16:8, 18:6) and periodic longer fasts (5:2, alternate-day fasting) have shown benefits. Early time-restricted eating — where you eat earlier in the day — has the strongest evidence for insulin sensitivity specifically. The best protocol is one that you can maintain consistently.

What breaks a fast in terms of insulin response?

Any caloric intake will stimulate some insulin release. Protein and carbohydrates provoke the largest insulin responses. Fat produces a minimal insulin response. Black coffee, plain tea, and water do not significantly affect insulin levels and are generally considered acceptable during fasting for metabolic purposes.

What to Read Next

This article is for informational purposes only and does not constitute medical advice. People with diabetes, hypoglycemia, or other metabolic conditions should consult their healthcare provider before starting any fasting protocol. Never adjust diabetes medications without medical supervision.